CSEC Human and Social Biology cheat sheet

Photosynthesis: 6CO2 + 6H2O → C6H12O6 + 6O2, driven by light energy trapped by chlorophyll in chloroplasts; light-dependent reactions occur in the thylakoid membranes, the Calvin cycle in the stroma.

Aerobic respiration: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy; glycolysis in cytoplasm, Krebs cycle and electron transport in mitochondria; ~38 ATP per glucose in theory.

Anaerobic respiration in humans: glucose → lactic acid + small energy yield (about 2 ATP); in yeast: glucose → ethanol + CO2 + energy; both release far less energy than aerobic respiration.

Enzymes are protein catalysts; activity peaks at optimum temperature and pH and is destroyed (denatured) above the optimum because the active site changes shape; reaction rate doubles roughly per 10 °C rise until denaturation.

Digestion: amylase converts starch to maltose (mouth, pancreas); protease converts proteins to amino acids (stomach pepsin, pancreas); lipase converts fats to fatty acids and glycerol (pancreas, small intestine).

Bile, made in the liver and stored in the gallbladder, emulsifies fats into small droplets, increasing surface area for lipase, and neutralises acidic chyme entering the duodenum.

Absorption occurs in the small intestine: glucose and amino acids pass into blood capillaries of villi; fatty acids and glycerol are absorbed into lacteals as chyle; villi increase surface area.

Metabolism includes assimilation (building absorbed nutrients into body substances) and excretion (removal of metabolic waste such as CO2 and urea); egestion of undigested food is not excretion.

Dietary deficiency rules: lack of vitamin C → scurvy; vitamin D or calcium → rickets; iron → anaemia; protein (kwashiorkor) or energy (marasmus) deficiency in children.

Transport in plants: xylem carries water and mineral ions upward (transpiration pull, cohesion-tension); phloem carries sucrose and amino acids bidirectionally in translocation.

Transpiration rate rises with higher temperature, wind, lower humidity and greater light intensity (stomata open); it is reduced by high humidity and closed stomata.

Blood flow: arteries carry blood away from the heart under high pressure, thick muscular walls; veins carry blood to the heart, thin walls with valves; capillaries are one cell thick for exchange.

Mammalian circulation is double: right side pumps deoxygenated blood to lungs (pulmonary), left side pumps oxygenated blood to the body (systemic); the septum separates the two sides.

Breathing mechanics: inhalation - diaphragm contracts and flattens, intercostal muscles raise ribs, thoracic volume increases, pressure falls, air enters; exhalation is the reverse.

Gas exchange in alveoli depends on a thin moist surface, large surface area, rich blood supply and a concentration gradient maintained by ventilation and blood flow.

Nervous coordination: receptor → sensory neurone → relay neurone in spinal cord → motor neurone → effector; a reflex arc bypasses conscious brain processing for speed.

Homeostasis uses negative feedback: a rise in blood glucose triggers insulin release, increasing glucose uptake and glycogen storage; a fall triggers glucagon release, raising blood glucose.

Osmoregulation: the kidney nephron filters blood at the glomerulus, reabsorbs useful glucose, amino acids and water, and excretes urea; ADH increases water reabsorption in collecting ducts.

Genotype = allele combination; phenotype = observable characteristic; homozygous = two identical alleles; heterozygous = two different alleles; dominant allele expressed when present.

Monohybrid cross: use a Punnett square of parental gametes; phenotypic ratio for heterozygous × heterozygous (Tt × Tt) is 3:1 dominant to recessive, genotypic ratio 1:2:1.

Test cross: crossing an unknown dominant phenotype with a homozygous recessive individual reveals whether the unknown is homozygous or heterozygous by the offspring ratio.

DNA structure: double helix of two strands held by hydrogen bonds between complementary base pairs - adenine with thymine, guanine with cytosine; sugar-phosphate backbone.

Protein synthesis: DNA is transcribed to messenger RNA in the nucleus, then translated at ribosomes where transfer RNA brings amino acids in the order coded by mRNA codons.

Human sex determination: females are XX, males are XY; the father's gamete determines sex because the mother contributes only X; sex-linked genes sit on the X chromosome.

Mutation is a change in DNA base sequence; it may be neutral, harmful (sickle-cell anaemia, cystic fibrosis) or occasionally beneficial; mutagenic agents include radiation and some chemicals.

Variation: continuous (height, mass - many genes plus environment) versus discontinuous (blood group, tongue rolling - single genes); natural selection acts on heritable variation.

Pathogen types: bacteria (cholera, tuberculosis), viruses (influenza, HIV), protozoa (malaria, caused by Plasmodium), fungi (athlete's foot); viruses cannot be treated with antibiotics.

Transmission routes: water and food (cholera, typhoid), airborne droplets (influenza, TB), vectors (Anopheles mosquito spreads malaria), direct contact and body fluids (HIV).

Control measures: chlorinated water and sanitation for cholera; mosquito nets, draining stagnant water and insecticides for malaria; vaccination, isolation and ventilation for airborne disease.

Immunity: white blood cells phagocytose pathogens or produce antibodies specific to antigens; memory cells give long-term immunity after infection or vaccination; breast milk and placenta pass antibodies.

HIV attacks helper T-lymphocytes, weakening the immune system and leading to AIDS and opportunistic infections such as TB; it is spread by unprotected sex, infected blood and from mother to child.

Drug resistance arises because antibiotics kill susceptible bacteria, leaving resistant mutants that multiply; therefore full courses are completed and antibiotics are not used for viral infections.

Pollution effects: sulphur dioxide causes acid rain that acidifies lakes and leaches soil nutrients; excess nitrates cause eutrophication - algal bloom, decomposition, oxygen depletion, fish death.

Conservation rules: habitat protection and corridors preserve biodiversity; deforestation raises atmospheric CO2, reduces transpiration and causes soil erosion and flooding.

Greenhouse gases (CO2, methane, nitrous oxide) trap outgoing infrared radiation; rising global temperature melts ice and raises sea level, and shifts species ranges.