All biological molecules are built from monomers joined by condensation reactions, which release water. Hydrolysis reactions add water back in to break bonds. This applies to carbohydrates, proteins and lipids alike.
Monosaccharides like glucose (C6H12O6) join via glycosidic bonds. Alpha-glucose forms starch and glycogen; beta-glucose forms cellulose.
Triglycerides = 1 glycerol + 3 fatty acids joined by ester bonds via condensation. Phospholipids replace one fatty acid with a phosphate group, giving a hydrophilic head and hydrophobic tails, which is exactly why they form bilayers in membranes. Saturated fatty acids have no C=C double bonds and pack tightly (solid at room temp); unsaturated fatty acids have double bonds causing kinks that lower melting point.
Amino acids share the general structure H2N-CHR-COOH, differing only in the R group. Peptide bonds form between the amine and carboxyl groups by condensation, releasing water. Levels of structure:
Common mistake: saying protein structure is 'held together by peptide bonds' at the tertiary/quaternary level — peptide bonds only form primary structure; the rest are held by weaker interactions.
Water is polar (due to unequal sharing of electrons between O and H) so it forms hydrogen bonds, giving it high specific heat capacity, high latent heat of vaporisation, cohesion and surface tension, and making it a good solvent for polar/ionic substances. Key ions to know: hydrogen ions (pH), iron ions (haemoglobin), sodium ions (co-transport).
Eukaryotic cells (animal, plant, fungal) have a nucleus and membrane-bound organelles; prokaryotic cells (bacteria) do not. Know the ultrastructure: nucleus (nucleolus makes rRNA, nuclear envelope has pores), mitochondria (double membrane, cristae, matrix, site of aerobic respiration), rough ER (ribosomes, protein transport), smooth ER (lipid synthesis), Golgi apparatus (modifies and packages proteins into vesicles), lysosomes (digestive enzymes), ribosomes (80S in eukaryotes, 70S in prokaryotes and mitochondria/chloroplasts - a key exam fact for endosymbiotic theory).
Magnification = image size divided by actual size. Always convert units to the same scale first (usually micrometres, 1 mm = 1000 micrometres). Resolution is the minimum distance two points can be apart and still be seen as separate; it is NOT the same as magnification. Electron microscopes have far higher resolution than light microscopes because electrons have a shorter wavelength. TEM gives 2D internal detail from thin sections; SEM gives 3D surface detail. Common mistake: students confuse magnification (how much bigger) with resolution (how clear/detailed).
Mitosis produces two genetically identical diploid daughter cells for growth and repair; phases are prophase, metaphase, anaphase, telophase (PMAT). Meiosis produces four genetically different haploid gametes through two divisions, involving crossing over and independent assortment for genetic variation.
Pathogens have antigens on their surface that the immune system recognises as non-self. Phagocytes (e.g. macrophages) engulf pathogens by phagocytosis and present antigens on their surface. T-helper cells bind to these antigens and release cytokines to activate B-cells and cytotoxic T-cells. B-cells undergo clonal selection and differentiate into plasma cells, which secrete specific antibodies, and memory cells, which give a faster, stronger secondary immune response on re-infection. Antibodies are Y-shaped proteins with a variable region that binds specifically to one antigen (complementary shape), causing agglutination or acting as opsonins.
Vaccination introduces antigens (weakened, dead, or subunit) to trigger primary response and memory cell formation without causing disease; herd immunity protects unvaccinated individuals when enough of the population is immune. HIV is a retrovirus that infects and destroys T-helper cells, weakening the whole immune response and eventually causing AIDS. Common mistake: students say antibodies 'kill' pathogens directly - they mark them for destruction or neutralise them, they do not lyse cells themselves in most cases.
As organisms get bigger, their surface area to volume (SA:V) ratio falls. A single-celled amoeba can exchange gases across its own membrane, but a mammal cannot, so specialised exchange surfaces evolved. All good exchange surfaces share: a large surface area, a thin barrier (short diffusion pathway), a steep concentration gradient maintained by blood/ventilation flow, and (in animals) selective permeability.
Humans ventilate using the intercostal muscles and diaphragm. On inspiration the diaphragm contracts and flattens, external intercostals contract, the ribcage moves up and out, thoracic volume increases, pressure falls below atmospheric, and air moves in. Expiration at rest is passive: the diaphragm relaxes and domes upward, volume falls, pressure rises above atmospheric.
Gas exchange happens across the alveolar epithelium. Alveoli have thin (one-cell-thick) squamous epithelium, a huge combined surface area (around 70 m2), and are surrounded by a dense capillary network, keeping the diffusion pathway under 1 micrometre. Alveoli also produce surfactant, which reduces surface tension and stops them collapsing.
Do not say 'oxygen is pumped into the blood' - it moves entirely by diffusion down a concentration gradient maintained by continuous blood flow and ventilation.
Mammals have a double closed circulatory system: the pulmonary circuit (heart to lungs and back) and the systemic circuit (heart to body and back). Double circulation keeps oxygenated and deoxygenated blood separate and lets blood be re-pressurised after the low-pressure lungs.
The cardiac cycle has three phases: atrial systole, ventricular systole, and diastole. The SAN (sinoatrial node) in the right atrium wall acts as the pacemaker, initiating a wave of excitation. This is delayed briefly at the AVN (atrioventricular node) so atria finish contracting before ventricles start, then passed down the Bundle of His and Purkyne fibres to the ventricle apex, causing contraction from the bottom up.
Arteries have thick muscular and elastic walls to withstand and smooth high pressure. Veins have thinner walls, a wider lumen, and valves to prevent backflow at low pressure. Capillaries are one cell thick with pores for exchange.
Xylem transports water and mineral ions upwards via the transpiration stream, powered by cohesion-tension: water evaporates from leaf mesophyll (transpiration), pulling a continuous water column up through dead, lignified xylem vessels via cohesion (hydrogen bonding between water molecules) and adhesion to vessel walls.
Phloem transports sugars (mainly sucrose) both up and down the plant via translocation, using the mass flow hypothesis: sucrose is actively loaded into phloem sieve tubes at the source (companion cells use ATP for active transport), lowering water potential and drawing water in by osmosis, raising hydrostatic pressure; at the sink, sucrose is removed, water follows by osmosis, and pressure falls, creating a mass flow from high to low pressure.
Xylem transport is passive (transpiration pull); phloem translocation requires active loading, so a metabolic poison stops phloem transport but not xylem transport.
Meiosis produces four genetically different haploid cells from one diploid cell, halving the chromosome number from 2n to n. Two rounds of division follow one round of DNA replication.
A mutation is a change in the base sequence of DNA. Types include substitution, insertion and deletion. Insertions and deletions usually cause a frameshift, altering every codon downstream, so they tend to be more damaging than substitutions. Substitutions may be silent (due to the degenerate genetic code, several codons code for the same amino acid), missense (one amino acid changes, e.g. sickle-cell anaemia from a single base substitution in the beta-globin gene) or nonsense (creates a premature stop codon, truncating the protein).
Changes in chromosome number arise from non-disjunction, where chromosomes fail to separate correctly at anaphase of meiosis. This produces gametes with an extra or missing chromosome. Common mistake: students confuse non-disjunction (whole chromosome error) with a gene mutation (base-sequence error) - keep them separate in exam answers.
Genetic diversity is the number of different alleles of genes in a population. It provides the raw material for natural selection. Directional selection shifts the mean value of a characteristic (e.g. antibiotic resistance in bacteria); stabilising selection favours the mean and reduces variation (e.g. human birth weight).
The Hardy-Weinberg equation predicts allele and genotype frequencies in a non-evolving population: p + q = 1 and p^2 + 2pq + q^2 = 1, where p is the frequency of the dominant allele and q the recessive allele. It only applies if there is no selection, mutation, migration or genetic drift, mating is random, and the population is large.
A species is a group of organisms with similar morphology, physiology and behaviour that can interbreed to produce fertile offspring. Courtship behaviour ensures individuals recognise members of their own species, find a mate capable of breeding, and stimulate the release of gametes.
Photosynthesis happens in the chloroplast and has two linked stages.