Negative & positive feedback (Edexcel Int. A-level Biology)

This lesson describes the meaning of positive feedback and explains how negative feedback control is involved in maintaining systems within narrow limits. The PowerPoint and accompanying resources have been designed to cover points 7.16 (i) and (ii) of the Edexcel International A-level Biology specification but also provide introductory details for upcoming topics such as the importance of homeostasis during exercise and the depolarisation of a neurone.

The normal ranges for blood glucose concentration, blood pH and body temperature are introduced at the start of the lesson to allow students to recognise that these aspects have to be maintained within narrow limits. A series of exam-style questions then challenge their recall of knowledge from topics 1 - 6 as they have to explain why it’s important that each of these aspects is maintained within these limits. The students were introduced to homeostasis at GCSE, so this process is revisited and discussed, so that students are prepared for an upcoming lesson on exercise, as well as for the next part of the lesson on negative feedback control. Students will learn how this form of control reverses the original change and biological examples are used to emphasise the importance of this system for restoring levels to the limits (and the optimum). The remainder of the lesson explains how positive feedback differs from negative feedback as it increases the original change and the role of oxytocin in birth and the movement of sodium ions into a neurone are used to exemplify the action of this control system.