IGCSE Biology: cells and microscopy, explained for the exam

The cell structures and what each one does

Both boards expect the structure and its function. Learn the function as a full phrase, because one-word answers rarely earn the mark.

Animal cells can have small temporary vacuoles; the large permanent vacuole is the plant feature examiners want.

Key ideas, board by board

Plant cells have everything an animal cell has plus a cellulose cell wall, a large permanent vacuole and, in cells exposed to light, chloroplasts. Root hair cells are plant cells without chloroplasts, which is a favourite trap question. Bacterial cells have no nucleus: their DNA is a single circular chromosome loose in the cytoplasm, with extra plasmids, and they have no mitochondria or chloroplasts.

Cambridge 0610 (topic 2.1) lists six specialised cells with set functions: ciliated cells move mucus in the trachea and bronchi, root hair cells absorb water and mineral ions, palisade mesophyll cells photosynthesise, neurones conduct electrical impulses, red blood cells transport oxygen, and sperm and egg cells are gametes for reproduction. You must also define cell, tissue, organ, organ system and organism, and state that new cells are produced by division of existing cells.

Edexcel 4BI1 groups the same organelles under statements 2.2 to 2.4 and adds levels of organisation (organelles, cells, tissues, organs, systems). Statements marked B, such as 2.5B on cell differentiation and 2.6B on stem cells in medicine, are only examined on Paper 2. Edexcel is not tiered, so every student sits both papers, while Cambridge students sit either Core or Extended papers.

Microscopy appears as a practical skill: Edexcel lists investigating plant and animal cells with a light microscope, and Cambridge practical papers can ask you to draw a specimen and then calculate its magnification from your own drawing.

Worked example 1: finding actual size

Question: a drawing of a cheek cell is 45 mm wide. The magnification is x 400. Calculate the actual width of the cell in micrometres.

Step 1: rearrange the formula. actual size = image size / magnification = 45 / 400 = 0.1125 mm.

Step 2: convert to micrometres. 1 mm = 1000 µm, so 0.1125 x 1000 = 112.5 µm. Give the answer as 112.5 µm or 113 µm if the question asks for three significant figures.

Notice the order: work in the same unit for image and actual size first, convert at the end. Mixing millimetres and micrometres inside the formula is the commonest way to lose both marks.

Worked example 2: finding magnification from a scale bar

Question: a photograph of a palisade cell shows a scale bar labelled 20 µm. On the page the scale bar measures 30 mm. Calculate the magnification.

Step 1: put both lengths in the same unit. 30 mm = 30 x 1000 = 30 000 µm.

Step 2: magnification = image size / actual size = 30 000 / 20 = 1500. Write it as x 1500. Magnification has no units, so writing 1500 mm or 1500 µm is marked wrong.

A quick check: if the image is bigger than the real object, the magnification must be greater than 1. If you get 0.0007, you divided the wrong way round.

Common mistakes that cost marks

• Saying mitochondria "make energy". Energy cannot be made; write that respiration in mitochondria releases energy.
• Saying the cell membrane "protects the cell". The mark is for controlling what enters and leaves.
• Giving bacteria a nucleus or mitochondria. They have neither.
• Saying all plant cells have chloroplasts. Root hair cells and cells deep inside stems do not.
• Forgetting to convert units before dividing in magnification questions, or giving magnification a unit.
• Confusing the cell wall (structural support) with the cell membrane (controls entry and exit). Plant cells have both.
• Drawing practical diagrams with shading, sketchy double lines or arrows instead of single clear label lines.

Exam technique and mark-scheme language

"Describe the differences" questions want a comparison in each point: "plant cells have a cellulose cell wall but animal cells do not". Two separate lists (one about plants, one about animals) often score less than half, because the mark scheme rewards a matched pair. Use the words the mark scheme uses: partially permeable, chlorophyll, aerobic respiration, protein synthesis, genetic material.

For specialised cell questions, link a feature to its function with "so that" or "which": "a root hair cell has a long extension, which increases the surface area for absorption of water". A feature on its own is usually only half the point. In biological drawings, use a sharp pencil, continuous outlines, no shading, label lines that touch the structure and do not cross, and draw the specimen large, filling most of the space provided.

How one-to-one lessons help with this topic

Cells is usually taught first and then assumed in every later topic, so gaps here show up in respiration, photosynthesis and transport. In a lesson a tutor would check the organelle table from memory, then run timed magnification questions on the shared whiteboard until unit conversions are automatic, and mark answers against real mark schemes so your child hears exactly which phrase earns the point. For Cambridge practical papers, the tutor can set a drawing task from a micrograph and mark it with the same criteria examiners use.

Self-check: can you do all of these?

1. Draw and label a plant cell, an animal cell and (Cambridge) a bacterial cell from memory.
2. Give one full-sentence function for each of the eight organelles.
3. Name three structures found in plant cells but not animal cells.
4. Match each Cambridge specialised cell to its function, and explain one adaptation.
5. Put organelle, cell, tissue, organ, organ system and organism in order with an example of each.
6. Rearrange magnification = image size / actual size for any unknown and convert mm to µm.
7. (Edexcel Paper 2) Give one advantage and one disadvantage of using stem cells in medicine.