GCSE Biology Cells & cell division revision

The names matter in exams. Eukaryotic means "true nucleus": the genetic material sits inside a membrane-bound nucleus. Prokaryotic means "before nucleus": bacteria keep a single loop of DNA loose in the cytoplasm and often carry plasmids — small rings of extra DNA that replicate independently and can be transferred between bacteria. Plasmids are why bacteria can pick up antibiotic resistance so quickly, and why genetic engineers use them as delivery vehicles. Bacterial cells are also much smaller than plant or animal cells, which is why a sterile-looking drop of pond water can still teem with millions of them.

Every function mark in the exam pairs a structure with what it does. The three plant extras are worth memorising as a trio: the cellulose cell wall for strength, the permanent vacuole full of cell sap for rigidity, and the chloroplasts for photosynthesis. A wilting lettuce is a vacuole story: when its cells lose water the vacuoles shrink and the cell walls stop being pushed rigid. Algal cells share the plant plan — they photosynthesise, so they carry chloroplasts too.

The exam question is almost always the same shape: *explain how the structure of cell X is related to its function*. Answer in the pattern feature → what it does → why that helps the job. Root hair cells are the plant favourite, and note the twist: they absorb mineral ions against the concentration gradient by active transport, which is why their mitochondria matter as much as their surface area.

Light microscopes shine light through a thin, often stained specimen and magnify up to about ×2 000 — enough to see cells and most nuclei, but not the fine detail inside them. Electron microscopes fire beams of electrons instead of light: they magnify far more and have a much better resolution (the ability to distinguish two close points), which is how ribosomes, chloroplasts and the double membrane of mitochondria were discovered. Every organelle map you memorise is a debt to electron microscopy.

1. 1Prepare the slide

   Peel a thin layer of onion epidermis (plant) or swab the inside of a cheek (animal); place the specimen flat on a clean slide.

2. 2Stain it

   Add a drop of iodine solution (plant) or methylene blue (animal) to make structures such as the nucleus visible.

3. 3Cover slip

   Lower a cover slip at an angle with a mounted needle to push out air bubbles — bubbles show up as dark rings and hide the specimen.

4. 4Start low

   Clip the slide on the stage, use the lowest objective lens, focus with the coarse adjustment, then sharpen with the fine adjustment.

5. 5Zoom in

   Turn to a higher objective and re-focus with the fine adjustment only — the coarse screw can crush the slide at high power.

6. 6Record

   Draw what you actually see with clear, unshaded outlines, label with straight horizontal lines, add the magnification used and a scale bar.

Variables to control in the practical: the stain and the observation time are kept the same between slides, and the magnification used is recorded for every drawing. Risks: glass slides and cover slips break — sweep up with a brush, not fingers; iodine and methylene blue stain skin and clothes; onion layers are a low-risk specimen, while cheek-cell sampling uses only a swab, never a scalpel. Typical results: onion cells show a rectangular brick pattern with a visible nucleus; cheek cells are rounder, thinner and fainter, so they need more stain.

The nucleus holds chromosomes — long strands of DNA carrying hundreds of genes, the instructions for making proteins. In body cells the chromosomes come in pairs: humans have 23 pairs, 46 in total — one chromosome of each pair from each parent. Growth and repair depend on cells dividing by the cell cycle, whose three overall stages you must know in order:

1. 1Stage 1 — growth and DNA copying

   The cell grows, increases its number of sub-cellular structures such as ribosomes and mitochondria, and the DNA replicates to form two copies of each chromosome.

2. 2Stage 2 — mitosis

   One full set of chromosomes is pulled to each end of the cell and the nucleus divides. (You do not need the sub-phases of mitosis — only the overall stage.)

3. 3Stage 3 — division

   The cytoplasm and cell membrane divide to form two genetically identical cells.

Because the DNA was copied first, each daughter cell gets a full, identical set of chromosomes — that is why mitosis produces cells for growth and development of multicellular organisms, and why a skin repair patch is genetically your own. Mitosis is happening all over you right now: in growing root tips, healing cuts, and replacing the lining of your gut every few days.

A stem cell is an undifferentiated cell that can give rise to many more cells of the same type and from which certain other cells arise by differentiation. Three sources matter at GCSE: embryonic stem cells, which can be made to differentiate into most cell types; adult stem cells, such as those in bone marrow, which mainly form blood cells; and meristem tissue in plants, which can differentiate into any type of plant cell throughout the plant’s life. Treatments with adult stem cells are established (bone-marrow transplants for blood disorders); embryonic stem cells could help conditions such as diabetes and paralysis — but are not yet routine therapy.

In therapeutic cloning, an embryo is produced with the same genes as the patient (a body cell’s nucleus is placed in a donor egg whose own nucleus has been removed). Stem cells made from that embryo carry the patient’s genes, so they are not rejected by the patient’s immune system and can be used for treatment. The arguments cut both ways, and both sides must appear in a six-mark answer. Benefits: rejected-free transplants, cures for currently untreatable conditions, learning how differentiation works. Risks and objections: possible transfer of viral infection, uncontrolled division (tumour risk), and ethical or religious objections to using human embryos, which some people regard as potential lives. Plant meristems sidestep the ethics entirely: growers clone rare plants or pest-resistant crops quickly and economically.