AQA A Level Biology Paper 2 - Organisms Respond to Changes in Environments COMPLETE SOLUTIONS

Draw the reflex arc. - ANSWER What is homeostasis? - ANSWER Homeostasis involves physiological control systems that maintain the internal environment despite changes in the external environment of an organism within restricted limits. What factors does our internal environment consist of? - ANSWER Temperature, water potential, pH and blood glucose levels, water levels, etc. Why is homeostasis important? - ANSWER If body temperature and blood pH change too much then enzymes will become denatured e.g. if body temperature rises above 37oC, or cells may burst/shrink if the water potential is dramatically altered. How do we regulate conditions in our internal environment? - ANSWER Negative and positive feedback are control mechanisms used, among others. Describe the differences between negative and positive feedback. - ANSWER Negative feedback restores systems to their original level - it's when the change produced by the control system leads to a change in the stimulus detected, turning the system off. Positive feedback occurs when a deviation from an optimum causes changes that cause a greater deviation from the optimum. Give an example of positive feedback. - ANSWER In neurones - when a stimulus leads to a small influx of Na+ ions and this influx increases the permeability of the neurone's membrane to Na+ ions. This causes even more Na+ ions to enter rapidly.