Cambridge IGCSE Biology (0610) · Organisation of the organism
Mini-Lesson
Organisation of the organism
This mini-lesson covers Topic 2 — Organisation of the organism: the structure and functions of plant, animal and bacterial cells, specialised cells, the levels of organisation (cell → tissue → organ → organ system → organism), and how to work out the size of specimens using the magnification formula.
Work through each screen, answer the questions as you go (some are calculations) and collect ⭐ stars. Watch for the Supplement flag. Press Start when you're ready.
Cell structure · plant & animal
Plant & animal cell structure
Both plant and animal cells are eukaryotic — they have a nucleus. They share several structures; a plant cell has three extras.
Nucleus — contains the DNA; controls the cell's activities.
Cytoplasm — jelly-like; site of many chemical reactions.
Cell membrane — controls what enters and leaves the cell.
Mitochondria — site of aerobic respiration.
Ribosomes — where protein synthesis happens.
Cell wall (plant) — made of cellulose; supports and strengthens.
Chloroplasts (plant) — contain chlorophyll for photosynthesis.
Vacuole (plant, permanent) — filled with cell sap; helps keep the cell firm.
Quick check
Match the function
?Which structure, found in plant cells but not animal cells, contains chlorophyll and is the site of photosynthesis?
Cell structure · bacteria
Bacterial cell structure
A bacterial cell has a cell wall, cell membrane, cytoplasm and ribosomes, but no nucleus. Its genetic material is a loop of circular DNA free in the cytoplasm, plus small extra rings called plasmids.
Prokaryotic cell: no nucleus — circular DNA plus plasmids (extra DNA rings).
Watch out: bacteria do have a cell wall and ribosomes, but they have no nucleus, no mitochondria and no chloroplasts.
Sort it
Whose feature is it?
Tap a feature, then tap the cell type it belongs to. (Some are shared by all.)
🔁 All three
🌿 Plant only
🦠 Bacteria only
Specialised cells
Specialised cells & their jobs
Cells become specialised — their structure is adapted to a specific function. IGCSE names these examples:
Ciliated cells — tiny hairs (cilia) sweep mucus along the trachea and bronchi.
Root hair cells — a long extension gives a huge surface area for absorption of water and mineral ions.
Palisade mesophyll cells — packed with chloroplasts for photosynthesis.
Neurones (nerve cells) — long and thin to conduct electrical impulses.
Red blood cells — no nucleus, biconcave, full of haemoglobin to transport oxygen.
Sperm and egg cells (gametes) — adapted for reproduction (fertilisation).
Exam tip: always link the structure to the function — e.g. "a root hair cell has a large surface area, so it can absorb more water".
Quick check
Why no nucleus?
?A red blood cell has no nucleus and a biconcave (dented) shape. What is the main advantage of these adaptations?
Levels of organisation
Cell → tissue → organ → system → organism
Specialised cells are organised into ever-larger structures:
Cell — the basic building block (e.g. a muscle cell).
Tissue — a group of similar cells working together (e.g. muscle tissue).
Organ — several tissues performing a function (e.g. the stomach).
Organ system — organs working together (e.g. the digestive system).
Organism — a whole living individual made of organ systems.
Match it
Match the example to its level
Tap an example on the left, then its level of organisation on the right.
Example
Level
Size of specimens
Magnification & size of specimens
Cells are too small to see with the eye, so we magnify them. The key formula is:
magnification = image size ÷ actual sizerearrange: actual size = image size ÷ magnification · measure in millimetres (mm)
Supplement — units: you may need to convert between millimetres and micrometres. Remember 1 mm = 1000 µm. So a cell that is 0.02 mm across is 20 µm across.
Worked example — keep the units the same!
A cell is really 0.05 mm wide. In an image it measures 20 mm wide.
magnification = 20 ÷ 0.05 = ×400
Misconception: magnification is image ÷ actual, not actual ÷ image. Always convert both lengths to the same unit first.
Calculate
Your turn — magnification
1A cell is really 0.1 mm long. Under the microscope its image is 50 mm long. Calculate the magnification.
×
Hint: magnification = image ÷ actual = 50 ÷ 0.1.
Supplement · Calculate
Your turn — find the actual size
2A cell's image is 30 mm wide at a magnification of ×1500. Calculate the actual width of the cell in micrometres (µm). (1 mm = 1000 µm)
µm
Hint: actual = image ÷ magnification = 30 ÷ 1500 = 0.02 mm, then × 1000 → µm.
Quick check
Which level is the leaf?
?A leaf is made of several tissues (palisade mesophyll, epidermis, xylem, phloem) working together to carry out photosynthesis. Which level of organisation is a leaf?