Transport and Circulation
Blood and its components
Blood transports substances around the body. Plasma carries dissolved glucose, amino acids, carbon dioxide, urea and hormones; red blood cells carry oxygen using haemoglobin; white blood cells defend against pathogens; platelets help clotting. A common trap is saying red cells 'contain haemoglobin to carry carbon dioxide'; haemoglobin binds oxygen, and most carbon dioxide travels dissolved in plasma.
The heart and blood vessels
Know the double circulation: the right side pumps deoxygenated blood to the lungs, the left side pumps oxygenated blood around the body, so the left ventricle wall is thicker. Arteries carry blood away from the heart at high pressure, with thick elastic walls; veins return blood at low pressure and have valves; capillaries are one cell thick for exchange. Questions often ask you to explain why capillaries are suited to exchange: thin walls and large total surface area slow flow and shorten diffusion distance.
Plants and transport
Xylem transports water and minerals upward, driven by transpiration from the leaves; phloem transports sugars made in photosynthesis to growing and storage regions. A frequent trap is mixing these up. Remember: xylem is dead, hollow tubes; phloem is living tissue. Transpiration rate rises with warmth, wind, low humidity and bright light.
Worked example
Explain why a plant wilts in hot sun. High transpiration removes water faster than roots absorb it, cells lose turgor and the plant droops. Rule of thumb: transpiration pulls water up the xylem; water loss outpaces uptake and the plant wilts.
Practical trap
When counting heart rate or pulse, state the unit (beats per minute) and repeat readings for reliability.