# Cell Biology (IB)
Cell biology forms the foundation of IB Biology (Topic 1). You need to understand cell theory, the differences between prokaryotic and eukaryotic cells, membrane structure and function, cell division, and the therapeutic use of stem cells.
1. Cell Theory
The three key tenets of cell theory:
- All living organisms are composed of one or more cells
- Cells are the smallest unit of life
- All cells arise from pre-existing cells
Evidence Supporting Cell Theory
- Robert Hooke (1665) observed cell walls in cork using a microscope
- Anton van Leeuwenhoek observed single-celled organisms
- Pasteur's experiments disproved spontaneous generation
Exceptions/Limitations
- Striated muscle fibres — very large cells with multiple nuclei (syncytium)
- Fungi — some have continuous cytoplasm with many nuclei (coenocytic hyphae)
- Giant algae (Acetabularia) — single cell up to 10 cm
- Viruses — not considered cells but have some characteristics of life
2. Prokaryotic vs Eukaryotic Cells
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No membrane-bound nucleus | Membrane-bound nucleus |
| DNA | Circular, in nucleoid region; plasmids | Linear chromosomes with histones |
| Ribosomes | 70S (smaller) | 80S (larger) |
| Organelles | No membrane-bound organelles | Membrane-bound organelles (mitochondria, ER, Golgi, etc.) |
| Cell wall | Present (peptidoglycan in bacteria) | Present in plants (cellulose) and fungi (chitin); absent in animals |
| Size | 0.1–5 μm | 10–100 μm |
| Reproduction | Binary fission | Mitosis/meiosis |
Structure of E. coli (model prokaryote)
- Cell wall (peptidoglycan), cell membrane, cytoplasm, ribosomes (70S), nucleoid (circular DNA), plasmids, flagella (some), pili (some)
3. Membrane Structure — The Fluid Mosaic Model
The cell membrane is described by the fluid mosaic model (Singer and Nicolson, 1972):
- Phospholipid bilayer — hydrophilic heads face outward (water), hydrophobic tails face inward
- Integral (transmembrane) proteins — span the entire bilayer; function as channels, carriers, receptors
- Peripheral proteins — attached to the surface; signalling, structural roles
- Cholesterol — found in animal cell membranes; regulates fluidity (reduces at high temp, prevents solidifying at low temp)
- Glycoproteins — proteins with carbohydrate chains; cell recognition, immune response
- Glycolipids — lipids with carbohydrate chains; cell-cell recognition
The membrane is fluid because phospholipids and proteins can move laterally, and mosaic because of the diverse array of proteins embedded in it.
4. Membrane Transport
| Type | Description | Energy Required? |
|---|---|---|
| Simple diffusion | Movement of molecules from high to low concentration through the bilayer | No |
| Facilitated diffusion | Movement through channel or carrier proteins, down concentration gradient | No |
| Osmosis | Net movement of water through a semi-permeable membrane from high to low water potential | No |
| Active transport | Movement against concentration gradient via carrier proteins + ATP | Yes |
| Endocytosis | Cell engulfs material by folding the membrane inward (phagocytosis = solids; pinocytosis = liquids) | Yes |
| Exocytosis | Vesicles fuse with membrane to release contents outside the cell | Yes |
5. Cell Division
Mitosis
- Produces two genetically identical diploid cells
- Stages: Prophase → Metaphase → Anaphase → Telophase
- Used for growth, repair, asexual reproduction
- Cytokinesis differs: cleavage furrow (animals) vs cell plate (plants)
The Cell Cycle
- Interphase: G₁ (growth) → S (DNA replication) → G₂ (preparation)
- Mitotic phase: mitosis + cytokinesis
- Regulated by cyclins and checkpoints
- Uncontrolled division → cancer (tumours)
Tumours
- Oncogenes (mutated proto-oncogenes) stimulate excessive division
- Tumour suppressors (e.g., p53) normally halt the cycle; mutations remove this control
- Metastasis: malignant tumour cells spread via blood/lymph
6. Stem Cells
| Type | Potency | Examples |
|---|---|---|
| Totipotent | Can form any cell type + placenta | Zygote, early embryo |
| Pluripotent | Can form any cell type (not placenta) | Embryonic stem cells, iPSCs |
| Multipotent | Limited range of cell types | Adult stem cells (bone marrow) |
Therapeutic Uses
- Bone marrow transplants for leukaemia
- Research into treating Parkinson's, diabetes, spinal cord injuries
- Induced pluripotent stem cells (iPSCs) — reprogrammed adult cells; avoid embryo destruction
Ethical Considerations
- Use of embryonic stem cells involves destroying embryos
- iPSCs reduce ethical concerns but may carry cancer risk
- Regulation varies by country
7. Drawing Skills (IB Specific)
IB requires you to draw and label:
- Prokaryotic cell (e.g., E. coli): cell wall, plasma membrane, cytoplasm, ribosomes, nucleoid, plasmid
- Animal cell: nucleus, mitochondria, rough ER, Golgi apparatus, lysosomes, ribosomes, cell membrane
- Plant cell: as animal cell + cell wall, chloroplasts, large central vacuole
Drawing rules: Use simple, clear lines; labels with straight lines; accurate proportions; no shading.
Worked Example
Question: Compare the structure of a prokaryotic cell with a eukaryotic cell. (4 marks)
Solution:
Prokaryotic cells lack a membrane-bound nucleus; their DNA is in a nucleoid region as a circular chromosome. Eukaryotic cells have a true nucleus enclosed by a nuclear envelope containing linear chromosomes associated with histones. Prokaryotes have 70S ribosomes while eukaryotes have larger 80S ribosomes. Prokaryotes lack membrane-bound organelles (no mitochondria, ER, Golgi), while eukaryotes have compartmentalised organelles. Prokaryotes may have plasmids (small extra DNA rings); eukaryotes generally do not. Both have a cell membrane, cytoplasm, ribosomes, and DNA.
Practice Questions
- State the three principles of cell theory and one exception. (3 marks)
- Draw and label a diagram of the fluid mosaic model. (4 marks)
- Compare simple diffusion and active transport. (3 marks)
- Explain how stem cells could be used to treat a named disease. (4 marks)
- Outline the stages of mitosis. (4 marks)
Answers
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Summary
- Cell theory: all organisms are made of cells; cells are the smallest unit of life; cells come from pre-existing cells.
- Prokaryotes lack a nucleus and membrane-bound organelles; eukaryotes have both.
- The fluid mosaic model describes the membrane as a phospholipid bilayer with embedded proteins.
- Transport: diffusion, osmosis, facilitated diffusion (passive) vs active transport, endo/exocytosis (active).
- Mitosis produces 2 identical diploid cells; regulated by the cell cycle and checkpoints.
- Stem cells have therapeutic potential but raise ethical issues, especially embryonic stem cells.