IGCSE Biology Term 4 Study Notes PDF

IGCSE Term 4 Biology Study Notes All about Inheritance Part 1 Gene- Carrying Chromosomes are in the Nuclei Long threads known as chromosomes can be found in all cell nuclei. These structures are very thin, making it difficult to visualise. An electron microscope is needed....

IGCSE Term 4 Biology Study Notes All about Inheritance Part 1 Gene- Carrying Chromosomes are in the Nuclei Long threads known as chromosomes can be found in all cell nuclei. These structures are very thin, making it difficult to visualise. An electron microscope is needed. A chromosome can be defined as a thread of DNA that is made up of a string of genes. A very long molecule of DNA is found in each chromosome. This DNA molecule carries a code of instructions telling the cell which type od protein the need to produce. There are many protein instructions that are carried by each chromosome. A gene refers to the part of the DNA molecule that codes for one protein. It is a unit of hereditary that is passes from one generation to the next. Genes are Unique to Each Specie Every specie of living organism has their own unique number and variety of genes.  We have 46 chromosomes. Many genes can be found on these chromosomes. The gene copy is the same in each of your cells. No person has the same sets of genes, unless you have an identical twin. Mitosis: Genetically Identical Daughter Cells are Produced The nuclei of the gametes (eggs cell and sperm cell) contain a completed single set of 23 chromosomes. We call them haploid cells because the chromosomes are still unpaired. A zygote forms when an egg cell and a sperm cell fuse or join. These nuclei contain two sets of chromosomes. We call then diploid cells because there are two sets, namely the chromosomes have paired. One chromosome of each pair has been inherited from the mother and one pair from the father. The two chromosomes in one pair are known as homologous chromosomes. The zygote cells divided to produce two new cells with identical set of chromosomes. All the new cells that continue to be produced are genetically identical.  This means that the new cells contain the same chromosomes sets as the original zygote cells. This type od cell division is known as mitosis. Mitosis can be defined as the division od the nuclei to form genetically identical cells where the number of chromosomes is maintained through the exact duplication of the chromosomes. Mitosis: Helps with growth and Asexual Reproduction Mitosis is used by all cells in plants and animals for growth and repair od damaged cells. Growth implies an increase in size. Growth takes place so that all parts of the cells obtain the optimal amount of oxygen and nutrition (trough diffusion). Growth also implies that the mass of the organism is increased. It is usually recorded as the dry mass. Young organisms grow as the develop. A zygote into an embryo. An embryo develops into a foetus. A foetus develops into an infant (baby) Each chromosome consists of two identical threads that have been joined together. The Division of Cells During Mitosis The two threads are known as chromatids. The centromere refers to the point where the chromatids are held together. Cell Division Through Meiosis Produces Gametes Gametes contain half the number of chromosomes compared to normal body cells. Gametes only contain one set of chromosomes. This occurs for a reason. The zygote formed when these two gametes join will have wo sets of chromosomes. Cell division in the ovaries and tests form the gametes. This cell division is meiosis. During meiosis is chromosomes are shared so that each cell only has one type. Meiosis is defined as reduction division as the number of chromosomes is halved diploid to haploid) One chromosome from the homologous chromosomes pair is inherited from the mother and the other chromosome is inherited from the father. Meiosis mixes these pairs. There are Two Copies of Each Gene in Each Cell Biologist believe there are approximately 20 000 human genes spread throughout our 46 chromosomes. The cells in our bodies contain identical genes. However, not all the genes are switched on. In other words, the genes do not send out their messages. Genes with the same characteristics are found in the same position on each chromosomes of the homologous pair. Proteins and Cystic Fibrosis Cystic fibrosis is the disease that results when the mucus production is abnormal. The mucus that is produced is much thicker and collects in the lungs. The build-up of mucus inhibits the movement of oxygen into the blood. In addition, this thick mucus is the ideal breeding ground for bacteria. Cystic fibrosis patients are therefore prone to lung infections. There are two variations for these proteins. The different variations of a gene are known as alleles. An allele can be defined as two or more alternate forms of a gene. The normal mucus- producing is represented as F and the abnormal mucus- producing protein is represented as f. There are therefore three possible combinations namely, FF, Ff and ff. The genotype FF and ff are known as homozygous because the alleles are the same. Homozygous can be defined as the position of two identical alleles of a gene. Heterozygous can be defined as the possession of two different alleles of a gene. There is no pure breeding when these two heterozygous individuals breed together. The Genotype Determines the Phenotype The genotype is determined as the genetic makeup of an individual based on the alleles that they possess.  The phenotype is defined as the physical features of an individual as prescribed by the genotype and the environment. Phenotype refers to your features or what you look like namely, eye colour and hair colour and so forth. Coordination and response in animals The nerves and hormones allow communication Changes in an organism’s environment is known as a stimulus (plural: stimuli). There are special cells in the body known as receptors that can sense these stimuli. The organism must respond to these stimuli, and they do that with the help of effectors. Muscles are one example of effectors because they respond by contracting. Another example of effectors is your glands. The stimulus is your sense of smell and the effector is your salivary glands secreting saliva. There needs to be fast and effective communication between the receptors and effectors, especially in animals. Animals need to be able to respond to their environment. A lion needs to respond to catching his food and the buck needs to respond to escaping the predator. To make sure that the right effector responds at the correct time, there needs to be communication between the receptor and effector. If you touch something hot, the pain receptors in your finger send a message (impulse) to your arm muscle, quickly pulling your hand away from the hot surface. Coordination is defined as the way in which the receptors pick up stimuli and then pass on the information to the effectors. In animals there are two ways to do this.  Fastest method → using the nerves. The receptors and the nerves form the nervous system Slower method → using hormones. These are chemicals that form part of the endocrine system Human nervous system The neurons carry the nerve impulses The nervous system consists of special cells known as neurons. Neurons are highly specializing cells that have adapted to carry messages very quickly. A nerve impulse travels through some parts of your body at 288km/h! The neurons are like the animal cells as they each have a nucleus, cytoplasm and a cell membrane. There are long, thin fibres of cytoplasm that stretch out from the cell body known as nerve fibres. The axon is the longest of the nerve fibres. These axons may be more than a metre in length. The shorter fibres are known as dendrons or dendrites. Electrical signals are picked up by the dendrons from other nearby neurons. The signal is then passed to the cell body and down the axon. From here, it might be passed on to another neuron. The myelinated neurons carry the impulses quicker Myelin refers to the layer of fat and protein that wraps around some of the nerve fibres.  There are some gaps that appear in this myelin sheath. Signals are in the form of electrical impulses. The myelin acts as an insulator for the nerve fibres. These nerves can then carry the message at a much faster rate. The myelinated nerve fibres in a cat’s body can transmit impulses at a speed of 100m per second. The nerve fibres that do not have myelin can only carry impulses at a speed of 5m per second. We have a central nervous system All mammals have a central nervous system (CNS) and a peripheral nervous system. o Central nervous system → include the brain and spinal cord o Peripheral nervous system → includes the nerves and receptors The CNS is made up of neurons. The function of the CNS is to coordinate all the messages (impulses) that are travelling through the nervous system. How the messaging works: 1. A message has been detected by the receptor 2. The receptor sends an electrical impulse to the brain and spinal cord 3. The brain and spinal cord receive this message 4. The brain and spinal cord send a new message along the relevant nerve fibre 5. This nerve fibre relays the message to the relevant effector Rapid response is achieved by a reflex arc You touch a hot plate on the stove, immediately an impulse is picked up by a sensory receptor in your finger. The impulse travels along the axon of the receptor cell to the spinal cord. This cell is known as a sensory neuron as it is carrying an impulse received from a sensory receptor. Once the impulse has reached the spinal cord, the impulse is relayed (passed along) to several other neurons known as relay neurons. They have this name because they relay all the impulses. The relay neurons are the neurons that pass the impulse to the brain. It is also their responsibility to pass the impulse to the effector. The effector in this example is your arm muscles. The impulse travels along the axon of the motor neuron to the muscle. The muscle reacts by contracting and pulling your hand away from the hot plate. This type of reaction is known as a reflex reaction. It is not necessary for you to think about it. Your brain has been made aware of the situation and you  only consciously realize what has happened after the impulse has been sent to the muscles. Reflex reactions are extremely helpful because the message in taken from the receptor to the effector very quickly. There is no time wasted by first thinking about it. The pathway that these neurons (sensory neurons, relay neurons and motor neurons) pass messages along is known as the reflex arc. Two examples of reflex actions: Knee-jerk reflex → a sharp tap is given just below the knee. The message moves along the sensory neuron to the spinal cord. From here it travels along a motor neuron to the thigh muscle. The muscle contracts and the lower leg is raised Iris reflex → the pupil of your eye adjusts to the brightness of the light. If it is bright, it becomes smaller. If it is dark, it becomes larger. Reflex actions are involuntary actions, as we do not have to think about doing them (not under conscious control). Action that we decide to do is known as voluntary actions. The receptors Receptors form part of the sense organs The part of the body that detects the stimuli (the receptors) may either be specialized cells or simply the endings of a sensory neuron. In animals these receptors generally form part of a sense organ, a group of receptor cells that respond to specific stimuli: ✓ Eyes → vision and light ✓ Ears → hearing and balance ✓ Skin → touch, temperature and pain ✓ Nose → smell and chemicals ✓ Tongue → taste ✓ An example of a sense organ is the eyes. The receptors inside the eyes are the rods and comes that form part of the retina. The eyes are protected  The receptor cells are in the rods and cones of the retina. The retina is sensitive to light. T The remaining parts of the eye are there to protect the retina and to focus the light. The eyes are set in a bony socket in the skull known as an orbit. Inside the orbit, the eyes are further protected by a very tough coat known as the sclera. The front of the eye is that only part that is not surrounded by bone. There is a thin, transparent membrane that covers the front of the eye known as the conjunctiva. This membrane protects all the parts of the eye that are behind it. The conjunctiva is constantly kept moist by a fluid produced in the tear glands. An enzyme known as lysozyme is present in this fluid and it kills any bacteria that may be present.  Every time you blink, this fluid is wiped across your eye. Dirt is prevented from landing in your eyes by the eyelids, eyebrows and eyelashes. Cells in the retina respond to light The retina is found at the back of the eye. Light falls on a receptor cell in the retina. This cell immediately sends an electrical impulse along the optic nerve to the brain. The brain begins to sort out all the impulses from each of the receptor cells and builds up an image. The closer the receptors cells are to one another, the clearer the image formed by the brain. The fovea is the part of the retina where the receptor cells are packed the closest. It is here where the light is focused when you look straight at an object. No receptor cells are found where the optic nerve leaves the retina. This part is known as the blind spot because no impulses will be sent to the brain if light falls here. There is a block layer called the choroid which is found behind the retina. The function of the choroid is to absorb all the light after it has been through the retina. This is to prevent it from scattering all around the inside of the eye. Its second function is to nourish the eye because it is rich in blood vessels. There are two types of receptors in the retina. The rods are sensitive to dim light, but unresponsive to colour. The cones can distinguish colours, but they function better in brighter light. Rods → can see in dim light, but only in black and white Cones → can see in bright light and give us coloured vision. The fovea contains mostly cones that are packed tightly together. Looking directly at an object, we use the cones to produce a sharp and colourful image. The rods are situated further out of the retina and are loosely packed. The iris controls how much light enters the eye The iris is a circular piece of tissue found in front of the lens. There are pigments in the iris that absorb the light, preventing it from reaching the retina. There is a gap in the middle of the iris, and this is known as the pupil. The pupil has an adjustable size. A wider pupil allows more light to reach the retina. If the light is strong, the iris closes in, making the pupil smaller. This prevents the light from damaging the retina. There are circular muscles in the iris that lie in circles around the pupil. This helps the pupil to adjust to the light. When the muscles contract, the pupil becomes smaller. There are also radial muscles that run outwards from the edge of the pupil. When the radial muscles contract the pupil dilates (gets bigger). This is known as the iris reflex. It may sometimes also be referred to as the pupil reflex. The responses of the iris are a reflex reaction. The nerve impulses do go to the brain, but we do have to think about it. This response is very quick because it must prevent the retina from being damaged if the light is too bright. Light is focused by the cornea and lens The brain will only see a clear image if there is a clear image on the retina. The light rays therefore need to be bent (refracted) so that they can focus exactly on the retina. The humours found within the eye are clear thus allowing the light to easily pass through them.  The cornea does most of the bending of the light while the lens does some minor adjustments. The brain then interprets the image and turns it the right way up. Focusing is adjusted by the lens The light rays that enter the eyes are bent at different degree. Light rays that are bouncing off a nearby object are diverging (going away from one another). So, to focus on the retina, they need to be bent inwards quite strongly. The shape of the lens can be adjusted to bend the light rays. The fuller (rounder) the lens, the more the light rays are bent. The thinner (flatter) the lens, the less the light rays will be bent. When the shape of the lens adjusts so that it can focus the light coming from a near or distant object is known as accommodation. The lens is held in place by a ring of suspensory ligaments. There are ciliary muscles that adjust the tension of the suspensory ligaments and this is what alters the shape of the lens. The suspensory ligaments are loosened when the ciliary muscles contract. The suspensory ligaments are pulled tight when the ciliary muscles relax. The lens is pulled thin (flattens) when the sensory ligaments are tight, and it becomes fuller (rounder) when the sensory ligaments are loosened. Homeostasis and Excretion Our internal environment is kept stable through homeostasis The surrounding environment in which a living organism live is always changing. The environment is always kept stable through a process known as homeostasis. It is crucial that the following remain constant: o Amount of tissue fluid surrounding our cells. o Amount of water. o Internal body temperature. o Glucose concentration.  Homeostasis can be defined as the maintenance of a constant internal environment. It is very important as it helps our cells to work as efficiently as possible. Reasons why homeostasis is so important: o Constant temperature allows for optimal working of enzymes. o A constant amount of water prevents cell damage through loss of water or excessive absorption (osmosis). o Glucose levels must be maintained as it is used as fuel for respiration. Homeostasis involves the nervous system, parts of the endocrine system and the kidneys. Controlling the body temperature Mammals and birds are both homoeothermic Some animals can control their body temperature. Their temperature remains constant, even though the outside temperature varies. Animals with this ability are known as homoeothermic animals. They may also be referred to as endothermic animals because they obtain their heat energy from within themselves. Animals that cannot control their own body temperatures are known as poikilothermic or ectothermic. Poikilothermic animal at 20˚c The animal is active because its metabolic rate has speeded up. Its body temperature is also 20˚C Poikilothermic animal at 0˚C The animal is inactive because its metabolic rate has slowed down. Its body temperature is also 0˚C  Homoeothermic animal at 20˚C The animal is active because its cells continue to produce heat using respiration to breakdown food. Its metabolic rate is maintained. The animal may become less active to prevent overheating. Homoeothermic animal at 0˚C The animal is active because its cells produce heat using respiration to breakdown food. Its body temperature remains constant and its metabolic rate is maintained. The metabolism remains active even when the outside temperature is low. In cold weather, the homoeothermic animal is still able to remain active. A poikilothermic animal in the same cold conditions must remain inactive, as it is too cold to move. Homoeothermic animals eat more than poikilothermic animals. Food is required for heat energy. The skin has two layers The skin of a mammal is the most important organ for temperature regulation. Our skin consists of two layers, namely the epidermis (top layer) and the dermis (lower layer). Deeper layers are protected by the epidermis The base of the epidermis is known as the Malpighian layer.  All skin cells are produced in this layer. The cells constantly divide through a process known as mitosis (a form of cell division). The new cells that have been generated gradually move towards the surface of the skin. As they move, they die and begin to fill up with a protein known as keratin. The top layer of skin consists of these cells and this layer of skin is referred to as the cornified layer. The cornified layer is hard and waterproof. This layer therefore protects the softer, living cells found beneath it. This layer is worn away constantly. New cells are replaced from beneath. This layer may even grow thicker in parts of the body that are worn the most. An example is your feet. The soles of our feet grow thicker. There are some cells in the epidermis that contain a dark brown pigment known as melanin. The function of the melanin is to absorb the harmful ultraviolet rays of the sun. These rays would damage the living cells that are found in the deeper skin layers. The epidermis is sporadically folded inwards to create hair follicles. A hair grows out from each follicle. Your hair is made from keratin A sebaceous gland can be found next to each hair follicle. These glands are responsible for the secretion of sebum. This is an oily substance the keeps the hair and skin supple, moist and soft. Functions of the dermis The main part of the dermis consists of connective tissue which contains elastic fibres and collagen fibres. The skin becomes loose and wrinkled as you age because these fibres lose their elasticity. Sweat glands are located in the dermis. The sweat glands secrete sweat. Water, tiny amounts of salt and urea can be found in sweat. The sweat travels up the sweat glands, through the sweat pores and out on to the surface of the skin. Sweat assists the body with temperature regulation.  Blood vessels and nerve endings are found in the dermis. The nerve endings are sensitive to pain, touch, pressure as well as temperature. These nerve endings help to keep you aware of your environment. A fatty layer (adipose tissue) is found under the dermis. Cells containing large droplets of oil make up this layer. This layer acts as an energy reserve and insulation for your body. Temperature control is regulated by the hypothalamus Our internal temperature is controlled by a part of our brain known as the hypothalamus. It coordinates the parts of the body that are responsible for adjusting to temperature changes. The hypothalamus is our thermostat. It uses the temperature of the blood as an indicator. If the blood temperature is above 37˚C, it signals the relevant parts of the body to take corrective action to regulate the body temperature. The body produces and saves heat when it is cold Our internal temperature has dropped to below 37˚C. The hypothalamus sends out impulses so that corrective action may be taken. This includes: The muscles work. Heat is generated when certain muscles in our bodies begin to contract and relax very quickly. This is known as shivering. The heat that has been generated by the muscles warms the blood as it flows through. The blood will then distribute this heat to all parts of the body. Our metabolism increases. The chemical processes (for example, respiration) begin to speed up, releasing heat. Our hair stands up. The hairs are pulled up by erector muscles in our skin. In humans, this just results in “goose flesh.” In other hairy animals it serves a purpose. When a cat’s hair is pulled up, a thick layer of warm air is trapped right next to the skin. This warm layer acts as an insulator and prevents heat loss.  Our blood system conserves heat. The arterioles supplying the blood capillaries near to the surface of the skin will become narrower (constrict). This is known as vasoconstriction. Only a small amount of blood can flow through these constricted arterioles. Most of the blood is passed through the deeper-lying capillaries of our skin. Heat loss is minimal in these capillaries because they lie so deep. We lose more heat when we get hot The following corrective action steps are taken by our bodies when we get too hot: Our hairs lie flat The skin’s erector muscles relax, and our hairs remain flat. Our blood system loses heat The arterioles supplying the blood capillaries near to the surface of the skin will become wider (dilate). This is known as vasodilation. More blood can flow through these dilated arterioles. Heat is lost because a much larger quantity of blood lies so near the surface of the skin. We begin to sweat Sweat is produced by the sweat glands and lies on the skin’s surface. The water in sweat will evaporate and take the heat of the body with it. The body will begin to cool down. Negative feedback is involved in temperature regulation The hypothalamus is constantly monitoring your body temperature by detecting small changes in the blood’s temperature. A rise in temperature alerts the hypothalamus. It responds by sending nerve impulses to your skin so that corrective action to cool your blood may be taken. As the cooled blood reaches your hypothalamus, it once again sends out nerve impulses to the skin. The cooling corrective action is stopped.  Further heat loss is prevented. Simultaneously, the muscles increase their heat production. This process is known as negative feedback. The term “feedback” is used because of the signals sent to the hypothalamus. The first signal is sent because a change in temperature was detected. The body proceeded to take steps to regulate this temperature. Once these steps were completed, a second signal was sent back to the hypothalamus with feedback regarding the effect of the steps taken. The hypothalamus then stops sending out signals to rectify the change in temperature. Blood Glucose and Excretion Controlling the blood glucose content Blood sugar levels are regulated by insulin and glucagon Controlling the concentration of glucose in the blood is another important part of homoeostasis. Cells need a constant supply of glucose to respire. Without glucose the cells are unable to release the energy they require for respiration. Our brain cells are the most dependent on glucose for respiration, they will die without it. An excessive amount of glucose in our blood is not optimal. Water moves from the cells into the blood by means of osmosis. The cells do not have enough water levels to continue with their normal metabolic processes. The pancreas and liver are responsible for the regulation of our blood glucose concentration. The pancreas is basically two glands in one. Most of the pancreas is responsible for producing pancreatic juice. This flows into the abdomen via the pancreatic duct. Groups of cells are scattered throughout the pancreas. These cell groups are known as the islets of Langerhans. These cells do not produce pancreatic juice. These cells are responsible for the production of two hormones, namely insulin and glucagon. These hormones assist the liver with controlling the amount of glucose in our blood. Insulin lowers our blood sugar and glucagon increases our blood sugar. This is what happens:  You have just eaten a tasty sugary desert which will increase the glucose concentration in your blood. The islets of Langerhans have detected this glucose increase so they secrete insulin into the blood. The insulin finds its way to the liver where it signals the liver to absorb the glucose from the blood. Some of the glucose is used for respiration. Some of the glucose is also stored in the liver as glycogen (an insoluble polysaccharide). Your blood sugar is too low. The pancreas secretes glucagon and the liver begins to breakdown the glycogen into glucose. The glucose is then released into the blood. All about excretion Metabolic waste products are excreted Our metabolic processes produce waste products. The cells do not use these waste substances. These waste products become toxic for the cell if they can remain within the cell. The products of respiration are water and carbon dioxide. Our cells can use the water. Carbon dioxide is not needed by our cells; therefore, it is a waste product. Carbon dioxide is used by plants for the process of photosynthesis. Carbon dioxide is produced by the plant during respiration. In the daytime, the carbon dioxide is used up. At night, the plant is unable to photosynthesize so the carbon dioxide becomes a waste product. A waste product that is generated by the cell because of a metabolic process is known as an excretory product. The process whereby the excretory product is removed is known as excretion. Excretion can be defined as the removal of toxic materials (waste products of metabolism) and substances more than requirements. Egestion is not excretion A different type of waste material that needs to be removed is undigested food. Humans are unable to digest cellulose. It passes straight through the alimentary canal and out by the anus in the form of faeces.  The cellulose has not been involved in any metabolic process. It has not been taken up by any of our cells. Removing the cellulose from our body can therefore not be classified as excretion, but rather egestion. The excretory products of animals Carbon dioxide waste is excreted from the lungs, gills or other gaseous surfaces. In animals nitrogenous waste is formed due to excess proteins and amino acids. Animals are unable to store an excessive amount of proteins and amino acids. They must be broken down into a form of nitrogen-containing excretory product known as urea. Mammals convert excess proteins to urea Digestive enzymes in the stomach, duodenum and ileum break the proteins down into amino acids. The amino acids are absorbed into the blood. The absorbed food is taken to the liver via the hepatic portal vein. The liver disperses the amino acids between the blood and other parts of the body. Amino acids must be removed if there is more than what your body requires. The amino acids themselves are not removed from the body because they contain energy. Even if this energy is not used immediately, it will be stored for future use. Liver enzymes split up each amino acid molecule. The part of the amino acid that contains the energy is converted into carbohydrates and then stored. The part of the amino acid that contains the nitrogen is converted into urea by a process known as deamination. The urea dissolves in the blood and it’s transported to the kidneys. It will be excreted here. A minimal amount of urea is excreted in our sweat. The main function of the liver is storage. The functions of the liver can be summarized as follows: Excess amino acids are converted into carbohydrates and urea by deamination. Blood glucose is controlled using the hormones insulin and glucagon.  Carbohydrates are stored as glycogen, a polysaccharide. Produces bile. Breaks down the old red blood cells. The iron component is stored. The remaining haemoglobin components are excreted as bile pigments. Breaks down harmful substances including alcohol. Storage for vitamins A, B, D, E and K. Storage for potassium. Produces cholesterol that is required for cell membrane repairs. The excretory system of humans The kidneys form part of the excretory system The kidneys are located behind the intestines at the back of the abdomen. When observing the longitudinal section through a kidney, you notice the three parts that make up the kidney. They are the cortex, medulla and pelvis. A tube known as the ureter connects the kidneys to the bladder. The urine produced in the kidneys is transported to the bladder via the ureter. The kidneys contain tubules The kidneys are made up of thousands of small tubules known as nephrons. Each nephron originates in the cortex. It then loops down into the medulla and back into the cortex. It then moves straight down again through the medulla and into the pelvis. Once the nephrons are in the pelvis, they join to the ureter. Urine is produced through filtration and selective reabsorption The blood is filtered as it passes through the kidneys. Most of the urea, excess salt and water are removed in this way. As this liquid pass through the kidneys, glucose that is still present is reabsorbed back into the blood. Most of the water and parts of the salt are also reabsorbed. The final liquid that is produced by the kidneys is a salt and urea solution known as urine. The urine flows out of the kidneys along the ureter and into  the bladder. The urine is stored in the bladder until it is released from the body through the urethra. Filtration occurs in the renal capsules The renal capsule is found in a branch of the renal artery. As the blood flows in this renal capsule, small molecules, water and all dissolved substances are squeezed out the blood. Each kidney contains thousands of renal capsules in its cortex. In the middle is a tangle of blood capillaries known as a glomerulus. The blood vessel that carries blood to the glomerulus is wide. The blood vessel that carries blood away from the glomerulus is narrow. This implies that once the blood is in the glomerulus, it cannot move away quickly. The blood in the glomerulus is squeezed against the capillary walls because of the high pressure build up. There are small holes along the walls of these capillaries and the renal capsules. This means that any molecule small enough to pass through these holes will move into the space in the renal capsule. The smaller molecules include water, glucose and urea. Larger molecules such as proteins are too big. These protein molecules remain in the blood along with the other blood cells. The useful substances are reabsorbed Fluid found in the renal capsules is a solution of glucose, salt and urea that have dissolved in water. Some of these substances are required by the body. The glucose, salt and water must remain in the blood. Each kidney tubule has blood capillaries wrapped around it. Substances that are useful to the body are reabsorbed from the fluid. They are then passed back into the blood through the capillaries. All the remaining fluid continues along the tubule. By the time it reaches the collecting duct, it contains mostly water with salts and urea dissolved in it. This is known as urine. The kidneys are very efficient at reabsorbing water. 99% of the water that has re-entered the tubules is reabsorbed. Our kidneys filter approximately 170dm3 of water each day. Only 1.5dm3 of urine is produced during this time. Urine is stored in the bladder Urine enters the bladder via the ureter. The bladder can hold rather large quantities of urine because of its stretchy walls. The urethra (a tube) leads out of the bladder. A sphincter muscle is situated at the top of the urethra and it remains tightly closed. When the bladder is full, the sphincter muscle opens. The urine is then able to flow along the urethra and out of the body. Adult mammals can control this sphincter muscle. Young mammals to do not have this ability and the sphincter muscle will open automatically when the bladder is full. Dialysis takes over the function of damaged kidneys There are times when a person’s kidneys do not function optimally. Infection is the leading cause of this. If the kidneys fail completely, a build-up of waste products occurs in the blood. This will lead to death if left untreated. The optimal treatment for kidney failure is a kidney transplant. However due to the “tissue type” challenges between the donor and recipient this is not always possible. The “tissue type” between these individuals must be a match or as close to a match as possible. This avoids rejection of the transplanted kidney. The healthy kidney is usually obtained from a deceased person (after a tragic vehicle accident). Treatment for kidney failure includes several sessions a week at the dialysis unit. Dialysis may also be referred to as a kidney machine because the blood flows from the person’s body through the machine and back into the body. There is a special fluid containing water, glucose, salts and other substances inside the machine. This dialysis fluid is separated from the blood by a partially permeable membrane (like Visking tubing). The patient’s blood passes through the tubes. The substances in the dialysis fluid diffuse through the membrane, down their concentration gradients. The dialysis fluid does not contain urea.  The urea from the patient’s blood will therefore diffuse from the blood and into the dialysis fluid. It is possible to control the amount of the other substances in the patient’s blood by controlling their concentration in the dialysis fluid. The proteins in the patient’s blood remain where they are as they are too big to pass through the membrane. Patients must undergo dialysis two to three times a week. Each session lasts for several hours. Our immune system can reject transplants The person who receives a transplant is known as the recipient. The person from whose body the organ was taken is known as the donor. Many people carry donor cards with them. This means that they voluntary donate their organs to be used for transplants. To prevent deterioration, the organs for transplant must be removed quickly from the deceased and kept cold. There are times when live donors are used. A brother may donate a kidney to his sister. It is possible to live with one kidney only. Surgeons have overcome most of the challenges that surround the actual transplant procedure. The biggest challenge occurs after the operation. The immune system of the recipient recognizes that the transplanted organ is “foreign” and begins to attack it. This is known as rejection. To decrease this possibility, the recipient is given immunosuppressants. This medication stops the white blood cells from working efficiently. However, these immunosuppressant’s affect the entire immune system. The recipient becomes more susceptible to other infectious diseases. The recipient must take this medication for the rest of their natural life. Rejection is reduced if the recipient and donor are close family relatives. These family members have the same antigens on their cells. The immune system of the recipient does not see the transplanted organ as “foreign”. If a relative is unable to donate an organ, the search is widened. A worldwide search to look for donors that share similar antigens as the recipient is performed. The uses of drugs The definition of drugs Drugs have always been used. A long time ago people discovered the healing properties of plants. It was also discovered that certain properties of the plants were able to change one’s perception of the world around you. These plant properties caused hallucinations and feelings of contentment or excitement Today we still make use of many plant-based drugs. A drug can be defined as any substance that is taken into the body and modifies or affects the body’s chemical reactions. Many people would have a shorter lifespan or a painful existence without the use of drugs. There are drugs that are used for medicinal purposes. Unfortunately, there is also misuse of drugs that proves harmful to the user and those around them. The medicinal drugs Antibiotics are used to kill the bacteria in the body There are times when your body needs help to fight an infection. Unfortunately, this help only arrived after 1944. Prior to that, many people succumbed to infections that are quite harmless by today’s standards. Antibiotics were discovered. This discovery had a tremendous effect on doctors’ ability to treat diseases. Antibiotics kill the bacteria without harming other living cells. Most antibiotics are made from fungi. It is generally thought that the fungi naturally produce these antibiotics to kill bacteria living around them. Bacteria and fungi are decomposers. Penicillin was the first antibiotic to be discovered and it is made from the fungus Penicillium. You often find this fungus growing on decayed fruit. Penicillin prevents the bacteria from making cell walls. Many other antibiotics have been discovered since penicillin.  The search for new antibiotics is ongoing because bacteria have evolved and developed a resistance to the antibiotics. The more we use antibiotics, the more selection pressure we place on the bacteria to evolve. This was not known at first and it led to the un- discriminatory use of antibiotics. They were even used for viral infections. Antibiotics do not harm viruses because a virus does not have a cell wall. Doctors have become more aware of the ability of the bacteria to evolve. Thus, they only use antibiotics when and if they are needed. The misuse of drugs Heroin: An addictive depressant Opium is produced in opium poppies. Opium contains many different chemicals. Two of these chemicals are morphine and codeine. As you know, these are used in medicine for pain management. Opium is also the raw material that is used in the production of heroin. Heroin is addictive. An addictive drug means that the person using it becomes dependent on it. When they stop taking it they suffer from severe psychological and physical symptoms. Heroin is also a very powerful depressant. This means that it can slow down many of our brain functions. It can reduce pain and decrease our rate of breathing. Heroin slows down the functioning of our hypothalamus. A person feels euphoric (excessive happiness) when taking heroin and for many people it is highly addictive. They begin to feel ill if they stop taking it. To such an extent, that they are willing to do anything to obtain the drug. A person begins to build up a tolerance for the heroin. This means that they need more heroin to have the same euphoric effect they experienced the first time they used it. Addiction to heroin occurs rapidly, within the first week of using it. However, there are people who do not become addicted to heroin. A heroin addict loses their ability to function normally in society. They become consumed with finding their next “fix” (dose). Addicts eventually lose their jobs and are unable to support their families. Many turn to crime as a means of obtaining money to buy heroin.  Heroin is usually injected into the veins. This becomes dangerous, especially when using dirty needles. These needles are not sterile and may contain pathogens such as the hepatitis virus. Needle sharing among addicts contributes to the spread of HIV. It is possible for a person to stop taking heroin. This however requires a strong support system and willingness and will power from the addict. The addict suffers from withdrawal symptoms after a few hours of not taking the drug. Withdrawal symptoms are extremely unpleasant and may even be life- threatening. Alcohol: A powerful depressant Alcohol is one of the most common drugs used worldwide. People may drink alcoholic drinks because they enjoy the effect that alcohol has on their brain. A person feels more relaxed and inhibitions are decreased so that it becomes easier to socialize. The wall of the stomach rapidly absorbs the alcohol. From here it is carried in the blood to all parts of the body. After some time, the alcohol is eventually broken down by the liver. Consuming alcohol is small quantities is not harmful. Alcohol affects the body in many ways which may become dangerous if care is not taken. The effects of alcohol are: Lengthened reaction time. Alcohol is a depressant. Even in small amounts, the actions part of the brain is slowed down. This means that it will take you longer to react to stimuli. The decreased reaction time may lead to death, especially if the driver is affected by alcohol consumption. There are a high proportion of accidents related to alcohol. Either the drivers are affected or pedestrians. Many countries have set legal limits for alcohol permitted in the bloodstream. Take note, that even a small amount of alcohol may affect the driver’s reaction time. Increases aggression in certain individuals.  People react differently to alcohol. Some people lose their inhibition to be less violent. They become violent, aggressive and are more likely to commit crimes. Often people become violent with their family members. Research has shown that in many countries and much as 50% of domestic violence is alcohol related. Furthermore, it has shown that in many countries as much as 60% of prisoners are in jail because of alcohol-related crimes. Potential to be lethal People die every year as a direct result of alcohol. Alcohol is a poison and consuming large quantities in a small period is not such a good idea. These large alcohol intakes result in unconsciousness, coma and possibly death. A person might die because of vomiting while still unconscious. The airway is blocked by the vomit and the person suffocates. Alcoholism: A dangerous disease Alcoholism is an addiction to alcohol. The person is unable to function without consuming alcohol. The cause of this disease is still not 100% understood. Many people consume large quantities of alcohol without becoming an alcoholic. There are many factors which determine a person’s predisposition to alcoholism. Factors include genetic make-up, personality and stress levels. An alcoholic must drink large quantities of alcohol at regular intervals. Many of the cells are damaged in the process. The liver is damaged because it's the function of the liver to break the alcohol down. The liver disease associated with alcohol damage is known as cirrhosis. Fibres begin to grow in the liver, and these may be fatal. The brain is also affected by consuming too much alcohol. After a long period of time, a person will experience memory loss and confusion. The alcohol present in the body fluids draws the water out of the cells by osmosis. If this happens to the brain cells, they begin to shrink and are irreversibly damaged. There is a hormone that is released to stop the kidneys from excreting too much water in urine. Alcohol prevents the release of the hormone thus  worsening the osmotic effect. Drinking alcohol produces a lot of diluted urine which results in the low levels of water in the blood. How smoking affects your health There are irritants and carcinogens in tobacco smoke People continue to smoke even though they know how damaging it is to their health. Young people continue to smoke. There are many components present in tobacco smoke. Research surrounding the components and their effect on health continues. The dangers affecting the health of smokers are also applicable to non- smokers. The non-smokers breathe in the smoke from the burning cigarette and the smoke that the smoker exhales. This is known as passive smoking. Many countries have banned public smoking and smoking near children. The addictiveness of nicotine The brain is affected by nicotine. The nicotine is a stimulant and makes the smoker feel more alert. However, it is also addictive. This is the reason why many smokers struggle to quit smoking. Nicotine also damages the circulatory system. The blood vessels begin to narrow, leading to increased blood pressure or hypertension. There is a higher risk of developing heart diseases amongst smokers than there is amongst non-smokers. The tar is made up of many different chemicals. These chemicals include carcinogens (cause cancer). The chemicals affect the cells in the respiratory passage and lungs. Their behaviour begins to change. These cells begin to divide uncontrollably. Eventually these dividing cells form a lump or tumour. The smoker has cancer if the tumour is malignant. New tumours grow in other parts of the body because cells break away from the original tumour. Most of the persons diagnosed with lung cancer are smokers, lived with a smoker or worked in an environment where they were passive smokers. Smoking increases the chance of obtaining other forms of cancer.  Carbon monoxide is a poisonous gas. It affects the blood. Once in the lungs, the carbon monoxide diffuses into the blood. Here it combines with the haemoglobin present in red blood cells. The red blood cells cannot carry as much oxygen. The cells throughout the body are deprived of oxygen. This is especially harmful to a baby in its mother’s uterus. As the mother smokes, the chemicals are transported to the baby. The carbon monoxide inhibits the normal growth of the baby. Smoke particles are small particles of carbon and other materials. These particles become trapped in the lungs. The white blood cells try to remove these particles by secreting chemicals that are usually intended to rid the body of these particles. A drawback to these secreted chemicals is that they can cause serious damage to the lungs. The wall of the alveoli is eventually broken down. There surface area for gaseous exchange is decreased. The person is diagnosed with emphysema. Emphysema patients struggle to breathe as they have insufficient oxygen in their blood. These patients eventually become totally inactive as they do not have enough energy. The chemicals in the cigarette smoke harm the cells that line the respiratory passages. These cells lose their ability to clean the air as it passes through..The effectors The muscles Muscles are effectors because they carry out the actions. There are several different types of muscles in the body: Cardiac muscles Only found in the heart and they make up the walls and the ventricles Smooth muscles These muscles are also known as the involuntary muscles, as you have no control over them. These muscles are found in the bladder, blood vessel walls and the alimentary canal walls. Striated muscles  These are all the muscles that are attached to a bone. These are also known as skeletal or voluntary muscles. You can control these muscles. The different muscles contract in different ways Our movement is caused when the muscles shorten or contract. Cardiac muscle Contracts and relaxes rhythmically for the duration of your life. It does not stop, and it does not need conscious messages from your brain to do this. The rate of contraction can however be changed by nerve impulses from the brain. Smooth muscles These muscles are also able to contract on their own, for example the smooth muscles in the alimentary canal wall during peristalsis. Smooth muscle in other parts of the body need nerve stimulation. Smooth muscles contract slower than cardiac muscles. Striated muscles These muscles only contract when electrical nerve impulses are sent along its nerves. Striated muscles can contract very strongly and very quickly. This muscle however does tire quicker the smooth or cardiac muscles. The bones meet at the joints A joint is formed when two bones meet. A synovial joint is formed when the two bones that have joined need to move freely. Examples of synovial joints are the shoulder and elbow joints. Ligaments hold the two bones together. These ligaments are very strong, but they can move when the bones move. The tendons and ligaments help the joints to function The muscles are attached to the bones by strong cords known as tendons.  When the muscle contracts, it pulls on the bone, making it move. The tendons do not stretch; they just transmit the force from the muscle to the bone. If the tendons stretch, the muscles would stretch the tendons and not make the bones move. The ligaments are strong cords with the ability to stretch. They attach bones at a joint to one another. The ligaments need to be stretchy so that the bones can move. The movements of the forearm There is a bone and two muscles in your arm. The elbow is a hinge joint because the arm can bend here. The biceps muscle is attached to the top of the scapula and the bottom of the radius. When this muscle contracts, the radius and ulna are pulled upwards towards the scapula, thus bending the arm. This is known as flexing your arm. The biceps muscle is your flexor muscle. Muscles do not push, they only pull. The biceps muscle can therefore not return your arm to its original position. Another muscle, the triceps muscles pull the arm back down. When this muscle contracts, your arm is straightened (extended). It is therefore known as an extensor muscle. The flexor and extensor muscles need to work together. So, when the biceps muscle contracts, the triceps muscle relaxes. These muscles are antagonistic because they work against one another. Endocrine system Hormones are produced by the endocrine glands The human body also makes use of chemicals to transmit information to its different parts. These chemicals are known as hormones and they are produced in the endocrine glands.  A hormone can be defined as a chemical substance that is produced by a gland and carried by the blood. It alters the activity of one or more specific target organ. It is then destroyed by the liver. There is a good supply of blood to these endocrine glands. The blood capillaries run right through them. When a hormone is produced, it can be released directly into the blood system. The endocrine glands are the only glands that secrete directly into the blood. Saliva is secreted by the salivary duct into the mouth. There are no ducts in endocrine glands, so they are also known as the ductless glands. Once the hormone has entered the blood, it is dissolved in the plasma and carried to all parts of the body. The hormones only affect specific parts of the body known as target organs. There is a small gland (the adrenal gland) above each of our kidneys and they produce the hormone adrenalin. Your brain sends impulses along the nerves to your adrenal glands every time you are frightened or excited. These glands then secrete adrenalin into your blood stream.  Adrenaline is designed to help you cope with danger: Your heart rate is increased which in turn pumps oxygen much faster to your brain and muscles. You now have more energy for fighting or running away. The blood vessels in the skin and digestive tract contract so that they carry a little blood only. This causes you to have the “butterflies in your tummy” sensation and you go a bit paler. The body is sending as much blood as possible to your brain and muscles in this state of emergency. The adrenaline also causes the liver to release more glucose so that there is enough energy for the muscles to use to contract. Table of comparison The nervous system The endocrine system Made up of Neurons Secretory cells Type of impulse Electrical impulses Hormones (chemicals) transmitted How impulse is Along nerve fibres (axon and Dissolved in blood transmitted dendrons) plasma Rate of transmission Very quickly Slow Lasting effect For a very short time Lasts longer Animal hormones are used in food production Some farmers give their animals hormones so that they grow faster or produce milk faster. The hormone that they use is called bovine somatropin (BST). This hormone is produced naturally by the cattle. When dairy cows are given this hormone, they produce milk faster, so fewer cows can produce the same yield. There are arguments about whether it is a good idea to feed the animals these hormones. Here are some of the aspects from people who are against using hormones in this manner: o Drinking milk containing BST may cause damage to our health. This is unlikely as the milk contains low concentrations of this hormone.  o Whether the BST is really required. According to the European Union, there is too much milk produced. This results in imposing milk quotas to stop the over production of milk. o BST is harmful to the cows. The amount of milk produced by the cows is unnatural. Their bodies have not been designed to contain this amount. This leads to udder infections and general discomfort. Coordination and responses in plants Some plants respond by growing Plants also respond to their environment, although not as rapidly as what animals do. Plants respond by changing their rate or direction of growth. The plant may grow towards the stimuli or away from it. Growing towards the stimulus response is considered a positive response while growing away from the stimulus is a negative response. The responses are known as tropisms. A tropism can be defined as a plant’s growth response to a stimulus, namely, the direction of the growth is affected by the direction of the stimulus. The most common growth response is to gravity and light: o Geotropism → plant grows either towards or away from gravity Plant shoots: Grow away from the pull of gravity Plant roots: Grow towards the pull of gravity o Phototropism → plant grows either towards or away from light Plant shoots: Grow towards the light Plant roots: Grow away from the light The most common growth response is to gravity and light. Consider the following in your observations and results: Response of plants in each petri dish Name the type of response Explain petri dish C results Which petri dish would act as the control? Growth movements assist in plant survival The direction of growth is vital to the survival of the plant. Shoots grow towards the light so that the leaves can photosynthesize. The flowers must be in the air to facilitate their pollination by insects, birds or wind. The roots grow downwards to absorb water, minerals and nutrients from the soil. They are also responsible for anchoring the plant. How shoots respond to light Plants have more simpler means of detecting stimuli from their environment. The shoot of the root (also known as the coleoptiles) can pick up the stimulus of light shining on to it. The tip of the shoot is the sensitive region because the receptors are found here. The part responding to the stimulus is the effector and it lies just below the tip. These two parts are communicating with one another by means of plant hormones. These hormones diffuse from the tip down into the rest of the shoot, promoting growth. Varying auxin concentrations promote growth Auxin is one of the hormones that plants produce. It is produced by the cells in the shoot tip from where it diffuses to the rest of the shoot. Auxin increases the length of the cells behind the tip. The rate of growth depends on the amount of auxin. Higher amounts of auxin promote faster growth. The auxin is distributed evenly throughout the tip when there is light coming from all sides. The shoot will grow straight. If the light is only shining on one side of the shoot tip, the auxin will concentrate in the shady side. The cells on the shady side grow faster, bending the plant towards the light. Placing a potted plant on its side in the dark causes an interesting response. The auxin collects on the lower side of the stem. This part grows faster and the stem curves upwards.  In seedlings, the auxin has a different growth response. The auxin collects on the lower surface of the root. The auxin slows down the rate of the growth, bending the radicle downwards. It does not matter which way the seed is planted, the roots (radicle) will always grow downwards. Plants are etiolated in the dark Seedlings that grow in the dark are pale, thin and tall. In the darkness the auxin is evenly distributed around the tip, so the shoot grows upwards. In the dark, chloroplasts cannot develop as they should. The plant grows to be light, tall, spindly and with small, far apart leaves. Plants like this are said to be etiolated. Placing these plants in the sunlight will allow chloroplasts to develop and the plant will eventually return to normal. However, if no light is reached, they will die because they are unable to photosynthesize. The use of plant hormones in food production Plant hormones are used by farmers and horticulturists to improve their gardens, increase the yield and increase the rate of production. Most weed killers contain plant hormones. A type of auxin is manufactured synthetically in a factory. These weed killers are selective. The weeds are the only plants that are affected. The weed killer speeds up the rate of growth for the weed. It eventually dies and can be replaced with grass. Similar weed killers are used by farmers for their crops of wheat, millet, sorghum and maize. Fruit growers use the plant hormones to increase the rate of fruit production. When fruits are growing, they are producing a gas known as ethene. This gas stimulates the surrounding fruit to grow too. Picking tomatoes while they are still green makes transport much easier. There is less damage and rotting. Once they are at the suppliers, they are exposed to ethene gas and they ripen. Asexual reproduction Offspring produced through asexual reproduction are genetically identical All living organisms reproduce. Each organism follows their unique method of reproducing. Reproduction falls into two categories namely, asexual and sexual. Asexual reproduction can be defined as the process that results in the production of genetically identical offspring from one parent. During reproduction each of new organism obtains a set of chromosomes from one or both its parents. Chromosomes are long threads of DNA. The chromosomes are found in the cell nucleus. Chromosomes contain genes which are sets of instructions. In asexual reproduction only one parent is involved. Some of the parent’s cells start to divide through a cell division process known as mitosis. This type of cell division produces new cells that contain identical genes to that of the parent. They are genetically identical. These cells will grow into new organisms. All the organisms are identical to one another and their parent. Bacteria reproduce using binary fission There is no chromosome-containing nucleus in bacterial cells. They do not divide through mitosis. They make use of binary fission, they basically split in two. Bacterial DNA is a single, circular molecule. An exact copy of the DNA is made when the bacterium is about to divide. The bacterium simply divides itself into two. One molecule of DNA is found in each half of the bacterium. The two new cells that have been formed are genetically identical to the parent cell. Bacteria reproduce remarkably quickly provided they are in suitable conditions (optimal temperature, enough water and nutrients). After 20 minutes, a new cell can divide again. In other words, if you start with one cell, after 20 minutes you will have 2 cells. After 40 minutes you will have  4 cells. After 1 hour, you will have 8 cells. After 5 hours, there will be 32 768 genetically identical cells. Fungal spores are produced asexually Fungi fall into a class of their own. They are not animals, plants, bacteria or viruses. Fungi are like plants, as their cells have cell walls. Fungal cell walls contain chitin (not cellulose). Fungi are unable to photosynthesize, and they feed on organic food materials. Mucor is the fungus that you often see growing on bread. It is also referred to as bread mould. It is possible to grow Mucor. Leave a piece of moistened bread for a few days. Place a loose cover over it to prevent drying out. The fungus will look like furry growth. The “fur” is known as mycelium. It is made up of many thread-like structures known as hyphae (singular: hypha). The hyphae are one cell thick. Some of the hyphae grow directly through the bread. Here they secrete enzymes that digest the starch, protein and fats present in the bread. From these digested substances, the fungus produces glucose, amino acids and fatty acids and glycerol. All these substances are then diffused into the hyphae. The other hyphae will grow upwards, forming aerial hyphae. At the top of each of these hyphae, swelling occurs. This swelling is known as a sporangium. Inside the sporangium cells divide asexually forming many genetically identical spores. A hard, dry coat surrounds each spore to prevent it from drying out. The fungal spores are very small and light. The spores are easily spread through air currents or by being stuck to the feet of horseflies. When these spores land on suitable food, they germinate and grow into new mycelium. Potatoes reproduce through stem tubers Plants can reproduce asexually. By reproducing asexually, plants can generate many genetically identical offspring.  The plant grower can use this to create larger flowers, good flavour or higher yield. Potatoes reproduce by means of stem tubers. There are plants that grow normal stems. In these cases, the stem grows above ground, produces leaves and photosynthesizes. In other plants the stems grow underground. Swellings known as tubers, form on the stem. Sucrose is transported to these tubers from the leaves. In the tubers, the sucrose is converted to starch and stored. The tubers increase in size. Many tubers are produced by each plant. The tubers are harvested and used as food. The farmer will save a portion of the tubers to be used to cultivate the following year’s crop. Many tubers are produced from one plant. To increase the yield, each tuber can be cut into several pieces. If each tuber piece contains a bud, it will grow into a completely new plant. Sexual reproduction Fertilization is involved in sexual reproduction Parent organism produce sex cells known as gametes. Examples of gametes are eggs and sperm. Two gametes will join, fusing their nuclei. This is known as fertilization. The new cell formed is known as a zygote. The zygote will divide continuously until it forms a new organism. The gametes contain half the normal number of chromosomes Gametes only contain half the number of chromosomes than usual. The reason for this is that once they join, the zygote contains the correct number of chromosomes. Humans have 46 chromosomes. Each gamete contains 23 chromosomes. The zygote formed after the joining of an egg and sperm will therefore have 46 chromosomes. The 46 chromosomes in humans consist of two sets of chromosomes, namely one set from the mother and one set from the father. A diploid cell is a cell that contains the full number of chromosomes, with two complete sets. A haploid cell is a cell that contains only one set of  chromosomes. Gametes will always be haploid cells. When the two gametes fuse, they form a diploid zygote. Sexual reproduction can be defined as the process that involves the fusion of haploid nuclei to form a diploid zygote and the production of offspring that are genetically dissimilar. Genetically different offspring are produced in sexual reproduction Gametes are produced when ordinary cells divide. Human sperm is produced when the cells found in the testes divide. Gametes must only contain half the number of chromosomes compared to the parent cell. Dividing through mitosis is therefore not possible. When gametes are formed, the cells divide through meiosis. After meiosis, new cells with half the number of chromosomes are formed. The male gametes move while the female gametes remain stationary There are two different types of gametes in humans. The larger, stationary gamete is the female gamete known as the egg. The smaller, mobile gamete is the male gamete known as the sperm. The sperm moves actively in search of the egg. In flowering plants, the pollen grains contain the male gamete. The male gamete reaches the female gamete through the pollen tube. Most organisms can only produce one type of gamete, namely male or female. There are organisms that can produce both male and female gametes. These organisms include earthworms and slugs. An organism that can produce both male and female gametes is known as a hermaphrodite. There are many flowering plants that are hermaphrodites. Human sexual reproduction Reproductive organs of the female The ovaries produce the female gametes (eggs). The oviducts or Fallopian tubes lead away from the ovaries. These do not connect to the ovaries. They have a funnel-shaped opening a short distance from the ovaries.  The two oviducts then lead to the uterus (also known as the womb). The uterus has thick, muscular walls. It is small, approximately the size of a clenched fist. The uterus can stretch large enough to accommodate a baby. There is a narrow opening that is guarded by muscles at the end of the uterus. This is known as the cervix (neck of the uterus). The cervix leads to the vagina. The vagina then opens to the outside. The urethra (opening from the bladder) runs in front of the vagina. The rectum can be found behind the vagina. These three tubes (urethra, vagina and rectum) open separately to the outside. Reproductive organs of the male The sperm (or spermatozoa) are produced in the testes. The testes are found outside the body in two skin sacs known as the scrotum. The sperm duct carries the sperm away from the testes. The sperm ducts join the urethra found just below the bladder. The urethra continues to open at the tip of the penis. The urethra can carry both sperm and urine (at different times). The prostate gland is situated where the sperm ducts join the urethra. The prostate gland is responsible for making the fluid in which the sperm swims. The seminal vesicles are situated behind then prostate gland. These vesicles also secrete fluid. The ovaries make the eggs The eggs begin to form inside a girl’s ovaries even before she is born. At birth her ovaries will have thousands of partially developed eggs. These partially developed eggs (follicles) begin to develop when the girl reaches puberty. These eggs develop one at a time. When the egg has reached maturity, it bursts out of the ovary, into the funnel connected to the oviduct. This is known as ovulation and it occurs once every month. The testes make the sperm The testes contain thousands of very narrow, coiled tubes or tubules. The sperm is produced in these tubules. The cells in the walls of these tubules divide by meiosis to form the sperm. Sperm production begins at puberty and it continues after puberty.  The production of sperm is heat sensitive. If the temperature is too hot, the tubule cells will not produce sperm cells. This is the reason why the testes are found outside the body. The testes remain cooler outside the body than if they were inside the body. Sperm is introduced into the vagina through mating The funnel of the oviduct is lined with cilia. These cilia beat rhythmically and sweep the egg (from ovulation) into the oviduct entrance. The egg travels slowly to the uterus. The egg continues to travel along the oviduct. It is assisted by the cilia found along the oviduct. Peristalsis of the muscles in the oviduct wall further assists this migration. The egg dies if it is not fertilized by the sperm within 8-24 hours. The egg has yet not travelled very far. For successful fertilization, the sperm must reach the egg while the egg is still near the top of the oviduct. Blood is pumped into spaces found within the penis when a man becomes aroused (sexually excited). The penis becomes erect. To bring the sperm as close to the egg as possible, the penis is placed into the vagina. This is known as sexual intercourse. The sperm is pushed out of the penis and into the vagina. This happens because the muscles of the tubule walls rhythmically contract. This contraction wave originates in the testes, and then travels through the sperm duct and into the penis. The sperm is squeezed out through the man’s urethra and into the vagina. This known as ejaculation. The fluid that contains the sperm is known as semen. When the sperm is ejaculated it is deposited close to the cervix of the vagina. Fertilization occurs in the oviduct The sperm have quite a distance to swim before they reach the egg. They use their tails to swim up through the cervix, to the uterus and into the oviduct. The swimming rate of the sperm is 4mm per minute. It will take some time before the sperm reaches the egg. Some of the sperm die before they reach the egg. An ejaculation deposits millions of sperm within the vagina. This increases the chance of sperm reaching the egg.  Only one sperm can enter the egg. The sperm head is the only part of the sperm that enters the egg. The tail remains outside the egg. The membrane of the egg becomes impenetrable once a sperm has entered it. No other sperm can enter the egg. Sperm that do not gain entry will die. Once inside, the nucleus of the sperm fuses with the nucleus of the egg. Fertilization has occurred. The zygote becomes embedded (implanted) within the uterus wall A zygote is formed after the egg and sperm nuclei have fused. The zygote travels slowly down the oviduct. As the zygote travels along the oviduct it undergoes cell division through mitosis. After several hours this cell division, a ball of cells known as an embryo is formed. The embryo obtains enough food from the egg yolk. The embryo reaches the uterus after several hours. By the time the embryo reaches the uterus it is a ball of approximately 16 or 32 cells. The embryo sinks into the thin, spongy lining of the uterus. This is known as implantation. The placenta is the life-support system of the embryo The cells of the embryo continue to divide after implantation. A placenta grows as the embryo grows. The placenta connects to the wall of the uterus. The placenta is soft and dark red. It has finger-like projections known as villi which fit closely to the wall of the uterus. After 11 weeks, the embryo has developed into a foetus. The umbilical cord joins the foetus to the placenta. There are two arteries and a vein inside the umbilical cord. Blood is taken from the foetus to the placenta via the arteries. The vein returns the blood to the foetus. Capillaries in the placenta are filled with blood from the foetus. The wall of the uterus has large spaces that are filled with the blood from the mother. The blood from the mother and foetus do not mix. The placenta wall keeps them separate. They are however brought very close to one another because the placenta walls are very thin. Food and oxygen from the mother’s blood diffuse across the placenta and into the blood of the foetus. Once here it is carried to the foetus along the umbilical cord. Carbon dioxide and other waste materials diffuse in the other  direction. These diffuse from the foetus’s blood into the blood of the mother to be carried away. The placenta grows as the foetus grows. At the time of birth, the placenta is a flat disc. It is 12cm in diameter and 3cm thick. The foetus is protected by the amnion The amnion is a strong membrane that surrounds the foetus. Amniotic fluid refers to the fluid found within the amnion. The amniotic fluid supports and protects the foetus. Birth is caused by muscular contractions The foetus turns over in the uterus a few weeks before birth. It is positioned downwards, and its head lies over the opening of the cervix. The birthing process starts when the muscles in the uterus wall begin to contract. This is known as labour. In the beginning stages of labour, the opening of the cervix is slowly stretched open when these muscle contract. After several hours have passed, the cervix is wide enough, and the baby’s head begins to push through. The contractions push the baby in a downwards direction until it passes through the cervix and the vagina. This part of the birthing process usually happens quite rapidly. Once the baby is born, it is still attached to the umbilical cord and the placenta. The baby can breathe independently and no longer needs the placenta. The placenta is dislodged from the uterus and passes through the vagina. This is known as afterbirth. The doctor cuts and clamps the umbilical cord just above the point where it joins the baby. The baby feels no pain; the umbilical cord does not have any nerves. The baby’s navel is the stump of the umbilical cord. It is extremely painful when the muscles of the uterus begin to contract. It feels like a severe cramp. The mother can decrease the amount of pain to a certain degree by preparing her body through exercises. Specific breathing exercises will also assist with pain management. The mother has the option of making use of pain killers. Pregnant women must be health conscience A pregnant woman must take extra care of her health because it will be to her benefit and that of the baby’s. This is known as ante-natal care (before birth care). The mother-to-be should be aware of the following: Additional vitamins and minerals. ✓ Calcium → formation of bones in the foetus. ✓ Iron → mother produces extra blood to help carry oxygen to the foetus. Iron is used in the production of haemoglobin. ✓ Carbohydrates → extra energy for carrying the growing foetus. ✓ Protein → form new cells in the growing foetus. ✓ Regular exercise. ✓ Gentle exercise (swimming or walking) → keeps her fit throughout the pregnancy and allows her to play an active role during the birth. Avoid harmful substances. ✓ Harmful substances can enter the blood of the foetus. ✓ Nicotine and carbon monoxide (smoking) → baby grows slowly and is usually smaller compared to a baby of a non-smoker. ✓ Alcohol → ability to harm the foetus. ✓ Drugs → only use drugs that are recommended by her doctor. These drugs will not harm the foetus. Avoid illnesses. ✓ Rubella (a virus) → causes a rash and fever. If the Rubella virus can cross the placenta, a great deal of harm may be done to the foetus. The baby may be born deaf or with other disabilities. Many countries immunize teenage girls against Rubella. ✓ AIDS → The virus responsible for AIDS is the HIV virus. It is able to cross the placenta and increases the risk of the baby being born HIV positive. Mammals are able to care for their young A baby is very helpless once it is born, despite having been developing for nine months. Both parents are responsible for the care of the baby.  The glands in the breasts of the mother have become enlarged throughout her pregnancy. Soon after the baby has been born, milk will develop. This is known as lactation. All mammals lactate. Not all animals lactate. Mother’s milk contains all the nutrients required by the baby. Most importantly, the milk contains antibodies which protect the new born baby from infection. It is important to keep the baby warm. The baby has a large surface area and heat is lost very quickly. The emotional well-being of the baby is just as important as its physical wellbeing. Babies must have close contact with both parents. Mammals care for their babies by feeding then and keeping them warm. In humans, the parental care includes teaching the baby and young child to look after themselves. Furthermore, the children are taught how-to live-in society. These teachings continue until the children reach puberty. Most mammals do not take care of their young until this age. The advantage of breast milk over formula milk Most people are of the opinion that breast milk is a better option to bottle- feeding the baby. Formula milk is in a powder form. It is mixed with sterilized water and placed in a sterile bottle. Bottle-feeding is an easier option for the mother, as other people can assist at feeding time. The father is able to connect with the baby if he can help with the feeding too. Formula milk is more expensive than breast milk. It is easier for bacteria to gain access to the formula milk, especially if the bottles and such are not cleaned properly. Breast milk contains antibodies from the mother. The composition of the breast milk changes as the baby grows. The baby therefore obtains all the essential nutrients that are needed for each stage of its development. The menstrual cycle is 28 days In an adult woman, one egg is released into the oviduct every month. The uterus prepares itself for the fertilized egg. Prior to the release of the egg, the  uterus lining becomes thick and spongy. It contains many blood vessels that will be responsible for supplying food and oxygen to the egg. The egg dies before it reaches the uterus if it is not fertilized. It simply continues down and out through the vagina. It does not implant itself into the uterus lining. The spongy tissue is not needed, so it will begin to disintegrate and be lost through the vagina. This is known as menstruation (or a period). A period usually lasts for 5 days. Once menstruation is complete, the lining of the uterus begins to build itself up again. It is ready to receive the next potentially fertilized egg. Puberty means that sexual maturity has been reached The stage of life when sexual maturity is reached is known as adolescence. It is the beginning of ovulation in girls and sperm production in boys. Secondary sexual characteristics develop during adolescence. In boys, facial and pubic hair begins to grow, voice breaking, and muscular developments take place. In girls, pubic hair begins to grow, breasts develop, and the pelvic girdle broadens. Hormones are responsible for these changes. Testosterone produced in the testes assists with the sexual development in boys. Oestrogen produced in the ovaries assists with the sexual development in girls. Girls reach puberty (sexual maturity) several years earlier than boys. Puberty does not mean that they person is an adult. At puberty the emotional development is as yet incomplete. The menstrual cycle is controlled by the female sex hormones Another female sex hormone produced by the ovaries is progesterone. This hormone is produced during certain stages of the menstrual cycle and pregnancy. The secretion of oestrogen and progesterone are controlled by the two hormones that are secreted by the pituitary gland. These hormones are LH (luteinizing hormone) and FSH (follicle-stimulating hormone). Males produce sperm continuously, whereas females only produce one egg once a month.  During ovulation, a follicle develops inside the ovary. As the follicle develops it secretes oestrogen, slowly increasing the concentration of oestrogen in the blood. The oestrogen is responsible for the thickening and spongy development of the uterus wall. During all this time, LH and FSH are being secreted by the pituitary gland. These two hormones stimulate the follicle to continue with its oestrogen secretion. Once the follicle has developed, a surge of LH production (and to a lesser extend FSH production) occurs and ovulation can now take place. The empty follicle no longer secretes oestrogen and it becomes a corpus luteum. The corpus luteum now secretes a different hormone, namely progesterone. The levels of the LH and FSH hormones begin to decrease. It is the responsibility of the progesterone to maintain the thick lining and sponginess of the uterus wall. It also ensures that the uterus wall is supplied with blood. All this is maintained in case the egg is fertilized. The corpus luteum will gradually disappear if the egg is not fertilized. The secretion of progesterone is stopped, and the uterus lining breaks down. Menstruation begins. The development of a new follicle occurs, and the cycle is repeated. The corpeus luteum does not degenerate rapidly if the egg is fertilized. It will continue to secrete the progesterone until the fertilized egg implants and the placenta develops. The placenta now becomes responsible for the secretion of progesterone and it will continue secreting it throughout the pregnancy. Progesterone maintains the uterus lining thus no menstruation occurs during pregnancy. Birth control prevents unwanted pregnancies Birth control can be used to help with family planning. It keeps the family sizes small and limits an increase in the human population. The responsible use of birth control further limits the amount of unwanted children. Methods of birth control include: Using natural methods Sexual abstinence (no sexual intercourse) is practiced during the time that an egg is present in the oviducts. However, this method is very risky as it is only successful if the menstrual cycle is regular and predictable. This method is particularly useful if couples prefer natural birth control methods because of religious beliefs. Women are able to determine their “safe period” when the egg is not in the oviducts by keeping record of her body temperature. There will be a slight rise in her body temperature during ovulation. Body temperature can be used if the cycle is regular. The woman will then abstain from sexual activities on day three and day four on either side of this date. There are few women who have such completely regular and predictable menstrual cycles. It is for this reason that this method of birth control is unreliable and not always successful. Using chemical methods These chemicals are known as spermicides and they kill the sperm. This method works better when it is used in conjunction with another method. Spermicides are inserted on to a diaphragm (cap) and then inserted into the vagina. A different chemical used is the contraceptive pill. This pill disrupts the hormones responsible for the menstrual cycle by stopping the egg production in the ovaries. The contraceptive pill contains oestrogen and progesterone. The pill is taken daily, or it may be in the form of a long-lasting contraceptive injection. In these methods, a physical barrier is placed between the sperm and the egg. The condom is the most widely used mechanical method. It is a thin sheath that is placed over the erect penis. This prevents any sperm from entering into the vagina. An added advantage of using condoms is its ability to prevent the transfer of pathogens. Condoms protect against sexually transmitted diseases such as gonorrhea and HIV/AIDS. A female version of the condom known as the femidom is available. The femidom is placed inside the vagina. It follows the same principles as a normal condom. Another form of mechanical barrier is the cap or diaphragm. It is a circular and slightly domed-shaped. It is inserted into the vagina to cover the cervix. The sperm is then stopped before they are able to cross into the uterus. To  ensure this, the diaphragm works best when a spermicidal cream is used with it. The next method used is the intra-uterine device (IUD). It is placed inside the uterus. The insertion procedure must be performed by a medical doctor (such as a gynecologist). The IUD interferes with the ability of the sperm to find and fertilize the egg. The IUD further prevents implantation and development of an egg that had been fertilized. Using surgical methods Surgical procedures are suitable for couples when they are content with the number of children that they already have. The man will undergo a vasectomy where the sperm ducts are cut and tied. This is a simple procedure that requires a local anaesthetic only. The woman undergoes are more invasive procedure that requires a hospital stay. The procedure is performed under general anaesthetic. Her oviducts are cut and tied. The table summarizes the birth control methods Method Basic principle Advantages Disadvantages Mechanical: Placed over the erect Safe if used Be aware that no Condoms penis. Ejaculated sperm is correctly. semen escapes trapped and prevented Prevents disease before being put from entering vagina. transfer such as on or after it is gonorrhoea and removed. HIV/AIDS. Mechanical: A circular sheet of rubber Effective method. Only effective if Diaphragm is placed over the cervix. placed correctly. Spermicidal cream is Fitted by a applied around its edges. doctor. Sperm present in the vagina cannot cross into the uterus. Method Basic principle Advantages Disadvantages Chemical: Pill containing oestrogen Effective method Must be taken at Contraceptive pill and progesterone. Pill the same time (oral taken daily. Hormones each day. contraception) are similar to pregnancy Unpleasant side hormones. Stops egg effects. production. Regular medical check-ups required. Surgical: Man undergoes Highly successful Procedures are Sterilization vasectomy. Sperm ducts birth control. not always are cut and tied. Sperm No side effects. reversible (tubes cannot travel to the penis. cannot be re- Woman undergoes cutting opened). and tying of oviducts. Egg Not cannot travel to the uterus. recommended for younger people. Natural Careful record is kept of Used by people who Highly unreliable. the woman’s menstrual prefer a more cycle. Ovulation is natural method of predicted and abstinence birth control. is practiced for several days around this time. Using hormones to increase fertility There are people who are unable to have children. The reason for this may lie with either (or both) the man and woman. For example, the sperm that is produced is unhealthy. In this case, the couple may choose to use the sperm from another man (sperm donor). The sperm is collected from the donor in a specialized clinic and stored for several months at extremely low temperatures. The woman will attend this clinic where a doctor places some of the sperm in her vagina. This is known as artificial insemination.  This method assists a couple to have a child that they may not have had otherwise. A major setback may be the difficulty surrounding the biological implications. Biologically, the baby will carry the genetic makeup of the sperm donor. Additional challenges may arise if the child begins to ask questions surround the biological father. Remember that one sperm donor could end up being the father to many children. It is for this reason that some people are of the opinion that child should know who the sperm donor was. Fertility drugs are another method to assist infertile couples. Generally, this method is used by a woman who does not produce sufficient eggs. The woman is given hormones, including FSH to increase her egg production. There are times w