Contents: 7 sections
Cambridge IGCSE Biology 0610 · Core and Extended
Syllabus points
- Describe and compare the structure of a plant cell and an animal cell.
- State the functions of the structures found in cells.
- Relate the structure of specialised cells to their functions.
- Describe the levels of organisation: cell, tissue, organ, organ system, organism.
- Calculate magnification and actual size using the formula, and convert between millimetres and micrometres.
Plant and animal cells
Both have a cell membrane, cytoplasm, a nucleus, mitochondria and ribosomes. Plant cells have three things animal cells do not.
| Structure | Animal | Plant | Function |
|---|---|---|---|
| Cell wall | no | yes | made of cellulose, gives support and stops the cell bursting |
| Chloroplast | no | some | contains chlorophyll, site of photosynthesis |
| Large permanent vacuole | no | yes | filled with cell sap, keeps the cell turgid |
Note the two qualifications, because questions rely on them. A cell wall is not a cell membrane: the wall is fully permeable and freely lets substances through, while the membrane is partially permeable and controls what enters. And not every plant cell has chloroplasts — root cells never do, since no light reaches them.
What each structure does
- Cell membrane — partially permeable, controls what enters and leaves.
- Cytoplasm — jelly-like, where most chemical reactions happen.
- Nucleus — contains the chromosomes, so it controls the cell's activities and inheritance.
- Mitochondria — site of aerobic respiration, so cells that need a lot of energy contain many.
- Ribosomes — where proteins are made.
- Vacuole — stores cell sap and supports the cell by pressing outwards.
- Chloroplast — absorbs light for photosynthesis.
A useful pattern: if a question asks why one cell has far more mitochondria than another, the answer is always that it needs more energy for a named active process, such as active transport in a root hair cell or contraction in a muscle cell.
Specialised cells
Structure follows function every time. Learn the link, not just the name.
- Ciliated cell — cilia sweep mucus, and the trapped dust and bacteria in it, away from the lungs.
- Root hair cell — a long extension gives a very large surface area for absorbing water and mineral ions; many mitochondria supply energy for active transport.
- Palisade mesophyll cell — packed with chloroplasts and at the top of the leaf, so it absorbs the most light.
- Neurone — very long, so it carries impulses over a distance quickly.
- Red blood cell — a biconcave disc for a large surface area, packed with haemoglobin, and with no nucleus so it can hold more.
- Sperm cell — a tail for swimming and many mitochondria to power it.
- Egg cell — large, with a store of food for the embryo.
- Xylem vessel — dead, hollow and strengthened with lignin, forming a continuous tube.
Levels of organisation
The sequence runs upward and questions often ask you to put it in order.
Cell → tissue → organ → organ system → organism.
- A tissue is a group of cells with similar structure working together on one function, such as muscle or xylem.
- An organ is several tissues working together, such as the stomach or a leaf.
- An organ system is several organs working together, such as the digestive system.
A leaf is a good worked example: palisade tissue, spongy tissue, xylem and phloem are the tissues; the leaf is the organ; the shoot system is the organ system.
Magnification
One formula does all of these questions:
magnification = image size ÷ actual size
Rearranged: actual size = image size ÷ magnification.
Magnification has no units. It is a ratio of two lengths, so the units cancel, which is why an answer written as "×3000" is right and one written as "3000 mm" is wrong.
The units are where marks are lost, so convert before dividing:
- 1 mm = 1000 µm
- 1 µm = 1000 nm
Worked example. A cell is drawn 21 mm wide at a magnification of ×3000.
actual size = 21 ÷ 3000 = 0.007 mm
Convert to micrometres: 0.007 × 1000 = 7 µm.
Always measure the image in millimetres first, divide, then convert the answer. Converting first is where the arithmetic usually goes wrong.
Sizes worth remembering
These let you sanity-check an answer and sometimes settle a question on their own.
- Bacterium: about 1 to 5 µm
- Plant or animal cell: about 10 to 100 µm
- Red blood cell: about 7 µm
- Human egg cell: about 100 to 120 µm, just visible without a microscope
If a calculation gives a "cell" of 5 mm, something has gone wrong by a factor of a thousand, which is almost always a missed conversion.