Eukaryotic and prokaryotic cells

All living things are made of cells. AQA splits them into two great groups:

• Eukaryotic cells have a nucleus that holds the DNA, plus membrane-bound organelles. Animal and plant cells are eukaryotic.
• Prokaryotic cells (e.g. bacteria) are much smaller and have no nucleus — their single loop of DNA floats free in the cytoplasm.

Scale (maths skill — standard form): a plant or animal cell is roughly 10–100 µm; a bacterium is about 1 µm. Remember 1 mm = 1000 µm and 1 µm = 1000 nm. So a bacterium (≈1 µm = 1 × 10⁻⁶ m) is about an order of magnitude smaller than an animal cell.

Animal & plant cell organelles

Every animal cell has these four shared parts. A plant cell has all four plus three extras:

• Nucleus — controls the cell; contains DNA (the genetic material).
• Cytoplasm — jelly where most chemical reactions happen.
• Cell membrane — controls what enters and leaves the cell.
• Mitochondria — site of aerobic respiration (release energy).
• Ribosomes — where protein synthesis happens.
• Cell wall (plant) — made of cellulose; strengthens the cell.
• Chloroplasts (plant) — contain chlorophyll for photosynthesis.
• Permanent vacuole (plant) — filled with cell sap; keeps the cell firm (turgid).

Match the function

Bacterial (prokaryotic) cell

A bacterium has a cell wall, cell membrane, cytoplasm and ribosomes, but no nucleus. Its genetic material is a single DNA loop, and it may carry small extra rings of DNA called plasmids.

Watch out: bacteria do have a cell wall and ribosomes, but it is not a cellulose wall and they have no mitochondria, chloroplasts or true nucleus.

Whose feature is it?

Tap a feature, then tap the cell type it belongs to. (Some are shared by all.)

Specialisation & differentiation

As an organism develops, cells become specialised to do a particular job. The process of a cell gaining its specialised features is called differentiation.

Animals vs plants: in animals, most cells differentiate early and can't re-specialise (some stay as stem cells for repair). Many plant cells keep the ability to differentiate throughout life.

Why a long tail?

Microscopy & magnification

Cells are too small to see, so we use microscopes. Electron microscopes have a much higher magnification and resolving power (resolution) than light microscopes, so they reveal tiny sub-cellular structures.

Misconception: magnification is image ÷ real, not real ÷ image. Always convert both lengths to the same unit first (e.g. mm and mm, or µm and µm).

Your turn — magnification

Your turn — find the real size

Culturing microorganisms

To study microbes we grow them on sterile nutrient agar in Petri dishes. To get an uncontaminated culture you must use aseptic technique:

• Sterilise the inoculating loop in a flame before use.
• Lift the Petri dish lid as little as possible and work near a Bunsen flame.
• Tape the lid (don't seal fully — to stop anaerobic pathogens), and incubate at 25 °C in school labs.

We can test antibiotics/antiseptics: a clear ring with no bacteria — the zone of inhibition — shows where growth was stopped. We compare its cross-sectional area using A = π r².

Your turn — zone of inhibition

Chromosomes, the cell cycle & mitosis

The nucleus holds DNA as structures called chromosomes, carrying genes. A human body cell has 23 pairs (46). Cells divide in a sequence called the cell cycle:

Key: before a cell divides it must copy its DNA and grow more sub-cellular structures. Mitosis itself is when the chromosomes separate into two new nuclei.

What does mitosis make?

Stem cells & their uses

A stem cell is an undifferentiated cell that can keep dividing and can differentiate into many cell types.

• Embryonic stem cells — from early embryos; can become almost any cell type.
• Adult stem cells — e.g. in bone marrow; form a limited range (mostly blood cells).
• Plant meristems — in root and shoot tips; can make any plant cell throughout the plant's life.

Uses: treating diabetes or paralysis; therapeutic cloning makes cells that won't be rejected. Cloned meristem cells let growers mass-produce identical, disease-resistant plants quickly and cheaply.

Issues to evaluate: ethical objections to using embryos; the risk of viral contamination being passed on; and limited supply. AQA wants you to discuss these, not just list them.

Where in a plant?

Diffusion, osmosis & active transport

Substances move in and out of cells in three ways. The first two need no energy; the third does.

• Diffusion — net movement of particles from higher to lower concentration (e.g. O₂, CO₂, urea). No energy needed.
• Osmosis — movement of water across a partially permeable membrane, from a dilute (high water) to a more concentrated (low water) solution. No energy needed.
• Active transport — moves substances against the gradient (low → high), so it needs energy from respiration (e.g. root hairs absorbing mineral ions; the gut absorbing glucose).

Misconceptions to nail: osmosis moves water only, down a concentration gradient, with no energy. Active transport is the opposite — against the gradient and energy-using.

Match each statement to its process

Tap a statement on the left, then its matching process on the right.

Required practical — osmosis

The osmosis required practical investigates the effect of sugar (or salt) solution concentration on plant tissue, usually potato cylinders:

• Cut equal-sized potato cylinders; record each starting mass.
• Leave each in a different concentration of sugar solution.
• Re-weigh and calculate the percentage change in mass.

A potato in pure water gains mass (water moves in by osmosis). In a concentrated solution it loses mass. Using % change fairly compares cylinders that began at slightly different masses.

Your turn — percentage change

Surface area : volume ratio

The rate of diffusion, osmosis and active transport depends partly on the surface area to volume (SA:V) ratio. As something gets bigger, its volume grows faster than its surface area, so the SA:V ratio falls.

Why it matters: single-celled organisms have a large SA:V and can rely on diffusion. Big animals can't, so they evolve specialised exchange surfaces (lungs, gills, gut villi) with huge surface areas.

Your turn — SA:V ratio

The big ideas to know

You've covered all three parts of AQA 4.1 — cell structure, cell division, and transport in and out of cells. Press Finish to see your score.