Life Science
Domain 3: Life Science
This domain tests cell structure and function, energy transformations, heredity, evolution, ecology, and human body systems. Questions often use diagrams, data tables, and scenarios to ask for mechanisms and evidence-based conclusions.
Cells and energy
Prokaryotic cells lack a membrane-bound nucleus; eukaryotic cells have one. Mitochondria carry out cellular respiration; chloroplasts carry out photosynthesis. Photosynthesis stores solar energy in glucose and releases oxygen; respiration releases energy from glucose and produces carbon dioxide and water. The cell membrane controls transport; diffusion moves substances from high to low concentration without energy, while active transport requires ATP.
Genetics
DNA codes for proteins. Genes occur in alleles; genotype is the allele combination, phenotype is the observable trait. In complete dominance, the dominant allele masks the recessive allele. Meiosis produces haploid gametes with genetic variation through crossing over and independent assortment; mitosis produces genetically identical body cells. Punnett squares predict offspring probabilities, not guaranteed outcomes.
Ecology and evolution
Energy flows through food chains; only about 10% transfers between trophic levels. Matter cycles through ecosystems. Natural selection acts on heritable variation: organisms with traits that improve survival and reproduction become more common over generations. It is not goal-directed and does not occur because an organism "needs" a trait.
Worked example
Two heterozygous pea plants (Tt) are crossed, where T is tall and t is short. What phenotypes are expected? Reasoning: Each parent forms T and t gametes. The Punnett square gives TT, Tt, Tt, tt. TT and Tt are tall; tt is short. Phenotype ratio is 3 tall : 1 short; genotype ratio is 1 TT : 2 Tt : 1 tt. With small offspring numbers, observed ratios may deviate because each fertilization is a chance event.
Traps that cost marks
Do not confuse mitosis with meiosis. Do not say dominant alleles are always more common; dominance describes expression, not frequency. Do not claim evolution is an individual change; populations evolve. Do not reverse photosynthesis and respiration. Do not call a food web's top predator the most energy-rich level.