Biology Course Notes Unit 1.3. Animals PDF

Biology Course Notes Unit 1.3. | Animals What is an animal? Animals are classified into Kingdom Animalia. The Latin word ‘animalia’ means ‘to breathe’. All animals require oxygen and excrete carbon dioxide. Animals are eukaryotic, multicellular, and heterotrophic. As heterotrophs food molecules ar...

Biology Course Notes Unit 1.3. | Animals What is an animal? Animals are classified into Kingdom Animalia. The Latin word ‘animalia’ means ‘to breathe’. All animals require oxygen and excrete carbon dioxide. Animals are eukaryotic, multicellular, and heterotrophic. As heterotrophs food molecules are obtained from other living things. The food is digested in an internal chamber. Animals can move, even if only for part of their life cycle. In animals, the embryo passes through a stage called the blastula, a hollow, fluid filled ball. UNIT 1.3. | ANIMALS Date | 01/12/2023 1 Revision | 7 BIOLOGY PROGRAM UNIT 3 TABLE OF CONTENTS We will explore the biology of animals through engaging with the following questions: Chapter Chapter title Page 1 How are animal bodies organised, and how are internal conditions 3 maintained? 2 How does internal transport work in different animals? 27 3 How does gas exchange work in different animals? 43 4 How do cells communicate and why? 64 5 How are human bodies protected from infection? 92 6 How can protection from disease be enhanced? 115 UNIT 1.3. | ANIMALS Date | 01/12/2023 2 Revision | 7 BIOLOGY PROGRAM Chapter 1 | How are animal bodies organised, and how are internal conditions maintained? Key Concepts Organisation - Multicellular animals are composed of many cells. These cells are organised into tissues, tissues into organs and organs into organ systems. Cells – intracellular, extracellular Extracellular matrix. Junctions -tight, adhesion and gap junctions Maintenance – replace and/or repair Tissues – Cells with a common function That process of trying to maintain your balance is the essence of Epithelial homeostasis: the tendency of a living system to maintain its internal stability as a result of the coordinated responses of its parts to any Connective stimulus that would tend to disturb its normal conditions. Muscle https://sciencemusicvideos.com/ap-biology/animal-homeostasis- Nervous and-regulation/homeostasis-1-key-concepts-thermoregulation/ Organs and Organ systems Homeostasis - the maintenance of a constant internal environment. UNIT 1.3. | ANIMALS Date | 01/12/2023 3 Revision | 7 BIOLOGY PROGRAM  Learning Outcomes On completion of this topic students should have achieved the following learning outcomes: Level of Section Outcome achievement Describe that animal bodies are made from cells, tissues, organs, and organ systems, and explain the relationship 1.1 between each of these levels of structure. 1.2 Briefly describe the extracellular matrix with regards to its structure and function. 1.2 Identify and describe the purpose of tight, adhesion and gap (communicating) junctions. 1.2 Compare and contrast regenerative and permanent tissues and provide one example of each. 1.3 Identify that the four tissue types found in the animal body is epithelial, connective, muscle and nervous. Briefly describe the structure for an epithelial tissue and label the apical surface, basal surface, epithelial cells, and 1.3 basement membrane. 1.3 Provide a specific name for an epithelial tissue based on (i) the number of layers of cells and (ii) the shape of the cells. 1.3 Identify that glands are epithelial tissue. 1.3 Distinguish between endocrine and exocrine glands and provide an example of a product produced by each. 1.3 Name the three main components of a connective tissue. UNIT 1.3. | ANIMALS Date | 01/12/2023 4 Revision | 7 BIOLOGY PROGRAM  Outcome Level of Section achievement 1.3 Identify each muscle type from an illustration and justify your response. 1.3 Name the two main cell types of nervous tissue and describe their general function. 1.3 Label a simple diagram of a motor neuron, indicating the direction of travel of an electrical impulse through the cell. 1.4 Describe the four features common to all organs. 1.5 Explain the role of a sensor, a control centre and an effector in a homeostatic response. 1.5 Describe a negative feedback system with respect to homeostasis. Describe the body’s response to change in temperature (too high or too low). State which organs or organ systems are 1.5 involved. UNIT 1.3. | ANIMALS Date | 01/12/2023 5 Revision | 7 BIOLOGY PROGRAM  Chapter 1 Index Section Section title Page 1.1 Organisation in animals 7 1.2 Cells 8 1.3 Tissues 11 1.4 Organs 18 1.5 Organ Systems 19 1..6 Homeostasis 20 UNIT 1.3. | ANIMALS Date | 01/12/2023 6 Revision | 7 BIOLOGY PROGRAM Section 1.1 | Organisation in animals When its size approaches the limits of efficiency a cell will divide. In unicellular organisms this results in two new individuals. In a multicellular organism the new cells remain associated with the individual. Animals (and plants) can grow to large sizes because they are composed of many cells. The larger an animal, the greater the number of cells. Multicellularity allows specialisation of cells. Organisation and function LEVEL OF ORGANISATION FUNCTION 1 Cells Basic unit of organisation 2 Tissues A group of similar, differentiated cells and their associated extracellular matrix, that work together to carry out a particular function. 3 Organs A structure composed of two or more tissues with a specific function. 4 Organ systems A group of organs with a common function. UNIT 1.3. | ANIMALS Date | 01/12/2023 7 Revision | 7 BIOLOGY PROGRAM Section 1.2 | Cells (i) The extracellular matrix - materials outside of the cell Animal cells have no cell wall and therefore the plasma (cell) membrane of neighbouring cells may come into direct contact. When not in direct contact cells are separated by a fluid (interstitial) containing proteins and polysaccharides. The proteins and polysaccharides are produced by the cells of the tissue. Some of the proteins may be adhesive (for example: elastic fibres, collagen). These adhesive proteins link cells to each other and so hold the cells in tissues together. Figure 1.1 | The extracellular matrix may contain proteins and polysaccharides in fluid (Mader, 2010). UNIT 1.3. | ANIMALS Date | 01/12/2023 8 Revision | 7 BIOLOGY PROGRAM  (ii) Junctions These are connections between cells. The plasma membranes of neighbouring cells are connected by junctions. Cell junctions have different functions. Tight junctions - the plasma membrane proteins of neighbouring cells attach to each other. This holds the two membranes together and forms a barrier. This prevents the movement of substances through the extracellular space. Adhesion junctions - cells are held together into strong sheets. Filaments from the cytoskeleton of each cell extend through and link to each other. Gap (communicating) junctions - cells are linked to each other by a channel. This allows ions, sugars and other small molecules to pass between cells. Figure 1.2 | Junctions between gut epithelial cells (Knox et al., 2005) UNIT 1.3. | ANIMALS Date | 01/12/2023 9 Revision | 7 BIOLOGY PROGRAM  (iii) Maintenance (replacing /repairing) The tissues need to be maintained for the lifetime of an animal. For regenerative tissues new cells can be produced while for permanent tissues no new cells are made. Regenerative tissue | In a regenerative tissue if damaged or worn out, the cells will be replaced. The rate at which cells are replaced depends on the function of the tissue. For example, skin cells undergo continual wear and tear. Red blood cells (move oxygen in the circulatory system) are constantly being bumped about and damaged. These cells must be replaced constantly. Some cells have become so specialised that they have lost their ability to divide. For example, mature red blood cells in humans have no nucleus and very few organelles, and function to carry oxygen in the circulatory system. They do not have the ability to divide yet they must be replaced. These types of cells are replaced by division of small populations of relatively unspecialised cells known as stem cells. Stem cells | are unspecialised groups of cells that are able to divide many times during the lifetime of an animal. Red blood cells are produced by stem cells that are present in the red marrow in the bone. Stem cell regeneration occurs in cases where there is a need for replacement of cells yet the fully differentiated cell cannot undergo cell division. E.g., red blood cells; mature sperm cells Permanent tissues | In some tissues NO new cells are produced. Cells are produced during development and must last the entire life of the animal. Examples include nerve cells, cells of the retina and lens, oocytes and cardiac muscle cells. Most permanent cells are able to repair themselves by repairing or producing new cell components. UNIT 1.3. | ANIMALS Date | 01/12/2023 10 Revision | 7 BIOLOGY PROGRAM Section 1.3 | Tissues (i) Epithelial Tissues 1. Covers all the body surfaces both internal and external, and forms glands. 2. Performs a wide variety of functions, including protection, absorption, secretion & sensation. 3. Consists of layer(s) of cells fitted tightly together to form a sheet of cells. 4. The number of layers of cells and the shape of cells varies. Epithelia are defined according to the number of layers of cells and the cell shape. a) simple - single layer of cells; stratified – more than one layer. b) cuboidal – cube shaped; columnar- column shaped; squamous – flattened shape. 5. Epithelial tissue that lines a body surface has a free surface called the apical surface. This surface is not attached to any structures. The basal surface is attached to the basement membrane. Apical Surface Figure 1.3 | The free surface is the apical surface. The attached surface is the basal surface, and the cells are attached to the basement membrane. (https://www.quora.com/What-are-the- functions-of-simple-cuboidal-epithelium) Epithelial cell Basal Surface Basement membrane UNIT 1.3. | ANIMALS Date | 01/12/2023 11 Revision | 7 BIOLOGY PROGRAM  Simple Cuboidal epithelial tissue Simple Columnar epithelial tissue Stratified squamous epithelial tissue Figure 1.4 | Examples of epithelial tissues (Knox et al., 2005). UNIT 1.3. | ANIMALS Date | 01/12/2023 12 Revision | 7 BIOLOGY PROGRAM The organisation of epithelial cells into glands Glands consist of one or more cells specialised to produce and secrete a product such as sweat, saliva, hormones, or enzymes. Exocrine glands secrete their products onto a free epithelial surface (usually through a duct to the surface). They are involved in the secretion of sweat, saliva. Endocrine glands secrete their product (hormones) directly into the surrounding tissue (do not have a duct). Usually, the hormone enters the circulatory system to be transported to its site of action. Figure 1.5 | Endocrine and exocrine glands (Mader, 2010) UNIT 1.3. | ANIMALS Date | 01/12/2023 13 Revision | 7 BIOLOGY PROGRAM  (ii) Connective Tissues Connective tissues provide structural, metabolic, and defensive support for other body tissues. Connective tissues are composed of cells, fibres, and a matrix. The consistency of the matrix has significant effect on the properties of the connective tissue. The matrix may be soft and fluid (adipose tissue), firm or hard (cartilage and bone) or liquid (blood). Non-specific connective tissues include loose and dense (fibrous) connective tissues. Their role is to connect tissues to each other. Specific connective tissues include cartilage, bone, blood, and adipose tissues. The specific connective tissue connects tissues to each other but also have a secondary role: o Cartilage provides structural support. It is firm yet elastic. o Bone provides structural support. Bone matrix consists of collagen and other organic molecules but also hydroxyapatite crystals (which are calcium salts). The calcium salts make bone very hard, but the collagen prevents it from being too brittle. o Blood provides defensive support and transports materials. o Adipose tissues provide metabolic support as it stores lipids. UNIT 1.3. | ANIMALS Date | 01/12/2023 14 Revision | 7 BIOLOGY PROGRAM  Figure1.6 | Some examples of connective tissues (Knox et al., 2005). UNIT 1.3. | ANIMALS Date | 01/12/2023 15 Revision | 7 BIOLOGY PROGRAM  (iii) Muscle Tissue Most animals move as a result of contraction of muscle cells. Muscle cells are referred to as fibres because the cells are elongated. Muscles cells contain myofilaments – these are highly organised structures made up of fibrous proteins. (The main proteins involved are actin and myosin.) Myofilaments produce contraction or shortening of muscle cells. Multinucleated = more than one nucleus per cell. Uninucleated = single nucleus per cell. Figure 1.7 | There are three types of muscle. These are skeletal, cardiac and smooth muscle (Mader, 2010). UNIT 1.3. | ANIMALS Date | 01/12/2023 16 Revision | 7 BIOLOGY PROGRAM  (iv) Nervous Tissue Nerve cells are neurons. They are specialised to carry information rapidly and precisely from one part of the animal body to another, often over long distances. Neurons have an enlarged cell body (containing the nucleus) and two types of extensions - dendrites and axons. Dendrites (several) receive impulses and transmit them to the cell body, while the single axon transmits impulses away from the cell. Neuroglia are cells that support and nourish the neurons. Schwann cells are neuroglia that encircle the axon and form the myelin sheath. Schwann cells are one type of neuroglia, there are others. Figure 1.8 | Nervous tissue (Mader, 2010). UNIT 1.3. | ANIMALS Date | 01/12/2023 17 Revision | 7 BIOLOGY PROGRAM Section 1.4 | Organs Organ - A structure composed of two or more different tissues with a specific function. All organs share four features: 1. Tissue - related to the main function of the organ. 2. Connective tissue - connects the tissues within the organ to each other and to the rest of the body. 3. Blood supply - transporting respiratory gases, nutrients etc. 4. Nerve supply - regulate functioning of the organ. 1. http://www.thehealthsite.com/diseases-conditions/tips-to-prevent-kidney-disease-sh11/ (KIDNEY) 2. https://medicalxpress.com/news/2017-02-immigrants-lungs-bacteria-disease.html (LUNGS) 3. http://pbiv.com/heart-health/ (HEART) UNIT 1.3. | ANIMALS Date | 01/12/2023 18 Revision | 7 BIOLOGY PROGRAM Section 1.5 | Organ Systems Organs usually do not work as independent units, but instead they work as part of a group of organs with a common function, called an organ system. Main organ systems in complex animals include digestive, muscular, integumentary, lymphatic, endocrine, nervous, skeletal, reproductive, urinary, and circulatory. Figure 1.9 | Simple representation of the organ systems (https://biologydictionary.net/body-systems/) UNIT 1.3. | ANIMALS Date | 01/12/2023 19 Revision | 7 BIOLOGY PROGRAM Section 1.6 | Homeostasis Homeostasis - the maintenance of a constant internal environment. Homeostatic mechanisms have three components: a sensor (receptor) - detects a change to ‘normal’ body condition, a control centre - organises a response and signals the effector, an effector - effects the change aimed at returning to ‘normal’ body condition. Homeostasis is maintained by negative feedback systems. Negative feedback A sensor detects a change in the internal environment and the control centre initiates an effect to bring conditions back to normal. Homeostasis of body temperature The core temperature is the temperature of body structures below the skin and subcutaneous layers. The set point is about 37°C. Slight temperature changes have a dramatic effect on the body’s metabolism. The hypothalamus regulates body temperature. (i) Temperature - HIGH FEEDBACK SYSTEM ACTION Sensor /Receptor Detects temperature increase – thermoreceptors on skin surface. Control centre Brain (Hypothalamus) Effectors Sweat gland activity increases. Blood vessels near skin surface dilate. Lowering of metabolic rate and muscle tone. Behavioural responses include moving into the shade and taking off some clothing. UNIT 1.3. | ANIMALS Date | 01/12/2023 20 Revision | 7 BIOLOGY PROGRAM What happens if temperature reduction mechanisms are not effective? Hyperthermia - If the core temperature reaches about 42°C (107°F), the heartbeat becomes irregular, oxygen levels in the blood drop, the liver ceases to function, unconsciousness and death soon follow. Fever – The set point (controlled by the hypothalamus) can be reset to a higher temperature. This is an effective way for the body to fight an infection. It occurs when white blood cells that fight infection release of chemicals (pyrogens) which travel to the hypothalamus and result in the set point being raised. The body then runs at a slightly higher temperature. Increased temperature allows the activity of the immune system to increase and is detrimental to many disease-causing organisms and viruses. A mild fever can therefore be beneficial. (ii) Temperate - LOW FEEDBACK SYSTEM ACTION Sensor /Receptor Detect a decrease in temperature - Thermoreceptors in the skin. Control centre Brain (Hypothalamus) Effectors Sweat gland activity decreases. Blood vessels near skin surface constrict. Metabolic activity increases, shivering. Behavioural responses may include folding arms across chest and putting on warmer clothes. UNIT 1.3. | ANIMALS Date | 01/12/2023 21 Revision | 7 BIOLOGY PROGRAM What happens if mechanisms for increasing temperature are not effective? Hypothermia occurs when the body’s heat preserving and generating mechanisms are not successful and the core temperature continues to drop. At 35°C, there is disruption of the nervous system and temperature regulation itself. At 33°C there is a loss of consciousness. At 30°C blood vessels are completely restricted and temperature regulating mechanisms are fully shut down. Death soon follows. Hypothermia can be treated. The most severe cases can be treated using dialysis machines to artificially warm blood and pump it back into the body. The organ systems cooperate to keep the body temperature around its set point. In the response to a change in body temperature the integumentary system, circulatory, nervous, and muscular systems are involved. UNIT 1.3. | ANIMALS Date | 01/12/2023 22 Revision | 7 BIOLOGY PROGRAM  Hyperthermia: Glossary for Animals Chapter 1. Write the definitions for these words yourself. Add any extra words that are helpful for you! Hypothermia: Cartilage: Intercellular: Collagen: Interstitial: Columnar: Intracellular: Epithelial: Metabolic: Exocrine: Neuron: Extracellular: Skeletal: Gland: Squamous: Homeostasis: UNIT 1.3. | ANIMALS Date | 01/12/2023 23 Revision | 7 BIOLOGY PROGRAM  ON REFLECTION, A BIGGER PICTURE: Some ways these topics connect into the wider world or to other topics, some points that caught my interest… UNIT 1.3. | ANIMALS Date | 01/12/2023 24 Revision | 7 BIOLOGY PROGRAM  Test your knowledge 1 What is the extracellular matrix? Some animal tissues have a large amount of extracellular matrix, while others have very little. Provide an example of a tissue that has very little extracellular matrix. Provide an example of a tissue in which there is a large amount of extracellular matrix. 2 When considering animal cells, what is a junction? Describe the function of each of the following junctions – tight junction, adhesion junction and gap junction. 3 On the diagram (left) label A, B and C as a tight, adhesion or gap junction. Test your knowledge UNIT 1.3. | ANIMALS Date | 01/12/2023 25 Revision | 7 BIOLOGY PROGRAM 4 What are the four types of tissues found in the animal body? Provide a general description of how the cells and extracellular matrix are organised for each tissue type. What is the general role of each tissue type in the animal body? 5 (a) What is homeostasis? (b) Most of the body’s homeostatic mechanisms are negative feedback systems. Describe a negative feedback system. (a) Describe the role of the receptors, control centre and effectors in the homeostatic response to an increase in temperature. (b) What organ systems are involved? Brilliant, Chapter 1 Animals completed! Now go back to the learning outcomes at the start of this chapter. What have you learnt? What might you need to revise? UNIT 1.3. | ANIMALS Date | 01/12/2023 26 Revision | 7 BIOLOGY PROGRAM Chapter 2 | How does internal transport work in different animals? Key Concepts Internal Transport In a multicellular animal the individual body cells require various Heart with the coronary arteries on the surface. These arteries supply substances and must be rid of wastes. In very small animals these the walls of the heart (cardiac muscle cells) with blood carrying oxygen requirements can be met by diffusion alone. Such animals do not and nutrients. In a heart attack one or more of the arteries is blocked, require a circulatory system. In larger animals a specialised circulatory so a section of cardiac muscle no longer receives a blood supply. If the system is required to supply the individual body cells. blood supply is not returned quickly cardiac muscle cells will die. ( https://www.geelongmedicalgroup.com.au/2018/03/5-signs-that- 1. Circulatory system - pumping organ, circulating fluid and vessels your-arteries-are-clogged) 2. No circulatory system - in very small animals 3. Open circulatory system - in insects, crustaceans and arachnids 4. Closed circulatory system – in some invertebrates, and all vertebrates. UNIT 1.3. | ANIMALS Date | 01/12/2023 27 Revision | 7 BIOLOGY PROGRAM  Learning Outcomes On completion of this topic students should have achieved the following learning outcomes: Level of Section Outcome achievement 2.1 Describe the three main components of a circulatory system. 2.1 List and explain the substances that may be transported by a circulatory system. 2.2 Provide an example of animal with no circulatory system and explain why some animals have no circulatory system. Describe the structure and functioning of an open circulatory system. Provide an example of an animal with an open 2.3 circulatory system. Describe the structure and functioning of a closed circulatory system. Provide an example of an animal with a closed 2.4 circulatory system. Compare the structure and functioning of open and closed circulatory systems, including differences in vessel 2.3-2.4 structure, circulating fluid and efficiency. 2.4 For a closed circulatory system describe the main components of the circulating fluid. For a closed circulatory system describe how gases, nutrients and wastes are exchanged between the circulatory 2.4 system and the cells of the body. Explain the role of lymphatic fluid and the lymphatic system as they relate to the closed circulatory system. 2.4 UNIT 1.3. | ANIMALS Date | 01/12/2023 28 Revision | 7 BIOLOGY PROGRAM Section Outcome Level of achievement 2.4 Identify and describe the components of the pulmonary and systemic circuits in the human. Label the right and left atria, right and left ventricles, pulmonary artery, pulmonary veins, vena cava and aorta on a 2.4 diagram of a human heart. Explain the action of the human heart in terms of the alternative contraction (systole) and relaxation (diastole) of its 2.4 chambers. UNIT 1.3. | ANIMALS Date | 01/12/2023 29 Revision | 7 BIOLOGY PROGRAM  Chapter 2 Index Section Section title Page 2.1 Circulation 31 2.2 No (specialised) circulatory system 32 2.3 Open circulatory system 33 2.4 Closed circulatory system 35 UNIT 1.3. | ANIMALS Date | 01/12/2023 30 Revision | 7 BIOLOGY PROGRAM Section 2.1 | Circulation What is circulation? Circulation is the bulk transport of substances throughout an animal’s body (It brings dissolved substances close enough to cells so that diffusion can be effective). What is a circulatory system? Many animals have specialised structures to transport materials – these structures make up the circulatory system. Circulatory systems typically consist of three components: (i) Circulating fluid, (ii) Pipes through which the fluid moves, (iii) Pumping organ. What types of substances are transported by the circulatory system? 1. Nutrients – digestive system to cells. 2. Oxygen – respiratory structures to cells (NOT IN INSECTS.) 3. Metabolic wastes – cells to excretory organs. 4. Hormones – carried to target sites. 5. Water - to maintain water balance in body fluids. 6. Immune system substances – carried in the circulatory system. 7. Heat – distributed throughout the body. UNIT 1.3. | ANIMALS Date | 01/12/2023 31 Revision | 7 BIOLOGY PROGRAM Section 2.2. | No (specialised) circulatory system In very small animals the cell requirements can be met by diffusion. (Animal body thickness needs to be smaller than about 2mm thick). Ameoba – single cell organisms. Cell requirements are met by diffusion. Materials diffuse from the environment to the cell and waste diffuses directly into the environment. Hydra – small aquatic animal The body is two cells thick; oxygen and nutrients circulate through the gastrovascular cavity and come into contact with the inner layer of body cells. Materials diffuse from the gastrovascular cavity to cells and wastes diffuses from cells out. Materials also diffuse to/from the water surrounding the Hydra directly to/from cells. Figure 2.1 | The unicellular ameoba (not an animal but a protist) and the Hydra have no circulatory system (Knox et al., 2005). UNIT 1.3. | ANIMALS Date | 01/12/2023 32 Revision | 7 BIOLOGY PROGRAM Section 2.3 | Open circulatory system Open circulatory systems are found in Arthropods. Phylum Arthropoda includes insects, crustaceans, and arachnids (spiders, mites, ticks). Figure 2.2 | Insects have an open circulatory system (Knox et al., 2005). Structures: The open circulatory system has heart, circulating fluid and vessels. The heart has openings on the surface (ostia). The circulating fluid is haemolymph. The vessels are open ended. UNIT 1.3. | ANIMALS Date | 01/12/2023 33 Revision | 7 BIOLOGY PROGRAM  Open circulatory systems – function: Heart pumps haemolymph through vessels. Vessels are open ended. Haemolymph empties into haemocoel (body cavity) and supplies cell requirements. Haemolymph moves back into heart via an ostium. An ostium is an opening on the heart surface. Figure 2.3 | Open circulatory system in a grasshopper (Mader, 2010). UNIT 1.3. | ANIMALS Date | 01/12/2023 34 Revision | 7 BIOLOGY PROGRAM Section 2.4 | Closed circulatory system The closed circulatory system is found in some invertebrates (for example, earthworms) and all vertebrates. The circulating fluid (blood) stays within the circuit of blood vessels at all times and is separated from the interstitial fluid which surrounds the cells. Figure 2.4 | The closed circulatory system (Knox et al., 2005). UNIT 1.3. | ANIMALS Date | 01/12/2023 35 Revision | 7 BIOLOGY PROGRAM The circulating fluid (blood) in which substances are dissolved or carried is made up of the following: (i) Plasma - fluid component of blood containing protein (albumins and globulins). It makes up 55-60% of the blood volume. (ii) Blood cells – make up 40-45% of the blood volume. Blood cells and platelets Red Blood Cells–transport oxygen (erythrocytes). White Blood Cells – protecting the body against disease and removing cellular debris (include lymphocytes, monocytes, and granulocytes). Platelets – disc shaped membrane bound cell fragments; take part in wound healing and clot formation. Figure 2.5 | Blood contains RBC, WBC and platelets (Mader, 2010). UNIT 1.3. | ANIMALS Date | 01/12/2023 36 Revision | 7 BIOLOGY PROGRAM Blood circulates through a system of pipes (blood vessels). Three types of blood vessels in humans (and other vertebrates): (i) Arteries – carry blood away from the heart toward the tissues/organs. (ii) Capillaries – once arteries reach tissues / organs they break up into smaller branches which form a network of vessels in the tissue / organ. Exchange of materials occurs. (iii) Veins – after blood has been through the capillaries in tissues/organs the capillaries merge into veins that transport blood back to the heart. Figure 2.6 | The blood vessels in a typical closed circulatory system (Knox et al., 2005). UNIT 1.3. | ANIMALS Date | 01/12/2023 37 Revision | 7 BIOLOGY PROGRAM How are gases and materials exchanged? Arteries and veins have thick walls therefore gases and materials do not pass through them. Capillaries are a single layer of cells, so the walls are very thin. Capillaries are where the exchange of materials occurs. Substances diffuse in/out of capillaries into interstitial fluid and from the interstitial fluid to/from cells. Capillary networks are so extensive that at least one capillary is located close to almost every cell in the body. Figure 2.7 | Exchange of materials between blood and interstitial fluid (Knox et al., 2005). UNIT 1.3. | ANIMALS Date | 01/12/2023 38 Revision | 7 BIOLOGY PROGRAM The Lymphatic System The lymph vessels extend into the interstitial space. They have closed ends however fluid can enter or exit the lymph vessels through spaces between the cells. Excess interstitial fluid is collected and transported through the series of lymph vessels (as lymph) until it is returned to the venous system (at subclavian veins). One role of the lymphatic system is to return excess interstitial fluid to the blood. The lymphatic system also has an important role in the immune system (see Immune system lecture) and in lipid collection from the digestive system. Figure 2.8 | Lymphatic system (Campbell et al., 2005). UNIT 1.3. | ANIMALS Date | 01/12/2023 39 Revision | 7 BIOLOGY PROGRAM Circulation in humans The circulatory system in the body is divided into two main parts: the pulmonary and systemic circuits. The pulmonary circuit is involved with exchange of gases at the lungs and involves a circuit of vessels that carries blood between the heart and lungs. The systemic circuit is involved with supplying the cells of the body with oxygen and nutrients and forms a circuit of vessels from the heart to the tissues of the body. Figure 2.9 | The pulmonary and systemic circuits (Mader, 2010). UNIT 1.3. | ANIMALS Date | 01/12/2023 40 Revision | 7 BIOLOGY PROGRAM Pumping organ (Heart) The heart propels circulating fluid around the animal body. The heart is a hollow muscular organ. The human heart (and bird and mammal hearts) has four chambers – right atrium; right ventricle; left atrium; left ventricle. The atria are thin walled and receive blood while the ventricles have thicker muscular walls to pump blood away from the heart. The chambers alternately contract – systole, and relax – diastole. Figure 2.10 Human heart (Campbell et al., 2005). UNIT 1.3. | ANIMALS Date | 01/12/2023 41 Revision | 7 BIOLOGY PROGRAM  Lymphatic: Glossary for Animals chapter 2. Write the definitions for these words yourself. Add any extra words that are helpful for you! Pulmonary: Aorta: Systemic: Artery: Systole: Atrium: Vein: Capillary: Vena cava: Cardiac: Ventricle: Diastole:.. Haemoglobin:.. Haemolymph: UNIT 1.3. | ANIMALS Date | 01/12/2023 42 Revision | 7 BIOLOGY PROGRAM  ON REFLECTION, A BIGGER PICTURE: Some ways these topics connect into the wider world or to other topics, some points that caught my interest… UNIT 1.3. | ANIMALS Date | 01/12/2023 43 Revision | 7 BIOLOGY PROGRAM  Test your knowledge 1 Why are circulatory systems necessary in larger animals but not in very small animals? 2 Name the types of substances that are transported in a typical circulatory system. 3 (i) What are the three components of a typical circulatory system? (ii) Describe two differences between an open and closed circulatory system. (iii) Provide an example of an animal with an open circulatory system. (iv) Provide an example of an animal with a closed circulatory system. 4 Describe the role of arteries, veins and capillaries. 5 Circulation in humans is divided into pulmonary and systemic circuits. Describe the role of each of these circuits. 6 The lymphatic system is an important secondary part of vertebrate circulatory system. What is the name of the transport vessels and the transport fluid of the lymphatic system? The lymphatic system helps to maintaining fluid balance in the body. Describe how this is achieved. Well done, you have completed another section. Go back to the learning outcomes at the start of this chapter. What have you learnt? What might you need to revise? UNIT 1.3. | ANIMALS Date | 01/12/2023 44 Revision | 7 BIOLOGY PROGRAM Chapter 3 | How does gas exchange work in different animals? Key Concepts To support cellular (aerobic) respiration, all cells require a supply of O2 and removal of CO2. In larger animals a specialised respiratory system is required. In such cases the respiratory system may work with the circulatory system to supply the body cells. The type of respiratory structure depends on the size of the animal, its level of activity and whether it is an air or water breather. 1. Respiration 2. Types of respiratory structures: (i) Cutaneous exchange (ii) Tracheal tubes The ventilation (breathing) rate in humans is regulated by the brain (iii) Gills (pons and medulla oblongata). If carbon dioxide in the blood (iv) Lungs increases above the normal level, then the ventilation rate increases in order to rid the body of excess carbon dioxide. Levels of carbon Animals may use one or more of these surfaces for gas exchange. dioxide are detected by receptor cells (chemoreceptors) in the walls of the aorta or carotid arteries. (Campbell et al., 2006). UNIT 1.3. | ANIMALS Date | 01/12/2023 45 Revision | 7 BIOLOGY PROGRAM  Learning Outcomes On completion of this topic students should have achieved the following learning outcomes: Level of Section Outcome achievement 3.1 Define the process of respiration. 3.1 Explain the two factors required for respiratory surfaces to work effectively. 3.1 Compare water and air as respiratory media. 3.2(i) Provide an example of an animal that uses cutaneous exchange. 3.2(i) For cutaneous exchange describe the exchange structure and how it works for an earthworm. 3.2(ii) Provide an example for an animal that uses tracheal tubes for gas exchange. 3.2(ii) For tracheal tubes describe the exchange structure and how it works to provide gases directly to cells. 3.2(iii) Provide an example of an animal that uses gills as respiratory structure. For gills, describe the exchange structure and the counter-current system to maximise gas exchange, and direction of 3.2(iii) ventilation. 3.2(iv) For lungs, describe the exchange structure and how it works for gas exchange, and including direction of ventilation. UNIT 1.3. | ANIMALS Date | 01/12/2023 46 Revision | 7 BIOLOGY PROGRAM  Outcome Level of Section achievement 3.2(iv) Describe how gases are transported in blood. Describe how changes in blood gas concentrations are returned to normal through homeostasis including the receptors, 3.2(iv) the control centre, and the effectors. UNIT 1.3. | ANIMALS Date | 01/12/2023 47 Revision | 7 BIOLOGY PROGRAM  Chapter 3 Index Section Section title Page 3.1 Respiration 49 3.2 Types of respiratory structures 52 UNIT 1.3. | ANIMALS Date | 01/12/2023 48 Revision | 7 BIOLOGY PROGRAM Section 3.1 | Respiration What is respiration? Respiration is the exchange of gases (oxygen and carbon dioxide) between an animal and its environment. The respiratory structure is the structure at which this exchange occurs. To work effectively a respiratory must have the following: A large surface area (for exchange) that is in contact with the respiratory media (air or water). The respiratory structure must be moist. To get from the atmosphere to body fluids, O2 must be dissolved in water. For water-breathers, air is dissolved in surrounding water anyway. For air-breathers, the respiratory surface is moist and O2 dissolves in this water layer. Respiratory media – air or water Air is less dense, and less viscous than water, so less energy is required to breathe air. The concentration of O2 in air is high and very stable (20.9%) compared to water. The amount of O2 available in water depends on several factors – temperature, salinity, and the distance from the air/water interface. On average water in equilibrium with air contains 20-40 times less O2 than air. Water breathers need to be very efficient at extracting available O2 from the water. O2 and CO2 diffuse faster through air than water. Seawater contains salt, and animals that breathe seawater must cope with the diffusion of salt into their bodies. UNIT 1.3. | ANIMALS Date | 01/12/2023 49 Revision | 7 BIOLOGY PROGRAM Section 3.2 | Types of respiratory structures (i) Body Surface (Cutaneous exchange) (ii) Tracheal tubes (iii) Gills (iv) Lungs The type of respiratory structure that an animal has depends on: Respiratory media – either water or air. Metabolic activity - more active animals have a higher metabolic rate so a greater need for oxygen and produce more carbon dioxide. UNIT 1.3. | ANIMALS Date | 01/12/2023 50 Revision | 7 BIOLOGY PROGRAM (i) Body Surface (Cutaneous exchange) In some organisms, gas exchange occurs across the body surface –cutaneous exchange. Cutaneous exchange most commonly occurs in aquatic organisms. It is sometimes used as well as some other type of respiratory structure. Frogs use both cutaneous exchange and lungs. The Earthworm (an annelid) is an example of a terrestrial organism that uses only cutaneous exchange. Gland cells secrete mucous. This means that the surface is kept moist. O2 in air pockets in the soil diffuses into the moist layer. O2 diffuses through the body wall and is collected by capillaries. Gases are transported to cells of the body by the circulatory system. CO2 diffuses out (via same path). Figure 3.1 | Earthworms use cutaneous exchange. (Mader, 2010). UNIT 1.3. | ANIMALS Date | 01/12/2023 51 Revision | 7 BIOLOGY PROGRAM (ii) Tracheal Tubes In insects the respiratory system consists of a network of tracheal tubes. Air enters tracheal tubes through a series of tiny openings called spiracles (up to 20) on body surface. Air passes through the branching tracheal tubes, which extend to all parts of the body. The tracheal tubes terminate at fluid filled tracheoles. Gases are exchanged directly between this fluid and the body cells. Tracheal tubes deliver air directly to the cells of the insect’s body, the circulatory system is not involved in transport of gases in the insect’s body. Figure 3.2 | Tracheal tubes (Campbell et al., 2006). UNIT 1.3. | ANIMALS Date | 01/12/2023 52 Revision | 7 BIOLOGY PROGRAM (iii) Vertebrate Gills Gills are delicate structures found in aquatic animals. Gills in Bony Fish Gills are internal. A bony plate called the operculum protects the gills. Water flow is unidirectional – through mouth, over gill surfaces and out through the gill slits. Most fish actively ventilate their gills. Each gill consists of many filaments. Water flows over the filaments. The gill filaments have a network of capillaries under the surface. Oxygen and carbon dioxide diffuse between blood and water. Gases are transported to the body cells by the circulatory system. UNIT 1.3. | ANIMALS Date | 01/12/2023 53 Revision | 7 BIOLOGY PROGRAM  Figure 3.3 | Vertebrate gills (Knox et al., 2005). Counter-current exchange In fish the water and blood flow in opposite directions across the gills. This is the counter-current exchange system. This maximises the difference in oxygen concentration between blood and water as they flow across the gills. As a consequence, it allows maximum extraction of oxygen from the water. UNIT 1.3. | ANIMALS Date | 01/12/2023 54 Revision | 7 BIOLOGY PROGRAM  Figure 3.4 | Water and blood flow across the gill surface in opposite directions. As a result, the concentration of oxygen in the water is higher than concentration of oxygen in blood across the gill surface, so oxygen will move from water to blood. (https://www.iitbmonash.org/using-fish-gill- design-to-solve-biomedical-challenges/). (v) Lungs The human respiratory system is typical of air breathing vertebrates. Lungs are internal. This provides protection from damage and reduces the effect of drying out. UNIT 1.3. | ANIMALS Date | 01/12/2023 55 Revision | 7 BIOLOGY PROGRAM  In mammals a single tube, the trachea, branches many times (23 times in human lungs) until it ends at small cup shaped chambers called alveoli. Gas exchange occurs in the alveoli. These are tiny air sacs composed of a single layer of very thin epithelial cells. The alveoli are surrounded by a network of capillaries. Gases diffuse freely between the alveoli and capillaries. Only two thin cells – one from the alveoli and one from the capillary wall separate air from blood. The distance between these cells is very small (in humans, 0.62μm). Oxygen from the air dissolves in the moist surface layer of the alveoli before transporting across the two cells and collected in the blood. Gases are transported to the cells of the body in the circulatory system. UNIT 1.3. | ANIMALS Date | 01/12/2023 56 Revision | 7 BIOLOGY PROGRAM  Figure 3.5 | The respiratory tract. The respiratory tract extends from the nose to the lungs. The lungs are composed of air sacs called alveoli. Gas exchange occurs between the air in the alveoli and the blood within the capillary network that surrounds the alveoli. (Campbell et al., 2005). UNIT 1.3. | ANIMALS Date | 01/12/2023 57 Revision | 7 BIOLOGY PROGRAM Ventilation of lungs Ventilation of the lungs in humans is bidirectional (in and out). Inhalation – diaphragm contracts (moves downwards) and this increases the volume of the lungs so air is drawn in. Exhalation – diaphragm relaxes (moves upwards), decreasing the volume of the lungs and air is forced out. Control of breathing The medulla of the brain (breathing centre) sets the basic breathing rhythm. Figure 3.6 | Ventilation of the lungs (Campbell & Reece, 2002). UNIT 1.3. | ANIMALS Date | 01/12/2023 58 Revision | 7 BIOLOGY PROGRAM Transport and exchange of gases Haemoglobin in the red blood cells carries oxygen. Haemoglobin is composed of four polypeptide chains – two α and two β chains. Each chain carries one heme group. Oxygen binds to the heme group so each haemoglobin molecule can carry four oxygen molecules. Haemoglobin (oxygen carrying pigment) binds oxygen and carries it to a region of lower concentration where the oxygen is released. Carbon dioxide is the by-product of respiration and must be removed. CO2 is mainly transported in the blood plasma as bicarbonate (HCO3-). Carbon dioxide diffuses out of the blood at the alveoli. Homeostasis for blood levels of CO2 and O2 It is important that levels of CO2 and O2 in the blood remain constant, and so there is homeostatic regulation of concentration of these gases in the body. The homeostatic response involves receptors, a control centre, and effectors. Receptors: CO2 levels are detected by chemoreceptors in the aorta and carotid arteries (aortic and carotid bodies). Control Centre: Medulla region of the brain. If the CO2 level is too high or too low the medulla (brain) organises a response (aimed at returning levels to normal). Effectors: Lungs and heart - rate of ventilation of the lungs (that is, the breathing rate changes), and the heart rate changes. UNIT 1.3. | ANIMALS Date | 01/12/2023 59 Revision | 7 BIOLOGY PROGRAM  Blood isn’t meant to be white, is it? “When bad things happen to good fish.” Can you imagine what might happen to us if we did not have red blood cells or haemoglobin? This is exactly what happened to the Antarctic Icefish. In 1928 a biologist caught one of these fish and thought it looked most strange. Its muscles were white, its gills were white. Its blood was also white. No colour of red anywhere! What had happened to these fish? First scientists thought they must have evolved this as an advantage to surviving in such extremely cold conditions (Antarctic ocean water is almost freezing!). After all, these fish lived in very high oxygen water so maybe they could pick oxygen up just by diffusion? Many other fish species that live in such cold water have less red blood cells to haemoglobin, and that in turn messed up their ability to make red help the blood stay thin enough to keep flowing through the cold fish blood cells. They had not evolved to be better for their environment. bodies. Did the Icefish lose their red blood cells as an adaptation to There was no advantage in this at all. It was a rather large genetic their environment? Icefish also have larger hearts and blood vessels mistake that somehow did not kill these fish. They can only carry 10% compared to other species. Did this help them to get rid of their of the oxygen in their blood that normal fish can manage so only the haemoglobin and red-blood cells to help survive in the cold? What fish with unusually large hearts and blood vessels could survive this had happened here? It was a mystery. mistake. This was an amazing example of having made a very big After very careful investigation, scientists discovered they had got mistake and through body adaptations and a favourable environment, the story of these fish completely backwards! Genetic testing these very unusual fish were able to survive. showed these fish had a mutation that messed up their gene for SIdell, B. and O’Brien, K. (2006). When bad things happen to good fish: The loss of hemoglobin and myoglobin expression in Antarctic icefishes. Journal of experimental Biology 209, 1791-1802. UNIT 1.3. | ANIMALS Date | 01/12/2023 60 Revision | 7 BIOLOGY PROGRAM  Glossary for Animals lecture 3. Write the definitions for these words yourself. Add any extra words that are helpful for you! Alveoli: Cutaneous: Gills: Tracheal: Ventilation:.... UNIT 1.3. | ANIMALS Date | 01/12/2023 61 Revision | 7 BIOLOGY PROGRAM  ON REFLECTION, A BIGGER PICTURE: Some ways these topics connect into the wider world or to other topics, some points that caught my interest… UNIT 1.3. | ANIMALS Date | 01/12/2023 62 Revision | 7 BIOLOGY PROGRAM  Test your knowledge 1 (i) What is the function of a respiratory structure? (ii) What features must a respiratory structure have? (iii) Think about the respiratory structure of an earthworm. Does it have the features listed in (ii)? Explain. 2 Respiratory media may be air or water. What are the advantages of breathing air compared to water? Are there any advantages to breathing water compared to air? 3 (i) In many animals the respiratory and circulatory systems work together to provide O2 and remove CO2. Explain this statement. (ii) Insects are an exception; there is no involvement of the circulatory system in providing O2 and removing CO2. Explain how the respiratory structures of insects provide O2 and remove CO2. 4 (i) In which types of respiratory structures would you expect a counter current exchange system? (ii) What is counter current exchange system? (iii) Why is it necessary? 5 (i) In human lungs at which structures does the exchange of gases occur? (ii) The structures named in (i) are found at the end of a series of pipes (bronchial tubes). What is the purpose of the bronchial tubes? 6 It is important that blood levels of O2 and CO2 are maintained a constant level (homeostasis!) How are blood gas levels detected? (receptors) What is the control centre in this homeostatic mechanism? What response occurs (effectors)? Congratulations! You have completed chapter 3 of the Animals unit. Which of the learning outcomes have you achieved? Check them off on your learning outcomes checklist at the start of this chapter. UNIT 1.3. | ANIMALS Date | 01/12/2023 63 Revision | 7 BIOLOGY PROGRAM Chapter 4 | How do cells communicate and why? Key Concepts 1. Cell Communication – why, and its importance 2. Cell Signalling 3. Chemical Signalling (i) Modes of transmission (travelling) (ii) Characteristics (iii) Examples of signalling molecules (iv) Hormones (animals and plants), Neurotransmitter, cytokines, Pheromones, Stimulus–Response Model, Insulin Myxobacteria fruiting body. Chemical signals released by individual 4. Apoptosis an overview cells stimulate cells to come together to form the fruiting body. (i) Why does it occur? (https://pinegreenwoods.blogspot.com.au/2015/01/montessori- (ii) How does it occur? biology-phylum) (iii) Intrinsic and extrinsic pathways. (iv) Malfunctions of apoptosis. UNIT 1.3. | ANIMALS Date | 01/12/2023 64 Revision | 7 BIOLOGY PROGRAM  Learning Outcomes On completion of this topic students should have achieved the following learning outcomes: Level of Section Outcome achievement 4.1 Describe why cell signalling is important and provide an example to support your reason. 4.2 Identify that cell signalling is the process whereby a cell releases a signalling molecule that moves to a target cell. 4.3 Describe the four methods of transmission (traveling) for the signalling molecule as autocrine, paracrine, endocrine and gap junctions. 4.3 Compare and contrast hydrophobic and hydrophilic signalling molecules. 4.3 Provide two examples of signalling molecules and describe the function of these signalling molecules. 4.3 For the stimulus response model describe the roles of reception, signal transduction and response, including explaining that transduction can be single-step or a multi-step process. 4.4 Define apoptosis, including reasons why it might occur. 4.4 Identify the caspases as the enzyme group responsible for apoptosis and provide two examples of their actions. 4.4 Outline the process of apoptosis. 4.4 Describe the intrinsic (mitochondrial) and extrinsic pathways for triggering apoptosis. 4.4 Provide two examples of apoptosis malfunction. UNIT 1.3. | ANIMALS Date | 01/12/2023 65 Revision | 7 BIOLOGY PROGRAM  Chapter 4 Index Section Section title Page 4.1 Cell communication – an overview 67 4.2 Cell signalling 69 4.3 Chemical signalling 70 4.4 Apoptosis - an overview 82 UNIT 1.3. | ANIMALS Date | 01/12/2023 66 Revision | 7 BIOLOGY PROGRAM Section 4.1 | Cell communication – an overview Cell to cell communication This is vital for the survival of all living things. In multicellular organisms, the billions of cells in an individual must communicate so that activities can be coordinated. Communication between single celled organisms can also occurs (see Figure 6.2). To maintain homeostasis, cells in a multicellular organism must be in communication with one another. Examples of homeostasis including temperature regulation and maintenance of blood levels of carbon dioxide have been considered, and in each case communication between cells in different parts of the body is the basis of any response. In plants cell communication can aid in defence. When attacked by an insect, for example a caterpillar eating a leaf, the plant may respond by producing volatile chemicals to attract wasps. These wasps will ultimately kill the caterpillar. The response occurs because the damaged plant leaf cells release chemical signals to which other cells in the plant respond. Figure 4.1 | Plant cell communication can aid with defence (Campbell et al., 2006). UNIT 1.3. | ANIMALS Date | 01/12/2023 67 Revision | 7 BIOLOGY PROGRAM  a b c Figure 4.2 | Chemical signals cause a. Myxococcus xanthes bacteria to come together, b. aggregate, and c. form a fruiting body (https://evo.ethz.ch/research/mycrobacteria.html). UNIT 1.3. | ANIMALS Date | 01/12/2023 68 Revision | 7 BIOLOGY PROGRAM Section 4.2 | Cell signalling This refers to the mechanisms by which cells communicate with each other. It involves a sequence of steps. These are summarised below and discussed in more detail in the following sections. Summary of steps: Chemical signalling | About the chemical signal. In response to a stimulus certain cells produce and release signalling molecules. The signalling molecules move to a target cell. (i) Modes of transmission, (ii) Characteristics of signalling molecules, (iii) Examples of signalling molecules. Stimulus-Response model | About the response to the chemical signal. (i) Reception - detection of the signalling molecule, (ii) Signal transduction - relay of the signal through the cell, (iii) Response - alter some cell process due to the signal. UNIT 1.3. | ANIMALS Date | 01/12/2023 69 Revision | 7 BIOLOGY PROGRAM Section 4.3 | Chemical signalling In chemical signalling, a cell produces and releases signalling molecules. These molecules must travel to target cells. Target cells respond to the signal. (i) Modes of transmission (Travelling) In multicellular organisms the main distinction between chemical signals is the distance that the signalling molecule travels. Autocrine | A cell targets itself. The signal is produced and secreted by the target cell and it will affect the target cell itself by binding to its receptors. In immune cells, this can be an important part of the immune response to pathogens. If a cell is infected by a virus it can signal itself to undergo programmed cell death (apoptosis) destroying the virus in the process. Gap junctions link animal cells (see Animals Course Notes Chapter 1, Organisation and Homeostasis) and plasmodesmata link plant cells (Plants Course Notes Section 1). These provide direct connections between neighbouring cells. Small signalling molecules can move between cells via gap junctions or plasmodesma but not larger molecules like proteins or DNA. This allows a signalling molecule to move from cell to cell in a tissue. Paracrine | Signalling cell targets a nearby cell. The signal targets cells in the local area of the emitting cell. An example is the transfer of signals across the synapse between nerve cells. The distance between cells (synaptic gap) is very small, and neurotransmitters rapidly cross this gap. Endocrine | The signalling cell targets a distant cell. Endocrine cells produce hormones that travel in the blood to reach distant target cells in the body. UNIT 1.3. | ANIMALS Date | 01/12/2023 70 Revision | 7 BIOLOGY PROGRAM  Figure 4.3 | Modes of transmission include autocrine, across junction between cells, paracrine and endocrine. (https://courses.lumenlearning.com/boundless- biology/chapter/signaling-molecules-and-cellular- receptors/) UNIT 1.3. | ANIMALS Date | 01/12/2023 71 Revision | 7 BIOLOGY PROGRAM (ii) Characteristics Signalling molecules may be hydrophobic or hydrophilic. Hydrophobic molecules are non-polar molecules such a lipid-based molecule and are insoluble in water. They are able to cross the cell membrane. If they cross the membrane the signalling molecule will bind to a receptor inside the cell. They are usually transported through the body fluids attached to a carrier protein. Hydrophilic molecules are polar ions and molecules like peptide/protein-based molecules that are relatively soluble in water. They cannot cross the cell membrane and will bind to a receptor on the surface. They can be transported through the body fluids and may not require a carrier protein. a. b. Figure 4.4 | a. A hydrophilic signalling molecule attaches to a cell surface receptor. b. A hydrophobic signal molecule enters the cell and binds to an intracellular receptor. (http://medbiology.meduniversity-plovdiv.bg/wp-content/uploads/2018/10/Lecture-7_Cell-Reproduction.- Regulation-of-the-Cell-Cycle.Pharmacy.pdf) UNIT 1.3. | ANIMALS Date | 01/12/2023 72 Revision | 7 BIOLOGY PROGRAM (iii) Examples of signalling molecules: Hormones Hormones are signalling molecules that are released in small amounts. Hormones help regulate metabolism, growth and reproduction in living things. Animal Hormones The endocrine system consists of glands and organs that produce and release hormones. The endocrine system is responsible for the regulation of many body functions including metabolism, growth, and reproduction. The three main groups of animal hormones are the steroids, peptide and protein hormones and amino acid derived hormones. Steroids are an example of hydrophobic signalling molecules. Figure 4.6 | The endocrine system is composed of several glands and organs that produce and secrete hormones. (https://www.pinterest.com.au/pin/665195807447370267/vis ual-search/?x=15&y=15&w=485&h=580) UNIT 1.3. | ANIMALS Date | 01/12/2023 73 Revision | 7 BIOLOGY PROGRAM  Insulin and glucagon are examples of peptide hormones. These peptide hormones help to regulate blood sugar levels. Insulin is a peptide hormone that is released from the pancreas. It attaches to a receptor on the cell surface and the response is to allow uptake of glucose by the cell, shown in the diagram to the right. Insulin and glucagon are hydrophilic hormones. Figure 4.7 | An insulin molecule (red, top) binding to the insulin receptor. The receptor is a large protein molecule (blue, purple) which extends across the cell membrane. Binding of insulin causes a change in the receptor protein inside the cell (https://pdb101.rcsb.org/motm/182). UNIT 1.3. | ANIMALS Date | 01/12/2023 74 Revision | 7 BIOLOGY PROGRAM Plant hormones Like animal hormones, plant hormones are produced in low concentrations and have a significant effect on plant development and growth. Plant cells are generally able to produce a variety of hormones. There are five main types of plant hormone. These may work independently or together to influence plant growth. Auxin (IAA) is involved in cell growth and cell expansion, so it is produced primarily in parts of the plant that are actively growing like the stem (specifically, the very top of the stem). Figure 4.9 | Auxin (IAA) structure Figure 4.10 | Plant exhibiting phototropism (Knox et al., 2005). (https://biologydictionary.net/auxin/) UNIT 1.3. | ANIMALS Date | 01/12/2023 75 Revision | 7 BIOLOGY PROGRAM Phototropism in plants is growth in response to light. If a plant receives light from one side only, the stem will bend and grow toward the light source. Auxin is responsible for the phototropic response in plants. Auxin is produced at the top of the stem and will move down to shaded side of the plant stem. As a result, the cells on the shaded side expand and grow longer, while the cells on the sunny side of the plant stay the same size. That will cause the plant to bend to one side – toward the sun! Figure 4.11 | Auxin molecules diffuse from the top of the plant. When a plant receives sunlight from one side auxin diffuses the shaded side only. (https://edu.glogster.com/glog/phototropism-and- auxin/25pf2uxu1bm?=glogpedia-source) UNIT 1.3. | ANIMALS Date | 01/12/2023 76 Revision | 7 BIOLOGY PROGRAM Neurotransmitters Neurotransmitters are hydrophilic signalling molecules produced by neurons. Neurotransmitters are produced and released at the axon terminal of the neuron and move to the target cell which may be the dendrite of the next neuron to elicit a response. Examples of neurotransmitters include serotonin which influences mood, sleep, and appetite. Figure 4.12 | (Above) Neuron structure. (https://owlcation.com/stem/Structure-of-a- Neuron). Figure 4.13 | (Right) Neurotransmitters are released at the axon terminal of the neuron. (https://courses.lumenlearning.com/wmopen- psychology/chapter/outcome-neurons/). UNIT 1.3. | ANIMALS Date | 01/12/2023 77 Revision | 7 BIOLOGY PROGRAM Cytokines These are hydrophilic signalling molecules involved communication between immune cells. For example, histamine may be released by immune system or other cells in response to allergic reactions, injury or infection. It causes blood vessels to dilate and capillaries become more permeable so molecules and cells can move from the blood to the site of infection (See Section 5.4 part (iii), Inflammatory response). Pheromones These signalling molecules are secreted into the external environment. They influence the behaviour and/or physiology of other individuals. They may trigger alarm and aggressive responses or, be used to mark food trails or attract a mate. Ants marks their paths with pheromones that are volatile hydrocarbons. Ants lay down the initial trail as they return to the colony with food. The trail attracts other ants and serves as a guide to the food. As ants use the trail, they continue to release pheromones. Because these are volatile compounds they will be lost to the atmosphere over time. When ants stop using the trail (because the food source is exhausted, for example) the trail disappears. Figure 4.14 | Ants release pheromones which mark the trail for other ants in the colony. UNIT 1.3. | ANIMALS Date | 01/12/2023 78 Revision | 7 BIOLOGY PROGRAM Stimulus-Response model The processes involved in detecting and responding to a signalling molecule are explained using a stimulus- response model. The stimulus-response model is a three-step process. (i) Reception – detecting the signalling molecule by the receptor. (ii) Transduction – the relay of the signal into the cell. (iii) Cellular response – the activation of a cellular activity. (i) Reception In a multicellular organism there may be hundreds of signalling molecules in the fluids surrounding the cells, how does a cell know which signals to respond to? Cells have receptors for specific signalling molecules. The type of receptors a cell has depends on the function of that cell. Reception is highly specific, only a signalling molecule that fits a specific receptor can influence the cell. Figure 6.7 shows the insulin receptor, while Figure 6.13 shows neurotransmitter receptors on the postsynaptic cell. Each of these receptors is specific to its signalling molecule. (ii) Transduction Transduction involves converting the signal into a form that can reach its final destination within the cell and bring about a cellular response. Transduction can involve a single step in which the signalling molecule binds to its receptor causing a change which directly produces a cellular response. This is observed for some cellular ion channels. Alternatively, transduction can be a multistep process in which a number of different molecules act sequentially. The cellular response occurs at the end of this cascade. UNIT 1.3. | ANIMALS Date | 01/12/2023 79 Revision | 7 BIOLOGY PROGRAM  Figure 4.15 | (Above) The signalling molecule binds to its receptor causing a change in the receptor molecule which then allow ions to enter the cell (https://courses.lumenlearning.com/boundless-biology/chapter/signaling- molecules-and-cellular-receptors/) Figure 4.16 | (Right) Signalling molecule binds to the receptor and the change results in a multistep cascade reaction which ultimately results in gene transcription(https://chronicml.weebly.com) UNIT 1.3. | ANIMALS Date | 01/12/2023 80 Revision | 7 BIOLOGY PROGRAM (iii) Cellular Response At the end of this process ultimately a response is elicited. The type of response varies greatly depending on the requirements of the cells. It could ultimately lead to opening of an ion channel on a cell surface (Figure 6.15) or to inducing protein production through an interaction with the genetic material of the cell (Figure 6.16). The response can occur on the cell membrane, cytosol or in the nucleus. Figure 4.17 | Insulin action 1.Reception -The peptide hormone insulin binds to the insulin receptor which is embedded in the cell membrane. 2.Transduction -The change in the insulin receptor causes a series of chemical reactions. 3. Response - The glucose transport protein opens to allow glucose into the cell. Individuals with Type I diabetes do not produce insulin. As a result, glucose cannot be taken up by the cells. If left untreated the individual will die. Individuals with Type II diabetes do produce insulin however insulin receptor does not respond to insulin binding. The following links to some current research relating to Type II diabetes: https://youtu.be/VbwRYFMPZS4 UNIT 1.3. | ANIMALS Date | 01/12/2023 81 Revision | 7 BIOLOGY PROGRAM Section 4.4 | Apoptosis an overview Apoptosis is programmed cell death. In a multicellular organism growth and development involves cell division, differentiation and also cell death. These processes are controlled by cell signalling. Apoptosis is a highly regulated process that involves cell signalling. In multicellular organisms, apoptosis is necessary so that the number of cells in the body is regulated, cells that are no longer required are removed or cells that contain damaged DNA that cannot be repaired are removed. Examples of normal cell death: In response to an infection, lymphocytes that produce antibodies are formed. The antibodies are released in response to the infection however once the pathogen is removed the lymphocytes are no longer required. As the lymphocytes are no longer required, they die by apoptosis. As human hands develop in the embryo they have webbing between the individual fingers, as development continues the webbing is removed by apoptosis. Figure 4.18 | The webbing cells between figures is removed by apoptosis. (https://badtzmarucy831.files.wordpress.co m/2010/12/webbed-feet.png) UNIT 1.3. | ANIMALS Date | 01/12/2023 82 Revision | 7 BIOLOGY PROGRAM The process of apoptosis Apoptosis is triggered by signalling molecules which can come from inside or outside of the cell. Cell aging, cell obsolesc