World of Living
This domain covers the processes that keep organisms alive, how plants and animals coordinate responses, how organisms reproduce, and how traits pass from parents to offspring. BSEH questions often require you to compare processes, interpret a Punnett square, and explain a hormone or a reflex pathway.
Core rules and concepts
- Autotrophic nutrition uses photosynthesis; heterotrophic nutrition includes saprophytic, parasitic and holozoic modes.
- Aerobic respiration releases more energy per glucose molecule than anaerobic respiration; anaerobic respiration in muscles produces lactic acid.
- Xylem transports water and minerals upward; phloem translocates food in both directions.
- Mammals have double circulation, which keeps oxygenated and deoxygenated blood separate.
- The nephron filters blood; useful substances are reabsorbed by the tubule.
- Tropisms are directional growth responses; plant hormones such as auxin control them.
- Reflex actions are rapid, involuntary and usually routed through the spinal cord.
Worked example
Cross two heterozygous tall pea plants, Tt × Tt. Each parent makes gametes T and t. A Punnett square gives TT, Tt, Tt, tt. The genotype ratio is 1:2:1 and the phenotype ratio is 3 tall : 1 short. The probability that an offspring is short is 1/4, or 25%. If 200 seeds are sown, the expected number of short plants is 50.
Traps that cost marks
- Saying all arteries carry oxygenated blood; the pulmonary artery carries deoxygenated blood.
- Reversing xylem and phloem functions.
- Treating a dominant allele as the more common allele.
- Forgetting that gametes are haploid.
- Describing a reflex as a decision made by the brain before the response.