Biology: Life Processes, Coordination and Heredity
Biology tests life processes - nutrition, respiration, transport and excretion - plus control and coordination, reproduction and heredity. Answers must name the structure, give its function and, where asked, a ratio with the reasoning shown.
Life processes
Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ in chloroplasts, using chlorophyll and light. Aerobic respiration releases far more energy than anaerobic respiration, which gives ethanol and CO₂ in yeast and lactic acid in muscles. Xylem carries water and minerals upward; phloem carries food in both directions. In the human heart, the right side pumps to the lungs and the left side to the body; arteries carry blood away from the heart, veins towards it. The nephron is the filtration unit of the kidney.
Control, coordination and heredity
A reflex arc runs receptor → sensory neuron → spinal cord → motor neuron → effector. Insulin from the pancreas lowers blood sugar, thyroxine from the thyroid controls metabolism, and auxin promotes growth in plants. In Mendel's monohybrid cross, Tt × Tt gives a genotypic ratio 1 TT : 2 Tt : 1 tt and a phenotypic ratio 3 tall : 1 short. Human sex chromosomes are XX in females and XY in males, so the father's sperm decides the sex of the child.
Worked example
Cross two heterozygous tall pea plants (Tt). Give the offspring ratios and explain why short plants reappear. Each parent produces T and t gametes, giving TT, Tt, Tt, tt. TT and Tt are tall because T is dominant; only tt is short. So phenotypes are 3 : 1 and genotypes 1 : 2 : 1. The recessive t allele was hidden in both parents but met in one quarter of the offspring.
Traps that cost marks
Swapping xylem and phloem; writing that all arteries carry oxygenated blood (the pulmonary artery does not); calling a reflex action voluntary; confusing genotype with phenotype; quoting 1 : 2 : 1 as a phenotypic ratio.