Biology

The seven characteristics that distinguish living organisms from non-living things — movement, respiration, sensitivity, growth, reproduction, excretion, nutrition (MRSGREN).

Why biologists classify, what a species is (with the fertile-offspring test), the binomial system, and how to construct and walk dichotomous keys.

The features that place organisms into the five kingdoms (animal, plant, fungi, prokaryote, protoctist), the five vertebrate classes (mammal, bird, reptile, amphibian, fish), and the four arthropod groups (insect, arachnid, crustacean, myriapod).

The structure and function of animal, plant and bacterial cells. Names and roles of the key organelles — cell membrane, nucleus, cytoplasm, mitochondria, ribosomes, plus plant-only extras (cell wall, chloroplasts, vacuole) and bacterial features (circular DNA, plasmids).

The magnification formula (M = I ÷ A) and its three rearrangements, plus unit conversion between millimetres and micrometres for expressing the size of biological specimens.

Diffusion as net movement from high to low concentration driven by random kinetic energy. Its role in cells (gas exchange, nutrient uptake, waste removal) and the four factors that influence rate (surface area, temperature, concentration gradient, distance).

Osmosis as the net movement of water across a partially permeable membrane. The effects of dilute, equal and concentrated solutions on plant cells (turgid, flaccid, plasmolysed) and animal cells (lysed, normal, crenated). HL extensions: water potential and turgor pressure.

Active transport as the movement of particles across a cell membrane against a concentration gradient, using energy from respiration. The role of protein carriers and the importance of active transport in the uptake of mineral ions by root hair cells (HL).

The three families of biological molecules — carbohydrates, proteins, and fats — and the small units (glucose, amino acids, fatty acids + glycerol) that build them. The five classic food tests (iodine, Benedict's, biuret, ethanol, DCPIP). HL extension: DNA structure (double helix, base pairing).

Enzymes as biological catalysts: proteins with active sites complementary to specific substrates (lock and key). The effect of temperature and pH on enzyme activity, optimum and denaturation. HL extension: enzyme-substrate complex and the molecular basis of specificity.

Photosynthesis as the process by which plants synthesise carbohydrates from CO₂ and water using light energy and chlorophyll. The word and balanced (HL) equations, the uses of glucose, the importance of nitrate and magnesium ions, and the limiting factors (light, CO₂, temperature) that control the rate.

How a leaf is engineered for photosynthesis. The whole-leaf adaptations (large surface area, thinness), the labelled cross-section (cuticle, upper and lower epidermis, palisade mesophyll, spongy mesophyll, air spaces, guard cells and stomata), and the vascular bundles (xylem and phloem) that supply water and remove sucrose.

What a balanced human diet is, the seven nutrient groups (carbohydrates, proteins, fats, vitamins C and D, mineral ions calcium and iron, fibre, water) with their dietary sources and roles, and the deficiency diseases scurvy and rickets.

The five stages of digestion (ingestion, digestion, absorption, assimilation, egestion). The organs of the alimentary canal in order (mouth, oesophagus, stomach, small intestine, large intestine, anus). The associated organs (salivary glands, liver, gall bladder, pancreas) and their digestive juices.

The two transport tissues in plants. Xylem carries water + mineral ions upward and provides support. Phloem carries sucrose + amino acids in any direction. Their positions in roots, stems, and leaves. The HL structure of xylem vessels — thick lignified walls, no cell contents, no cross walls.

What a circulatory system is — pump, vessels, and valves for one-way blood flow. Single circulation in fish (HL): blood passes through the 2-chamber heart once per circuit. Double circulation in mammals (HL): 4-chamber heart, two separate loops (pulmonary + systemic), high-pressure delivery to body tissues.

The mammalian heart — 4 chambers, valves between every section, septum keeps oxygenated and deoxygenated blood separate, and tiny coronary arteries on the surface feed the heart muscle. Coronary heart disease (CHD) results from blockage of coronary arteries; risk factors split into modifiable (diet, exercise, smoking, stress) and non-modifiable (age, sex, genetics). HL extends to AV vs semilunar valves, why the LV wall is thicker than the RV, and the role of the septum.

The three blood vessel types — arteries (thick wall, narrow lumen, high pressure, away from heart), veins (thin wall, wide lumen, valves, low pressure, back to heart) and capillaries (1 cell thick, exchange site). Named vessels of the heart (aorta, vena cava), lungs (pulmonary artery + vein), kidneys (renal artery + vein) and (HL) liver (hepatic artery, hepatic vein, hepatic portal vein). HL extends to explaining how each structure matches its pressure or exchange function.

The four components of blood — red blood cells (haemoglobin transports O₂), white blood cells (defence: phagocytosis + antibody production), platelets (clotting), plasma (transport of cells, ions, glucose, urea, hormones, CO₂). Two roles of clotting: prevent blood loss + prevent entry of pathogens. HL extends to distinguishing phagocytes (multi-lobed nucleus, engulf) from lymphocytes (single round nucleus, antibodies), and the clotting reaction: soluble fibrinogen → insoluble fibrin mesh.

How humans take in oxygen and remove carbon dioxide. The breathing system (lungs, trachea, bronchi, bronchioles, alveoli, ribs, intercostal muscles, diaphragm). Four features that make alveoli effective: thin (1-cell) walls, large surface area (~70 m²), good blood supply, and good ventilation. Inhaled vs exhaled air composition (O₂, CO₂, water vapour). Effect of physical activity on rate and depth of breathing. HL extends to the ventilation mechanism (volume + pressure changes), cartilage in the trachea, and the role of goblet cells, mucus, and ciliated cells in airway defence.

Why living organisms need energy — the seven Cambridge uses (muscle contraction, protein synthesis, cell division, active transport, growth, nerve impulses, body temperature) — and the practical investigation of how temperature affects respiration rate in yeast (using a hydrogencarbonate indicator + water bath at varied temperatures, with a rate-vs-temperature curve showing the optimum near body temperature and enzyme denaturation above ~50 °C).

Aerobic respiration as the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy. Word equation: glucose + oxygen → carbon dioxide + water (+ energy). HL extends to the balanced chemical equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.

Anaerobic respiration as chemical reactions in cells that break down glucose without oxygen, releasing much less energy than aerobic respiration. Two pathways: yeast (glucose → alcohol + CO₂ — basis of bread, beer) and muscle during vigorous exercise (glucose → lactic acid — the burn). HL extends to the balanced chemical equation for yeast (C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂), the build-up of lactic acid causing oxygen debt, and the three-step removal mechanism (fast heart rate transports lactic acid to liver, deeper breathing supplies O₂, liver aerobically respires lactic acid).

How the human body removes metabolic waste — CO₂ excreted via the lungs, urea + excess water + ions excreted via the kidneys (with the four-organ urinary system: kidneys → ureters → bladder → urethra). HL extends to the kidney's internal anatomy (cortex + medulla) and the nephron's two-stage operation (filtration at the glomerulus, reabsorption of all glucose, most water, and some ions in the tubule), plus the liver's role in assimilating amino acids into proteins and deaminating excess amino acids to form toxic urea.

The mammalian nervous system as the body's high-speed coordination network — central nervous system (brain + spinal cord) plus peripheral nervous system (all nerves outside the CNS), with electrical impulses travelling along three types of neurones: sensory (input to CNS), relay (inside CNS), and motor (output to effectors). The reflex arc (stimulus → receptor → sensory → relay → motor → effector → response) bypasses the brain to give a fast, automatic, life-saving response. HL extension: synapse structure (vesicles, neurotransmitter, synaptic gap, receptor proteins), the four-step events (impulse arrival → vesicle release → diffusion → binding → new impulse), and one-way directionality.

Sense organs as receptor cells responding to specific stimuli (light, sound, touch, temperature, chemicals) — focused on the human eye. Identify the seven Cambridge parts (cornea, iris, pupil, lens, retina, optic nerve, blind spot) and state each function. Explain the pupil reflex (bright = small, dim = large) as protection of the retina. HL extension: antagonistic action of circular and radial iris muscles, plus accommodation (ciliary muscles + suspensory ligaments + lens shape) to focus on near vs far objects, and the distribution + functions of rods (night vision, periphery), cones (three types for colour vision), and the fovea (cone-packed pit for sharpest vision).

Hormones as chemical messengers — substances produced by endocrine glands, carried in the blood, that alter the activity of one or more specific target organs. Identify the four Cambridge gland-hormone pairs (adrenal→adrenaline, pancreas→insulin, testes→testosterone, ovaries→oestrogen). Adrenaline as the fight-or-flight hormone with three named effects (increased heart rate, breathing rate, pupil diameter). Comparison of nervous vs hormonal control on speed of action and duration of effect. HL extension: glucagon secreted by the pancreas (opposite of insulin), and the role of adrenaline in metabolic activity (increasing blood glucose concentration and increasing heart rate).

Drugs as substances taken into the body that modify or affect chemical reactions. Antibiotics for treating bacterial infections — what they do (kill bacteria) and what they do NOT do (affect viruses). Some bacteria are resistant to antibiotics, which reduces antibiotic effectiveness. HL extension: how using antibiotics only when essential can limit the development of resistant bacteria such as MRSA — through reducing selection pressure on resistance genes.

Flower structure of insect-pollinated flowers (sepals, petals, stamens — anther + filament — and carpels — stigma + style + ovary + ovules) and the functions of each named part. Pollination as transfer of pollen from anther to stigma. Fertilisation as the fusion of a pollen nucleus with a nucleus in an ovule. Distinction between insect- and wind-pollinated flowers — their structural adaptations and pollen grain features. Environmental conditions affecting germination (water, oxygen, suitable temperature). HL extension: self-pollination vs cross-pollination and their effects on population variation; growth of the pollen tube down the style into the ovule.

The named parts of the male reproductive system (testes, scrotum, sperm ducts, prostate gland, urethra, penis) and female reproductive system (ovaries, oviducts, uterus, cervix, vagina) with their functions. Fertilisation as the fusion of sperm and egg nuclei in the oviduct. Adaptive features of sperm (flagellum, mitochondria, acrosome enzymes) and egg cells (energy stores, jelly coat that changes at fertilisation). Comparison of male and female gametes by size, motility, and number. Embryonic development from zygote to a ball of cells that implants in the uterus lining. Fetal support structures: umbilical cord, placenta, amniotic sac, and amniotic fluid. HL extension: detailed function of the placenta and umbilical cord in materials exchange (O₂, glucose, amino acids in; CO₂, urea out — without blood mixing) and the warning that some pathogens and toxins can also cross the placenta.

Chromosomes are DNA molecules. A gene is a length of DNA that codes for a protein. An allele is an alternative form of a gene. Inheritance of sex via X and Y chromosomes (XX female, XY male). HL extension: base sequence in a gene determines amino acid sequence in the protein, which determines shape and therefore function. Protein synthesis flow — gene stays in nucleus, mRNA copies the gene, exits to cytoplasm, ribosome assembles amino acids into the protein. Selective gene expression. Haploid vs diploid nuclei — humans are diploid (46 = 23 pairs) in body cells; gametes are haploid (23).

Variation as differences between individuals of the same species. Continuous variation (range — body length, body mass) caused by genes + environment vs discontinuous variation (distinct categories — ABO blood groups, seed shape, seed colour) caused by genes only. Mutation as a genetic change — the source of new alleles. Ionising radiation and certain chemicals increase the mutation rate. HL extension: gene mutation defined as a random change in the DNA base sequence; four sources of genetic variation in populations (mutation, meiosis, random mating, random fertilisation).

Mitosis as nuclear division giving rise to genetically identical daughter cells (details of stages not required). Chromosomes are exactly replicated BEFORE mitosis; during mitosis the copies (sister chromatids) separate, maintaining the chromosome number in each daughter cell. The four roles of mitosis: growth, repair of damaged tissues, replacement of cells, and asexual reproduction. Stem cells defined as unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions. Entire topic is HL only.

Meiosis as the nuclear division involved in the production of gametes. Meiosis described as a reduction division — the chromosome number is halved from diploid (2n) to haploid (n), resulting in genetically different cells (details of the stages not required). In humans: 2n = 46 → n = 23 in each gamete; fertilisation restores diploid (23 + 23 = 46). Includes explicit pedagogical contrast with mitosis on the four mark-scheme dimensions: number of daughter cells (2 vs 4), chromosome count (maintained vs halved), genetic identity (identical vs different), and purpose (body cells vs gametes). Entire topic is HL only.

The vocabulary and prediction of single-gene inheritance. Genotype (alleles present) vs phenotype (observable feature). Homozygous (two same alleles, pure-breeding) vs heterozygous (two different alleles). Dominant alleles expressed when present; recessive only when no dominant. Punnett squares predict offspring genotype + phenotypic ratios — Cambridge core ratios 1:1 (Bb × bb) and 3:1 (Bb × Bb). Pedigree diagram conventions (squares = males, circles = females, filled = affected). HL extensions: test cross to identify unknown dominant genotype; codominance with ABO blood groups (alleles I^A, I^B, I^O — I^A and I^B codominant); sex linkage (gene on X chromosome) explaining why traits like red-green colour blindness are more common in males than females.

An adaptive feature defined as an INHERITED feature that helps an organism SURVIVE and REPRODUCE in its environment. Cambridge keywords: inherited + survive + reproduce + environment. Skill of interpreting images to describe the adaptive features of a species. HL extension: explain the adaptive features of XEROPHYTES (cacti — spines, thick waxy cuticle, succulent stem, deep roots — all to minimise water loss in dry environments) and HYDROPHYTES (water lilies — wide floating leaves, stomata on upper surface, air spaces in stem, short roots — all to float and access oxygen in water).

Natural selection described in 5 Cambridge steps: (a) genetic variation in populations, (b) overproduction of offspring, (c) struggle for survival including competition for resources, (d) better-adapted individuals reproduce more, (e) alleles passed to next generation. Selective breeding (artificial selection) described in 3 steps: (a) select desirable individuals, (b) cross them, (c) select offspring with desired feature — repeated over many generations to improve crops and domesticated animals. HL extension: adaptation defined as the PROCESS by which populations become more suited to environment; antibiotic resistance in MRSA as a real-time example of natural selection; differences between natural and artificial selection (selecting agent: environment vs human; trait value: survival vs human-desired; timescale).

The Sun is the principal source of energy input to biological systems. Plants capture light energy via photosynthesis and convert it to chemical energy. Animals (consumers) get their energy by feeding on plants or other animals. Energy flows in three forms: LIGHT (from Sun) → CHEMICAL (in organisms — glucose, fat, protein) → HEAT (eventually lost via respiration, body warmth, movement). Unlike carbon and nitrogen, energy is NOT recycled — it flows one-way through living systems and dissipates as heat.

Food chains as sequences showing energy transfer between organisms — arrow points in the direction of energy flow, beginning with a producer. Producers (plants) make their own organic nutrients via photosynthesis; consumers (animals) get energy by feeding; decomposers (bacteria/fungi) break down dead or waste organic material. Trophic levels: producer (1) → primary consumer/herbivore (2) → secondary/carnivore (3) → tertiary (4) → quaternary (5). Food webs as networks of interconnected chains. Pyramids of numbers (count) vs pyramids of biomass (total dry mass) — biomass preferred because it's never inverted. Human impact through overharvesting and introducing foreign species disrupts food webs. HL extension: pyramids of energy (kJ/m²/yr), the ~90% loss per trophic level (respiration + heat + indigestible material), why food chains have fewer than 5 levels, and why eating crops is ~10× more energy-efficient than eating livestock fed on crops.

Carbon cycle limited to six Cambridge processes: photosynthesis (CO₂ removed by plants), respiration (CO₂ released by all organisms), feeding (carbon transferred between organisms), decomposition (dead matter broken down by bacteria/fungi releasing CO₂), formation of fossil fuels (dead organisms compressed over 100M+ years), and combustion (burning fossil fuels releases stored CO₂). HL extension: nitrogen cycle with four named microorganism roles — decomposition (protein → NH₄⁺), nitrification (NH₄⁺ → NO₃⁻), nitrogen fixation (N₂ → NH₄⁺), and denitrification (NO₃⁻ → N₂). Plants need nitrogen for proteins and DNA but cannot use atmospheric N₂ directly because the triple bond is too strong; bacteria do the work.

Population (one species, same area, same time), community (all populations together), ecosystem (community + non-living environment) — three terms that NEST. Sigmoid growth curve with four phases (lag, log/exponential, stationary, death) and four limiting factors (food supply, competition, predation, disease) that produce the characteristic S-shape. HL extension: explaining each phase mechanistically — lag = adapting + few breeders, log = no limits yet, stationary = limiting factors balance growth (birth = death at carrying capacity), death = limiting factors overwhelm.

Five Cambridge-named methods humans use to increase food production: agricultural machinery, chemical fertilisers, insecticides, herbicides, and selective breeding. Trade-offs of two intensive farming systems — large-scale monoculture (yield + efficiency, but disease vulnerability through genetic uniformity, soil degradation, and biodiversity loss) and intensive livestock production (cheap meat from controlled high-density rearing, but disease transmission, animal welfare, and antibiotic resistance concerns).

Biodiversity (number of different species in an area) and the three Cambridge-named reasons humans destroy habitats: increased area for housing and crop/livestock production, extraction of natural resources, and freshwater + marine pollution. Altering food webs has a negative impact on habitats — removing one species cascades through chains. Deforestation case study: five undesirable effects to explain — reducing biodiversity, extinction, loss of soil (no roots to hold it), flooding (no rainfall absorption), and increase of atmospheric CO₂ (no photosynthesis + stored carbon released).

Effects of untreated sewage and excess fertiliser on aquatic ecosystems (oxygen-collapse pathway via algal bloom + decomposer respiration). Effects of non-biodegradable plastics in aquatic and terrestrial ecosystems (entanglement, ingestion, microplastic accumulation). Sources and effects of methane and carbon dioxide air pollution — enhanced greenhouse effect and climate change. HL extension: the six-step eutrophication mechanism — increased nitrate availability, increased producer growth, increased decomposition after producer death, increased aerobic respiration by decomposers, reduction in dissolved oxygen, and death of oxygen-requiring organisms.

Sustainable resources defined as those produced as rapidly as they are removed so they do not run out. Forests and fish stocks as the two Cambridge-named resources that can be conserved and managed sustainably. Six causes of organisms becoming endangered or extinct: climate change, habitat destruction, hunting, overharvesting, pollution and introduced species. Four conservation methods for endangered species: monitoring and protecting species and habitats, education, captive breeding programmes, and seed banks (e.g., Svalbard Global Seed Vault). HL extensions on forest + fish stock specific conservation, AI/IVF in captive breeding, and genetic-variation risks in small populations covered in Lesson 2.

Bacteria are useful in biotechnology and genetic modification because of their RAPID REPRODUCTION RATE (dividing every ~20 minutes → trillions in 12 hours) and their ABILITY TO MAKE COMPLEX MOLECULES (proteins, hormones, antibiotics) using ribosomes that can read inserted human genes — exemplified by recombinant human insulin produced in E. coli since 1982. HL extension: two additional reasons — FEW ETHICAL CONCERNS (bacteria are not sentient, no welfare laws apply) and the PRESENCE OF PLASMIDS (small circular DNA outside the chromosome that scientists can cut open with restriction enzymes, insert a target gene, ligate, and transform back into bacteria).

Yeast (Saccharomyces cerevisiae) anaerobic respiration produces ETHANOL + CO₂ from glucose — used in biofuel production (ethanol distilled from sealed fermenters → renewable fuel) and bread-making (CO₂ trapped by gluten makes dough rise; ethanol evaporates during baking). Industrial enzymes: pectinase digests pectin in fruit cell walls to release more juice and clarify it; biological washing powders contain a cocktail of enzymes — protease (protein stains: blood, sweat, egg), lipase (fat stains: butter, oil), amylase (starch stains: pasta sauce, gravy) — that work at cool temperatures (30–40 °C) to save energy. HL extensions: lactase (immobilised on alginate beads → lactose-free milk), industrial fermenters producing insulin (genetically modified E. coli), penicillin (Penicillium fungus) and mycoprotein (Fusarium fungus → Quorn), and the 5 controlled fermenter conditions (temperature, pH, oxygen, nutrient supply, waste products).

GENETIC MODIFICATION = changing an organism's DNA by REMOVING, CHANGING or INSERTING individual genes. 4 named examples: (a) human genes inserted into bacteria → human proteins (e.g. insulin in E. coli); (b) genes inserted into crops → herbicide resistance; (c) genes inserted into crops → insect-pest resistance (Bt); (d) genes inserted into crops → improved nutritional quality (e.g. golden rice + vitamin A). HL extension: 6-step recombinant-DNA workflow — RESTRICTION ENZYMES cut human DNA forming STICKY ENDS; same enzyme cuts bacterial PLASMID forming complementary sticky ends; DNA LIGASE seals the join → RECOMBINANT PLASMID; plasmid into bacterium; bacteria multiply; expression → human protein. HL discussion of GM crop advantages (higher yields, less spraying, better nutrition) and disadvantages (gene escape via pollen, biodiversity loss, pest resistance, ethics + corporate control of patented seeds), with examples from soya, maize and rice.