Edexcel A Level (IAL) Biology -3.18 Cell Specialisation- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -3.18 Cell Specialisation- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -3.18 Cell Specialisation- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 3.18 understand how cells become specialised through differential gene expression, producing active mRNA, leading to the synthesis of proteins which, in turn, control cell processes or determine cell structure in animals and plants
Cell Specialisation & Differential Gene Expression
📌 Introduction
All cells in an organism contain the same DNA and therefore the same genes.
However, not all genes are active in every cell.
The difference in which genes are switched on or off leads to cell specialisation – forming nerve cells, muscle cells, xylem, phloem, etc.
1. What is Differential Gene Expression?
Definition:
Differential gene expression means that different genes are expressed (activated) in different cells, even though they have the same DNA.
Only specific genes are transcribed into mRNA, which is then translated into proteins.
These proteins control the structure and function of each cell type.
📘 Simple Idea:
Same book (DNA), but each cell reads different chapters (genes)!
2. Step-by-Step Process
| Step | Description | Result |
|---|---|---|
| 1. DNA contains all genes | Every cell has full genome | Genetic potential for any cell type |
| 2. Certain genes switched “on” | By transcription factors & cell signals | Only some genes expressed |
| 3. Active genes produce mRNA | Transcription of active DNA regions | mRNA carries code to ribosome |
| 4. mRNA translated into proteins | Ribosomes synthesise proteins | Proteins build cell structure or regulate functions |
| 5. Proteins determine cell’s characteristics | Proteins = enzymes, receptors, hormones, structural elements | Cell becomes specialised for a specific role |
3. Example: Specialisation in Animals
| Cell Type | Key Active Genes | Resulting Specialisation |
|---|---|---|
| Muscle cell | Actin, myosin, ATP synthase | Contracts to cause movement |
| Nerve cell (neuron) | Ion channel & neurotransmitter genes | Transmits electrical impulses |
| Red blood cell | Haemoglobin gene | Transports oxygen efficiently |
4. Example: Specialisation in Plants
| Cell Type | Key Active Genes | Function |
|---|---|---|
| Xylem vessel | Lignin synthesis genes | Forms hollow, rigid tubes for water transport |
| Guard cells | Ion transport & turgor-regulating proteins | Control opening and closing of stomata |
| Root hair cell | Membrane protein genes for ion uptake | Absorbs water and minerals efficiently |
5. Control of Gene Expression
- Transcription factors: Proteins that bind to DNA and turn specific genes on/off.
- Epigenetic changes: Chemical modifications like DNA methylation or histone modification alter gene activity without changing DNA sequence.
- Cell signals (e.g., hormones): Can trigger activation/inactivation of certain genes.
📌 Example:
Hormone auxin in plants or growth factors in animals can activate sets of genes for growth or differentiation.
6. Why It Matters
- Ensures that multicellular organisms develop different tissues (muscle, nerve, blood).
- Allows efficient division of labour each cell performs a specific function.
- Enables development, growth, and tissue repair.
⚡ Quick Recap
All cells have the same DNA, but express different genes.
Active genes → mRNA → proteins → specialised functions.
Proteins control structure & processes → cell type identity.
Example: Nerve cell → neurotransmitter proteins; Xylem → lignin proteins.
Controlled by transcription factors, epigenetics, and signals.
Differential expression = foundation of cell differentiation in animals & plants.