Cell biology — section overview

Cell biology B1 covers the microscopic world of cells — how they are structured, how they are studied and how they divide. It is the foundation for everything else in GCSE Biology.

What B1 covers

Sub-topic	Key ideas
B1.1 — Cell structure	Eukaryotic vs prokaryotic cells; plant vs animal cells; cell organelles
B1.2 — Cell division	Mitosis (growth/repair); meiosis (sexual reproduction); chromosomes
B1.3 — Transport in cells	Diffusion, osmosis, active transport; surface area : volume ratio

Why cell biology underpins the rest of the course

Every organ system you study later (B2 Organisation, B5 Homeostasis, B7 Ecology) relies on understanding what cells are and how they work. Osmosis links directly to kidney function B5. Cell division links to cancer and genetics B6.

Core vocabulary

• Cell membrane — selectively permeable; controls what enters/leaves
• Mitochondria — site of aerobic respiration (ATP production)
• Ribosome — site of protein synthesis (both prokaryotes and eukaryotes)
• Cell wall — plant/fungal cells only; made of cellulose (plants) or chitin (fungi)
• Nucleus — contains DNA (eukaryotes only); controls cell activity
• Prokaryote — no true nucleus; DNA is circular and free in cytoplasm; smaller (~1–10 μm)
• Eukaryote — membrane-bound nucleus; larger (~10–100 μm)

Microscopy essentials

Magnification formula: $$\text{Magnification} = \frac{\text{Image size}}{\text{Actual size}}$$

Always check units (convert μm → mm → cm as needed).

• Light microscope — maximum ~×2000 magnification; limited resolution; live specimens possible; cheap
• Electron microscope — maximum ~×500,000; much higher resolution; specimens must be dead (vacuum); reveals organelle detail

Diffusion, osmosis, active transport

Process	Movement	Energy	Against gradient?
Diffusion	High → low concentration	None	No
Osmosis	High → low water potential	None	No
Active transport	Low → high concentration	ATP	Yes

Osmosis definition: movement of water molecules through a partially permeable membrane from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution).

Common exam mistakes in B1

1. Saying nucleus contains chromosomes — true, but chromosomes are only visible during cell division; otherwise DNA is as chromatin
2. Confusing mitosis (2 identical daughter cells) and meiosis (4 genetically unique cells)
3. Osmosis — forgetting it is only water molecules — solute cannot cross the partially permeable membrane
4. Magnification rearrangement — most errors come from unit mismatch (image in mm, actual in μm)