Transport and respiration
Biology · WAEC and JAMB · SS2 and SS3
Two systems that examiners love to compare. Keep the plant and animal versions clearly separate in your mind.
What you need to know
- Xylem carries water and mineral salts upward only; vessels are dead, hollow and lignified.
- Phloem carries manufactured food in both directions; sieve tubes are living.
- Transpiration pull, capillarity and root pressure together move water up the stem.
- Blood has plasma, red cells, white cells and platelets.
- Red cells have no nucleus and are biconcave, which maximises surface area for oxygen.
- Arteries carry blood away from the heart with thick muscular walls; veins carry it back and have valves.
- Aerobic respiration releases about 2,900 kJ per mole of glucose; anaerobic releases far less.
- In muscle, anaerobic respiration produces lactic acid; in yeast it produces ethanol and carbon dioxide.
Key terms
- Transpiration
- Loss of water vapour from the aerial parts of a plant, mostly through stomata.
- Oxygen debt
- The extra oxygen needed after exercise to break down the lactic acid built up anaerobically.
- Double circulation
- Blood passes through the heart twice per complete circuit — once to the lungs, once to the body.
Worked example
Why does an athlete continue breathing heavily after a race has ended?
- During the race oxygen supply could not meet demand, so muscles respired anaerobically.
- That produced lactic acid, which accumulated.
- Extra oxygen is now needed to oxidise the lactic acid.
Answer: To repay the oxygen debt by breaking down accumulated lactic acid.
The mistake to avoid
Respiration is not breathing. Breathing is the physical movement of air; respiration is the chemical release of energy in cells. Examiners penalise the confusion every year.
In the exam
When comparing xylem and phloem, give both sides of each point. "Xylem is dead, phloem is living" scores; "xylem is dead" alone does not.