7.9 - 7.10B - Why homeostasis matters

7.9 - A stable internal environment

Cells carry out many enzyme-controlled reactions. For those reactions to continue at suitable rates, the conditions around the cells must remain within limits. However, the body is continually disturbed by external changes, such as the air becoming hotter, and internal changes, such as muscle respiration releasing more energy during vigorous exercise or water being lost in sweat.

"Stable" does not mean perfectly unchanging. A homeostatic variable can fluctuate within a narrow normal range. The important point is that the body detects a deviation and produces a response that moves the variable back towards that range.

A homeostatic control pathway has a consistent causal order:

1. An internal or external change causes a variable to move away from its normal range.
2. Receptors detect the change.
3. A coordination centre processes the information and coordinates a response.
4. Effectors, such as muscles or glands, act to oppose the change.
5. As the variable returns towards its normal range, the original stimulus becomes smaller, so the response is reduced.

This is negative feedback: the response opposes the deviation. It does not mean that the response is harmful or that the value becomes negative.

Both parts of this lesson apply to Foundation and Higher Biology. The B suffix on 7.10B identifies separate Biology content.

7.10B - Why homeostasis matters to cells

Two examples show why a stable internal environment matters.

Thermoregulation and enzymes

Thermoregulation is the control of core body temperature within a narrow range. At low temperatures, enzyme and substrate particles have less kinetic energy and collide less frequently, so enzyme-controlled reactions become slower. If temperature becomes too high, heat can change an enzyme's three-dimensional shape. Its active site is no longer complementary to its substrate, so activity decreases; the enzyme is denatured.

Homeostasis therefore keeps body temperature close to conditions in which the body's enzymes can work effectively. Low temperature does not usually denature an enzyme, and 37°C is not an identical optimum for every human enzyme.

Osmoregulation and animal cells

Osmoregulation is the control of the water concentration of body fluids. Water moves by osmosis across the partially permeable cell membrane from a region of higher water concentration to a region of lower water concentration.