Biology: Cell Biology, Human Physiology, Genetics and Ecology
Biology carries the largest weight in the exam and spans cell biology, human physiology, genetics, reproduction, ecology and basic biotechnology. Questions reward precise recall of structures, hormones and pathways.
Core rules and concepts
- Cell: mitochondria (ATP), rough ER (protein synthesis), Golgi (packaging), lysosomes (hydrolytic enzymes); prokaryotes lack a nucleus and membrane-bound organelles.
- Physiology: cardiac output = stroke volume × heart rate; most tubular reabsorption occurs in the proximal convoluted tubule; insulin comes from beta cells of the islets of Langerhans, glucagon from alpha cells.
- Genetics: Mendel's law of segregation; a monohybrid Tt × Tt gives 3:1 phenotypic and 1:2:1 genotypic ratios; blood group O is a universal donor, AB universal recipient.
- Reproduction: meiosis halves chromosome number (2n → n); spermatogenesis yields four sperm, oogenesis one ovum plus polar bodies.
- Photosynthesis: light reactions in thylakoid membranes, Calvin cycle in the stroma.
Worked example
A person has a stroke volume of 70 mL and a heart rate of 72 beats per minute. Cardiac output = 70 × 72 = 5040 mL/min, about 5 L/min. If heart rate rises to 100/min while stroke volume stays at 70 mL, output becomes 7 L/min. The reasoning is direct multiplication, but note that at very high rates stroke volume falls because filling time shortens.
Traps that cost marks
- Confusing mitosis (2n → 2n) with meiosis (2n → n).
- Mixing up insulin and glucagon sources, or ADH and aldosterone actions.
- Placing the site of maximum reabsorption in the loop of Henle instead of the PCT.
- Swapping light and dark reaction locations.
- Forgetting that the 10% law applies to energy transfer between trophic levels.