Cell structureOxford AQA IGCSE Biology: Revision notes
Section 1
What are the key structures in animal cells and their functions?
Animal cells contain several essential structures, each with specific roles:
| Structure | Function |
|---|---|
| Cell membrane | Controls entry and exit of substances; selectively permeable barrier between cell and environment |
| Cytoplasm | Gel-like substance where chemical reactions occur; contains organelles |
| Nucleus | Controls cell activities; contains genetic material (DNA) |
| Mitochondria | Site of aerobic respiration; releases energy in the form of ATP |
| Ribosomes | Site of protein synthesis |
The nucleus is the largest organelle and is surrounded by a nuclear membrane. The cytoplasm is the main site of metabolic activity. Mitochondria are often described as the 'powerhouse' of the cell because they produce energy. Ribosomes are the smallest organelles and can be free-floating or attached to rough endoplasmic reticulum.
Examiners expect you to name both the structure AND describe its specific function. Don't just say 'mitochondria makes energy' – explain that it's the site of aerobic respiration where glucose is broken down to release ATP.
Think of the nucleus as the 'control centre' or 'office manager' of the cell, directing all activities. Mitochondria are the 'power plant', constantly producing energy for the cell to use.
Section 2
What additional structures do plant cells have and why?
Plant cells contain all the structures found in animal cells, plus three additional structures:
| Structure | Function | Why plants have it |
|---|---|---|
| Cell wall | Provides structural support and rigidity; protects the cell | Plants are stationary and need rigid support to stand upright without a skeleton |
| Vacuole (large central) | Stores water, minerals, and other substances; maintains turgor pressure | Plants need water storage for survival during dry periods; turgor pressure keeps plant tissues firm and prevents wilting |
| Chloroplasts | Site of photosynthesis; contains chlorophyll for light absorption | Plants are autotrophic and must produce their own food using light energy |
Animal cells do not have these structures because:
- Animals are motile and do not require rigid cell walls (flexibility aids movement)
- Animals obtain energy heterotrophically (by consuming other organisms) and do not need chloroplasts
- Animals maintain water balance differently and do not require large central vacuoles
The cell wall is made of cellulose and sits outside the cell membrane. Chloroplasts contain the green pigment chlorophyll. Vacuoles in plant cells are typically very large, occupying up to 90% of the cell volume.
Questions often ask 'why do plant cells have X but animal cells do not?' Always link your answer to the lifestyle differences: plants are stationary autotrophs, animals are mobile heterotrophs.
A common error is saying animal cells have small vacuoles. Animal cells either have no vacuoles or very small ones; the large central vacuole is a distinctly plant cell feature.
Section 3
How do prokaryotic and eukaryotic cells differ?
Eukaryotic cells (found in animals and plants) and prokaryotic cells (bacteria and archaea) have fundamental differences:
| Feature | Eukaryotic | Prokaryotic |
|---|---|---|
| Nucleus | Present; DNA enclosed in nuclear membrane | Absent; DNA in nucleoid region, not membrane-bound |
| Organelles | Present (mitochondria, chloroplasts, etc.) | Absent |
| Cell wall | Present in plants (cellulose); absent in animals | Present in bacteria; made of peptidoglycan |
| Size | Generally larger (10–100 μm) | Generally smaller (1–10 μm) |
| DNA structure | Linear chromosomes within nucleus | Circular chromosome in nucleoid; plasmids present |
| Ribosomes | Larger (80S) | Smaller (70S) |
| Complexity | More complex; compartmentalised functions | Less complex; simpler organisation |
Key distinction: The presence or absence of a membrane-bound nucleus is the defining feature separating these two cell types.
When comparing cells, be precise: say 'eukaryotic cells have a membrane-bound nucleus containing linear chromosomes, whereas prokaryotic cells have no nucleus and DNA in the nucleoid region.' This level of detail gains full marks.
A human cell is eukaryotic (has nucleus + organelles). A bacterium is prokaryotic (no nucleus, just nucleoid). The presence of the nuclear membrane is the absolute differentiator.
Section 4
What is the structure of a bacterial cell?
Bacterial cells are prokaryotic and contain the following structures:
| Structure | Description |
|---|---|
| Cell membrane | Selectively permeable; controls substance movement |
| Cell wall | Made of peptidoglycan; provides rigidity and protection |
| Cytoplasm | Contains ribosomes and enzymes for metabolic reactions |
| Ribosomes | Smaller (70S) than eukaryotic ribosomes; sites of protein synthesis |
| Chromosomal DNA | Single, circular chromosome located in nucleoid region (not membrane-bound) |
| Plasmids | Small, circular loops of DNA; often carry useful genes (e.g. antibiotic resistance) |
| Flagellum/Flagella | Whip-like structures for cell movement |
Note: Bacterial cells lack a nucleus, mitochondria, and chloroplasts. Energy production occurs in the cytoplasm, not in dedicated organelles. The flagellum (singular) or flagella (plural) rotate to propel the bacterium through liquid.
Examiners test knowledge of all seven structures. Ensure you can name and describe each one. Plasmids are often overlooked—remember they carry genes and are used in genetic engineering.
Do not confuse the bacterial cell wall (peptidoglycan) with plant cell wall (cellulose). They are chemically different and serve bacteria and plants respectively.
Section 5
How do you use a light microscope to observe and measure cells?
Observing cells with a light microscope:
- Prepare a specimen (fresh cell sample or stained slide)
- Place on stage and focus using objective lenses (typically 4×, 10×, or 40× magnification)
- Draw observations clearly, labelling all visible structures
- Record the magnification used
Calculating magnification:
Use the formula: Magnification = Image size ÷ Actual size
Rearranged:
- Image size = Magnification × Actual size
- Actual size = Image size ÷ Magnification
Worked example: A cell has an actual width of 50 micrometres (μm). Under the microscope at ×400 magnification, the image width is 20 mm.
- Magnification = 20 mm ÷ 50 μm
- First, convert to same units: 50 μm = 0.05 mm
- Magnification = 20 ÷ 0.05 = 400× ✓
Converting units (remember the sequence):
- 1 metre (m) = 1000 millimetres (mm)
- 1 millimetre (mm) = 1000 micrometres (μm)
- 1 micrometre (μm) = 1000 nanometres (nm)
Quick conversion table:
| From | To | Multiply by |
|---|---|---|
| m to mm | × 1000 | |
| mm to μm | × 1000 | |
| μm to nm | × 1000 | |
| m to μm | × 1,000,000 | |
| mm to m | ÷ 1000 | |
| μm to mm | ÷ 1000 | |
| nm to μm | ÷ 1000 |
Key tips for microscopy:
- Always convert units before using the magnification formula
- State units in all calculations (mm, μm, etc.)
- Scale drawings accurately using a ruler
In exam questions, always show unit conversions separately before calculating magnification. Examiners award marks for showing working, even if the final answer is wrong. Write out: '50 μm = 0.05 mm, then magnification = ...'
Convert 200 μm to mm: 200 ÷ 1000 = 0.2 mm. Convert 3 mm to μm: 3 × 1000 = 3000 μm. Always divide when going from smaller to larger units, multiply when going from larger to smaller.
Must Know
- Animal cells contain: cell membrane, cytoplasm, nucleus, mitochondria, and ribosomes. Each has a distinct function in cell survival and metabolism.
- Plant cells additionally contain: cell wall (cellulose, provides support), large vacuole (stores water, maintains turgor), and chloroplasts (photosynthesis). These reflect plants' stationary, autotrophic lifestyle.
- Prokaryotic cells (bacteria) lack a nucleus and organelles. DNA is circular in the nucleoid; small plasmids may be present. Peptidoglycan cell wall differs chemically from plant cellulose.
- Eukaryotic cells have a membrane-bound nucleus containing linear chromosomes and possess compartmentalised organelles. This is the defining distinction from prokaryotes.
- Magnification formula: Image size ÷ Actual size. Always convert units to the same measurement before calculating. Common mistake: forgetting unit conversion.
- Unit conversion hierarchy: 1 m = 1000 mm = 1,000,000 μm = 1,000,000,000 nm. Divide when converting smaller units to larger; multiply for larger to smaller.
That's the notes covered.
Carry on to the next subtopic.