Continuity and Change

Water potential ($\psi$) measures water's tendency to move out of a solution: pure water is $0$, and solutes always make it more negative. Water moves from high $\psi$ to low $\psi$ across a partially permeable membrane — memorise this single rule and most exam questions solve themselves. Total water potential is the sum of solute potential (${\psi }_s$, always negative) and pressure potential (${\psi }_p$, positive in a turgid walled cell). At incipient plasmolysis, exactly 50% of cells show plasmolysis, meaning the external solution's $\psi$ equals the cell's $\psi$ at that point.

DNA replication is semi-conservative: each new double helix keeps one original template strand and gains one newly synthesised strand. Helicase unwinds the double helix at the replication fork, and DNA polymerase adds nucleotides only in the 5' to 3' direction — forcing continuous synthesis on the leading strand and fragmented synthesis (Okazaki fragments) on the lagging strand. Short RNA primers give DNA polymerase a starting point, and DNA ligase later joins the Okazaki fragments together.

Genotype (the allele combination) determines phenotype through dominance relationships, and Punnett squares let you predict offspring ratios from known parental genotypes. A cross between two heterozygotes (Aa × Aa) gives a 3:1 phenotype ratio, while a test cross with a homozygous recessive (Aa × aa) gives 1:1 if the tested parent is heterozygous. Sex-linked genes on the X chromosome behave differently in males (XY) and females (XX), so recessive X-linked conditions show up more often in males.

Sexual reproduction combines gametes from two parents via meiosis and fertilisation, producing genetically variable offspring through independent assortment, crossing over, and random fertilisation. Asexual reproduction copies a single parent's genome using mitosis alone, giving identical offspring fast but no new variation for natural selection to act on. Fertilisation restores the diploid chromosome number after haploid gametes fuse.

Greenhouse gases — CO2, CH4, N2O, and water vapour — absorb and re-radiate long-wave infrared radiation, warming the lower atmosphere. Human activity such as fossil fuel combustion, deforestation, and agriculture has pushed these gas concentrations beyond natural background levels, driving the enhanced greenhouse effect. Evidence includes ice core CO2/temperature records, glacial retreat, rising global temperatures, and ocean acidification, with biological consequences like range shifts, altered phenology, and coral bleaching.