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This lesson covers cell differentiation and specialised cells as required by the Edexcel GCSE Biology specification (1BI0), Topic 1: Key Concepts in Biology. You need to understand how cells become specialised, know the key examples of specialised cells and their adaptations, and understand the basics of stem cells.
Cell differentiation is the process by which a cell becomes specialised to perform a particular function. During differentiation, a cell develops specific structural and chemical features that allow it to carry out its role efficiently.
Exam Tip: Differentiation does not involve a change in the DNA itself — the genes are the same in every cell. What changes is which genes are expressed (turned on). This is a subtle but important distinction.
| Feature | Animal Cells | Plant Cells |
|---|---|---|
| When does differentiation occur? | Mostly in the early stages of embryo development | Can occur throughout the plant's life |
| Can cells re-differentiate? | Most mature animal cells cannot re-differentiate (they are permanently specialised) | Many plant cells can re-differentiate — they retain the ability to become different cell types |
| Where are undifferentiated cells found? | In embryos and in some adult tissues (e.g. bone marrow — adult stem cells) | In meristems (growing regions at root tips and shoot tips) |
Plant cells retain their ability to differentiate because they contain meristematic tissue (in meristems). Cells in meristems are undifferentiated and can divide and become any type of plant cell. This is why:
Exam Tip: The ability of plant cells to differentiate throughout life (from meristems) is a key difference from animal cells. This is often tested in comparison questions.
You need to know several examples of specialised cells for the Edexcel specification. For each one, learn the adaptations and explain how each adaptation helps the cell carry out its function.
Function: To reach and fertilise the egg cell (ovum).
| Adaptation | How It Helps |
|---|---|
| Streamlined shape (elongated head) | Reduces drag, allowing the sperm to swim efficiently through the female reproductive tract |
| Long tail (flagellum) | Propels the sperm forward — enables movement towards the egg |
| Many mitochondria (in the middle section) | Provide a large amount of energy (ATP) by aerobic respiration for the tail to move |
| Acrosome (enzyme-filled cap on the head) | Contains digestive enzymes that break down the outer layer of the egg cell, allowing the sperm to penetrate and fertilise it |
| Haploid nucleus (23 chromosomes) | Carries the male genetic information; fuses with the egg's haploid nucleus to form a diploid zygote (46 chromosomes) |
| Minimal cytoplasm | Keeps the cell lightweight and streamlined for efficient swimming |
Function: To carry electrical impulses (nerve signals) rapidly around the body.
| Adaptation | How It Helps |
|---|---|
| Long axon | Can carry impulses over long distances (some neurones extend from the spinal cord to the toes — over 1 metre) |
| Myelin sheath | A fatty insulating layer around the axon that speeds up the transmission of electrical impulses (impulses "jump" between gaps in the myelin) |
| Many dendrites | Branching extensions at each end that form connections with many other neurones, allowing signals to be passed on efficiently |
| Synaptic knobs (at the axon terminals) | Release neurotransmitters (chemical signals) into the synapse (gap) to transmit the signal to the next neurone |
| Many mitochondria | Provide energy for the production and release of neurotransmitters at synapses |
| Cell body contains many ribosomes | For producing the proteins (e.g. neurotransmitters) needed for nerve function |
Function: To transport oxygen from the lungs to the body's tissues.
| Adaptation | How It Helps |
|---|---|
| Biconcave disc shape | Increases the surface area to volume ratio for faster oxygen absorption and release. Also makes the cell flexible to squeeze through narrow capillaries |
| No nucleus | More space for haemoglobin — the protein that binds to oxygen, forming oxyhaemoglobin. This maximises the amount of oxygen each cell can carry |
| Contains haemoglobin | A red pigment that reversibly binds to oxygen: haemoglobin + oxygen ⇌ oxyhaemoglobin. Picks up oxygen in the lungs and releases it in the tissues |
| Small and flexible | Can squeeze through the narrowest capillaries (some as narrow as 5 μm) to deliver oxygen close to every cell |
Exam Tip: Red blood cells have no nucleus — this is a key adaptation. Do not confuse "no nucleus" with "no DNA". The cell had a nucleus originally but expelled it during differentiation to make more room for haemoglobin.
Function: To absorb water (by osmosis) and mineral ions (by active transport) from the soil.
| Adaptation | How It Helps |
|---|---|
| Long, thin root hair extension | Greatly increases the surface area in contact with soil particles and soil water for more efficient absorption |
| Thin cell wall | Short diffusion distance — water and dissolved minerals can enter the cell more quickly |
| Large permanent vacuole | Contains dilute cell sap, creating a low water potential inside the cell, which draws water in by osmosis |
| Many mitochondria | Provide energy (ATP) for active transport of mineral ions against the concentration gradient |
| No chloroplasts | Root hair cells are underground and do not receive light, so they do not carry out photosynthesis |
Function: To transport water and dissolved mineral ions from the roots to the leaves and provide structural support.
| Adaptation | How It Helps |
|---|---|
| Dead cells (at maturity) | The cells die and lose their contents, forming hollow tubes with no obstruction to water flow |
| No end walls | Cells are stacked end to end with no cross-walls, forming a continuous, uninterrupted tube for efficient water transport |
| Walls reinforced with lignin | Lignin is a strong, waterproof substance that provides structural support (prevents the vessel from collapsing) and makes the walls impermeable to water |
| Narrow lumen | Helps draw water upward through capillary action and the transpiration stream |
Function: To carry out photosynthesis in leaves.
| Adaptation | How It Helps |
|---|---|
| Packed with chloroplasts | Contains a very large number of chloroplasts for maximum photosynthesis. Chloroplasts contain chlorophyll, which absorbs light energy. |
| Chloroplasts can move within the cell | Chloroplasts cluster near the top of the cell (closest to light) when light intensity is low, or spread out when light is abundant |
| Located near the upper surface of the leaf | Positioned to receive the maximum amount of light that passes through the transparent upper epidermis |
| Tall, column-shaped cells | Allows many cells to be packed closely together, and maximises the number of chloroplasts per cell |
| Thin cell walls | Allow carbon dioxide to diffuse easily into the cell from the air spaces for photosynthesis |
Exam Tip: When describing specialised cells, always follow this structure: (1) state the adaptation, (2) explain how it helps the cell perform its function. Simply listing features without explaining their purpose will not get full marks.
While the cells above are the most commonly tested, you should also be aware of:
| Cell | Function | Key Adaptations |
|---|---|---|
| Ciliated epithelial cell | Moves mucus along airways | Hair-like cilia on surface that beat in waves; many mitochondria for energy |
| Muscle cell | Contraction and movement | Many mitochondria for energy; contains protein fibres (actin and myosin) that can contract; elongated shape |
| White blood cell (phagocyte) | Engulfs and destroys pathogens | Can change shape to engulf bacteria (phagocytosis); contains many lysosomes with digestive enzymes |
| Guard cell | Controls opening/closing of stomata | Kidney-shaped — uneven wall thickness allows them to change shape; contain chloroplasts for photosynthesis |
| Egg cell (ovum) | Female gamete — fuses with sperm at fertilisation | Large cell with nutrient-rich cytoplasm to nourish the developing embryo; haploid nucleus (23 chromosomes); zona pellucida (outer layer that hardens after fertilisation to prevent other sperm entering) |
Stem cells are undifferentiated cells that have the ability to divide and differentiate into specialised cell types.
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