Biology
Biology
Biology covers cell structure and function, enzymes, photosynthesis and respiration, genetics, homeostasis, and ecology. Questions often ask you to predict a result and justify it with a mechanism, not just name a term.
Core rules
- Membranes are selectively permeable; movement depends on concentration gradient and transport protein availability.
- Enzymes lower activation energy; activity depends on temperature, pH, substrate concentration, and inhibitors.
- Photosynthesis converts light energy to chemical energy; respiration transfers energy to ATP.
- DNA replication is semi-conservative; transcription makes RNA; translation uses mRNA codons.
- In meiosis, homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II.
- Energy flows through ecosystems; nutrients cycle.
Worked example
In a monohybrid cross Aa×Aa, assuming complete dominance and independent assortment, gametes are A and a at equal frequency. The Punnett square gives AA:Aa:aa=1:2:1. Phenotypes are dominant:recessive=3:1, so the probability of a recessive phenotype is 1/4; the probability of a heterozygote is 1/2. If the question asks for a homozygous dominant offspring, that is 1/4. State the dominance assumption; without it the phenotype ratio is not defined.
Traps that cost marks
- Confusing genotype frequency with phenotype frequency.
- Applying 9:3:3:1 to linked genes without recombination data.
- Saying enzymes are “used up” in reactions.
- Mixing up meiosis I and meiosis II separation events.
- Treating energy as cycling in an ecosystem.