BGCSE Human & Social Biology cheat sheet

Seven life processes: movement, respiration, sensitivity, growth, reproduction, excretion, nutrition (MRS GREN).

Plant cells have cellulose cell walls, chloroplasts and a large permanent vacuole; animal cells lack all three.

Both cell types share a nucleus, cytoplasm, cell membrane, mitochondria and ribosomes.

Magnification = image size ÷ actual size; convert units so both are in the same scale (1 mm = 1000 µm).

Diffusion moves particles from high to low concentration down a gradient without energy input.

Osmosis is water movement across a partially permeable membrane from dilute to concentrated solution.

Active transport moves substances against a concentration gradient and requires energy from respiration.

Enzymes are biological catalysts; each has an active site specific to its substrate, and high temperature or wrong pH denatures it.

Balanced diet needs carbohydrates, proteins, fats, vitamins, minerals, water and fibre (roughage).

Carbohydrates and fats are energy sources; proteins supply amino acids for growth and repair.

Vitamin C deficiency causes scurvy; vitamin D or calcium deficiency causes rickets; iron deficiency causes anaemia.

Physical digestion (chewing, churning, emulsification) increases surface area for enzyme action.

Amylase digests starch to maltose; protease digests protein to amino acids; lipase digests fat to fatty acids and glycerol.

Stomach acid (hydrochloric acid) kills bacteria and gives the low pH pepsin needs.

Bile from the liver is stored in the gall bladder; it emulsifies fats and neutralises stomach acid.

Villi in the small intestine increase absorption by having thin walls, large surface area and a good blood supply.

Egestion is removal of undigested food; excretion is removal of metabolic waste such as carbon dioxide and urea.

Aerobic respiration: glucose + oxygen → carbon dioxide + water + energy (about 38 ATP per glucose in cells).

Anaerobic respiration in humans: glucose → lactic acid + small energy release; causes oxygen debt.

Anaerobic respiration in yeast: glucose → ethanol + carbon dioxide + energy (fermentation).

Word equation for aerobic respiration must be distinguished from photosynthesis, which is the reverse.

Inhaled air is about 21% oxygen and 0.04% carbon dioxide; exhaled air is about 16% oxygen and 4% carbon dioxide.

Inspiration: intercostal muscles and diaphragm contract, thorax volume rises, pressure falls, air enters.

Expiration is passive at rest: muscles relax, thorax volume falls, pressure rises, air is forced out.

Alveoli are efficient gas exchange surfaces because they are thin, moist, numerous and surrounded by capillaries.

Limewater turns milky with carbon dioxide; this tests for the gas produced by respiring organisms.

Double circulation: blood passes through the heart twice per circuit (pulmonary then systemic).

Blood flow order: vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta.

Arteries carry blood away from the heart at high pressure; veins carry blood back and contain valves.

Capillaries have walls one cell thick to allow exchange of gases, nutrients and waste with tissues.

Left ventricle wall is thickest because it pumps blood around the whole body at the highest pressure.

Red blood cells contain haemoglobin, which combines with oxygen in the lungs and releases it in tissues.

Phagocytes engulf pathogens; lymphocytes produce antibodies; platelets help clot blood.

Transpiration pull and root pressure move water up xylem; translocation moves sugars in phloem.

Stomata open and close via guard cells to control water loss and gas exchange in leaves.

Excretion removes metabolic waste: carbon dioxide from lungs, urea from kidneys, bile pigments in faeces.

The kidney removes urea by filtering blood in the glomerulus and reabsorbing useful substances in tubules.

Selective reabsorption returns all glucose, most water and some salts to the blood.

Urine composition depends on water intake, salt intake, temperature and hormone levels.

ADH (antidiuretic hormone) increases water reabsorption, producing small concentrated urine.

Dialysis uses a partially permeable membrane to remove urea when kidney function fails.

Kidney transplant replaces failed kidney function and avoids repeated dialysis but requires immunosuppressant drugs.

Excess protein or salt in the diet increases urea production and stresses the kidneys.

Lungs, skin (sweat) and kidneys are the main excretory organs; the liver deaminates amino acids to make urea.

Homeostasis maintains a constant internal environment: temperature, blood glucose, water and ion balance.

Negative feedback reverses a change; for example, high blood glucose triggers insulin release from the pancreas.

Insulin lowers blood glucose by converting glucose to glycogen in the liver; glucagon raises it.

Type 1 diabetes is insulin deficiency treated with injections; type 2 involves reduced sensitivity to insulin.

Vasodilation and sweating cool the body; vasoconstriction, shivering and hair erection conserve heat.

A reflex arc order: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response.

Synapses are gaps between neurones where a chemical transmitter diffuses across to carry the impulse.

The eye: the iris controls pupil size; the lens focuses light; the retina contains rods and cones.

Hormones travel in the blood and act more slowly than nerve impulses but for longer periods.

Sexual reproduction involves gametes and meiosis, producing genetically varied offspring.

Asexual reproduction involves mitosis only and produces genetically identical clones.

Male gamete is the sperm (motile, small); female gamete is the egg (large, food store, non-motile).

Fertilisation fuses haploid gametes to form a diploid zygote; human chromosome number is 46 (23 pairs).

Menstrual cycle: ovulation around day 14; oestrogen thickens the uterus lining; progesterone maintains it.

Pregnancy: placenta exchanges nutrients and wastes; amniotic fluid protects the fetus; gestation is about 38–40 weeks.

Contraception methods include barrier (condom), hormonal (pill), surgical (vasectomy, sterilisation) and intrauterine devices.

Flower structure: sepals protect, petals attract pollinators, stamens are male, carpels are female.

Pollination transfers pollen to the stigma; fertilisation follows when the pollen tube reaches the ovule.

Growth is a permanent increase in size, mass or cell number; development is increase in complexity.

Plants grow by cell division in meristems and by cell elongation, unlike animals which stop growing at maturity.

Growth curves: sigmoid (S-shaped) curves show slow, fast then slowing growth phases.

Human growth shows a rapid infant phase, a slow childhood phase and an adolescent growth spurt.

Metamorphosis is a change of body form; insects may be complete (egg, larva, pupa, adult) or incomplete.

Seed germination requires water, oxygen and a suitable temperature; light is not always required.

Cell differentiation produces specialised cells with particular shapes and functions.

Measuring growth by height or mass is affected by nutrition, disease and genetic factors.

Genes are sections of DNA on chromosomes that code for a characteristic; alleles are alternative forms of a gene.

Genotype is the allele combination; phenotype is the visible characteristic produced.

Dominant alleles are expressed with one copy; recessive alleles only when two are present.

Homozygous means two identical alleles; heterozygous means two different alleles.

Punnett squares predict offspring ratios, e.g. heterozygous × heterozygous gives 3:1 phenotype ratio.

Sex determination: XX is female, XY is male; the father determines the sex of the child.

Co-dominance and incomplete dominance produce intermediate phenotypes, e.g. AB blood group.

Sex-linked conditions such as colour blindness are carried on the X chromosome and are commoner in males.

Mutation is a random change in DNA sequence and is the source of new alleles.

Continuous variation shows a range of values (height, mass); discontinuous variation shows distinct categories (blood group).

Genetic variation arises from meiosis, random fertilisation and mutation; environmental variation from diet, climate and lifestyle.

Natural selection: more offspring are produced than survive; those with advantageous alleles reproduce more.

Adaptations may be structural (thick fur), behavioural (migration) or physiological (water retention).

Fossils, homologous structures and DNA comparisons provide evidence for evolution.

Speciation occurs when populations become reproductively isolated and diverge over time.

Selective breeding by humans produces desired traits but reduces genetic variation.

Evolution changes allele frequencies in a population over generations, not individuals.

Pathogens include bacteria, viruses, fungi and protozoa; each causes disease in specific ways.

Bacteria can be treated with antibiotics; viruses cannot, since antibiotics do not affect them.

Antibiotic resistance develops because bacteria with resistant alleles survive and reproduce.

Vaccination introduces weakened or dead pathogens to stimulate antibody production and memory cells.

Active immunity is produced by the body after infection or vaccination; passive immunity comes from antibodies given or passed via placenta.

Barriers to infection include skin, mucus, cilia, stomach acid and tears.

Non-specific defences include phagocytosis and inflammation; specific defences involve lymphocytes.

Risk factors for disease include smoking, alcohol, poor diet, lack of exercise and obesity.

Coronary heart disease involves fatty deposits narrowing arteries supplying the heart muscle.

Habitat is where an organism lives; population is all individuals of one species in an area; community is all populations together.

An ecosystem includes the community plus the non-living environment (abiotic factors).

Food chains begin with a producer; arrows show the direction of energy transfer.

Only about 10% of energy passes to the next trophic level; the rest is lost as heat, movement and waste.

Pyramids of number can be inverted; pyramids of energy are always broad at the base.

Decomposers break down dead matter and recycle carbon and nitrogen back to the environment.

The carbon cycle includes photosynthesis, respiration, feeding, decomposition and combustion.

Abiotic factors include light, temperature, water and soil pH; biotic factors include predation and competition.

Sampling with quadrats estimates population size; capture-recapture estimates animal numbers.

Burning fossil fuels releases carbon dioxide, increasing the greenhouse effect and global warming.

Sulfur dioxide causes acid rain, damaging leaves and acidifying lakes and soil.

Eutrophication: fertiliser runoff raises nitrate levels, algae bloom, decompose, oxygen falls and fish die.

Deforestation reduces carbon dioxide absorption and causes soil erosion and habitat loss.

The greenhouse gases include carbon dioxide, methane, nitrous oxide and water vapour.

Conservation measures include protected areas, recycling, renewable energy and sustainable fishing quotas.

Water pollution indicators include sewage, industrial chemicals and plastic; treatment removes pathogens and solids.

Ozone depletion is caused by CFCs and increases ultraviolet radiation reaching the surface.

Human population growth increases demand for food, water, energy and land, intensifying pollution and resource depletion.

Sustainable development meets present needs without compromising resources for future generations.

Reduce, reuse and recycle lessen waste going to landfill and conserve raw materials.

Renewable energy sources include solar, wind, hydroelectric, tidal and biomass; fossil fuels are non-renewable.

Biodiversity is the variety of species and genetic variation within an ecosystem; it increases ecosystem stability.

Intensive farming can cause soil erosion, water pollution and reduced genetic variety in crops.

Biological control uses natural predators instead of pesticides but may introduce non-native species.

Indicator species such as lichens signal air quality; mayfly larvae signal clean, oxygen-rich water.

Carbon footprint measures greenhouse gas emissions from an activity, person or product.

Conservation of endangered species requires habitat protection, captive breeding and legal protection.

Energy flows one way through ecosystems; nutrients cycle repeatedly.

Photosynthesis equation: carbon dioxide + water → glucose + oxygen (requires light and chlorophyll).

Respiration equation: glucose + oxygen → carbon dioxide + water + energy.

Nitrogen-fixing bacteria convert nitrogen gas to nitrates; nitrifying bacteria convert ammonium to nitrates; denitrifying bacteria release nitrogen gas.

Competition occurs for light, water, minerals, food and space; the best-adapted organisms survive.

Predator–prey cycles show a lag: rising prey numbers are followed by rising predator numbers.

Human activities that reduce biodiversity include deforestation, pollution, overfishing and urbanisation.

Recycling aluminium saves far more energy than producing it from ore.

Environmental monitoring uses physical, chemical and biological indicators to assess ecosystem health.

Receptors detect stimuli: rods and cones in the retina, chemoreceptors in the tongue and nose, thermoreceptors in skin.

The nervous system is divided into the central nervous system (brain, spinal cord) and peripheral nerves.

A synapse delays transmission and ensures impulses travel one way only.

Reflex actions are rapid, involuntary and protective because they bypass conscious brain processing.

Plant tropisms: phototropism is growth towards light; geotropism is growth in response to gravity.

Auxins are plant hormones that promote cell elongation on the shaded side, bending shoots towards light.

The pancreas acts as both an endocrine gland (insulin, glucagon) and an exocrine gland (digestive enzymes).

The pituitary gland controls other glands and releases ADH, which acts on the kidney tubules.

Blood glucose regulation involves the liver storing glycogen and releasing glucose when needed.