Life Science
The Life Science domain tests cell structure and function, heredity, evolution, ecology, and the flow of energy and matter through living systems. Expect questions that ask you to interpret a cross, a food web, or a feedback loop.
Core rules and concepts
- Cell theory: all living things are made of cells; cells arise from existing cells. Prokaryotes lack a membrane-bound nucleus; eukaryotes have organelles, including mitochondria and chloroplasts.
- Photosynthesis stores energy in glucose; cellular respiration releases it. Both involve electron transfer and ATP.
- DNA is transcribed to mRNA, which is translated to protein. Mitosis produces genetically identical diploid cells; meiosis produces haploid gametes with genetic variation.
- In Mendelian genetics, alleles segregate, and assort independently when on different chromosomes. Dominant alleles mask recessive alleles in heterozygotes.
- Natural selection acts on heritable variation: individuals with traits that improve survival and reproduction leave more offspring.
- Energy flows through trophic levels and is lost as heat at each step; matter cycles.
Worked example
In pea plants, tall (T) is dominant to short (t). Cross two heterozygotes, Tt × Tt. Each parent produces T and t gametes in equal proportion. A Punnett square gives TT, Tt, Tt, tt: genotype ratio 1 TT : 2 Tt : 1 tt. Phenotypes are 3 tall : 1 short, so the probability of a short offspring is 1/4, or 25%. The dominant phenotype is not "more common" in general; it is simply expressed in heterozygotes.
Traps that cost marks
- Describing acquired traits as inherited (Lamarckian evolution).
- Confusing mitosis with meiosis or diploid with haploid.
- Assuming dominant alleles are always more frequent in a population.
- Forgetting that energy is lost at each trophic level, while matter cycles.
- Treating natural selection as goal-directed or as "trying" to adapt.