A-Level · Biology · AQA · Mark scheme decoded

AQA A-Level Biology: Stem Cells and Their Applications in Treating Human Disorders — mark scheme explained

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

The short answer

In A-Level Biology, understanding the different types of stem cells and their applications is crucial for grasping how gene expression is controlled during development and how this knowledge can be applied to treat human disorders.

The question

Explain the difference between totipotent and pluripotent cells.

[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 explanation questions, marks are typically awarded for providing a detailed description that includes relevant principles and relationships. For comparison questions, marks are given for identifying both similarities and differences between the concepts. For evaluation questions, marks are awarded for assessing the advantages and disadvantages of a particular approach or therapy. For descriptive questions, marks are given for giving a clear and concise account of a process or phenomenon.

What the command words demand

Explain: Provide a detailed description of the concept, including relevant principles and relationships.
Compare: Identify similarities and differences between two or more concepts.
Evaluate: Assess the advantages and disadvantages of a particular approach or therapy.
Describe: Give a clear and concise account of a process or phenomenon.

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 answers, and check your work.

Define totipotent cells.2 marks
Totipotent cells can divide and differentiate into any cell type in the body, including extra-embryonic tissues. They are only present for a limited time during early embryonic development.
Define pluripotent cells.1 mark
Pluripotent cells can differentiate into any cell type within the three primary germ layers (ectoderm, mesoderm, and endoderm). They are found in embryos and can divide indefinitely in culture.
Compare the differentiation potential of totipotent and pluripotent cells.1 mark
Totipotent cells have a broader differentiation potential than pluripotent cells. While both can differentiate into various cell types, totipotent cells can also form extra-embryonic tissues.

Final answer: Totipotent cells can divide and differentiate into any cell type in the body, including extra-embryonic tissues, and are only present for a limited time during early embryonic development. Pluripotent cells can differentiate into any cell type within the three primary germ layers (ectoderm, mesoderm, and endoderm) and can divide indefinitely in culture.

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

Another worked example

Describe how induced pluripotent stem cells (iPS cells) are produced from adult somatic cells.

5 marks

Identify the starting material.1 mark
Adult somatic cells, such as skin fibroblasts, are used as the starting material.
Introduce specific protein transcription factors.2 marks
Specific protein transcription factors, such as Oct4, Sox2, Klf4, and c-Myc, are introduced into the adult somatic cells.
Reprogramming process.1 mark
The introduction of these transcription factors reprograms the adult somatic cells to regain pluripotency, transforming them into iPS cells.
Characteristics of iPS cells.1 mark
iPS cells have the ability to differentiate into any cell type within the three primary germ layers and can divide indefinitely in culture.

Final answer: Induced pluripotent stem cells (iPS cells) are produced from adult somatic cells by introducing specific protein transcription factors, such as Oct4, Sox2, Klf4, and c-Myc. This reprogramming process transforms the adult somatic cells into iPS cells, which have the ability to differentiate into any cell type within the three primary germ layers and can divide indefinitely in culture.

Work through every step correctly and you earn all 5 marks.

Common mistakes

Confusing the differentiation potential of totipotent and pluripotent cells.
Why it happens: Students may not fully understand the differences in the breadth of cell types that each type can differentiate into.
Fix: Remember that totipotent cells can form any cell type, including extra-embryonic tissues, while pluripotent cells are limited to the three primary germ layers (ectoderm, mesoderm, and endoderm).
Misidentifying the source of multipotent and unipotent cells.
Why it happens: Students may confuse where these cell types are found in the body.
Fix: Multipotent cells are found in mature mammals and can differentiate into a limited number of cell types within the same tissue lineage. Unipotent cells are also found in mature mammals but can only differentiate into one specific cell type.
Forgetting the process of producing iPS cells from adult somatic cells.
Why it happens: The steps involved in reprogramming adult cells to become pluripotent may be overlooked.
Fix: Always remember that iPS cells are produced by introducing specific protein transcription factors, such as Oct4, Sox2, Klf4, and c-Myc, into adult somatic cells. This reprogramming process transforms the cells into pluripotent stem cells.
Not considering the ethical concerns associated with embryonic stem cell use.
Why it happens: Students may focus solely on the therapeutic potential without addressing the ethical implications.
Fix: When evaluating stem cell therapies, always consider the ethical concerns related to the destruction of embryos. iPS cells provide an alternative that avoids these issues.
Overlooking the safety and efficacy considerations in stem cell therapies.
Why it happens: Students may not fully appreciate the risks associated with uncontrolled cell growth, immune rejection, and genetic instability.
Fix: Always discuss the potential risks and challenges of ensuring the safety and efficacy of stem cell therapies. This includes addressing issues such as uncontrolled cell growth, immune rejection, and genetic instability.
Failing to explain the advantages of iPS cells over embryonic stem cells.
Why it happens: Students may not fully understand the unique benefits of iPS cells.
Fix: iPS cells can be derived from a patient's own cells, reducing the risk of immune rejection. They also avoid ethical concerns associated with the use of embryonic stem cells.

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
Compare Stem Cells	Explain how totipotent and pluripotent cells differ in their differentiation potential.	4
Describe iPS Production	Describe how adult somatic cells are reprogrammed into induced pluripotent stem cells.	5
Evaluate Stem Cell Therapy	Assess the advantages and disadvantages of using pluripotent stem cells to treat spinal cord injuries.	6
Compare Stem Cells	Compare how many cell types multipotent and unipotent cells can each differentiate into.	4
Total across these question types		19

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