A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Homeostasis in Mammals: Negative Feedback and Stability — mark scheme explained
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.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the initial change.
- S2
Blood glucose levels rise above the normal range.
- S3
Step 2: Describe the role of sensors.
- S4
Sensors in the pancreas detect the increase in blood glucose levels.
- S5
Step 3: Explain the response from the control center.
- S6
The pancreas (control center) responds by releasing insulin into the bloodstream.
- S7
Step 4: Detail the action of effectors.
- S8
Insulin acts on cells to increase glucose uptake and storage, reducing blood glucose levels back to normal.
- S9
Step 5: Summarize the negative feedback loop.
- S10
The rise in blood glucose triggers a response that lowers it back to the set point, maintaining homeostasis.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the initial change.
- S2
Blood glucose levels rise above the normal range.
- S3
Step 2: Describe the role of sensors.
- S4
Sensors in the pancreas detect the increase in blood glucose levels.
- S5
Step 3: Explain the response from the control center.
- S6
The pancreas (control center) responds by releasing insulin into the bloodstream.
- S7
Step 4: Detail the action of effectors.
- S8
Insulin acts on cells to increase glucose uptake and storage, reducing blood glucose levels back to normal.
- S9
Step 5: Summarize the negative feedback loop.
- S10
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.
Common mistakes
- Confusing positive feedback with negative feedback. — 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. — 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. — 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. — 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. — 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. — 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
- Full worked solution (all marking points)4 marks