Eukaryotic and prokaryotic cells

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

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

Scale (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 & bacterial cells

Every animal cell has these shared parts. A plant cell has all of them plus a cell wall, chloroplasts and a permanent vacuole:

• Nucleus — contains the DNA and controls the cell's activities.
• Cytoplasm — jelly where most chemical reactions happen.
• Cell membrane — controls what enters and leaves the cell.
• Mitochondria — site of aerobic respiration.
• 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).
• Bacterial cell — has a cell wall, membrane, cytoplasm and ribosomes, a single loop of chromosomal DNA and often plasmids, but no nucleus and no mitochondria or chloroplasts.

Which structure?

Specialised cells

As an organism develops, cells become specialised (they differentiate) so their structure fits their function:

• Sperm cell — a tail (flagellum) to swim, many mitochondria for energy, and enzymes in the acrosome to penetrate the egg.
• Egg cell — large store of nutrients in the cytoplasm; membrane changes after fertilisation to stop more sperm entering.
• Ciliated epithelial cell — hair-like cilia that waft mucus (e.g. up the airways).
• Root hair cell — a long projection giving a large surface area to absorb water and mineral ions.
• Xylem carries water up the plant; phloem carries dissolved sugars around it.

Link it up: a feature is only “specialised” if you can say how it helps the job — e.g. “many mitochondria release energy for the tail to beat”.

Whose feature is it?

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

Light vs electron microscopes

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

Misconception: magnification is image ÷ actual, not actual ÷ 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 actual size

Your turn — unit conversion

Your turn — magnifying a bacterium

Enzymes and the active site

Enzymes are biological catalysts — proteins that speed up reactions without being used up. Each enzyme has a specific active site shaped to fit one substrate, described by the “lock and key” model.

Denaturation: if the shape of the active site changes, the substrate no longer fits and the enzyme stops working. This is caused by high temperature or the wrong pH.

Temperature, pH & substrate concentration

• Temperature: rate rises as temperature increases (more kinetic energy, more collisions) up to an optimum (about 37 °C in humans). Above this the enzyme denatures and rate falls sharply.
• pH: each enzyme has an optimum pH. Too acidic or too alkaline changes the active site's shape and denatures the enzyme.
• Substrate concentration: more substrate means a faster rate — until all active sites are working flat out, when the rate levels off.

Too hot to work

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 a higher to a lower concentration (e.g. O₂, CO₂). 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).

Nail it: osmosis moves water only, down its 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.

Core practical — osmosis in potato

The osmosis core practical investigates how sugar (or salt) solution concentration affects 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 enters by osmosis). In a concentrated solution it loses mass. Using % change fairly compares cylinders that began at slightly different masses.

Your turn — percentage change

The big ideas to know

You've covered the core of Edexcel Topic 1 — Key concepts in biology. Press Finish to see your score.