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

The question

Explain the difference between totipotent and pluripotent cells. [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: Define totipotent cells.

• S2

  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.

• S3

  Step 2: Define pluripotent cells.

• S4

  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.

• S5

  Step 3: Compare the differentiation potential of totipotent and pluripotent cells.

• S6

  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.

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   Step 1: Define totipotent cells.

2. S2

   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.

3. S3

   Step 2: Define pluripotent cells.

4. S4

   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.

5. S5

   Step 3: Compare the differentiation potential of totipotent and pluripotent cells.

6. S6

   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.

7. 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.

Common mistakes

• Confusing the differentiation potential of totipotent and pluripotent cells. — 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. — 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. — 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. — 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. — 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. — 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

• Full worked solution (all marking points)4 marks

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