Biology: Cells, Inheritance, and Energy Flow
EJU Biology covers cell structure and function, metabolism, genetics and reproduction, homeostasis, immunity, ecology, and evolution. Questions often combine a pathway diagram with a ratio or a feedback prediction.
Core rules and concepts
- Cells: prokaryotes lack a nucleus and membrane-bound organelles; transport may be passive or active.
- Metabolism: photosynthesis has light reactions and the Calvin cycle; respiration runs glycolysis, the Krebs cycle, and oxidative phosphorylation.
- Genetics: Mendel's law of independent assortment applies to unlinked genes; meiosis produces haploid gametes with recombination; DNA replication is semiconservative.
- Homeostasis: negative feedback reverses a deviation; the kidney regulates water and solute balance.
- Ecology: energy decreases along a food chain; gross primary production minus respiration gives net primary production.
- Evolution: natural selection changes allele frequencies; genetic drift is random.
Worked example
Cross AaBb × AaBb with independent assortment. Find the probability of offspring showing both dominant traits. For Aa × Aa, P(A_)=3/4. For Bb × Bb, P(B_)=3/4. Independent events multiply: 3/4×3/4=9/16. The 9:3:3:1 ratio is the phenotype ratio when both parents are double heterozygotes and traits are unlinked.
Traps that cost marks
Confusing mitosis with meiosis; counting recombinant gametes incorrectly; reversing dominant and recessive phenotypes; mixing gross and net primary production; assuming negative feedback increases the original deviation; treating linked genes as independent.