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AQA A-Level Biology: Homeostasis in Mammals: Negative Feedback and Stability — mark scheme explained

Machine-verifiedchecked against the AQA A-Level Biology specificationlast verified 3 July 2026

The short answer

Homeostasis is a fundamental biological process that ensures the internal environment of an organism remains stable despite external changes. In mammals, this involves various physiological control systems that maintain key parameters within restricted limits.

The question

Explain how a rise in blood glucose concentration triggers a negative feedback mechanism.

[Paraphrased for study — not reproduced from any exam paper.]

4 marks

Mark scheme, decoded

How the examiner actually awards the marks on this topic.

Gradora's own decode of the marking approach — not the exam board's published mark scheme.

How marks are awarded

For descriptive questions, marks are typically awarded for clear and concise explanations that demonstrate a deep understanding of the topic. For data interpretation questions, marks are given for accurately analyzing the data and drawing appropriate conclusions. Ensure your answers include all relevant components and use specific examples where applicable.

What the command words demand

Explain: Provide a detailed description of the concept, including relevant principles and relationships.
Describe: Give a clear account of the components and processes involved in a homeostatic mechanism.
Compare: Highlight the similarities and differences between two or more related concepts or mechanisms.
Interpret: Analyze data or graphs to draw conclusions about how homeostatic mechanisms respond to changes.

Model answer

A full-mark response to the question above, worked through step by step.

Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, provide detailed explanations, and check your work.

Identify the initial change.0 marks
Blood glucose levels rise above the normal range.
Describe the role of sensors.1 mark
Sensors in the pancreas detect the increase in blood glucose levels.
Explain the response from the control center.1 mark
The pancreas (control center) responds by releasing insulin into the bloodstream.
Detail the action of effectors.1 mark
Insulin acts on cells to increase glucose uptake and storage, reducing blood glucose levels back to normal.
Summarize the negative feedback loop.1 mark
The rise in blood glucose triggers a response that lowers it back to the set point, maintaining homeostasis.

Final answer: A rise in blood glucose concentration is detected by sensors in the pancreas. The pancreas responds by releasing insulin, which acts on cells to increase glucose uptake and storage, thereby reducing blood glucose levels back to normal.

Work through every step correctly and you earn all 4 marks.

Another worked example

Describe how a drop in core temperature triggers a negative feedback mechanism.

4 marks

Identify the initial change.0 marks
Core body temperature drops below the normal range.
Describe the role of sensors.1 mark
Thermoreceptors in the skin and hypothalamus detect the decrease in temperature.
Explain the response from the control center.1 mark
The hypothalamus (control center) responds by initiating mechanisms to generate heat and reduce heat loss.
Detail the action of effectors.1 mark
Effectors such as muscles initiate shivering to generate heat, and blood vessels constrict (vasoconstriction) to reduce heat loss from the skin.
Summarize the negative feedback loop.1 mark
The drop in core temperature triggers a response that increases heat production and reduces heat loss, restoring body temperature to normal.

Final answer: A drop in core temperature is detected by thermoreceptors in the skin and hypothalamus. The hypothalamus responds by initiating shivering to generate heat and vasoconstriction to reduce heat loss from the skin, thereby restoring body temperature to normal.

Work through every step correctly and you earn all 4 marks.

Common mistakes

Confusing positive feedback with negative feedback.
Why it happens: Students sometimes mix up the definitions of positive and negative feedback, leading to incorrect explanations in problems involving homeostatic mechanisms.
Fix: Remember that negative feedback reverses changes to restore a system to its original state, while positive feedback amplifies changes. Practice identifying which type of feedback is involved in different scenarios.
Failing to explain the importance of stable internal conditions for enzyme activity and metabolic processes.
Why it happens: Students may not fully understand how deviations from optimal temperature or pH can affect enzyme function and overall metabolism.
Fix: Practice explaining why maintaining a stable core temperature, blood pH, and blood glucose concentration is crucial for optimal enzyme activity and metabolic efficiency. Use specific examples to illustrate the importance of these conditions.
Misunderstanding the direction of change in negative feedback.
Why it happens: Students may not grasp that negative feedback works by reversing the direction of change, leading to incorrect explanations of how homeostatic mechanisms function.
Fix: Always ensure that your explanation of negative feedback includes a clear description of how the response counteracts the initial deviation. Use diagrams or flowcharts to visualize the process if needed.
Confusing the roles of different hormones in maintaining blood glucose levels.
Why it happens: Students may mix up the functions of insulin and glucagon, leading to incorrect answers when comparing their roles in homeostasis.
Fix: Practice describing the specific actions of insulin (lowering blood glucose) and glucagon (raising blood glucose). Use examples to illustrate how these hormones work together to maintain a stable blood glucose concentration.
Failing to identify the components of a negative feedback loop.
Why it happens: Students may not clearly distinguish between sensors, control centers, and effectors in their explanations of homeostatic mechanisms.
Fix: Always include all three components (sensors, control centers, and effectors) when describing a negative feedback loop. Use specific examples to show how each component contributes to the overall process.
Incorrectly interpreting data on homeostatic responses.
Why it happens: Students may struggle to analyze graphs or data sets and draw accurate conclusions about how homeostatic mechanisms respond to changes in internal conditions.
Fix: Practice analyzing different types of data, such as graphs showing changes in blood glucose levels over time. Identify the initial change, the response, and the final outcome to ensure a clear understanding of the homeostatic process.

Where the marks go

The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.

Question type	What you’re asked to do	Marks
Negative Feedback Control	Explain how rising blood glucose triggers a corrective negative feedback response toward normal.	4
Negative Feedback Mechanism	Describe how the body detects and corrects a fall in core temperature back to normal.	4
Negative Feedback Mechanism	Describe how the body detects and corrects a decrease in blood pH back to normal.	4
Compare Hormone Roles	Compare how insulin and glucagon act antagonistically to regulate blood glucose concentration.	4
Total across these question types		16

Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.