Energy flow, ecosystems and the environment
Natural selection and population genetics
Variation may be discontinuous (distinct categories, few genes, e.g. blood group) or continuous (range, many genes plus environment, e.g. height). Natural selection: individuals vary; a selection pressure gives some a survival and reproductive advantage; they pass on their alleles, so allele frequencies change over generations. Stabilising selection favours the average; directional favours one extreme; disruptive favours both extremes. Genetic drift changes allele frequencies by chance, strongest in small populations (founder effect, bottleneck). Hardy–Weinberg: in a large, randomly mating population with no selection, mutation or migration, and , letting you estimate allele and carrier frequencies. Artificial selection is selective breeding by humans.
≈ 50 minLight-dependent reactions
Chloroplasts have thylakoids (stacked into grana) where the light-dependent reactions occur, surrounded by stroma (light-independent). Photosystems contain chlorophyll a and b and accessory pigments (carotenoids) that absorb different wavelengths; the absorption spectrum matches the action spectrum. Non-cyclic photophosphorylation: light excites electrons in chlorophyll (photoionisation) of PSII; they pass along an electron transport chain to PSI, then to NADP, forming reduced NADP; energy pumps H⁺ into the thylakoid space and ATP synthase makes ATP by chemiosmosis. Photolysis of water replaces PSII's electrons: . Cyclic photophosphorylation (PSI only) makes ATP but no reduced NADP or O₂. Pigments are separated by chromatography ().
≈ 50 minSpeciation and evolution
A species (biological concept) is a group of organisms that can interbreed to produce fertile offspring. Speciation requires reproductive isolation, which stops gene flow. Allopatric speciation: a geographical barrier (river, mountain, sea) separates populations; different mutations, selection pressures and genetic drift make their allele frequencies diverge until they can't interbreed. Sympatric speciation happens in the same area through ecological, behavioural or temporal isolation, or polyploidy in plants. Isolating mechanisms may be prezygotic (different breeding seasons, courtship, incompatible genitalia) or postzygotic (hybrid inviability or sterility, e.g. mules). Molecular evidence (DNA base sequences, mitochondrial DNA, amino acid sequences of proteins such as cytochrome c) shows relationships: fewer differences mean more recent common ancestry.
≈ 45 minCalvin cycle and limiting factors
The Calvin cycle (stroma) uses ATP and reduced NADP from the light-dependent stage. Fixation: RuBisCO combines CO₂ with ribulose bisphosphate (RuBP, 5C) to form two glycerate 3-phosphate (GP, 3C). Reduction: GP → triose phosphate (TP) using ATP and reduced NADP. Regeneration: most TP (5 of every 6) is used to remake RuBP using ATP. TP is used to make glucose, sucrose, starch, cellulose, lipids and amino acids. Six turns fix six CO₂ to make one hexose. Limiting factors: light intensity, CO₂ concentration and temperature; the factor in shortest supply limits the rate. In the dark, GP accumulates and RuBP falls; with low CO₂, RuBP accumulates and GP falls. C4 plants (maize) fix CO₂ in mesophyll cells and release it to bundle-sheath cells, keeping RuBisCO away from O₂.
≈ 50 minEcosystems, energy and nutrient cycles
Producers capture light energy: gross primary production (GPP) is the total chemical energy fixed; net primary production (respiratory losses) is available to consumers. For consumers, (ingested minus faeces/urine and respiration). Only about 10% passes between trophic levels: energy is lost as heat in respiration, in uneaten parts and in faeces. Farming increases efficiency by reducing respiratory losses (warm, restricted animals) and removing competitors and pests. Nitrogen cycle: nitrogen fixation (Rhizobium in root nodules; lightning), ammonification (saprobionts), nitrification (NH₄⁺ → NO₂⁻ → NO₃⁻, aerobic bacteria), denitrification (anaerobic bacteria → N₂). Phosphorus cycles through rocks, soil and organisms; mycorrhizae aid uptake. Fertilisers that leach cause eutrophication: algal bloom → light blocked → plants die → decomposers use O₂ → fish die. Succession: pioneer species → climax community. Rising CO₂ and methane drive climate change.
≈ 50 min