Homeostasis and feedbackEdexcel A-Level Biology B: Revision notes

Homeostasis is the maintenance of a stable internal environment within narrow limits. It is a state of dynamic equilibrium, because conditions are not fixed. They fluctuate around a set point (for example 37 °C for core body temperature), and are continually corrected by control systems.

A control system has a receptor that detects a change in a condition, a coordinator (such as the hypothalamus) that processes the information, and effectors that bring about a response.

Enzymes control metabolic reactions, and their activity depends on temperature.

• Too high: hydrogen bonds in the tertiary structure break, so the shape of the active site changes and the enzyme denatures. The substrate no longer fits.
• Too low: enzyme and substrate molecules have less kinetic energy, so there are fewer collisions and fewer enzyme-substrate complexes, and reactions are slow.

The pH of blood and tissue fluid depends on the concentration of hydrogen ions. A change in pH alters the ionic and hydrogen bonds between the R groups of amino acids in enzymes, which changes the tertiary structure and the shape of the active site. The enzyme denatures and stops working. Haemoglobin and other proteins are also affected, so the transport of oxygen would be less effective.

In exercise, lactate production lowers blood pH, so the body must correct it to keep enzymes working near their optimum.

Water potential is the tendency of water to move; pure water has the highest value and solutions have negative values. Water moves by osmosis from a higher (less negative) to a lower (more negative) water potential across a partially permeable membrane.

• If blood water potential becomes too low (more negative), water leaves cells by osmosis. They shrink and cannot function.
• If it becomes too high, water enters animal cells by osmosis and they may burst (lyse), because they have no cell wall.

In negative feedback, a change in a condition is detected by receptors, and the response of the effectors reverses the change, so the condition returns towards the set point. When it returns, the stimulus falls and the response is reduced. Negative feedback keeps conditions within narrow limits and is the basis of homeostasis.

Example: when core temperature rises, thermoreceptors in the hypothalamus detect it. Impulses go to the sweat glands and skin arterioles, which respond with more sweating and vasodilation. Heat is lost and temperature falls to the set point.

In positive feedback, the response amplifies the change, so the condition moves further away from its starting point. It does not maintain a stable condition and needs an outside event to stop it.

Example: during labour, stretching of the cervix causes oxytocin release, which increases uterine contractions, which stretch the cervix more. This continues until the baby is born. Positive feedback is used in short-lived events and would be harmful if it controlled body temperature, as a rising temperature would increase metabolic heat production.