Water is polar, forming hydrogen bonds. This gives it high specific heat capacity (buffers temperature), high latent heat of vaporisation (cooling via sweat), cohesion (surface tension, transpiration pull) and it acts as a good solvent for polar/ionic substances.
Monosaccharides (e.g. glucose, C6H12O6) join by condensation reactions, forming glycosidic bonds and releasing water. Hydrolysis breaks these bonds by adding water.
Triglycerides = 1 glycerol + 3 fatty acids joined by ester bonds via condensation. Phospholipids = 2 fatty acids + phosphate group, giving a hydrophilic head and hydrophobic tails, crucial for membrane bilayers. Saturated fatty acids have no C=C double bonds (solid at room temp); unsaturated have one or more, causing kinks that lower melting point.
Amino acids share the general structure NH2-CHR-COOH, differing only in the R group. Peptide bonds form by condensation between the amine and carboxyl groups. Primary structure = amino acid sequence; secondary = alpha helices/beta pleated sheets (hydrogen bonds); tertiary = 3D folding (ionic, hydrogen, disulfide bonds, hydrophobic interactions); quaternary = multiple polypeptide chains (e.g. haemoglobin's 4 subunits).
Enzymes are globular proteins with a specific tertiary-structure active site (induced fit model). Rate increases with temperature until the optimum (denaturation above this breaks hydrogen/ionic bonds in the active site). Extreme pH also denatures enzymes. Competitive inhibitors resemble the substrate and bind the active site; non-competitive inhibitors bind elsewhere, changing active site shape.
As organisms get bigger, their surface area to volume (SA:V) ratio falls. Diffusion alone (Fick's Law) is too slow over large distances, so big organisms need specialised exchange surfaces plus mass transport systems.
Air enters via the trachea, down bronchi, bronchioles, into ~480 million alveoli. Alveolar walls and capillary walls are each one cell thick (squamous epithelium), giving a diffusion pathway of under 1 micrometre. Gas exchange follows Fick's Law: rate is proportional to (SA x concentration gradient) / diffusion distance. Ventilation uses the diaphragm and intercostal muscles, with pressure changes driven by Boyle's Law.
Don't say alveoli 'absorb' oxygen — they are the site of diffusion, not absorption (that word is for the small intestine). Also remember it is the concentration gradient, not oxygen concentration alone, that drives diffusion rate.
Mammals have a double circulatory system: pulmonary (heart to lungs) and systemic (heart to body). This is needed because a single circuit would lose too much pressure after passing through capillaries. The cardiac cycle has three stages: atrial systole, ventricular systole, diastole, controlled by the SAN (pacemaker), which triggers the AVN, then the Bundle of His and Purkyne fibres.
Haemoglobin's oxygen dissociation curve is S-shaped (sigmoidal) due to cooperative binding. A higher CO2 concentration shifts the curve right (the Bohr effect), meaning haemoglobin releases oxygen more readily to respiring tissues. Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, shifting its curve to the left, so oxygen transfers from mother to fetus across the placenta.
Xylem transports water via the cohesion-tension theory: transpiration pulls water up through cohesion (hydrogen bonding between water molecules) and adhesion to xylem walls, under tension, with no ATP required. Phloem transports sugars (mainly sucrose) via mass flow (translocation), from source (e.g. leaves) to sink (e.g. roots, fruits), which does require active transport (companion cells load sucrose into sieve tubes).
Genes are lengths of DNA that code for polypeptides. Each amino acid is coded by a triplet of DNA bases (a codon on mRNA). The genetic code is degenerate (61 codons for 20 amino acids), non-overlapping and (with rare exceptions) universal across organisms.
A gene mutation is any change to the base sequence of DNA. Types include substitution, deletion, insertion, inversion and duplication. Deletions and insertions usually cause a frame shift, changing every codon downstream of the mutation, so they tend to have a bigger effect than a single substitution. Substitutions can be silent (same amino acid, due to degeneracy), missense (different amino acid) or nonsense (creates a premature stop codon).
Meiosis is the division that produces four genetically different haploid gametes from one diploid cell, halving the chromosome number. Two processes generate variation: independent assortment (chromosomes line up randomly at metaphase I, giving 2^n combinations, where n is the haploid number) and crossing over (homologous chromatids exchange sections at chiasmata during prophase I). Random fertilisation adds further variation by combining any one of millions of possible sperm with any one of millions of possible eggs.
Use the chi-squared test to check if observed genetic ratios (e.g. from a dihybrid cross) differ significantly from expected ratios. Formula: sum of (O-E)^2 / E. Compare your calculated value to the critical value at p=0.05 for the correct degrees of freedom (categories minus 1). If your value exceeds the critical value, reject the null hypothesis; the difference is significant and not due to chance.
Natural selection acts on existing variation created by mutation. Alleles that increase survival and reproduction become more frequent in a population over generations. Speciation happens when populations become reproductively isolated (geographically - allopatric, or by behaviour/habitat/timing - sympatric) and diverge until they can no longer interbreed to produce fertile offspring.
Photosynthesis happens in the chloroplast. Light energy hits chlorophyll in photosystems II and I, embedded in the thylakoid membrane. Photoionisation splits water (photolysis) into protons, electrons and oxygen (the oxygen is a waste product, released through the stomata).
Electrons pass down an electron transport chain, releasing energy used to pump protons into the thylakoid space, building a proton gradient. Protons flow back through ATP synthase (chemiosmosis) making ATP. NADP picks up protons and electrons to form reduced NADP.
This happens in the stroma and does not need light directly, but needs the ATP and reduced NADP made in the light-dependent stage.
Glycolysis happens in the cytoplasm, does not need oxygen, and splits glucose (6C) into two pyruvate (3C) molecules, giving a net gain of 2 ATP and 2 reduced NAD.
The link reaction (in the mitochondrial matrix) converts pyruvate to acetate, releasing CO2 and reducing NAD; acetate combines with coenzyme A.
The Krebs cycle (matrix) combines acetyl CoA with a 4C compound, releasing CO2 and generating reduced NAD, reduced FAD and 1 ATP per turn (it turns twice per glucose).
Oxidative phosphorylation (inner mitochondrial membrane, cristae) uses the electron transport chain: electrons from reduced NAD/FAD release energy to pump protons, creating a gradient used by ATP synthase to make ATP by chemiosmosis. Oxygen is the final electron acceptor, forming water.
Without oxygen, glycolysis still runs but pyruvate cannot enter the link reaction. In animals, pyruvate is reduced to lactate, regenerating NAD so glycolysis can continue (only 2 ATP per glucose overall). In yeast and plants, pyruvate is decarboxylated to ethanal then reduced to ethanol, also regenerating NAD.