Cell structures and microscopy (B1.1)

OCR Gateway places this topic at the very start of the biology course because every other topic in B1–B6 depends on knowing what's inside a cell and how we see it. Expect at least one short multiple-choice question on cell components, a label-the-diagram task, and a longer 6-mark question on either microscopy or specialised cells.

Eukaryotic vs prokaryotic cells

A eukaryotic cell has a true nucleus enclosed by a membrane and contains membrane-bound organelles. Animal cells, plant cells, fungi and protists are all eukaryotic.

A prokaryotic cell has no nucleus — its DNA is a single circular chromosome free in the cytoplasm. Bacteria are prokaryotic. Prokaryotic cells are typically 1–10 μm across; eukaryotic cells are 10–100 μm.

What every animal cell contains

Sub-cellular structure	Function
Nucleus	Contains the DNA; controls cell activity
Cytoplasm	Jelly-like matrix where most reactions take place
Cell membrane	Controls what enters and leaves the cell
Mitochondria	Site of aerobic respiration; energy released
Ribosomes	Site of protein synthesis

Plant cells — three additional structures

In addition to the animal-cell features above, plant cells have:

• Cell wall (made of cellulose) — gives the cell rigid shape.
• Permanent vacuole — filled with cell sap; supports the cell.
• Chloroplasts — site of photosynthesis; contain chlorophyll.

Bacterial (prokaryotic) cells

Bacteria contain:

• A cell wall (different chemistry from plant cell walls — it's made of peptidoglycan, not cellulose).
• A cell membrane.
• Cytoplasm.
• Ribosomes (smaller than eukaryotic ribosomes).
• A single circular DNA chromosome in a region called the nucleoid (NOT a nucleus).
• Often plasmids — small extra rings of DNA carrying genes for antibiotic resistance.
• Sometimes flagella (tail-like structures for movement) and pili (for attaching to surfaces).

⚠ Common error: writing that bacteria have no DNA. They do — it's just not in a nucleus.

Specialised cells you must know

Specialised cell	Adaptation	Function
Sperm cell	Long tail / streamlined / many mitochondria / acrosome enzymes	Swim to fertilise egg
Egg (ovum) cell	Large food store / haploid nucleus / cell membrane changes after fertilisation	Provide nutrients for the embryo
Ciliated epithelial cell	Cilia on surface	Sweeps mucus / debris up the airway
Red blood cell	Biconcave shape / no nucleus / haemoglobin	Carry oxygen
Root hair cell	Long projection — increases surface area	Absorb water and mineral ions
Nerve cell (neurone)	Long axon / branched dendrites / myelin sheath	Transmit electrical impulses
Muscle cell	Many mitochondria / contractile proteins	Contract to produce movement

Microscopy

A light microscope uses visible light and can magnify up to about ×2,000 with a resolution of ~200 nm — good enough to see cells, nuclei and large organelles like chloroplasts.

An electron microscope uses a beam of electrons and can magnify up to ×2,000,000 with a resolution of ~0.1 nm — good enough to see ribosomes, mitochondria and the internal structure of organelles.

The magnification equation

magnification = image size ÷ actual size

Always work in the same units. If image is 4 cm and actual is 0.5 mm, convert: 4 cm = 40 mm. Magnification = 40 / 0.5 = ×80.

Required practical: making a slide

1. Add a drop of water to a slide.
2. Place a thin onion-epidermis specimen onto the water.
3. Add a drop of iodine stain (highlights starch / nuclei).
4. Lower a coverslip from one edge using a mounted needle to avoid air bubbles.
5. View on low power first; switch to high power once focused.

Common Gateway-paper mistakes

1. Saying bacteria have a nucleus (they have a nucleoid).
2. Calling chloroplasts "the place where respiration happens" — that's mitochondria.
3. Using "magnification" when you mean "resolution".
4. Forgetting unit conversion in the magnification equation.
5. Drawing a plant cell that lacks chloroplasts in a question about a leaf cell — for photosynthesising plant cells, ALWAYS include chloroplasts.