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BiologyGCSEAQA & EdexcelYears 9–11

GCSE Biology Organisation Notes

Cells are organised into tissues, organs and systems. This topic covers digestion, the heart and circulatory system and transport in plants.

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Key concepts

Organisation hierarchy
Cells to tissues to organs to organ systems to organism.
Enzyme
Biological catalyst that speeds up specific reactions via a 'lock and key' active site.
Double circulation
Blood passes through the heart twice per circuit: right side to lungs, left side to body.
Transpiration
Loss of water vapour from the leaves that pulls water up the xylem from the roots.
  • Digestive enzymes: amylase → starch → sugars; proteases → protein → amino acids; lipase → lipids → fatty acids + glycerol.
  • Enzymes have an optimum pH and temperature; too hot and they denature.
  • Blood: red blood cells (oxygen), white blood cells (defence), platelets (clotting), plasma (transport).
  • Xylem carries water/minerals up; phloem transports sugars up and down (translocation).

Worked example

Explain why an enzyme stops working at 60 °C.

  1. 1High temperature increases vibrations in the enzyme molecule.
  2. 2Bonds holding the active site's shape break.
  3. 3The active site changes shape (denatures).
  4. 4The substrate no longer fits, so the reaction stops.

Answer: The enzyme's active site denatures so the substrate no longer fits.

Common mistakes

✗ Saying enzymes are 'killed' at high temperatures.

✓ Enzymes are not alive — they denature.

✗ Mixing up xylem and phloem.

✓ Xylem = water up; phloem = sugars up and down (translocation).

✗ Confusing arteries and veins.

✓ Arteries away from heart (thick walls, high pressure); veins to heart (valves, low pressure).

Quick check

Try these, then reveal the answers.

Which enzyme digests protein?Reveal answer

Protease.

Which side of the heart pumps blood to the lungs?Reveal answer

The right side.

What is transpiration?Reveal answer

The loss of water vapour from leaves, mostly through stomata.

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Exam tips

  • Learn the 'lock and key' model — including denaturation — as a diagram and a description.
  • Use precise vessel names when describing blood flow: vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta.
  • Link plant transport structures to their functions with adaptations (e.g. xylem has lignin for support).

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Related topics

Diffusion, Osmosis & Active TransportBioenergetics

Key terms explained

Short, exam-ready definitions with a formula, a worked example and an examiner tip.

What is active transport?What is diffusion?What is dna?What is enzyme?What is homeostasis?What is mitosis?Full glossary

Frequently asked questions

What is the difference between arteries, veins and capillaries?+

Arteries carry high-pressure blood away from the heart; veins return low-pressure blood with valves; capillaries are thin-walled vessels where exchange occurs.

How does temperature affect enzyme activity?+

Rate rises with temperature to an optimum, then falls sharply as the enzyme denatures and its active site changes shape.

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