Biology · Homeostasis
Homeostasis and Control Systems
Your cells work properly only when the conditions around them are kept steady. This lesson explains why a constant internal environment matters, what is controlled, and how receptors, coordination centres and effectors use negative feedback to keep conditions in balance.
Learning Objectives
- 1Define homeostasis and name the conditions that the body controls.
- 2Explain the importance of maintaining a constant internal environment, so that metabolic reactions proceed at appropriate rates.
- 3Describe a control system in terms of receptor, coordination centre and effector.
- 4Explain how negative feedback keeps a condition near its normal level.
Retrieval practice
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1
What is an enzyme?
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A biological catalyst that speeds up a reaction.
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2
What happens to an enzyme at a high temperature?
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It denatures, so its active site changes shape and it stops working.
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3
What is osmosis?
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The movement of water across a partially permeable membrane from a dilute to a concentrated solution.
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4
What is a hormone?
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A chemical messenger carried in the blood.
Keeping the balance
A fish can live in a pond that is freezing in winter and warm in summer, but a human cannot. Your body temperature stays at about 37 °C whether it is a snowy day or a heatwave. The same is true of your blood sugar and the amount of water in your blood. Keeping these conditions steady is called homeostasis, and it depends on control systems that detect changes and reverse them.
What is homeostasis?
Homeostasis is the regulation of the internal conditions of the body to keep them constant.
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Internal environment
The conditions inside the body, in the blood and in the fluid that surrounds the cells.
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Body temperature
Kept at about 37 °C.
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Blood glucose concentration
Kept within narrow limits, so cells always have a supply of glucose.
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Water content
The balance of water and salts in the blood and body fluids is controlled.
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Automatic
The control is automatic. You do not have to think about it.
Why a constant internal environment matters
Cells and enzymes work well only within narrow limits.
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Enzymes
The enzymes that control metabolic reactions work best at one optimum temperature and pH. If the temperature gets too high, enzymes denature. If it is too low, reactions are too slow.
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Metabolic reactions
A constant internal environment allows the metabolic reactions of the cells to proceed at appropriate rates.
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Water
If the water content of the blood is wrong, osmosis moves water into or out of the cells, which may swell and burst or shrink.
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Glucose
Too little glucose means cells cannot release energy, and too much can damage the body.
A control system
Every system that keeps a condition steady has the same three parts.
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Receptor
Detects a change in the condition, such as a temperature receptor in the skin.
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Coordination centre
Receives the information, compares it with the normal level and decides on the response. It may be the brain, the spinal cord or the pancreas.
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Effector
Carries out the response, such as a muscle or a gland.
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Nervous and hormonal
The signals may be nerve impulses or hormones, or both.
A negative feedback loop
In negative feedback the response reverses the change.
Negative feedback
Most homeostatic systems work by negative feedback.
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The idea
A change in a condition causes a response that reverses the change and brings the condition back to normal.
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Too high
If the condition rises above normal, the response makes it fall.
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Too low
If it falls below normal, the response makes it rise.
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The result
The condition goes up and down a little but stays close to its normal level.
Nervous and hormonal control
Nervous control
- Nerve impulses
- Very fast response
- Used for reflex actions and quick changes
- Example: sweating when you get hot
Hormonal control
- Hormones in the blood
- Slower response, but longer-lasting
- Used for slower changes
- Example: insulin controlling blood glucose
Case study
A thermostat
A room is kept warm by a thermostat. If the temperature falls below the set value, the thermostat switches the heating on. When the room warms up past the set value, the thermostat switches the heating off. The temperature stays close to the set value. A thermostat is a model of negative feedback: the sensor is the receptor, the control unit is the coordination centre and the heater is the effector. Your body works in the same way, with the hypothalamus as the thermostat.
The big idea
Homeostasis keeps the internal environment constant using receptors, coordination centres and effectors in negative feedback loops, so that the metabolic reactions in cells can go on at the right rates.
Homeostasis
Homeostasis
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Controlled
- temperature
- blood glucose
- water content
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Why
- enzymes and cells work only within narrow limits
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Receptor
- detects the change
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Coordination centre
- decides the response
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Effector
- carries out the response
- negative feedback
Summary and exam focus
- Homeostasis is the regulation of the internal conditions of the body to keep them constant.
- Body temperature, blood glucose and water content are controlled.
- A constant internal environment lets metabolic reactions proceed at appropriate rates.
- A control system has a receptor, a coordination centre and an effector.
- In negative feedback, the response reverses the change.
Exam focus
Explain why it is important that the body keeps its internal conditions constant. (3 marks) (3 marks)
Link to enzymes and metabolic reactions: if the temperature or pH changes, enzymes stop working properly, so reactions in cells go too slowly or stop. You can add that water balance affects cells by osmosis.
Key terms
The words this lesson expects you to use. Each one is linked from the first place it appears above.
- Homeostasis
- The regulation of the internal conditions of the body to keep them constant.
- Internal environment
- The conditions inside the body, such as the blood and tissue fluid.
- Receptor
- A cell or organ that detects a change.
- Coordination centre
- A part of the body, such as the brain, that decides the response.
- Effector
- A muscle or gland that carries out the response.
- Negative feedback
- A response that reverses a change and returns a condition to normal.
- Metabolic reaction
- A chemical reaction in a cell, controlled by an enzyme.
- Optimum
- The conditions in which an enzyme works best.
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