ZIMSEC O Level · Biology · Form 3
The cell is the basic unit of life. Organisms are made of one cell (unicellular, e.g. Amoeba) or many cells (multicellular).
Organelles and their functions
- Nucleus: contains chromosomes made of DNA; controls cell activities.
- Cytoplasm: jelly where most chemical reactions take place.
- Cell membrane: partially permeable; controls what enters and leaves.
- Mitochondria: site of aerobic respiration.
- Ribosomes: make proteins.
- Plant cells only: cellulose cell wall, chloroplasts and a large sap vacuole.
Organisation
Specialised cells → tissues → organs → organ systems → organism. A palisade cell is packed with chloroplasts for photosynthesis; a ciliated cell in the trachea moves mucus.
Size and magnification
Magnification = image size ÷ actual size. 1 mm = 1000 µm.
Worked example: A drawing of a cell is 40 mm long. The cell is 0.1 mm long. Find the magnification.
Magnification = 40 ÷ 0.1 = ×400
Exam tip: Drawings should be large, in pencil, with clean single lines and ruled label lines that do not cross.
Check yourself
- State the function of ribosomes.
- Name the organelle where aerobic respiration takes place.
- An image is 25 mm long at ×500. Find the actual length in µm.
Show answers
- They make (synthesise) proteins.
- Mitochondrion.
- 25 ÷ 500 = 0.05 mm = 50 µm
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