Cell Structure & Organisation
What cells are made of, how they differ, and how many cells build an organism, plus how to measure what the microscope shows.
Plant and Animal Cells
Plant and animal cells are built to the same plan. Eight structures are on the syllabus, and the differences between the two kinds come down to three.
| Structure | Plant cell | Animal cell |
|---|---|---|
| Cell wall | Present, made of cellulose | Absent |
| Chloroplasts | Present in cells exposed to light | Absent |
| Vacuole | One large central vacuole | None, or only small ones |
| Present in both | Nucleus, cytoplasm, cell membrane, mitochondria, ribosomes |
Worked example (comparing cells): State two structural differences between a plant cell and an animal cell. Step 1. Name the structure and say which cell has it: a plant cell has a cell wall; an animal cell does not. Step 2. Repeat for a second structure: a plant cell has chloroplasts; an animal cell does not. Step 3. Each difference names both cells, so each earns its mark. Answer: Cell wall; chloroplasts.
What Each Structure Does
Each structure has one main job. Learn the pairing of name and job together, because exam questions ask for either.
| Structure | Function | Found in |
|---|---|---|
| Cell wall | Rigid layer outside the membrane; supports the cell and gives it shape | Plant, bacterial |
| Cell membrane | Controls which substances enter and leave the cell | All three |
| Cytoplasm | Jelly-like fluid in which many chemical reactions take place | All three |
| Nucleus | Holds the DNA and controls the activities of the cell | Plant, animal |
| Mitochondria | Where aerobic respiration releases energy | Plant, animal |
| Ribosomes | Where proteins are made | All three |
| Chloroplasts | Contain chlorophyll; where photosynthesis takes place | Plant |
| Vacuole | Stores cell sap; the sap’s pressure keeps the cell firm | Plant |
Worked example (naming the cell): A cell has a nucleus, a cell wall, a large vacuole and chloroplasts. Is it a plant cell, an animal cell or a bacterial cell? Step 1. A nucleus is present, which rules out a bacterial cell. Step 2. A cell wall is present, which rules out an animal cell. Step 3. The large vacuole and chloroplasts are found only in plant cells, so the identification does not depend on a single structure. Answer: Plant cell.
Worked example (matching jobs to structures): Name the structure that (a) controls which substances enter and leave the cell, and (b) is where proteins are made. Step 1. Control of entry and exit is a boundary job, so it belongs to the cell membrane. Step 2. Making proteins is the job of the ribosomes, found in all three cell types. Answer: (a) Cell membrane; (b) ribosomes.
Bacterial Cells
A bacterial cell is much simpler than a plant or animal cell. It has six structures on the syllabus, and no membrane-bound compartments inside.
| Feature | Plant | Animal | Bacterial |
|---|---|---|---|
| Cell wall | Cellulose | None | Present, not cellulose |
| Nucleus | Yes | Yes | None |
| DNA | In nucleus | In nucleus | Circular, free in cytoplasm |
| Plasmids | None | None | Often present |
| Mitochondria, chloroplasts | Mitochondria and chloroplasts | Mitochondria only | Neither |
Bacteria are far smaller than most plant and animal cells, so they can be seen only under a microscope.
Worked example (plant or bacterium?): A cell has a cell wall, ribosomes and DNA but no nucleus. Is it a plant cell or a bacterial cell? Step 1. Plant cells keep their DNA in a nucleus; this cell has none, so it is not a plant cell. Step 2. The DNA must therefore lie free in the cytoplasm as a loop, which is how bacterial DNA is arranged. Step 3. Small extra rings of DNA, plasmids, would support this. Answer: Bacterial cell.
Specialised Cells
A multicellular organism uses cells of many shapes, each suited to one job. Six are named on the syllabus.
| Cell | Job | Feature that helps |
|---|---|---|
| Ciliated cell | Moves mucus along the trachea and bronchi | Hair-like cilia on the surface that beat together |
| Root hair cell | Absorbs water and mineral ions | Long thin extension gives a large surface area |
| Palisade mesophyll cell | Photosynthesis | Many chloroplasts; tall cells packed near the top of the leaf |
| Neurone | Conducts electrical impulses | Very long, thin fibre with branched ends |
| Red blood cell | Transports oxygen | Biconcave disc full of haemoglobin; no nucleus |
| Sperm and egg cells (gametes) | Reproduction | Sperm: small, with a tail. Egg: large, with a food store |
Worked example (linking feature to function): Explain how the shape of a root hair cell helps it to do its job. Step 1. The job is absorption of water and mineral ions from the soil. Step 2. A long, thin extension reaches between soil particles, giving a large surface area. Step 3. A larger surface in contact with the soil means more water and ions can be absorbed. Answer: Large surface area.
From Cell to Organism
Cells never work alone in a large organism. Each level is built from the one below, and a new level is only reached when different parts work as one unit.
| Level | What it is | Example |
|---|---|---|
| Cell | The basic unit of life | A neurone |
| Tissue | Similar cells doing one job | Nervous tissue |
| Organ | Different tissues working as one structure | The brain; a leaf |
| Organ system | Organs working together on a major function | The nervous system |
| Organism | A complete living individual | A human; a plant |
Where new cells come from
New cells are produced only by the division of existing cells. A body grows and replaces worn-out cells because its cells divide, and every cell in an organism descends from earlier cells in this way.
Worked example (ordering levels): Put these in order, smallest first: leaf, palisade cell, plant, palisade layer. Step 1. A palisade cell is a single cell, the smallest unit. Step 2. Many palisade cells form the palisade layer, a tissue. Step 3. The leaf is an organ made of several tissues; the plant is the whole organism. Answer: Cell, tissue, organ, organism.
Size of Specimens
An image shows a specimen enlarged. Magnification tells you by how much, and it links the three quantities in one relationship.
Formula: magnification = image size ÷ actual size (image and actual size in the same unit·magnification has no unit)
Rearranged, image size = magnification × actual size and actual size = image size ÷ magnification. Both sizes must be in the same unit before dividing; at Core level that unit is the millimetre.
Worked example (finding magnification): A drawing of a cell is 60 mm long. The real cell is 0.12 mm long. Calculate the magnification. Step 1. Both lengths are in millimetres, so no conversion is needed. Step 2. magnification = 60 ÷ 0.12. Step 3. 60 ÷ 0.12 = 500, and magnification has no unit. Answer: ×500.
Worked example (finding actual size): A photograph shows a cell as 45 mm long at a magnification of ×150. Find the real length. Step 1. Rearrange: actual size = image size ÷ magnification. Step 2. actual size = 45 ÷ 150. Step 3. 45 ÷ 150 = 0.3 mm. Answer: 0.3 mm.
ExtendedConverting units
1 mm = 1000 µm, so multiply by 1000 to go from mm to µm and divide by 1000 to go back. A cell 25 µm long is 0.025 mm long. A drawing 75 mm long of that cell has magnification 75 ÷ 0.025 = ×3000.
Exam advice
Common mistakes
Model answer
Recall checklist
- Name the structures of plant and animal cells.
- State the function of each structure.
- List the structures of a bacterial cell.
- State where new cells come from.
- Match six specialised cells to their jobs.
- Order cell, tissue, organ, organ system, organism.
- Use magnification = image ÷ actual size.
- Convert between mm and µm (Extended)
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