Life Science
The Life Science domain tests cells, heredity, evolution, classification, ecosystems, and human body systems. Items emphasize structure-function relationships, energy flow, reproduction, and evidence for natural selection.
Core rules and concepts
Cell theory states that all living things are made of cells, cells are the basic unit of life, and new cells arise from existing cells. Prokaryotes lack a nucleus; eukaryotes have membrane-bound organelles. Mitochondria release energy from food, chloroplasts carry out photosynthesis, ribosomes build proteins, and the cell membrane regulates transport. Diffusion and osmosis move substances down concentration gradients; active transport requires energy.
Photosynthesis stores energy: carbon dioxide + water + light → glucose + oxygen. Cellular respiration releases energy: glucose + oxygen → carbon dioxide + water + ATP. The two processes are complementary. DNA carries hereditary information; genes are DNA segments; alleles are alternative versions. Genotype is genetic makeup; phenotype is observable trait. Mitosis produces identical cells for growth and repair; meiosis produces gametes with half the chromosome number and creates variation.
Natural selection requires variation, heritability, and differential reproductive success. Fossils, homologous structures, embryology, and DNA similarities provide evidence for evolution. In ecosystems, energy flows through food webs and is lost as heat at each trophic level; matter cycles. Homeostasis maintains stable internal conditions.
Worked example
Cross two heterozygous tall pea plants (Tt × Tt). Each parent can donate T or t. The Punnett square gives TT, Tt, Tt, tt. Genotypes: 1 TT : 2 Tt : 1 tt. Phenotypes: 3 tall : 1 short, because T is dominant and masks t in heterozygotes. The probability of a short offspring is 1/4, or 25%. This assumes independent assortment and equal fertilization.
Traps that cost marks
Do not confuse mitosis with meiosis or photosynthesis with respiration. Dominant does not mean more common; it means expressed when present. Energy is lost as heat along food chains, so food webs rarely have many trophic levels. Correlation in field data does not prove causation.