Communication and homeostasis (5.1.1)

Multicellular organisms contain specialised cells, tissues and organs that must coordinate their activities.

Communication systems enable different parts of the body to work together and respond to changes in internal and external conditions.

By detecting stimuli and coordinating responses, they help maintain stable conditions for survival (homeostasis).

Cell signalling is how cells communicate with each other to coordinate their activities.

A signalling molecule (like a hormone or neurotransmitter) is released by a cell and travels to a target cell (either locally to adjacent cells or through the bloodstream to distant cells).

The target cell has receptor proteins on its surface that are specific to the signal. When the molecule binds to the receptor, it triggers a response inside the cell.

Homeostasis is the maintenance of a stable internal environment by physiological control systems that detect and respond to changes in the internal or external environment.

It is important because enzymes and cells function best within narrow ranges of temperature, and water potential. If conditions move outside these limits, metabolic processes may be disrupted and cells may be damaged.

Examples of conditions regulated by homeostasis include:

• Body temperature
• Blood glucose concentration
• Water potential of the blood
• of blood and tissues

Positive feedback is a control mechanism that amplifies a change by increasing the effect of the original stimulus.

This response is usually not homeostatic, as it does not restore conditions back to the normal level.

Childbirth is an example of positive feedback.

Stretching of the cervix stimulates the hypothalamus to release more oxytocin from the pituitary gland. Oxytocin strengthens uterine contractions, causing further stretching of the cervix and the release of more oxytocin. This cycle continues until birth occurs.

Blood clotting is another example of positive feedback.

Platelets adhere to damaged vessel walls, releasing chemicals to attract more platelets. This accelerates clot formation until the wound is sealed.

Negative feedback is a control mechanism that maintains homeostasis by reversing changes in the internal environment, restoring conditions back to normal (the optimal range).

1. A change is detected (e.g., body temperature rises).
2. Receptors send signals to a coordination centre (e.g., hypothalamus).
3. The coordination centre activates effectors (e.g., sweat glands).
4. Effectors respond to reverse the change (e.g., cooling the body down).
5. Once normal conditions are restored, the system switches off.

Ectotherms (e.g., amphibians, reptiles) rely mainly on external sources of heat to regulate body temperature. Therefore, the majority of changes are behavioural, such as:

• Basking in the sun to warm up.
• Hiding in shade or burrowing to cool down.
• Changing body orientation to increase/decrease heat absorption.
• Altering activity level (e.g., active during warm times).

Ectotherms have little physiological control of body temperature, although some can increase blood flow or heart rate to distribute heat more effectively.

Endotherms (e.g., mammals, birds) control body temperature internally using metabolic heat.

Endotherms show behavioural responses such as seeking shade, sweating / panting, or bathing to cool. They may curl up, huddle, or move to sunny spots to warm up.

Physiological responses also help regulate their internal temperature:

• If too hot, vasodilation, sweating, panting or reducing metabolic rate increase heat loss.
• If too cold, vasoconstriction, shivering, raised hairs / feathers and an increased metabolic rate help conserve or generate heat, returning conditions toward the optimal range.