Learn · Biology · O Level

Cells & Organisation

Cell structure, the plant/animal differences, specialised cells and magnification, the foundation the whole syllabus is built on, and an easy source of full marks.

7 parts
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Plant cellAnimal cellcell wall — cellulose, support, plant onlynucleus — controls the cell, bothchloroplast — photosynthesis, plant onlymitochondrion — respiration, bothvacuole — large and permanent in plants
Three structures a plant cell has that an animal cell does not.
🧩 The partsSpecialised cells: name the feature, then its consequencePart 2 of 7
🔍 In detailLevels of organisation, and the magnification formulaPart 3 of 7
🌍 In the real worldUsing a microscope, and drawing what you seePart 4 of 7
⚠️ Common mistakesWhere marks goPart 5 of 7
✅ SummaryWhat to rememberPart 6 of 7
✏️ Worked exampleWatch one worked throughPart 7 of 7
1📘 DefinitionPart 1 / 7

The parts of a cell and what each one does

Learn each structure with its function attached; the paper almost never asks for one without the other.

  • Nucleus: contains DNA, controls the cell's activities.
  • Cytoplasm: jelly where most chemical reactions happen.
  • Cell membrane: partially permeable; controls what enters and leaves.
  • Mitochondria: site of aerobic respiration, releasing energy.
  • Ribosomes: protein synthesis.

Plants have three more: a cell wall of cellulose (support and shape, and fully permeable, not the same job as the membrane), chloroplasts containing chlorophyll for photosynthesis, and a large permanent vacuole of cell sap that keeps the cell turgid.

A bacterial cell has a wall, cytoplasm and a membrane, but no nucleus (its DNA lies free in the cytoplasm) and no mitochondria or chloroplasts.

2🧩 The partsPart 2 / 7

Specialised cells: name the feature, then its consequence

Every adaptation question wants two things: the structural feature, and what it lets the cell do. "It is long" scores nothing; "it is long, giving a large surface area for absorption" scores two.

  • Root hair cell: long extension → large surface area for absorbing water and mineral ions; many mitochondria → energy for active transport.
  • Red blood cell: biconcave → large surface area to volume ratio for fast oxygen diffusion; no nucleus → more room for haemoglobin.
  • Palisade mesophyll: packed with chloroplasts and near the upper surface → absorbs the most light.
  • Xylem vessel: dead, hollow, no end walls, lignified → an uninterrupted tube that also gives support.
  • Ciliated epithelium: cilia beat → sweeps mucus with trapped dust and bacteria away from the lungs.
  • Sperm: tail → swims; many mitochondria → energy for the tail; acrosome → enzymes to digest through the egg membrane.
  • Neurone: very long → carries impulses over distance.
3🔍 In detailPart 3 / 7

Levels of organisation, and the magnification formula

Cell → tissue → organ → organ system → organism. A tissue is a group of similar cells with a shared function (xylem, muscle). An organ is several tissues working together (leaf, heart, stomach). An organ system is several organs (the digestive system).

Magnification = image size ÷ actual size. Rearranged: actual = image ÷ magnification. The single most common error is mixing units, convert everything to the same unit first, and remember 1 mm = 1 000 µm.

Worked: a cell drawn 60 mm long at ×400. Actual = 60 ÷ 400 = 0.15 mm = 150 µm. Worked the other way: a cell 0.05 mm long drawn 40 mm. Magnification = 40 ÷ 0.05 = ×800. Magnification has no unit, writing "×800 mm" loses the mark.

4🌍 In the real worldPart 4 / 7

Using a microscope, and drawing what you see

A light microscope magnifies to about ×1 500; an electron microscope reaches hundreds of thousands and is needed to see ribosomes and membrane detail. Resolution, not just magnification, is why: beyond a point a light microscope gives a bigger but blurrier image.

Total magnification = eyepiece × objective. A ×10 eyepiece with a ×40 objective gives ×400.

Biological drawings are marked on convention, not artistry: clear continuous single lines with no shading or sketching, drawn in pencil, large enough to fill the space, with label lines drawn with a ruler that touch the structure and do not cross. Include a magnification or scale if asked.

5⚠️ Common mistakesPart 5 / 7

Where marks go

  • Cell wall and cell membrane confused. The wall is fully permeable and gives support; the membrane is partially permeable and controls entry.
  • "All plant cells have chloroplasts." Root cells do not. There is no light.
  • Adaptation given without its consequence. Always add "so that…".
  • Magnification given a unit. It has none.
  • Units not converted. 1 mm = 1 000 µm.
  • Bacteria described as having a nucleus. Their DNA is free in the cytoplasm.
6✅ SummaryPart 6 / 7

What to remember

  • Every structure with its function; wall, chloroplast and permanent vacuole are plant-only.
  • Specialised cell answers = feature + consequence.
  • Cell → tissue → organ → organ system → organism.
  • Magnification = image ÷ actual, no unit; 1 mm = 1 000 µm.
  • Light microscope ×1 500; electron microscope for organelles.
7✏️ Worked examplePart 7 / 7

Watch one worked through

Here is one done slowly, with the reason for every move. Then one for you.

Watch one first

Put these in order from smallest to largest and give an example of each: tissue, organ, cell, organ system, organism.

  1. Start with the smallest unit that is alive.
    cell → a muscle cell
    The cell is the basic unit of life. Nothing smaller counts as living on its own.
  2. Many similar cells working together form a tissue.
    tissue → muscle tissue
    The word similar matters. A tissue is one kind of cell doing one job together.
  3. Different tissues together form an organ.
    organ → the heart (muscle + nerve + blood tissue)
    Different is what distinguishes an organ from a tissue. The heart is more than muscle alone.
  4. Organs working together form a system, and systems form the organism.
    system → circulatory system organism → a human
    Each level is built from several of the level below. That one sentence is the whole hierarchy.

Answer: Cell, tissue, organ, organ system, organism

Now you try

A group of similar cells doing the same job is called what? One word.

Show the answer

tissue

Practise Cells & Organisation

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🧠 5 quizzes on Cells & Organisation (2 easy, 2 medium, 1 hard)

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🖨️ Printable worksheet

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