Life Science
Life Science covers the chemistry and structure of cells, energy transformations, heredity and molecular genetics, evolution by natural selection, and ecology. Expect items that ask you to trace information from DNA to trait, predict offspring ratios, or explain energy and matter movement through an ecosystem.
Cells and energy
- Cell theory: all organisms are made of cells, cells are the basic unit of life, and cells arise from existing cells. Prokaryotes lack a membrane-bound nucleus; eukaryotes have organelles.
- Photosynthesis (chloroplasts): 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. Cellular respiration (mitochondria) reverses it, releasing usable energy.
- Enzymes are specific catalysts that lower activation energy and denature at extreme temperature or pH.
Heredity
- DNA → RNA → protein: transcription occurs in the nucleus, translation at the ribosome.
- Mitosis yields two genetically identical diploid cells; meiosis yields four haploid gametes after crossing over and independent assortment.
- Genotype is the allele combination; phenotype is the observable trait. A dominant allele is expressed in the heterozygote.
Evolution and ecology
- Natural selection requires heritable variation and differential reproductive success.
- Energy flows one way through trophic levels, with roughly 10% transferred to the next level; matter cycles.
- Population size is limited by biotic and abiotic factors and carrying capacity.
Worked example
Cross two heterozygous tall pea plants (Tt × Tt). Each parent passes one allele at random, giving TT, Tt, Tt, tt. The genotypic ratio is 1 TT : 2 Tt : 1 tt, so 75% of offspring are tall and 25% short (3:1), and half are heterozygous. The short phenotype appears only in tt because T masks t.
Traps that cost marks
- Believing "dominant" means more common in the population.
- Confusing the products of mitosis and meiosis.
- Attributing traits acquired in a lifetime to inheritance.
- Reversing the 10% energy rule or treating energy as recycled.