Life Science
Key concepts
Life science questions test levels of organization-cell, tissue, organ, organ system, organism, population, community, ecosystem, biosphere-and the processes that sustain life. Know cell structures and functions, including the cell membrane, nucleus, mitochondria, chloroplasts, ribosomes, and cell wall, and distinguish prokaryotic from eukaryotic cells. Photosynthesis stores energy in glucose using sunlight, carbon dioxide, and water; cellular respiration releases energy from glucose, using oxygen in aerobic organisms. Genetics requires fluency with DNA, genes, alleles, dominant and recessive traits, genotype versus phenotype, and Punnett squares. Evolution by natural selection follows from variation, heritability, differential survival and reproduction, and adaptation; it acts on populations, not individuals. Ecosystems involve food webs, energy pyramids, carrying capacity, and cycling of matter; energy flows and is lost as heat, while matter cycles.
Traps: mitosis produces genetically identical body cells for growth and repair; meiosis produces genetically varied gametes with half the chromosome number. Photosynthesis and respiration are not opposites in every detail-both use electron transport, but only photosynthesis captures light energy. “Dominant” does not mean common or stronger; it means expressed when at least one allele is present. Natural selection is not “organisms trying to adapt.”
Worked example: Cross two heterozygous pea plants (Bb × Bb). Each parent gives B or b with equal probability. A Punnett square yields BB:Bb:bB:bb = 1:2:1 genotypes and 3:1 dominant:recessive phenotypes. Rule of thumb: multiply probabilities for independent events; add probabilities for mutually exclusive outcomes.
Rule of thumb: follow energy with arrows in food webs, but remember only about 10% typically moves to the next trophic level.