GCSE BIOLOGY • SPECIFICATION 1.5

Homeostasis

Aligned to AQA GCSE Biology (8461/8464). Wording consistent with AQA mark schemes.

Regulating the internal environment: Nervous and Hormonal coordination.

1. Principles of Homeostasis

Exam Definition (Homeostasis):

"The regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes."

Why is it necessary? To maintain optimal conditions for enzyme action and all cell functions.

The Three Key Control Elements:

Receptors: Detect stimuli (changes in the environment).
Coordination Centres: Receive and process information. These include the nervous system (brain and spinal cord) and endocrine system (e.g. pancreas).
Effectors: Muscles or glands which bring about responses.

2. The Human Nervous System

The Reflex Arc (Logic Chain)

Stimulus → Receptor → Sensory Neurone → Relay Neurone (Spinal Cord) → Motor Neurone → Effector → Response

Reflex actions are rapid, automatic responses that protect the body from harm and are coordinated by the spinal cord (not the conscious brain).

Q: Explain how information passes across a synapse (4 Marks).

An electrical impulse arrives at the end of the first neurone.
A chemical neurotransmitter is released.
The chemical diffuses across the gap.
This triggers an electrical impulse in the next neurone. (Note: Synapses ensure impulses travel in one direction only).

3. Hormonal Coordination (Endocrine System)

Comparison: Nervous vs Endocrine

Comparison	Nervous System	Endocrine System
Speed	Very Rapid	Slower
Duration	Short-lived	Longer-lasting
Targeting	Specific (one muscle/gland)	Widespread (all cells with receptors)
Signal	Electrical Impulse	Chemical (Hormone)

[Image of the human endocrine system glands including pituitary, thyroid, adrenal, pancreas, ovaries, and testes]

Control of Blood Glucose

Blood Glucose Too High: The pancreas produces insulin. Insulin causes glucose to move from blood into cells and be converted to glycogen mainly in the liver and muscles.

Negative Feedback: Glucagon

If blood glucose is too low, the pancreas produces glucagon that stimulates glycogen to be converted back into glucose and released into the blood.

4. Hormones in Reproduction

The Menstrual Cycle Hormones

Hormone	Function (AQA Phrasing)
FSH	Stimulates maturation of an egg in an ovary; stimulates estrogen production.
Estrogen	Thickens the uterus lining; inhibits FSH; stimulates LH.
LH	Stimulates ovulation (release of the egg).
Progesterone	Maintains uterus lining; inhibits FSH and LH.

Explain how oral contraceptives (The Pill) prevent pregnancy (2 Marks).

Contains hormones that inhibit FSH production, preventing egg maturation and therefore ovulation.

5. Water Balance & ADH (Triple Only)

Water levels in the blood are controlled by negative feedback via the hormone ADH, released by the Pituitary Gland.

Blood Too Concentrated:

More ADH released.
Kidney tubules become more permeable to water.
More water is reabsorbed; small volume of concentrated urine produced.

Blood Too Dilute:

Less ADH released.
Kidney tubules become less permeable to water.
Less water reabsorbed; large volume of dilute urine produced.

Final Exam Guardrail

• Negative Feedback: Always mention that it reverses the change to return conditions to normal.
• Diabetes: Type 1 (Pancreas stops making insulin); Type 2 (Cells no longer respond to insulin).
• Glucagon vs Glycogen: Remember: Glucagon is the hormone that is "gone" from the pancreas to help the blood.
• FSH Phrasing: Never just say "makes an egg"; say "stimulates maturation of an egg in an ovary."

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