A stable internal environment
Definition
Internal environment
The conditions inside the body that surround cells, especially the temperature and composition of the blood and tissue fluid.
- The internal environment is formed mainly by the blood and tissue fluid surrounding the body’s cells.
- Its conditions include temperature, water content, ion concentration, blood glucose concentration and pH.
- An external change, such as hot weather, cold weather or reduced water intake, can disturb these conditions.
- An internal change, such as exercise, eating a meal or increased respiration, can also disturb them.
- Control systems correct these disturbances before they move far enough to damage cells.
Stable conditions protect cells
- Enzyme activity depends on temperature and pH because each enzyme has an active site with a specific shape.
- A large temperature or pH change alters the rate of enzyme-controlled reactions, and extreme conditions can change the active site so the substrate no longer fits.
- Water and ion balance controls the net movement of water across cell membranes by osmosis.
- If too much water enters an animal cell, it swells and may burst because it has no cell wall.
- If too much water leaves an animal cell, it shrinks and its chemical reactions and membrane processes are disrupted.
- Blood glucose concentration must stay high enough to supply cells with glucose for respiration without rising to a level that damages tissues.
Common Mistake
- Do not write that internal conditions stay exactly unchanged.
- They fluctuate within narrow limits, and corrective responses bring them back towards the normal range.
Control responses reverse change
- Receptors detect a change in an internal condition or in the external environment.
- A coordination centre receives the information and organises a response.
- Effectors, such as muscles or glands, produce a response that opposes the original change.
- The response becomes smaller or stops as the condition returns towards its normal range.
Exam technique
- For an explain question, connect the disturbance to a cellular effect rather than naming a condition on its own.
- A complete chain is change in condition, effect on cells or enzymes, effect on body function, then the benefit of correction.
- Use linking words such as because, so and therefore to make each biological consequence explicit.
Changes come from inside and outside
- Hot surroundings and exercise both raise body temperature, but one begins outside the body and the other begins with increased muscle activity.
- Drinking water, sweating and urine production alter the amount of water in the body.
- Eating, fasting and respiration alter the supply and use of glucose.
- Maintaining the internal environment allows cells to keep functioning while the body faces all of these changes.
Self review
- What is the internal environment of the body?
- Give one internal change and one external change that can disturb it.
- Why can a large change in temperature disrupt metabolism?
- Why must the water content around animal cells be controlled?
