MECEE-BL Biology: mechanisms, ratios and human physiology
What the biology paper rewards
MECEE-BL biology is set at grade 11–12 standard and spans botany and zoology: cell structure and division, biomolecules, genetics, plant physiology, human physiology, diversity, reproduction, ecology and biotechnology. Questions often ask for a mechanism or a ratio rather than a definition, so link each fact to a process.
Core rules to hold
- Cell cycle: DNA replicates in S phase; meiosis I separates homologous chromosomes, meiosis II separates sister chromatids. Crossing over occurs in prophase I.
- Mendelian genetics: independent assortment applies to genes on different chromosomes; a test cross reveals an unknown genotype.
- Photosynthesis: light reactions split water, produce ATP and NADPH; the Calvin cycle fixes CO2 using RuBisCO.
- Human physiology: filtration occurs at the glomerulus; most reabsorption in the proximal tubule; ADH increases collecting-duct water permeability.
- Ecology: energy transfer between trophic levels is about 10%; pyramids of energy are always upright.
Worked example
Cross AaBb × AaBb with independent assortment. For each gene, A_ = 3/4 and aa = 1/4. The A_B_ phenotype is 3/4 × 3/4 = 9/16. The aabb phenotype is 1/4 × 1/4 = 1/16. The full phenotypic ratio is 9:3:3:1. If the genes were linked, the recombinant classes would be fewer than 3/16 each.
Traps
Do not confuse mitosis with meiosis when counting chromosome number; do not say meiosis produces identical cells. In osmosis, water moves toward higher solute concentration, not toward higher water potential. Learn the source of each hormone (insulin from beta cells, glucagon from alpha cells). In ecology, a pyramid of numbers can be inverted; a pyramid of energy cannot.