Eukaryotic & prokaryotic cells

Living things are built from two great groups of cell:

• Eukaryotic cells have a nucleus enclosing the DNA, plus membrane-bound organelles. Animal, plant, fungal and protist cells are eukaryotic.
• Prokaryotic cells (bacteria) have no nucleus — a single loop of DNA sits free in the cytoplasm, often with small extra rings called plasmids.

Scale (maths skill): a plant/animal cell is roughly 10–100 µm; a bacterium about 1 µm. Remember 1 mm = 1000 µm and 1 µm = 1000 nm, so 1 µm = 1 × 10⁻⁶ m.

Sub-cellular structures & their jobs

• Nucleus — holds DNA; controls the cell's activities.
• Cytoplasm — where most chemical reactions happen.
• Cell membrane — controls what enters and leaves.
• Mitochondria — site of aerobic respiration (release energy).
• Ribosomes — where protein synthesis happens.
• Cell wall (plant, made of cellulose) — supports the cell.
• Chloroplasts (plant) — contain chlorophyll for photosynthesis.
• Permanent vacuole (plant) — sap-filled; keeps the cell firm (turgid).

Bacteria have a cell wall (not cellulose), cell membrane, cytoplasm and ribosomes, but no nucleus, mitochondria or chloroplasts.

Which organelle?

Sort each into its group

Tap a feature, then tap the group it belongs to.

Microscopy & magnification

Cells are too small to see, so we use microscopes. Electron microscopes have far higher magnification and resolution than light microscopes, revealing tiny sub-cellular structures.

Misconception: magnification is image ÷ real, not real ÷ image. Convert both lengths to the same unit first.

Your turn — magnification

Your turn — real size

Enzymes — biological catalysts

Enzymes are proteins that act as biological catalysts: they speed up reactions without being used up. Each enzyme has a specific active site that fits only one substrate — the lock-and-key idea.

• Rate rises with temperature — until the enzyme denatures (active site changes shape) at high temperatures.
• Each enzyme has an optimum pH; too acidic or alkaline denatures it.
• Human enzymes usually work best near 37 °C.

Key words: a denatured enzyme's active site no longer fits the substrate — so it stops working. Denaturing is not the same as "killing" the enzyme.

What happens at high temperature?

Diffusion, osmosis & active transport

• Diffusion — net movement of particles from higher to lower concentration (e.g. O₂, CO₂). No energy needed.
• Osmosis — movement of water across a partially permeable membrane, from a dilute to a more concentrated 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).

Nail it: osmosis moves water only, down its gradient, using no energy. Active transport is the opposite — against the gradient, energy-using.

Match each statement to its process

Tap a statement, then its matching process.

Your turn — surface area : volume

DNA & protein synthesis (basics)

DNA is a polymer made of two strands coiled into a double helix. A gene is a short section of DNA that codes for a particular protein (a sequence of amino acids).

• DNA is made of repeating units called nucleotides.
• The order of bases in a gene sets the order of amino acids in the protein.
• The correct amino acid order makes the protein fold into a shape that lets it do its job (e.g. an enzyme's active site).

Big idea: genes carry the instructions; ribosomes build proteins by joining amino acids in the order the gene specifies.

What is a gene?

Spot the prokaryote

The big ideas to know

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