Cells, living processes and biodiversity
Cell structure
All living organisms are made of cells. CCEA Biology distinguishes two fundamental cell types.
Animal cells (typical)
| Component | Function |
|---|---|
| Cell membrane | Controls what enters and leaves the cell (partially permeable) |
| Nucleus | Contains DNA; controls cell activities and protein synthesis |
| Cytoplasm | Jelly-like fluid where most chemical reactions occur |
| Mitochondria | Site of aerobic respiration; release energy (ATP) |
| Ribosomes | Site of protein synthesis |
Plant cells — additional structures
| Extra Component | Function |
|---|---|
| Cell wall (cellulose) | Gives structural support and shape; fully permeable |
| Chloroplasts | Contain chlorophyll; site of photosynthesis |
| Permanent vacuole | Filled with cell sap; maintains turgor pressure |
Prokaryotic cells (e.g. bacteria)
Bacteria are prokaryotes: no membrane-bound nucleus, no mitochondria, no chloroplasts. They have:
- A cell wall (made of peptidoglycan, not cellulose)
- A cell membrane
- Cytoplasm with ribosomes (smaller than eukaryotic ribosomes)
- Circular DNA (no nucleus)
- Sometimes a plasmid (extra circular DNA)
- Sometimes a capsule (slime layer) and flagella
Microscopy
The light microscope uses visible light and glass lenses. It can magnify up to ~×1,500 and has a resolution of ~200 nm. It can view living specimens.
The electron microscope uses a beam of electrons. It magnifies up to ×500,000 with a resolution of ~0.1 nm. It reveals ultrastructure (e.g. mitochondrial cristae, endoplasmic reticulum) but kills specimens.
Magnification formula
Magnification = Image size ÷ Actual size
Rearranged: Actual size = Image size ÷ Magnification.
Units: always convert to the same unit before calculating. 1 mm = 1,000 µm; 1 µm = 1,000 nm.
Example: A cell appears 30 mm long in a drawing at ×500 magnification. Actual size = 30 mm ÷ 500 = 0.06 mm = 60 µm.
The seven characteristics of life (MRS GREN)
Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.
All living organisms carry out all seven. Viruses are not considered living because they cannot reproduce independently, respire, or carry out nutrition — they require a host cell.
Biological molecules
| Molecule | Monomer | Function | Test |
|---|---|---|---|
| Carbohydrates (starch) | Glucose | Energy storage (plants) | Iodine → blue-black if starch present |
| Carbohydrates (glucose) | — | Immediate energy source | Benedict's → brick-red precipitate if reducing sugar |
| Proteins | Amino acids | Enzymes, structure, antibodies | Biuret → purple/violet |
| Lipids (fats/oils) | Glycerol + fatty acids | Energy store, insulation, cell membranes | Emulsion test — white emulsion in water |
Enzyme action
Enzymes are biological catalysts — proteins that speed up chemical reactions without being used up.
- Active site: the specific region that fits the substrate (lock and key model).
- Optimum temperature: most human enzymes work best at ~37 °C. Above this, the enzyme denatures — the active site changes shape permanently.
- Optimum pH: enzymes are sensitive to pH changes. Pepsin (stomach) works best at pH 2; amylase (mouth/pancreas) at pH 7–8.
⚠Common mistakes— Common mistakes (CCEA examiner traps)
- Confusing cell wall and cell membrane — the wall is rigid (plant only); the membrane is present in ALL cells.
- Writing "mitochondria makes energy" — the correct phrase is "site of (aerobic) respiration, releasing energy."
- Using the magnification formula wrongly — always divide IMAGE by ACTUAL (or MAGNIFICATION).
- Stating viruses are living — CCEA mark schemes explicitly require candidates to explain that viruses lack the cellular machinery to reproduce independently.
- Confusing Benedict's and Biuret reagents — learn which colour each gives and what it tests for.
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