Unity and Diversity: IB DP Biology Theme A Essentials

Oxygen is more electronegative than hydrogen, so each water molecule carries a permanent dipole — $\delta^-$ on oxygen, $\delta^+$ on the hydrogens. This lets molecules hydrogen-bond to each other, and almost every unusual property of water traces back to that one fact. Cohesion (water molecules attracting each other) supports the water column in xylem; adhesion (water attracting other polar surfaces) drives capillary action; the dipole also lets water form hydration shells around ions, making it an excellent solvent.

High specific heat capacity means water absorbs a lot of energy before its temperature rises much, buffering cells and water bodies against rapid swings. High latent heat of vaporization means converting liquid to vapour requires breaking all the hydrogen bonds holding a molecule in the liquid, so evaporation removes a large amount of heat — this is exactly why sweat cools your skin. These two formulas, $Q=mc\Delta T$ and $Q=mL$, are tested with real numbers, not just definitions.

Osmosis is the net movement of water across a selectively permeable membrane, from higher to lower water potential ($\Psi$). Water potential is made up of solute potential ($\Psi_S$, always zero or negative) and pressure potential ($\Psi_P$, positive under turgor or negative under tension): $\Psi = \Psi_S + \Psi_P$. Pure water has $\Psi = 0$; everything else is usually negative unless under positive pressure.

Evolution is cumulative, heritable change in a population's characteristics across generations. Natural selection drives it through a specific chain: overproduction of offspring → heritable variation → a struggle for survival under limited resources → differential reproductive success of individuals with advantageous traits → a shift in allele frequency across generations. Evidence includes the fossil record, selective breeding, homologous structures (shared ancestry, e.g. the pentadactyl limb in whales, bats and humans) versus analogous structures (shared function only, e.g. insect and bird wings, from convergent evolution), and molecular sequence comparison.

Speciation happens when reproductive isolation stops gene flow between diverging populations long enough for genetic differences to build up until they can no longer interbreed to produce fertile offspring. Allopatric speciation requires a geographic barrier like a mountain range or river; sympatric speciation occurs without one, for example via polyploidy in plants or disruptive selection within the same range. On a continuously varying trait, selection can be directional (mean shifts), stabilising (extremes removed, variance narrows) or disruptive (middle removed, population splits toward two extremes).