UNIT THREE

1. UNIT THREE BIOLOGY AREA OF STUDY #2: DETECTING & RESPONDING EXAM REVISION LECTURE CHP 5: HOMEOSTASIS - INTRODUCTION

2. CHP 5TOPIC 1: HOMEOSTASIS - INTRODUCTION In order for cells to function properly they require an environment in which their requirements are met and conditions remain stable. This is true for all life forms! Homeostasis is: the maintenance of the internal environment in a relatively stable state despite changes in either the external or internal environment. • is the condition of a relatively stable internal, maintained within narrow limits • when changes occur in the internal environment, homeostatic mechanisms act to restore it to the ‘normal’ state • if the body deviates too far from the normal steady state of a variable, death can occur. Factors Controlled Include: • core body temperature • blood glucose concentration • water levels in body tissues • ph (hydrogen ion concentration) • ions, such as sodium, calcium and chloride ions • blood oxygen concentration • carbon dioxide concentration • blood volume • blood pressure • In this Petri dish, tiny green Volvox colonies have moved towards the light source, allowing increased levels of photosynthesis.

3. CHP 5TOPIC 1: HOMEOSTASIS - INTRODUCTION Key Body Systems Contributing To Homeostasis • nervous system • endocrine system • respiratory system • circulatory system • digestive system • excretory system • integumentary (skin)system

4. CHP 5TOPIC 1: HOMEOSTASIS - INTRODUCTION Defining The Internal Environment • the body’s internal environment consists of the tissue fluid (surrounding cells) and blood plasma • plasma is the liquid part of the blood. • both tissue fluid and plasma are located outside of cells. Together they are called extracellular fluid • the composition of the extracellular fluid is regulated so that body cells can operate at their optimum • fluids located outside cells are extracellular. • fluids located inside the cells are intracellular.

5. CHP 5TOPIC 2: STIMULUS – RESPONSE MODEL In order to be able to maintain stable conditions, organisms need to be able to detect changes in the internal or external environmenst, transmitinformation about those changes to a control centre , coordinate a plan to deal with the challenge and carry out an appropriate response. A simple model can dmonstrate this principle: Stimulus-Response Model • a change in the internal or external environment acts as a stimulusthat is detected by receptors • if the intensity of the stimulus is sufficient (threshold), messages are transferred to a control centre • messages are then passed to effectors which produce a response

6. CHP 5TOPIC 2: STIMULUS – RESPONSE MODEL Receptors are able to detect changes in the internal and external environments. Most act as misalignment or disturbance receptors, detecting a change from normal conditions. Some detect changes in the internal environment, and others, changes in the external environment which have the potential to alter the internal environment.

7. CHP 5TOPIC 2: STIMULUS – RESPONSE MODEL Think about the example of your body’s response to raised CO2 levels! Receptors: brain (medulla), aorta and carotid artery Control Centre: the brain (hypolthalamus) Effector: Respiratory muscles in lungs Consider -High CO2 levels: your response – faster and deeper breathing, but it doesn’t go on for ever. Eventually your breathing rate returns to normal. This is because increased breathing rate tends to lower your CO2 levels; that is, it produced an effect opposite to the original stimulus. This constitutes negative feedback. Your body detects that the original stimulus has been reversed and switches off the response.

8. Topic CHP 5.1 Questions: • Homeostasis is: the____________ of the ______________environment is a relatively ____________ state despite _______________ in either the external or internal environment. • Five factors maintained by homeostatic mechanisms include: ____________, ___________ ______________, ____________________ & __________________ • Another name for the integumentary system is the ______________ • Fluid outside the cell is known as:________________ and includes: ___________ & ________________. This is also known as the internal environment. Topic CHP 5.2 Questions: • Fluxuations within tolerance levels is known as maintaining the _________________ state • As stimulus is: ________________________________________ • True / False: A stimulus, no matter how minor will trigger a response from the control centre • A receptor is: _________________________________________ • Receptors that detect light are known as: ___________________receptors • Receptors that detect pressure and movement are known as: ________________receptors

9. CHP 5TOPIC 3: FEEDBACK Negative Feedback: The maintenance of a stable internal environment (homeostasis) relies on negative feedback systems. In the stimulus-response model, negative feedback occurs when the effector brings about a response that counteracts the original stimulus, so that the variable within the internal environment is returned to its optimal level. Transmission to nerves Stimulus Increase in blood carbon dioxide Feedback Response Decreased carbon dioxide In blood Transmission to nerves or hormones (Negative feedback is normal, good!) Receptor In arteries and brain ControlCentre Respiratory centre in brain Effector Respiratory muscles in lungs (increased ventilations)

10. CHP 5TOPIC 3: FEEDBACK • Negative Feedback • Despite the fact that it is called negative feedback its effects are positive • Without negative feedback homeostatic mechanisms would not work • The word negative comes from the fact that the receptors are receiving signals opposite to those caused by the original stimulus • Positive Feedback • Positive Feedback systems do exist in nature, but they are not mechanisms which maintain homeostasis • The feedback is positive as the response causes an increase in the intensity of the original stimulus, not a reversal of it, as in negative feedback • Example: production of oxytocin • produced by the pituitary gland during childbirth • once birthing process commences, oxytocin stimulates uterine contractions that help push • baby out of the uterus • also acts on the pituitary to produce more oxytocin • continues until baby is born

11. CHP 5TOPIC 4: SIGNALLING MOLECULES SIGNALLING MOLECULES • Body cells communicate with each other via chemical signals • are chemical signals, produced in cells, which affect the function of other body cells • they include: • Hormones • Neurotransmitters • Pheromones • Plant hormones

12. CHP 5TOPIC 4: SIGNALLING MOLECULES

13. CHP 5TOPIC 4: SIGNALLING MOLECULES Hormone Facts continued … • Some hormones cause an increase in a cellular function • Others may cause a decrease or turn the function off • Hormones are usually produced in very small quantities, yet exert large effects on target cells – this is due to signal amplification - the ability of one molecule of hormone to cause the production of many molecules inside the cell • Responses to signals include: activation of DNA to produce proteins • Once hormones have delivered their signal to target cells and the desired effect has occurred, they are degraded by cell enzymes and excreted via the kidneys or faeces

14. CHP 5TOPIC 4: SIGNALLING MOLECULES SIGNAL TRANSDUCTION: • Cascade of events linking an external signal (such as a hormone) to a particular cellular response, examples include: • Signal Transduction: Lipid Soluble Hormones (Hydrophobic) - example: sex hormones, • - steroid hormones enter cell through phospholipidbilayer (not possible for other hormones) • - in target cells, there are specific receptors forming a hormone-receptor complex – in cytoplasm • - hormone-receptor complex enters nucleus – it attaches to a specific region on a chromosome • - hormone-receptor complex activates a particular gene, causing production of mRNA • - mRNA travels to ribosomes – assembles a new protein (many copies) • - protein produced (enzyme, growth factor or structural protein) affects cell function • - typically, steroid hormones regulate long-term development

15. CHP 5TOPIC 4: SIGNALLING MOLECULES Signal Transduction: Peptide Hormones (Hydrophillic/ Lipophobic) Remember: The change from an extracellular (outside) signal to an intracellular (inside) signal happens by a process known as signal transduction Unlike lipids, peptide hormones cannot pass through the phospholipidbilayer of cell membranes. • The hormone (first messenger) binds with a specific receptor molecule on the cell surface (specific!) • Some have binding site on outside of cell and an enzyme site on the inside • The binding of receptor, hormone (first messenger) stimulates the enzymatic end of the protein molecule • The enzyme activates the formation of many molecules which act as second messengers. The signal is amplified • Second messengers cause a change in cell functioning- a response In another type of Signal Transduction, an enzyme doesn’t exist beneath the membrane protein but a nearby protein molecule known as a G protein is activated which in turn activates a third protein, an enzyme. This enzyme (as in 4 above) activates the formation of many molecules which act as second messengers. The signal is amplified . As in 5, second messengers cause a change in cell functioning- a response Once a hormone response is initiated, the hormone is degraded

16. CHP 5TOPIC 4: SIGNALLING MOLECULES Pheremones • signalling molecules that are released by animals into their environment enabling them to communicate with members of their own species • pheromones can be used for: • attracting mates • inducing mating activity • marking territories • signallng alarm • marking food trails Plant Hormones • The growth of plants is regulated by environmental factors and plant hormones • They are similar to animal hormones in that : • They are produced in response to a stimulus • They are produced in particular parts of the plant and travel to cells, which will respond, in a different part of the plant • They bind with specific receptors which cause a response within cells • They also differ from animal hormones in an number of ways: • The same hormone can cause many different hormones (animal hormones are very specific) • A plant hormone may cause one response at low concentrations and the opposite at high concentrations • The actions of plant hormones overlap a lot, with many hormones contributing to one observed response

17. Topic CHP 5.3 & 5.4 Questions: • True/ False: All homeostatic mechanisms maintain homeostasis with ‘negative feedback’ • Positive feed back mechanisms cause: an increase/ a decrease in the initial stimulus. • List four signalling molecules: ________________, ________________, _____________________, _______________ • Where are neurotransmitters produced? ______________ • How are hormones transported around the body: _______________ • What cells to signalling molecules react with? _______________ • What is signal amplification? _____________________________________ ________________________________ • True/ False: hormones can direct the activation of DNA to produce proteins • What happens to hormones once they have delivered their signal to target cells? ______________________ • What is signal transduction: ____________________________ • How does signal transduction differ between hydrophillic and hydrophobic messengers?___________________________________________________________ • Name two functions of pheremones: ________________________

18. CHP 5TOPIC 5: PLANT HORMONES Plant Hormones Classifications of plant hormones: • Auxins- regulate growth – elongates cells in stems, roots • Cytokinins – growth promoting – promote cell reproduction - shoots • Gibberellins- speed growth, germination • abscic acid- dormanct, close stomata, falling leaves and falling fruit • Ethylene (gas)- ripens fruit • Florigen- control of flowering? • Jasmonates • Brassinosteroids Plant Hormones & Growth Response Tropisms • is a growth response towards (positive) or away from (negative) a given stimulus Examples: - Positive Phototropism(Light): is growth towards a stimulus (light) - Thigmotropism(Touch):change in the direction of growth because of contact another object - Geotropism(Gravity): is a growth response due to gravity (positive (grows down!- with) - Hydrotropism (Water): growth response due to stimulus of water

19. CHP 5TOPIC 5: PLANT HORMONES Gravity sensing mechanism Auxin in higher amounts on “lower” side of organ Root more sensitive to auxin - inhibits elongation Statoliths at bottom of cells of pea root

20. CHP 5TOPIC 5: PLANT HORMONES

21. Topic CHP5.5 Questions: • True/ False: Auxins regulate growth through cell elongation • True/ False: Auxins always promote cell growth • True/ False: Auxins are one plant hormone that is responsible for plant tropism • What is a tropic response: ________________________________________________ • Define positive phototropism______________________________________________ • The effect of cutting a growing coleoptile tip from a growing shoot would be? ______________________________________________________ • What is apical dominance? __________________________________ • Gibberelins are responsible for: _________________________________________ • Gibberelins are transported through the plant within:________________________ • What is the role of the hormone ethylene? ___________________________________ • How is ethylene transported through a plant?_______________________________- • High levels of Abscisic Acid in a plant could indicate: _________________________________________________________________