Cell Structure and Organisation
Summary: All living organisms are made of cells. Animal cells contain a nucleus, cytoplasm, cell membrane, mitochondria, and ribosomes. Plant cells have these plus a cellulose cell wall, a large permanent vacuole, and chloroplasts. Specialised cells are adapted for specific functions. Cells are organised into tissues, organs, organ systems, and ultimately organisms. Tags: igcse biology cell-structure cell-organisation specialised-cells Created: 2026-07-16 Last Updated: 2026-07-16
1. Cell Theory
The cell theory states:
- All living organisms are made of one or more cells — the cell is the basic unit of life
- All cells arise from pre-existing cells — cells divide to produce new cells (mitosis)
- The cell is the smallest unit that can carry out all life processes (MRS GREN)
Viruses are an exception — they are not made of cells, which is why they are not classified as living organisms.
2. Animal Cell Organelles
| Organelle | Description | Function |
|---|---|---|
| Nucleus | Large, spherical, surrounded by a double membrane (nuclear envelope) with pores | Contains genetic material (DNA), controls all cell activities |
| Cytoplasm | Jelly-like fluid (mostly water) filling the cell | Site of chemical reactions (metabolism); contains dissolved substances and organelles |
| Cell membrane | Thin, flexible layer around the outside of the cell (made of phospholipids and proteins) | Partially permeable — controls which substances enter and leave the cell |
| Mitochondria (sing. mitochondrion) | Oval-shaped, double membrane, inner membrane folded (cristae) | Site of aerobic respiration — release energy (ATP) from glucose |
| Ribosomes | Very small organelles (not membrane-bound), found free in cytoplasm or attached to rough ER | Site of protein synthesis — amino acids are joined together to make proteins |
Mitochondria is where energy is released (not “produced” or “made” — energy cannot be created). Ribosomes are where proteins are made (protein synthesis).
3. Plant Cell Organelles
Plant cells contain all the organelles found in animal cells, plus three additional structures:
| Organelle | Description | Function |
|---|---|---|
| Cell wall | Rigid outer layer outside the cell membrane, made of cellulose | Provides structural support and prevents the cell from bursting when turgid; fully permeable |
| Large permanent vacuole | A large, fluid-filled sac surrounded by a membrane (tonoplast) | Contains cell sap (water, dissolved sugars, salts); maintains turgor pressure to keep the cell rigid |
| Chloroplasts | Oval-shaped, double membrane, contains stacks of membranes (grana) containing chlorophyll (green pigment) | Site of photosynthesis — absorbs light energy to convert CO2 and H2O into glucose |
Important: The cell wall is fully permeable — it lets everything through. The cell membrane is partially permeable — it controls what enters and leaves. These are different structures with different properties.
4. Comparing Animal and Plant Cells
| Feature | Animal Cell | Plant Cell |
|---|---|---|
| Nucleus | Yes | Yes |
| Cytoplasm | Yes | Yes |
| Cell membrane | Yes | Yes |
| Mitochondria | Yes | Yes |
| Ribosomes | Yes | Yes |
| Cell wall (cellulose) | No | Yes |
| Large permanent vacuole | No (small temporary vacuoles sometimes) | Yes |
| Chloroplasts | No | Yes (in photosynthetic cells) |
| Shape | Irregular / rounded | Regular / rectangular (due to cell wall) |
| Carbohydrate storage | Glycogen | Starch |
5. Specialised Cells
Specialised cells have adaptations (structural features) that help them perform their specific function.
| Specialised Cell | Structure | Function | How Structure Relates to Function |
|---|---|---|---|
| Red blood cell | Biconcave disc shape, no nucleus, contains haemoglobin (red protein) | Transport oxygen from lungs to tissues | Biconcave shape → large surface area for oxygen diffusion; no nucleus → more space for haemoglobin; flexible → can squeeze through narrow capillaries |
| Nerve cell (neurone) | Long axon (fibre), dendrites (branched endings), myelin sheath (insulation) | Transmit electrical impulses | Long axon → impulses travel long distances; dendrites → connect to many other neurones; myelin sheath → speeds up impulse transmission |
| Muscle cell | Long, contains many mitochondria, contains contractile protein fibres (actin and myosin) | Contract to produce movement | Many mitochondria → release lots of energy (ATP) for contraction; protein fibres → can shorten (contract) to pull on bones |
| Root hair cell | Long, thin extension (root hair) protruding from the root | Absorb water and mineral ions from soil | Long extension → greatly increases surface area for absorption; thin cell wall → short diffusion distance; many mitochondria → provide energy for active transport of mineral ions |
| Xylem vessel | Long, hollow tube, walls thickened with lignin (a waterproof substance), no cytoplasm or organelles, no end walls between cells | Transport water and dissolved mineral ions from roots to leaves; also provides structural support | Hollow tube with no cytoplasm → no obstruction to water flow; lignin → strong, waterproof (prevents collapse under tension); continuous columns → efficient water transport |
| Palisade mesophyll cell | Cylindrical shape, packed with many chloroplasts near the top surface of leaves | Main site of photosynthesis in leaves | Many chloroplasts → maximum light absorption; located near upper leaf surface → closer to light; tightly packed → no wasted space |
| Ciliated cell | Has many tiny hair-like projections called cilia on the surface | Move substances along tubes (e.g. sweep mucus-trapping dust and bacteria up the trachea towards the throat) | Cilia beat in a co-ordinated wave → mucus moved towards throat to be swallowed; many cilia → more efficient sweeping |
| Sperm cell | Has a flagellum (tail) for swimming, an acrosome (contains enzymes) at the head, and many mitochondria in the mid-piece | Deliver the male nucleus to the egg cell for fertilisation | Flagellum → swimming towards the egg; acrosome → enzymes digest outer layer of egg for penetration; mitochondria → provide energy (ATP) for swimming |
6. Levels of Organisation
Cells are organised into increasingly complex structures:
Cells -> Tissues -> Organs -> Organ Systems -> Organism
| Level | Definition | Example |
|---|---|---|
| Cell | The basic structural and functional unit of life | Muscle cell |
| Tissue | A group of similar cells working together to perform a shared function | Muscle tissue (many muscle cells together) |
| Organ | A structure made of different tissues working together to perform a specific function | Heart (muscle tissue + nervous tissue + connective tissue + blood) |
| Organ system | A group of organs working together to carry out a major body function | Circulatory system (heart + blood vessels + blood) |
| Organism | A complete living individual | Human |
Full example chain — Humans: Muscle cell → muscle tissue → heart → circulatory system → human
7. Magnification and Size Calculations
Formula:
Magnification = Image size / Actual size
M = I / A
Formula triangle:
I
-----
M | A
- Cover the one to find
- I = M x A
- A = I / M
Unit conversions:
- 1 mm = 1000 um (micrometres)
- 1 um = 1000 nm (nanometres)
| To convert… | Multiply/divide |
|---|---|
| mm → um | x 1000 |
| um → mm | / 1000 |
| um → nm | x 1000 |
| nm → um | / 1000 |
Always convert to the same units before using the formula. Questions often give image size in mm and actual size in um, so one must be converted first.
Sources
- BBC Bitesize GCSE Biology — Cell structure, BBC (free educational resource)
- OpenStax Biology 2e — Ch. 4 Cell Structure, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Syllabus 2: Organisation of the organism, Cambridge Assessment International Education
- CK-12 Biology for High School — Cell Structure and Function, CK-12 Foundation (free, CC BY-NC 3.0)
Related Notes
- Characteristics and Classification of Living Organisms — MRS GREN and how life is defined
- Movement Into and Out of Cells — Diffusion, osmosis, and active transport across the cell membrane
- Biological Molecules — Carbohydrates, proteins, lipids, and DNA inside cells
- Enzymes — The enzymes that control metabolic reactions in cells
- IGCSE-Bio-Index — Full IGCSE Biology index
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Plant cells don’t have mitochondria” | Plant cells have mitochondria — they also respire aerobically to release energy |
| ”The cell wall controls what enters and leaves” | The cell wall is fully permeable. The cell membrane is partially permeable and controls entry/exit |
| ”Mitochondria produce/make energy” | Energy cannot be created or destroyed. Mitochondria release energy (in the form of ATP) through respiration |
| ”All plant cells have chloroplasts” | Only plant cells exposed to light have chloroplasts — root cells, for example, do not |
| ”Magnification = actual size / image size” | Magnification = Image / Actual. It’s M = I/A, not A/I |
| ”Vacuoles in animal cells are the same as those in plant cells” | Animal cells may have small, temporary vacuoles; plant cells have a large, permanent vacuole |