2.5.1Additional lessons
Stem Cells — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 26 of 142
Part 1 of 5Properties of Stem Cells
Self-renewal
- A stem cell can divide by mitosis many times, producing new stem cells.
Differentiation
- Daughter cells can develop into specialised cell types, such as blood cells or muscle cells.
Gene expression
- Differentiation happens because different genes are switched on, not because genes are lost from the cell.
Niches
- Some stem cells stay in specific tissue locations, such as bone marrow and hair follicles, where they can repair and replace cells.
Niche conditions
- A niche keeps stem cells inactive until needed, then signals them to divide and differentiate.
Part 2 of 5Potency
Potency
Totipotent
- Can become any cell type, including the cells of the placenta; found in the early embryo.
Pluripotent
- Can become almost any cell type of the body, such as embryonic stem cells.
Multipotent
- Can become a limited range of cell types within one tissue, such as bone marrow stem cells.
Unipotent
- Can produce only one cell type, such as muscle satellite cells that form only muscle fibres.
| Potency | Range of cell types | Example |
|---|---|---|
| Totipotent | All, including extra-embryonic | Zygote |
| Pluripotent | All body cell types | Embryonic stem cell |
| Multipotent | A limited group | Bone marrow stem cell |
| Unipotent | One type | Muscle stem cell |
Part 3 of 5Specialisation and Cell Size
1
Specialised cells differ in shape, organelles and proteins, which suit them to a particular function.
2
Cell size is limited because oxygen and nutrients enter through the surface, while demand rises with volume.
3
As a cell grows, its volume increases faster than its surface area, so the surface area to volume ratio falls.
4
A low ratio means diffusion cannot supply the cell quickly enough and waste cannot be removed fast enough.
5
Multicellular organisms use gas exchange, digestive and circulatory systems to supply cells and avoid this limit.
Part 4 of 5Worked Example
Question
- A cell is modelled as a cube.
- Calculate the surface area to volume ratio for cubes with sides of 1 µm, 2 µm and 3 µm, and state the trend.
- Surface area of a cube = 6 × side², and volume = side³.
- Side 1 µm: 6 µm² / 1 µm³ = 6:1. Side 2 µm: 24 / 8 = 3:1. Side 3 µm: 54 / 27 = 2:1.
- The ratio falls each time the side increases.
Answer The ratios are 6:1, 3:1 and 2:1, so the surface area to volume ratio decreases as cell size increases.
Part 5 of 5Exam Checklist & Tips
2.5.1 – Key Points for the Exam
- Stem cell An undifferentiated cell that can divide repeatedly and differentiate.
- Differentiation Development of a cell into a specialised type by selective gene expression.
- Potency The range of cell types that a stem cell can become.
- Stem cell niche A tissue location that supports and regulates stem cells.
- Surface area to volume ratio Surface area divided by volume, which falls as cell size increases.
Common Mistakes
- Saying differentiated cells lose genes; they keep the genome but express only some genes.
- Mixing up pluripotent (many types) and multipotent (limited types).
- Stating that larger cells have a larger surface area to volume ratio.
- Forgetting that a stem cell needs to divide by mitosis to renew itself.
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Cambridge 9700 AS Biology
Core Formula
Magnification Triangle & Unit Conversion
Formula: M = I / A ⇔ A = I / M ⇔ I = M × A
Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.