Biology Test 2 Oct 2024 PDF

MRS GREN Movement: an action by an organism or part of an organism causing a change of position or place Respiration: the chemical reactions that break down nutrient molecules in living cells to release energy for metabolism Sensitivity: the ability to detect or sense stimuli in t...

MRS GREN Movement: an action by an organism or part of an organism causing a change of position or place Respiration: the chemical reactions that break down nutrient molecules in living cells to release energy for metabolism Sensitivity: the ability to detect or sense stimuli in the internal or external environment and to make appropriate responses Growth and development: a permanent increase in size and dry mass by an increase in cell number or cell size or both Reproduction: the processes that make more of the same kind of organism Excretion: the removal from organisms of toxic materials,the waste products of metabolism (chemical reactions in cells including respiration) and substances in excess of requirements Nutrition: the taking in of materials for energy, growth and development; plants require light, carbon dioxide, water and ions; animals need organic compounds, ions and usually need water How Organisms are Classified There are millions of species of organisms on Earth A species is defined as a group of organisms that can reproduce to produce fertile offspring These species can be classified into groups by the features that they share e.g. all mammals have bodies covered in hair,feed young from mammary glands and have external ears (pinnas) The Binomial System Organisms were first classified by a Swedish naturalist called Linnaeus in a way that allows the subdivision of living organisms into smaller and more specialised groups The species in these groups have more and more features in common the more subdivided they get  He named organisms in Latin using the binomial system where the scientific name of an organism is made up of two parts starting with the genus (always given a capital letter) and followed by the species (starting with a lowercase letter) When typed binomial names are always in italics (which indicates they are Latin) e.g. Homo sapiens The sequence of classification is: Kingdom, Phylum, Class, Order, Family, Genus, Species Dichotomous Keys Keys are used to identify organisms based on a series of questions about their features Dichotomous means ‘branching into two’ and it leads the user through to the name of the organism by giving two descriptions at a time and asking them to choose Each choice leads the user onto another two descriptions in order to successfully navigate a key You need to pick a single organism to start with, or you may be presented with an unfamiliar one as part of an exam questions Follow the statements from the beginning. Each statement or question you should be able to answer using the information provided in the question or an image given as part of the question. Eventually there will be no more statements or questions left and you will have the name of the organism. You then pick another organism and start at the beginning of the key again, repeating until all organisms are named Using DNA to Classify Organisms (extended) Organisms share features because they originally descend from a common ancestor. Example: all mammals have bodies covered in hair, feed young from mammary glands and have external ears (pinnas). Originally, organisms were classified using morphology (the overall form and shape of the organism, e.g. whether it had wings or legs) and anatomy (the detailed body structure as determined by dissection) As technology advanced, microscopes, knowledge of biochemistry and eventually DNA sequencing allowed us to classify organisms using a more scientific approach Studies of DNA sequences of different species show that the more similar the base sequences in the DNA of two species, the more closely related those two species are (and the more recent in time their common ancestor is) This means that the base sequences in a mammal’s DNA are more closely related to all other mammals than to any other vertebrate groups The Five Kingdoms The first division of living things in the classification system is to put them into one of five kingdoms. Animals Plants Fungi Protoctists Prokaryotes AMNCCO Main features of all animals: they are multicellular their cells contain a nucleus but no cell walls or chloroplasts they feed on organic substances made by other living things PMNCCP Main features of all plants: they are multicellular their cells contain a nucleus, chloroplasts and cellulose cell walls they all feed by photosynthesis FMNCCSPN Main features of all fungi: (e.g. moulds, mushrooms, yeast) usually multicellular cells have nuclei and cell walls not made from cellulose do not photosynthesize but feed by saprophytic (on dead or decaying material) or parasitic (on live material) nutrition PUMNCCPO Main features of all Protoctists: (e.g. Amoeba, Paramecium, Plasmodium) most are unicellular but some are multicellular all have a nucleus, some may have cell walls and chloroplasts meaning some protoctista photosynthesise and some feed on organic substances made by other living things PUCINM Main features of all Prokaryotes: (bacteria, blue-green algae) often unicellular cells have cell walls (not made of cellulose) and cytoplasm but no nucleus or mitochondria The Animal Kingdom Several main features are used to place organisms into groups within the animal kingdom Vertebrates All vertebrates have a backbone There are 5 classes of vertebrates Invertebrates Invertebrates do not possess a backbone One of the morphological characteristics used to classify invertebrates is whether they have legs or not All invertebrates with jointed legs are part of the arthropod phylum The Plant Kingdom (extended) At least some parts of any plant are green, caused by the presence of the pigment chlorophyll which absorbs energy from sunlight for the process of photosynthesis The plant kingdom includes organisms such as ferns and flowering plants Ferns Have leaves called fronds  Do not produce flowers but instead reproduce by spores produced on the underside of fronds Flowering plants Reproduce sexually by means of flowers and seeds Seeds are produced inside the ovary found at the base of the flower Can be divided into two groups – monocotyledons and dicotyledons How do you distinguish between monocotyledons and dicotyledons? Flowers Flowers from monocotyledons contain petals in multiples of 3 while flowers from dicotyledons contain petals in multiples of 4 or 5 Leaves Leaves from monocotyledons have parallel leaf veins while leaves from dicotyledons have reticulated leaf veins Leaves from monocotyledons are narrow and grass-like while leaves from dicotyledons tend to have broad leaves that come in a wide range of shapes Viruses Viruses are not part of any classification system as they are not considered living things They do not carry out the seven life processes for themselves, instead they take over a host cell’s metabolic pathways in order to make multiple copies of themselves Virus structure is simply genetic material (RNA or DNA) inside a protein coat Animal & plant cells Animal cell structure NMCCCG The main features of animal cells: They contain a nucleus with a distinct membrane Cells do not have cellulose cell walls Their cells do not contain chloroplasts (so they are unable to carry out photosynthesis) They contain carbohydrates stored as glycogen Plant cell structure NMCCCS The main features of plant cells: They contain a nucleus with a distinct membrane Cells have cell walls made out of cellulose They contain chloroplasts (so they can carry out photosynthesis) Carbohydrates are stored as starch or sucrose Plant and animal cell structure and function Structure Function Nucleus Contains the DNA (genetic material) which controls the activities of the cell Cytoplasm A gel like substance composed of water and dissolved solutes Supports the internal structures of the cell Site of many chemical reactions Ne  (including anaerobic respiration) Cell membrane Holds the cell together separating the inside of the cell from the outside Controls which substances enter or leave the cell Ribosomes Found in the cytoplasm The site of protein synthesis Mitochondria The site of aerobic respiration Plant cell structure and function Structure Function Cell wall Made of cellulose (a polymer of glucose) Gives the cell extra support, defining its shape Chloroplast Contains the green chlorophyll pigment that absorbs light energy for photosynthesis Permanent vacuole Contains cell sap: a solution of sugar and salt Used for storage of certain materials Helps to support the shape of the S  cell Bacteria cells Bacteria cell structure Bacteria, which have a wide variety of shapes and sizes, all share the following biological characteristics: They are microscopic single-celled organisms V CCCRNDFPL Possess a cell wall (made of peptidoglycan, not cellulose), cell membrane, cytoplasm and ribosomes Lack a nucleus but contain a circular chromosome of DNA A that floats in the cytoplasm Plasmids are sometimes present - these are small rings of DNA (also floating in the cytoplasm) that contain extra genes to those found in the chromosomal DNA They lack mitochondria, chloroplasts and other membrane-bound organelles found in animal and plant cells Some bacteria also have a flagellum (singular) or several flagella (plural). These are long, thin, whip-like tails attached to bacteria that allow them to move Examples of bacteria include: Lactobacillus (a rod-shaped bacterium used in the production of yoghurt from milk) Pneumococcus (a spherical bacterium that acts as the pathogen causing pneumonia) Identifying cell structures & function Within the cytoplasm, the following organelles are visible in almost all cells except prokaryotes when looking at higher magnification (ie using an electron microscope): Mitochondria (singular: mitochondrion) are organelles found throughout the cytoplasm Ribosomes are tiny structures that can be free within the cytoplasm or attached to a system of membranes within the cell known as Endoplasmic Reticulum  Endoplasmic reticulum studded with ribosomes looks rough under the microscope; this gives rise to its name of Rough Endoplasmic Reticulum (often shortened to R.E.R.) Vesicles can also be seen using a higher magnification - these are small circular structures found moving throughout the cytoplasm Producing New Cells The cells in your body need to be able to divide to help your body grow and repair itself Cells grow and divide over and over again New cells are produced by the division of existing cells Specialised Cells Specialised cells in animals Specialised cells are those which have developed certain characteristics in order to perform particular functions. These differences are controlled by genes in the nucleus Cells specialise by undergoing differentiation: this is a process by which cells develop the structure and characteristics needed to be able to carry out their functions Cell Function Adaptations Ciliated cell Movement of mucus in Extensions of the cytoplasm at the surface of the trachea and the cell from hair - like structures called cilia bronchi which beat to move mucus and trapped particles up to the throat Nerve cell Conduction of Long so that nerves can run to and from impulses different parts of the body to the central nervous system The cell has extensions and branches, so that it can communicate with other nerve cells, muscles and glands The axon (extension of cytoplasm away from the cell body) is covered with a fatty stealth, which insulates the nerve cell and speeds up the nerve impulse Red blood cell Transport of oxygen Biconcave disc shape increases surface area for more efficient diffusion of oxygen Contains haemoglobin which joins with oxygen to transport it Contains no nucleus to increase amount of space available for haemoglobin inside cell Sperm cell Reproduction The head contains the genetic material for fertilisation in a haploid nucleus (containing half of the normal number of chromosomes) The acrosome in the head contains digestive enzymes so that sperm can penetrate an egg The mid-piece is packed with mitochondria to release energy needed to swim and fertilise the egg The tail enables the sperm to swim Egg cell (ovum) Reproduction Contains a lot of cytoplasm which has nutrients for the growth of the early embryo Haploid nucleus contains the genetic material for fertilisation Cell membrane changes after fertilisation by a single sperm so that no more sperm can enter Ciliated cell Nerve cell Red blood cell Sperm cell  Sperm cell Examples of specialised cells in plants: Root hair cell Absorption of water Root hair increases surface area of and mineral ions cell to ensure maximum absorption from soil of water and mineral ions Walls are thin to ensure water moves through quickly No chloroplasts present Xylem vessel Conduction of water No top and bottom walls between through the plant; xylem vessels, so there is a support of the plant continuous column of water running through them Cells are dead without organelles of cytoplasm to allow free passage of water Their walls become thickened with a substance called lignin which means they are able to help support the plant Palisade Photosynthesis Column shaped to maximise mesophyll cell absorption of sunlight and fit as many in a layer under the upper epidermis of the leaf as possible Contains many chloroplasts from maximum photosynthesis Root hair cell Xylem structure  Palisade mesophyll cell Levels of Organisation in an Organism Organ system Organs Tissue examples Shoot system Leaf, stem, flower, fruit Epidermis mesophyll Xylem Phloem Root system Root, tuber Xylem Phloem Ground tissue Digestive system Oesophagus, stomach, small Muscle  intestine, large intestine Connective Nerve Epithelial Circulatory system Heart, veins, arteries Muscle Connective Nerve Epithelial Immune system Thymus, spleen Bone marrow Respiratory system Trachea, bronchi, lungs Connective Muscle Epithelial Excretory system liver , kidney, skin, lungs Muscle Connective Epithelial Nerve Nervous system Brain, spinal cord Nerve Reproductive system Ovary, cervix, uterus, vagina, Muscle testes, penis Connective Nervous Erectile Calculating magnification and specimen size using millimetres as units Magnification is calculated using the following equation: Magnification = Image size ÷ Actual size A better way to remember the equation is using an equation triangle: Rearranging the equation to find things other than the magnification becomes easy when you remember the triangle - whatever you are trying to find, place your finger over it and whatever is left is what you do, so: Magnification = image size / actual size Actual size = image size / magnification  Image size = magnification x actual size Remember magnification does not have any units and is just written as ‘x 10’ or ‘x 5000’ To find the actual size of the cell: Using millimetres and micrometres as units The table below shows how millimetres are related to two other measures of length What this basically means is that 1mm = 1000µm and 1cm = 10,000µm This usually comes up in questions where you have two different units and you need to ensure that you convert them both into the same unit before proceeding with the calculation For example: The human nervous system consists of the: central nervous system (CNS) - the brain and the spinal cord peripheral nervous system (PNS) - all of the nerves in the body It allows us to Make sense of our surroundings and respond to them Coordinate and regulate body functions Information is sent through the nervous system as nerve impulses - electrical signals that pass along nerve cells known as neurons A bundle of neurons is known as a nerve Neuron Diagrams There are three main types of neurons: sensory, relay and motor Sensory neurons carry impulses from sense organs to the CNS (brain or spinal cord) Relay neurons (also known as intermediate neurons) are found inside the CNS and connect sensory and motor neurons Motor neurons carry impulses from the CNS to effectors (muscles or glands) Neurones have a long fibre (axon) This means that less time is wasted transferring the impulse from one cell to another The axon is insulated by a fatty sheath with small uninsulated sections along it (called nodes) This means that the electrical impulse does not travel down the whole axon, but jumps from one node to the next Their cell body contains many extensions called dendrites This means they can connect to many other neurons and receive impulses from them, forming a network for easy communication Sensory neurons are long and have a cell body branching off the middle of the axon  Relay neurons are short and have a small cell body at one end with many dendrites branching off it Motor neurons are long and have a large cell body at one end with long dendrites branching off it Identifying the types of neurons: The Reflex Arc Voluntary Responses A voluntary response is one where you make a conscious decision to carry out a particular action therefore it starts with your brain An example is reaching out to pick up a cup of coffee An involuntary (or reflex) response does not involve the brain as the coordinator of the reaction and you are not aware you have completed it until after you have carried it out Involuntary actions are usually ones which are essential to basic survival and are rapid, whereas voluntary responses often take longer as we consider what the consequences might be before doing it Reflex Responses An involuntary (or reflex) response does not involve the brain as the coordinator of the reaction and you are not aware you have completed it until after you have carried it out  This is an automatic and rapid response to a stimulus such as touching something sharp or hot As it does not involve the brain, a reflex response is quicker than any other type of nervous response This helps to minimise the damage to the body A reflex action 1. The pin (the stimulus) is detected by a pain/pressure/touch receptor in the skin 2. Sensory neuron sends electrical impulses to the spinal cord (the coordinator) 3. Electrical impulse is passed on to relay neurone in the spinal cord 4. Relay neuron connects to motor neurone and passes the impulse on 5. Motor neuron carries impulse to a muscle in the leg (the effector) 6. The muscle will contract and pull the foot up and away from the sharp object (the response) Synapses Where two neurons meet or join, they do so at a junction called a synapse Synapses allow junctions between neurons so are important in the nervous system being a connected network of neurons Nerve impulses can transmit across synapses and be directed along the appropriate route by them eg. to the correct part of the brain Think about the analogy of railway points that guide the trains onto the appropriate tracks based on that train's destination. Reflex actions are: 1. Automatic 2. Fast 3. Protective Structure of a Synapse (Extended) The junction between two neurons is known as a synapse Synapses & Neurotransmitters (Extended) Neurons never touch each other The junctions (gaps) in between them are called synapses The electrical impulse travels along the first axon This triggers the nerve-ending of the presynaptic neuron to release chemical messengers called neurotransmitters from vesicles which fuse with the presynaptic membrane The neurotransmitters diffuse across the synaptic gap (or cleft) and bind with receptor molecules on the membrane of the second neuron (known as the postsynaptic membrane) This stimulates the second neuron to generate an electrical impulse that travels down the second axon The neurotransmitters are then destroyed to prevent continued stimulation of the second neuron which would cause repeated impulses to be sent Synapses ensure that impulses only travel in one direction, avoiding confusion within the nervous system if impulses were travelling in both directions As this is the only part of the nervous system where messages are chemical as opposed to electrical, it is the only place where drugs can act to affect the nervous system - eg. this is where heroin works # Sense Organs as Receptors Receptors are groups of specialised cells They detect a change in the environment and stimulate electrical impulses in response Sense organs contain groups of receptors that respond to specific stimuli  Once the receptor cell in the sense organ has been stimulated, it generates an electrical impulse This is passed on to a sensory neuron which carries the impulse to the central nervous system Here a response will be decided on and the impulse will be passed to a motor neuron (via a relay neurone) The motor neuron carries the impulse to the effector (muscle or gland) The effector carries out the response Eye structure The eye is a sense organ containing receptor cells that are sensitive to light The structure of the eye allows it to carry out its function; important structural features include the: Cornea Iris Lens Retina optic nerve + The pupil reflex The pupil reflex is an example of a reflex action; its role is to control the light that enters the eye by altering the pupil diameter In dim light the pupil dilates in order to allow as much light into the eye as possible In bright light the pupil constricts in order to prevent too much light entering the eye and damaging the retina Iris muscles (extended) The pupil reflex occurs due to changes in the iris muscles The iris muscles work together to regulate the amount of light entering the eye The iris contains circular muscles and radial muscles The circular muscles form circles around the pupil The radial muscles radiate outwards from the pupil The circular and radial muscles of the iris are antagonistic, meaning that they work against each other When one set of muscles contracts the other relaxes, and vice versa Iris muscles in dim light When light levels are low the pupil reflex acts to dilate the pupil and maximise the light entering the eye; this is achieved as follows: light receptors in the eye detect low light levels the radial muscles contract and the circular muscles relax the pupil dilates Iris muscles in bright light When light levels are high the pupil reflex acts to constrict the pupil to reduce light entering the eye and protect the retina; this occurs as follows: light receptors in the eye detect bright light the radial muscles relax and the circular muscles contract the pupil constricts The iris muscles and light levels Eye accommodation (extended) Accommodation is the term used to describe the way in which the eye focuses on near or distant objects During eye accommodation the shape of the lens is changed, altering the extent to which light is refracted; this change is brought about by: contraction or relaxation of the ciliary muscles adjustment of tension in the suspensory ligaments Eye accommodation and near objects When an object is close up: the ciliary muscles contract the suspensory ligaments loosen  the suspensory ligaments exert less pull on the lens, allowing the lens to become more rounded light is refracted more Eye accommodation and near objects diagram When an object is far away: the ciliary muscles relax the suspensory ligaments tighten the suspensory ligaments pull on the lens, causing it to become thinner light is refracted less Eye accommodation Rods & cones (extended) Rods and cones are the two types of receptor cell present in the retina of the eye Rod cells and cone cells have different roles in detecting light stimuli: Rods can detect light at low levels, so play an important role in night vision Three different types of cones can detect light at three different wavelengths, enabling colour vision Rods and cones are not distributed evenly across the retina: Rod cells are found all over the retina, with the exception of the blind spot Cone cells are concentrated in the fovea, the region of the eye onto which light is focused by the process of accommodation The fovea enables the brain to form sharp, coloured images when light is effectively focused by the eye Hormones & Their Associated Glands What is a Hormone? A hormone is a chemical substance produced by a gland and carried by the blood  The hormone alters the activity of one or more specific target organs i.e. they are chemicals which transmit information from one part of the organism to another and bring about a change The glands that produce hormones in animals are known collectively as the endocrine system Transport around the body Endocrine glands have a good blood supply as when they make hormones they need to get them into the bloodstream (specifically the blood plasma) as soon as possible so they can travel around the body to the target organs to bring about the response Hormones only affect cells with target receptors that the hormone can bind to. These are either found on the cell membrane, or inside cells. Receptors have to be complementary to hormones for there to be an effect. The liver regulates levels of hormones in the blood; transforming or breaking down any that are in excess. Important hormones in the human body: Comparison of Nervous & Hormonal Control Glucagon (Extended) Blood glucose levels are controlled by a negative feedback mechanism involving the production of two hormones - insulin and glucagon Both hormones which control blood glucose concentration are made in the pancreas Insulin is produced when blood glucose rises and stimulates liver and muscle cells to convert excess glucose into glycogen to be stored Glucagon is produced when blood glucose falls and stimulates liver and muscle cells to convert stored glycogen into glucose to be released into the blood The Hormone Adrenaline Adrenaline is known as the 'fight or flight' hormone as it is produced in situations where the body may be in danger Flight = remove oneself rapidly from a dangerous situation eg. run away Fight = if flight is not possible, resort to physical combat to overcome danger  It causes a range of different things to happen in the body, all designed to prepare it for movement (ie fight or flight). These include: Increasing blood glucose concentration for increased respiration in muscle cells Increasing pulse rate and breathing rate so glucose and oxygen can be delivered to muscle cells, and carbon dioxide taken away, from muscles cells more quickly Diverting blood flow towards muscles and away from non-essential parts of the body such as the alimentary canal; again to ensure the reactants of respiration are as available as possible Dilating pupils to allow as much light as possible to reach the retina so more information can be sent to the brain More on Adrenaline More on Adrenaline (Extended) Additional effects of adrenaline include; Increasing the concentration of glucose in the blood This help deliver more important glucose to muscles for respiration Increasing heart rate This has the same effect, to ensure that all muscles are well prepared for high levels of activity in a flight or fight situation Homeostasis: Definition Homeostasis is defined as the maintenance of a constant internal environment Role of Insulin in Homeostasis Homeostasis means that internal conditions within the body (such as temperature, blood pressure, water concentration, glucose concentration etc) need to be kept within set limits in order to ensure that reactions in body cells can function and therefore the organism as a whole can live When one of these conditions deviates far away from the normal if not brought back within set limits the body will not function properly and the eventual consequence without medical intervention will be death The Role of Insulin Insulin is secreted into the blood at times when blood glucose levels are high This is (most often) directly after a meal The kidneys can only cope with a certain level of glucose in the blood If the level gets too high, glucose gets excreted and is lost in the urine This is like running a car with a hole in the petrol tank; valuable fuel is being wasted  To avoid this, insulin temporarily converts excess glucose into glycogen in the liver and muscles Insulin decreases blood glucose concentration The glycogen is converted back to glucose several hours later when blood glucose levels have dipped due to respiration in all tissues The Concept of Negative Feedback (Extended) Negative feedback occurs when conditions change from the ideal or set point and returns conditions to this set point It works in the following way: if the level of something rises, control systems are switched on to reduce it again if the level of something falls, control systems are switched on to raise it again Negative feedback mechanisms are usually a continuous cycle of bringing levels down and then bringing them back up so that overall, they stay within a narrow range of what is considered ‘normal’ Type 1 Diabetes (Extended) Type 1 diabetes is a condition where the blood glucose levels are not able to be regulated as the insulin-secreting cells in the pancreas are not able to produce insulin This means that blood glucose levels are often far too high It can be treated by injecting insulin The extra insulin causes the liver to convert glucose into glycogen, which reduces the blood glucose level Symptoms of diabetes include extreme thirst, weakness or tiredness, blurred vision, weight loss and loss of consciousness in extreme cases People with Type 1 diabetes have to monitor their blood glucose levels throughout the day as their levels of physical activity and their diet affect the amount of insulin needed They can help to control their blood glucose level by being careful with their diet - eating foods that will not cause large increases in blood glucose level, and by exercising, which can lower blood glucose levels due to increased respiration in the muscles The Skin & Homeostasis (Extended) Control of body temperature is a homeostatic mechanism Homeostasis is the maintenance of a constant internal environment This means that internal conditions within your body (such as temperature, blood pressure, water concentration, glucose concentration etc) need to be kept within set limits in order to ensure that reactions in body cells can function and therefore the organism as a whole can live The human body maintains the temperature at which enzymes work best, around 37°C If body temperature increases over this temperature, enzymes will denature and become less effective at catalysing reactions such as respiration The Structure of the Skin Temperature Regulation by the Skin Regulation is controlled by the brain which contains receptors sensitive to the temperature of the blood The skin also has temperature receptors and sends nervous impulses to the brain via sensory neurons The brain responds to this information by sending nerve impulses to effectors in the skin to maintain the temperature within a narrow range of the optimum, 37°C Fatty tissue under the dermis acts as a layer of insulation to prevent too much body heat being lost through the skin Responses to changes in temperature: Vasoconstriction & Vasodilation (Extended) When we are cold blood flow in capillaries slows down because arterioles leading to the skin capillaries get narrower - this is known as vasoconstriction This reduces the amount of heat lost from blood by radiation as less blood flows through the surface of the skin When we are hot blood flow in capillaries increases because blood vessels to the skin capillaries get wider - this is known as vasodilation This cools the body as blood (which carries heat around the body) is flowing at a faster rate through the skin’s surface and so more heat is lost by radiation Gravitropism & Phototropism Plants can respond to changes in environment (stimuli) for survival, e.g. light, water, gravity Their responses are usually much slower than animals They grow either towards a stimulus (known as a positive response) or away from a stimulus (known as a negative response) The responses are known as tropisms It is very important to a plant that its roots and shoots grow in the right directions Shoots must grow upwards, away from gravity and towards light, so that leaves are able to absorb sunlight This means that shoots have a positive phototropic response and a negative gravitropic response Roots need to grow downwards into the soil, away from light and towards gravity, in order to anchor the plant and absorb water and minerals from the soil particles. This means that roots have a negative phototropic response and a positive gravitropic response Investigating Tropisms Investigating Phototropisms Three identical plants are set up as shown below (A, B and C)  The seedlings in A grow towards the light source In B the effect of the light only coming from one direction has been cancelled out by using a clinostat (it revolves slowly and repeatedly, so the shoots are evenly exposed to light) This means all sides of the seedlings get an equal amount of light so they do not curve towards the light source but grow straight up In C the seedlings grow straight up looking for light and the plant becomes tall and slender with yellowing leaves due to the lack of light Investigating Gravitropisms Add some damp cotton wool to two petri dishes Place 3 bean seedlings in the cotton wool in each petri dish A - radicle facing downwards B - horizontally C - radicle (root grows from here) facing upwards Cover each dish with a lid Attach one petri dish to a support so that it’s on its side Attach the second petri dish to a clinostat (as shown in the diagrams above). Place both in a light-proof box (so that the seedlings are in complete darkness), leave for two days and then observe growth of the seedlings Investigating the gravitropic response (results) In the first petri dish all radicles (roots) have grown downwards (positive gravitropic response) regardless of which way they were initially facing (horizontal, up or down) and all plumules (shoots) have grown upwards (negative gravitropic response) In the second petri dish, all radicals and all plumules have all grown neither up nor down but straight outwards in whichever direction they were placed as the effect of gravity has been cancelled out by the revolving of the clinostat - they have shown no gravitropic response at all The experiment needs to be done in a light proof box in order to cancel out the effect of light on the growth of the seedlings Auxins: Chemical Control of Tropisms (Extended) Plants respond to stimuli by producing a growth hormone called auxin which controls the direction of growth of roots or stems Therefore we say plants control their growth chemically Auxin is mostly made in the tips of the growing stems and roots and can diffuse to other parts of the stems or roots; spreading from a high concentration in the shoot tips down the shoot to an area of lower concentration Auxin stimulates the cells behind the tip to elongate (get larger); the more auxin there is, the faster they will elongate and grow Only the region behind the tip of a shoot is able to contribute to growth by cell division and cell elongation This part of a shoot is called the meristem How does phototropism occur in plants? If light shines all around the tip, auxin is distributed evenly throughout and the cells in the meristem grow at the same rate - this is what normally happens with plants growing outside When light shines on the shoot predominantly from one side though, the auxin produced in the tip concentrates on the shaded side, making the cells on that side elongate and grow faster than the cells on the sunny side This unequal growth on either side of the shoot causes the shoot to bend and grow in the direction of the light The role of auxin can be tested using seedlings placed in a box that has a slit on one side, only allowing light in from one direction: Investigating the phototropic response results How does gravitropism occur in plants? Auxin plays a role in a plant's response to gravity, affecting plant shoots and roots in different ways When shoots grow away from gravity it is known as negative gravitropism Gravity modifies the distribution of auxin so that it accumulates on the lower side of the shoot As seen in the phototropic response, auxin increases the rate of growth in shoots, causing the shoot to grow upwards When roots grow towards gravity it is known as positive gravitropism In roots, higher concentrations of auxin results in a lower rate of cell elongation The auxin that accumulates at the lower side of the root inhibits cell elongation As a result, the lower side grows at a slower rate than the upper side of the root This causes the root to bend downwards What are drugs? Drugs definition A drug is any substance taken into the body that modifies or affects chemical reactions in the body Some drugs are medicinal drugs that are used to treat the symptoms or causes of a disease - for example, antibiotics The liveris the primary site for drug metabolism Antibiotics What are antibiotics? Antibiotics are chemical substances made by certain fungi or bacteria that affect the working of bacterial cells, either by disrupting their structure or function or by preventing them from reproducing. Antibiotics are effective against bacteria but not against viruses. Antibiotics target processes and structures that are specific to bacterial (prokaryotic) cells; as such they do not generally harm animal cells. Antibiotic resistance (extended) Commonly prescribed antibiotics are becoming less effective due to a number of reasons: overuse and being prescribed when not really necessary patients failing to complete the fully prescribed course by a doctor large scale use of antibiotics in farming to prevent disease when livestock are kept in close quarters, even when animals are not actually sick  This has led to the effectiveness of antibiotics being reduced, and the incidence of antibiotic resistance increasing These bacteria are commonly known as superbugs and the most common is MRSA How to prevent antibiotic resistance Ways individuals can help prevent the incidence of antibiotic resistance increasing include: only taking antibiotics when absolutely essential when prescribed a course of antibiotics, ensure that the entire course is completed even if you feel better after a few days