A-Level · Biology · AQA · Mark scheme decoded

AQA A-Level Biology: Osmoregulation and Nephron Function — mark scheme explained

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

The short answer

Osmoregulation is a critical process that helps maintain the water potential of blood, ensuring that cells function optimally. This section delves into the roles of the hypothalamus, posterior pituitary gland, and antidiuretic hormone (ADH) in osmoregulation, as well as the structure and functions of the nephron in the kidney.

The question

Explain how the hypothalamus and posterior pituitary gland work together to maintain the water potential of blood.

[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 providing accurate and detailed information about the roles of different structures and processes. For functional questions, marks are given for explaining how these structures and processes work together to achieve a specific outcome. Ensure your answers are clear, concise, and well-organized.

What the command words demand

Explain: Provide a detailed description of the concept, including relevant principles and relationships.
Describe: Give a clear and concise account of the process or structure.
Identify: Name or point out specific structures or processes.
Summarize: Briefly outline the key points or main ideas.

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 role of osmoreceptors in the hypothalamus.1 mark
Osmoreceptors detect changes in the water potential of the blood.
Describe what happens when the blood becomes too concentrated (low water potential).1 mark
When the blood is too concentrated, osmoreceptors are activated, triggering the release of ADH from the posterior pituitary gland.
Explain the effect of ADH on the kidney.1 mark
ADH acts on the distal convoluted tubule and collecting ducts to increase water reabsorption, diluting the blood and restoring its water potential to normal levels.
Describe what happens when the blood is too dilute (high water potential).1 mark
When the blood is too dilute, less ADH is released, leading to decreased water reabsorption and more dilute urine.

Final answer: The hypothalamus contains osmoreceptors that detect changes in blood water potential. When the blood becomes too concentrated, these receptors trigger the release of ADH from the posterior pituitary gland. ADH increases water reabsorption in the kidney, diluting the blood and restoring its water potential to normal levels. Conversely, when the blood is too dilute, less ADH is released, leading to decreased water reabsorption and more dilute urine.

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

Another worked example

Describe the process of glomerular filtration in the nephron.

3 marks

Identify the key structures involved in filtration.0 marks
The glomerulus is a ball of capillaries, and Bowman's capsule surrounds it.
Explain how blood enters the glomerulus.1 mark
Blood enters the glomerulus under high pressure.
Describe what happens during filtration.2 marks
High pressure forces water and small molecules (such as glucose, amino acids, and ions) out of the blood and into Bowman's capsule. Large molecules like proteins and cells are retained in the blood.
Summarize the result of glomerular filtration.0 marks
The filtrate collected in Bowman's capsule contains water, small molecules, and some ions but lacks large molecules and cells.

Final answer: In the nephron, blood enters the glomerulus under high pressure. This high pressure forces water and small molecules (such as glucose, amino acids, and ions) out of the blood and into Bowman's capsule. Large molecules like proteins and cells are retained in the blood. The filtrate collected in Bowman's capsule contains water, small molecules, and some ions but lacks large molecules and cells.

Work through every step correctly and you earn all 3 marks.

Common mistakes

Confusing the roles of the hypothalamus and posterior pituitary gland in osmoregulation.
Why it happens: Students may mix up which part detects changes in blood water potential and which part releases ADH.
Fix: Remember that the hypothalamus contains osmoreceptors that detect changes in blood water potential, while the posterior pituitary gland releases ADH in response to these changes.
Misunderstanding how ADH affects water reabsorption in the kidney.
Why it happens: Students may think that ADH decreases water reabsorption instead of increasing it.
Fix: ADH increases the permeability of the distal convoluted tubule and collecting ducts to water, leading to more water reabsorption and more concentrated urine.
Confusing the functions of the descending and ascending limbs of the loop of Henle.
Why it happens: Students may mix up which limb is permeable to water and which is permeable to solutes.
Fix: The descending limb is permeable to water but not solutes, while the ascending limb is permeable to solutes but not water. This creates a concentration gradient in the medulla.
Failing to explain how the loop of Henle contributes to the concentration gradient in the medulla.
Why it happens: Students may not fully understand the processes involved in creating the concentration gradient.
Fix: Practice explaining that the descending limb reabsorbs water, making the filtrate more concentrated, while the ascending limb actively transports solutes out of the filtrate into the medulla, further concentrating the filtrate.
Misidentifying the key structures of the nephron and their functions.
Why it happens: Students may confuse the roles of different parts of the nephron.
Fix: Review the structure of the nephron, including the glomerulus, Bowman's capsule, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting ducts. Understand the specific functions of each part.
Failing to describe the process of glomerular filtration in detail.
Why it happens: Students may provide a vague or incomplete description of filtration.
Fix: Practice explaining that blood enters the glomerulus under high pressure, forcing water and small molecules out into Bowman's capsule. Large molecules like proteins and cells are retained in the blood.

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
Osmoregulation Explanation	Explain how the hypothalamus and posterior pituitary regulate blood water potential via ADH.	4
Describe Glomerular Filtration	Describe how blood is filtered in the glomerulus to form the filtrate in Bowman's capsule.	3
Countercurrent Multiplier Mechanism	Explain how the loop of Henle establishes a medullary concentration gradient using ion movement and permeability.	4
Describe Hormonal Control	Describe how ADH increases membrane permeability to raise water reabsorption in the tubule and ducts.	3
Total across these question types		14

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