SPM Biology cheat sheet

Photosynthesis summary: 6CO2 + 6H2O → C6H12O6 + 6O2, driven by light energy trapped by chlorophyll in chloroplast grana; the light-dependent stage occurs on thylakoid membranes and the light-independent (Calvin) stage in the stroma.

Chlorophyll absorbs mainly red and blue-violet light and reflects green; therefore green light is the least effective wavelength for photosynthesis.

Limiting factors of photosynthesis are light intensity, CO2 concentration and temperature; raising a factor that is not limiting does not increase the rate.

Transpiration pull: water evaporates from mesophyll cell surfaces into air spaces, diffuses out through stomata, and cohesion between water molecules plus adhesion to xylem walls pulls a continuous column up the stem.

Xylem vessels are dead, hollow, lignified tubes transporting water and mineral ions upward; phloem sieve tubes are living cells with companion cells transporting sucrose and amino acids bidirectionally.

Stomata open when guard cells take up water and become turgid (inner walls thicker, so cells bow outward); they close when guard cells lose water and become flaccid, reducing water loss.

Translocation moves assimilates from source (photosynthesising leaf) to sink (root, fruit, growing shoot) using energy from companion cells; it is independent of transpiration.

Plant hormones: auxin promotes cell elongation and causes phototropism (accumulates on the shaded side); gibberellin promotes stem elongation and seed germination; abscisic acid closes stomata and induces seed dormancy.

Mineral deficiency rules: nitrate → stunted growth and yellowing (needed for amino acids and proteins); magnesium → yellowing between veins (needed for chlorophyll); phosphate → poor root growth.

Prokaryotic cells lack a nucleus and membrane-bound organelles and have 70S ribosomes; eukaryotic cells have a true nucleus, membrane-bound organelles and 80S ribosomes.

Plant cells have a cellulose cell wall, chloroplasts and a large permanent vacuole; animal cells have none of these but may have centrioles and glycogen granules.

Magnification = image size ÷ actual size; convert all lengths to the same unit before dividing (1 mm = 1000 µm).

Surface area to volume ratio decreases as a cell or organism increases in size; small cells and flattened shapes exchange substances faster by diffusion.

Diffusion moves particles down a concentration gradient passively; osmosis is diffusion of water across a partially permeable membrane from dilute to concentrated solution; active transport moves substances against a gradient using ATP.

Enzymes are biological catalysts that lower activation energy; they are specific because the substrate fits the active site, and they are denatured (active site changed) by high temperature or extreme pH.

Mitosis produces two genetically identical diploid cells for growth and repair; meiosis produces four genetically different haploid gametes through two divisions with crossing over and independent assortment.

Levels of organisation: cell → tissue → organ → organ system → organism; cells specialise by developing specific structures, e.g. root hair cells have a long extension for absorption.

Human digestion: amylase (mouth, pancreas) digests starch to maltose; protease (stomach, pancreas) digests protein to amino acids; lipase (pancreas, small intestine) digests fat to fatty acids and glycerol; bile emulsifies fats and neutralises stomach acid.

Alveoli maximise gas exchange through a large total surface area, thin one-cell walls, a moist surface, and a dense capillary network maintaining a steep diffusion gradient.

Blood transport: arteries carry blood away from the heart in thick elastic muscular walls under high pressure; veins carry blood back with valves preventing backflow; capillaries are one cell thick for exchange.

The heart's cardiac cycle: atrial systole fills ventricles, ventricular systole pumps blood to lungs and body, diastole relaxes chambers; the left ventricle has the thickest wall because it pumps blood around the whole body.

Neurone function: a stimulus creates a nerve impulse travelling along the axon; at a synapse a neurotransmitter diffuses across the gap to the next neurone, so impulses travel in one direction only.

Homeostasis rules: negative feedback reverses a change; insulin lowers blood glucose by converting glucose to glycogen in the liver, glucagon raises it, and ADH increases water reabsorption in kidney tubules.

Thermoregulation: sweating and vasodilation increase heat loss; shivering and vasoconstriction reduce heat loss; the hypothalamus coordinates these responses.

Excretion and kidney: the glomerulus filters small molecules (water, glucose, ions, urea) under pressure; useful substances are reabsorbed by the tubules, and urine contains urea, excess ions and water.

Monohybrid cross rules: a cross of two heterozygotes (Tt × Tt) gives a 3:1 phenotype ratio and 1:2:1 genotype ratio; a test cross with a homozygous recessive reveals an unknown genotype.

Codominance and incomplete dominance mean both alleles are expressed or a blended phenotype appears, so heterozygotes are not identical to either homozygote.

Sex determination in humans: XX female, XY male; the father's sperm determines the sex of the offspring.

Mutations are random changes in DNA base sequence; substitution may change one amino acid, while insertion or deletion causes a frameshift affecting all subsequent codons.

Genetic engineering steps: identify the desired gene, cut it out with a restriction enzyme, insert it into a vector (e.g. plasmid) using DNA ligase, transfer into a host cell, then clone and select successful cells.

Gel electrophoresis separates DNA fragments by size; smaller fragments travel further through the gel, and banding patterns are used in DNA profiling.

Pedigree decision rule: a trait appearing in a child of unaffected parents indicates a recessive allele; a trait in every generation affecting both sexes suggests dominant inheritance.

Selective breeding crosses individuals with desired characteristics over generations, but reduces genetic variation and can concentrate inherited defects.

Food chain rules: arrows show the direction of energy flow; producers occupy the first trophic level; decomposers are not part of the grazing food chain.

Energy transfer between trophic levels is about 10%; the rest is lost as heat from respiration, in uneaten parts, and in excreted waste, so food chains rarely exceed four or five links.

Pyramids of numbers can be inverted (e.g. one tree supporting many insects), but pyramids of energy are always upright.

The carbon cycle exchanges carbon between atmosphere, organisms, soil and oceans through photosynthesis, respiration, feeding, decomposition and combustion of fossil fuels.

Eutrophication sequence: excess nitrate and phosphate fertiliser leaches into water → algal bloom → algae die → decomposers multiply and use up oxygen → fish and other aerobic organisms die.

Greenhouse effect: carbon dioxide and methane absorb and re-emit infrared radiation, trapping heat; rising concentrations raise global temperature, melt ice and raise sea levels.

Biodiversity conservation measures include protected areas, seed banks, captive breeding, and corridors linking habitats; habitat destruction is the main cause of species extinction.

Indicator species and rules: lichens indicate clean air, mayfly larvae indicate clean oxygen-rich water, and rat-tailed maggots indicate polluted low-oxygen water.

Sustainability principle: use renewable resources at or below their replacement rate, limit pollution, and conserve finite resources such as fossil fuels and minerals.