Homeostasis Starter what can you remember from GCSE

Homeostasis Starter: what can you remember from GCSE? How would you define homeostasis?

Homeostasis is the maintenance of a constant internal environment within a living organism. Homeo = same Stasis = state Internal environment – tissue fluids that surrounds cells Protects cells from external changes Organism is in balanced equilibrium Q: What needs to be kept ‘constant’ or regulated?

So why is homeostasis important? • • Water potential of blood and tissue fluid PH of blood and tissue fluid Temperature Blood glucose concentration What would happen if these things were not kept constant?

Importance of homeostasis • Enzyme activity is affected by changes in temperature and p. H • Cells could shrink or burst due to changes in water potential • Allows organisms to occupy a greater range of habitats

Endotherms and Ectotherms

Control mechanisms • • • Optimum point Receptors – detects change Coordinator Effector – causes a response Feedback mechanism

Feedback mechanisms • Negative feedback – this occurs when the stimulus causes the corrective measures to be turned off • Used in homeostasis • Tends to return the system to its original level

Feedback mechanisms • Positive feedback occurs when the feedback causes the corrective measures to remain on • Causes the system to deviate further away from original level • Less common than negative feedback • Used to create a rapid change in one direction • May be associated with the breakdown of a system e. g. hypothermia

Positive and Negative Feedback • Negative feedback: a change is detected by receptors, and then effectors return the system to its original state. • Positive feedback: the corrective mechanism stays on, which causes the system to deviate even more from the original level.

Example 1: Childbirth Positive Feedback In the process of childbirth in mammals, when the baby pushes against the cervix, a hormone called oxytocin stimulates uterine contractions. This causes more oxytocin to be released, hence more contractions. This continues until the baby is born.

Example 2: Blood pressure Negative Feedback While the heart is pumping, baroreceptors detect the pressure of the blood going through the arteries. If the pressure is too high or too low, a chemical signal is sent to the brain. The brain then sends a chemical signal to the heart to adjust the rate of pumping: if blood pressure is low, heart rate increases, while if blood pressure is high, heart rate decreases.

Example 3: Body Temperature Normal human body temperature is approximately 37 o. C. When body temperature rises above this, sweating and vasodilation occur. As more heat is lost from the body, normal temperature is regained. Should these cooling mechanisms continue, the body will become cold. Vasoconstriction, raising hairs and shivering will then cause more heat to be retained and normal temperature is regained Negative Feedback

Example 4: Apple Trees Positive Feedback On an apple tree with many apples, seemingly overnight they all go from unripe to overripe. This will begin with the first apple to ripen. Once ripe, it gives off a gas known as ethene (C 2 H 4) through its skin. When exposed to this gas, the apples near to it also ripen. Once ripe, they too produce ethylene, which continues to ripen the rest of the tree in an effect much like a wave.

Example 5: Blood clotting Positive Feedback When tissue is torn or injured, a chemical is released. This chemical causes platelets in the blood to activate. Once these platelets have activated, they release a chemical which signals more platelets to activate, until the wound is clotted. http: //blausen. com/en/video/positive-feedback-blood-clotting/

Exam Style Questions 1. What is homeostasis? (1) 2. Using one example, explain why homeostasis is important to mammals. (2) 3. Explain the term positive feedback. (1) 4. Cold water swimmers may suffer from muscle fatigue during which the contraction mechanism is disrupted. One factor thought to contribute to muscle fatigue is a decrease in the availability of calcium ions within muscle fibres. Explain how a decrease in the availability of calcium ions could disrupt the contraction mechanism in muscles. (3)

1. maintaining a constant internal environment; • 1 2. one mark for example of factor kept constant; one mark for explaining its importance; e. g. temperature / p. H; optimum for enzymes / effect of p. H / temperature on enzyme activity; OR • water potential / blood glucose; effect of osmotic / blood glucose imbalance on cells; • 2 max

3. a change away from the set point leads to further change; 5. cannot interact with / move tropomyosin from binding sites on actin; (reject active sites) myosin(heads) do not bind / actinomyosin not formed; does not activate ATPase / energy not released from ATP; • 3