BGCSE BSSE Biology 1428 cheat sheet

Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles; their DNA is a single circular chromosome in the nucleoid, and they may carry plasmids.

Eukaryotic cells have a nucleus, mitochondria, and (in plants and fungi) other membrane-bound organelles; ribosomes are present in both cell types.

Plant cells uniquely possess a cellulose cell wall, a large permanent central vacuole, and chloroplasts; animal cells have no wall and may have small temporary vacuoles.

Mitochondria are the site of aerobic respiration; chloroplasts are the site of photosynthesis; ribosomes are the site of protein synthesis.

Magnification = image size ÷ actual size; use the same units for both before dividing, then convert to µm or mm as required.

Diffusion moves particles down a concentration gradient without energy input; osmosis is diffusion of water across a partially permeable membrane from dilute to concentrated solution.

Active transport moves substances against a concentration gradient and requires energy from ATP plus a carrier protein in the membrane.

Cells specialise by developing organelle and shape features matching function, e.g. root hair cells have elongated projections, red blood cells lose their nucleus.

Carbohydrates contain C, H, O; proteins contain C, H, O, N (sometimes S); lipids contain C, H, O with a lower oxygen proportion than carbohydrates.

Benedict's test detects reducing sugars (blue to brick-red on heating); iodine detects starch (yellow-brown to blue-black); Biuret detects protein (blue to purple); ethanol emulsion test detects lipid (cloudy white).

Enzymes are protein catalysts that lower activation energy; they are specific because the active site shape is complementary to one substrate.

Rate of enzyme action increases with temperature to an optimum, then falls sharply as the enzyme denatures and the active site loses shape.

Enzyme activity is highest near the optimum pH; extreme pH alters ionic bonds holding the active site, denaturing the enzyme.

Increasing substrate concentration raises rate until all active sites are occupied (saturation); further substrate gives no increase.

A competitive inhibitor resembles the substrate and binds the active site; a non-competitive inhibitor binds elsewhere and changes active site shape.

Amino acids join by peptide bonds in condensation reactions releasing water; hydrolysis uses water to break these bonds.

Balanced diet requirements: carbohydrates for energy, proteins for growth and repair, lipids for energy storage and insulation, vitamins and minerals for health, fibre for gut movement, water as a solvent.

Deficiency diseases: vitamin C → scurvy; vitamin D or calcium → rickets; iron → anaemia; protein-energy deficiency → kwashiorkor or marasmus.

Mechanical digestion (chewing, churning) increases surface area; chemical digestion uses enzymes to hydrolyse large molecules into absorbable ones.

Carbohydrases (amylase) convert starch to maltose then glucose; proteases convert proteins to amino acids; lipases convert lipids to fatty acids and glycerol.

Stomach conditions: hydrochloric acid kills bacteria and provides low pH for pepsin; mucus protects the stomach lining from acid and enzyme attack.

Bile is made in the liver, stored in the gall bladder, and emulsifies fats, increasing surface area for lipase; bile also neutralises acidic chyme.

Villi in the small intestine increase surface area; thin walls, a rich blood supply and lacteals allow rapid absorption of glucose, amino acids and lipids.

Water is reabsorbed mainly in the colon; the colon also absorbs mineral ions, leaving undigested fibre to form faeces.

Aerobic respiration equation: glucose + oxygen → carbon dioxide + water (+ ATP energy); it releases far more energy per glucose molecule than anaerobic respiration.

Anaerobic respiration in muscle: glucose → lactic acid (+ small energy); in yeast: glucose → ethanol + carbon dioxide.

Oxygen debt is the extra oxygen needed after exercise to break down accumulated lactic acid in the liver.

Breathing mechanics: intercostal muscles and diaphragm contract → thorax volume increases, pressure falls, air enters (inhalation); relaxation reverses this.

Alveoli maximise gas exchange through a large total surface area, thin walls, moist surface, and a steep concentration gradient maintained by blood flow.

Smoking damages cilia and alveoli, causing tar build-up, reduced surface area (emphysema) and increased risk of lung cancer.

Gas exchange in plants occurs mainly through stomata in leaves and lenticels in stems; stomata close at night or in drought to reduce water loss.

Respiration rate rises with exercise to supply more oxygen and remove carbon dioxide; heart rate and breathing rate increase together.

Blood components: red blood cells carry oxygen via haemoglobin; white blood cells defend against pathogens; platelets clot blood; plasma transports dissolved substances and heat.

Double circulation: blood passes through the heart twice per circuit - pulmonary (heart → lungs → heart) and systemic (heart → body → heart).

Heart pathway: vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta.

The left ventricle wall is thicker than the right because it pumps blood at higher pressure around the whole body.

Arteries carry blood away from the heart, have thick muscular elastic walls and narrow lumens; veins carry blood to the heart and have valves.

Capillaries are one cell thick, allowing rapid diffusion of gases and nutrients between blood and tissues.

Transpiration is water loss as vapour from leaves; it is driven by evaporation and cohesion-tension in xylem, and is faster in hot, dry, windy, bright conditions.

Translocation moves sugars in phloem from source (leaf) to sink (root, fruit, growing tip) and requires energy from respiration.

Excretion removes metabolic waste: kidneys remove urea and excess ions and water; lungs remove carbon dioxide; skin removes water and salts in sweat.

The nephron filters blood at the glomerulus, reabsorbs all glucose, some ions and most water in the tubule, and secretes wastes into urine.

ADH from the pituitary increases water reabsorption in the collecting duct, producing concentrated urine when the body is dehydrated.

Homeostasis maintains a stable internal environment around a set point using negative feedback: a change triggers a response that reverses the change.

Blood glucose control: insulin lowers glucose by promoting uptake and glycogen storage; glucagon raises glucose by releasing glycogen from the liver.

Thermoregulation: sweating and vasodilation cool the body; shivering and vasoconstriction conserve heat.

Diabetes mellitus: type 1 involves insufficient insulin production; type 2 involves reduced cell response to insulin and is linked to obesity.

Osmoregulation in freshwater fish and in plants relies on water balance across partially permeable membranes; stomatal closure reduces water loss in drought.

A stimulus is detected by a receptor; the impulse travels via a sensory neurone to the CNS, then via a motor neurone to an effector (muscle or gland).

A reflex arc bypasses conscious decision-making and involves a relay neurone in the spinal cord, giving a rapid protective response.

The synapse is a gap between neurones; a neurotransmitter diffuses across it and triggers an impulse in the next neurone, allowing one-way transmission.

The eye: the iris controls pupil size; the lens focuses light by changing shape (accommodation); rods detect dim light, cones detect colour.

Hormones are chemical messengers secreted by endocrine glands into the blood and act on target organs with specific receptors.

Adrenaline raises heart rate and blood glucose in the fight-or-flight response; insulin and glucagon regulate blood glucose.

Plant tropisms: phototropism is growth towards light (auxin accumulates on the shaded side); geotropism is growth in response to gravity.

Auxin promotes cell elongation; uneven auxin distribution causes bending of shoots (negative geotropism) and roots (positive geotropism).

Asexual reproduction produces genetically identical offspring from one parent (e.g. runners, tubers, binary fission); sexual reproduction involves gametes and produces genetic variation.

Male gamete = sperm (motile, many mitochondria); female gamete = egg (large, food store); fertilisation fuses haploid nuclei to form a diploid zygote.

Meiosis halves chromosome number, producing four haploid cells and creating variation by crossing over and independent assortment.

Human male reproductive system: testes produce sperm and testosterone; sperm pass through the sperm duct and urethra.

Human female system: ovaries release eggs; the oviduct is the site of fertilisation; the uterus lining thickens for implantation.

The menstrual cycle: FSH stimulates egg maturation, oestrogen builds the uterus lining, LH triggers ovulation, progesterone maintains the lining.

Pregnancy: the placenta exchanges nutrients, oxygen and wastes between mother and fetus; the amniotic fluid cushions the fetus.

Plant reproduction: pollination transfers pollen to the stigma; fertilisation forms a zygote, and the ovary becomes the fruit while ovules become seeds.

A gene is a length of DNA coding for a protein; an allele is a version of a gene; a genotype is the allele combination; a phenotype is the observable characteristic.

Homozygous means two identical alleles; heterozygous means two different alleles; dominant alleles are expressed even when heterozygous.

A monohybrid cross uses a Punnett square; a cross of two heterozygotes (Aa × Aa) gives a 3:1 phenotype ratio and 1:2:1 genotype ratio.

Sex determination: females are XX, males are XY; the father's sperm determines the sex of the offspring.

Codominance occurs when both alleles are expressed in the heterozygote (e.g. AB blood group); incomplete dominance gives an intermediate phenotype.

DNA structure: a double helix of two strands held by hydrogen bonds between complementary bases A–T and C–G, with sugar-phosphate backbones.

Protein synthesis: DNA is transcribed to mRNA in the nucleus; mRNA is translated at the ribosome using triplet codons to assemble amino acids.

Mutation is a random change in DNA sequence that can alter the protein produced and may be inherited if it occurs in gametes.

A population is all organisms of one species in an area; a community is all populations; an ecosystem includes the community plus the physical environment.

Food chains show energy flow: producers → primary consumers → secondary consumers; arrows show the direction of energy transfer.

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

Pyramids of biomass and numbers can be inverted; pyramids of energy are always upright because energy is lost at each level.

The carbon cycle: photosynthesis removes CO₂; respiration, combustion and decomposition release it; fossil fuels store carbon long-term.

The nitrogen cycle: nitrogen fixation, nitrification, assimilation by plants, and denitrification return nitrogen to the atmosphere.

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

Human impacts: deforestation, pollution, and greenhouse gas emissions cause habitat loss, eutrophication, acid rain and climate change.

Classification groups organisms by shared features: kingdom, phylum, class, order, family, genus, species (broadest to narrowest).

Binomial nomenclature uses genus and species, e.g. Homo sapiens; the genus is capitalised and the species is lower case, both italicised.

The five-kingdom system: Monera (prokaryotes), Protista, Fungi, Plantae, Animalia; viruses are not classified in this system as they are non-cellular.

Vertebrate classes: fish, amphibians, reptiles, birds, mammals - distinguished by skin covering, respiration, reproduction and temperature regulation.

Evolution by natural selection: variation exists, more offspring are produced than survive, and those with advantageous traits reproduce more successfully.

Fossils provide evidence of evolution; they show gradual change and extinct intermediate forms, and are dated by rock strata.

Antibiotic resistance in bacteria is an example of natural selection occurring rapidly because bacteria reproduce quickly.

Adaptations are features that increase survival: structural (shape), behavioural (migration), and physiological (enzymes, water conservation).

Pathogens include bacteria, viruses, fungi and protozoa; they cause disease by damaging cells or releasing toxins.

Transmission routes include airborne droplets, contaminated food and water, vectors (e.g. mosquitoes), and direct contact.

Defences: skin and mucus are physical barriers; phagocytes engulf pathogens; lymphocytes produce antibodies specific to antigens.

Vaccination introduces antigens (or weakened pathogens) to stimulate antibody production and memory cells, giving immunity.

Antibiotics kill bacteria but not viruses; overuse promotes antibiotic resistance, so full courses must be completed.

Practical skills: identify independent, dependent and control variables; repeat measurements; calculate means; use appropriate units and significant figures.

Valid conclusions require reliable data; anomalies should be identified and excluded, and a control group is needed to show the effect of the treatment.

Microscopy drawing rules: use a sharp pencil, label with straight lines that do not cross, and state the magnification used.

Cell membrane structure: a phospholipid bilayer with embedded proteins and cholesterol; it is described as a fluid mosaic model.

The membrane is partially permeable, controlling which substances enter and leave by diffusion, osmosis, active transport and facilitated diffusion.

Enzymes in cells are located in specific compartments: digestive enzymes in lysosomes, respiratory enzymes in mitochondria, photosynthetic enzymes in chloroplasts.

Cell division by mitosis produces two genetically identical diploid cells for growth, repair and asexual reproduction.

The cell cycle: interphase (growth and DNA replication) followed by mitosis (nuclear division) and cytokinesis (cytoplasm division).

Stem cells are undifferentiated cells that can divide and differentiate into specialised cell types; plant meristems provide a source of plant stem cells.

Surface area to volume ratio decreases as cells get larger, limiting diffusion; this explains why large organisms need transport systems.

Practical: use a calibrated eyepiece graticule and stage micrometer to measure cell size; magnification = image size ÷ actual size.

Monosaccharides (glucose, fructose) are single sugars; disaccharides (sucrose, maltose, lactose) are two sugars joined by a glycosidic bond; polysaccharides include starch, glycogen and cellulose.

Starch is a storage polysaccharide of glucose; glycogen is the animal storage form; cellulose forms plant cell walls and is indigestible to humans.

Proteins are polymers of amino acids; the sequence of amino acids determines the shape and function of the protein.

Lipids are made of glycerol and fatty acids; they are insoluble in water and store more energy per gram than carbohydrates.

Enzyme activity can be measured by rate of product formation or rate of substrate disappearance; plot rate against the variable and read the optimum from the peak.

Denaturation is irreversible loss of active site shape caused by high temperature or extreme pH; it differs from inhibition, which is reversible.

Water is a solvent for metabolic reactions and has a high specific heat capacity, helping to buffer temperature changes in organisms.

Tests must include a control: e.g. a water sample with Benedict's reagent should remain blue to confirm the test is working.

Energy requirements vary with age, sex, activity level and pregnancy; a growing teenager needs more protein per kilogram than an adult.

Food tests: Benedict's (reducing sugar, heat), iodine (starch), Biuret (protein), ethanol emulsion (lipid), DCPIP (vitamin C decolourisation).

Digestive system order: mouth → oesophagus → stomach → small intestine (duodenum, ileum) → large intestine → rectum → anus.

Peristalsis is rhythmic muscular contraction that moves food along the gut; it is involuntary smooth muscle activity.

Absorption in the ileum: glucose and amino acids pass into blood capillaries; fatty acids and glycerol are absorbed into lacteals as chyle.

Tooth decay is caused by acid produced by bacteria feeding on sugar on tooth surfaces; fluoride and reduced sugar intake help prevent it.

Malnutrition includes undernutrition (too few nutrients) and overnutrition (excess energy), both affecting health.

Practical: use a colorimeter or compare colour intensity against a standard to estimate reducing sugar concentration.

ATP is the energy currency of cells; respiration releases energy that is used to synthesise ATP from ADP and phosphate.

Aerobic respiration stages: glycolysis in the cytoplasm, then reactions in the mitochondria; carbon dioxide is released in the later stages.

Lactic acid is broken down in the liver using oxygen; this explains why breathing remains fast after exercise stops.

The breathing rate is controlled by the medulla in response to carbon dioxide concentration in the blood.

In fish, gills use counter-current flow so water and blood flow in opposite directions, maintaining a concentration gradient for oxygen.

In insects, gas exchange occurs through spiracles and tracheae; in plants, stomata and lenticels serve this role.

Investigating respiration: use a respirometer to measure oxygen uptake, or limewater to detect carbon dioxide production.

Exercise increases tidal volume and breathing rate; vital capacity is the maximum volume of air that can be exhaled after a maximum inhalation.

Cardiac cycle: atrial systole fills ventricles; ventricular systole pumps blood into arteries; diastole relaxes the heart and fills the atria.

Valves (atrioventricular and semilunar) prevent backflow of blood; their closure produces heart sounds.

Blood pressure is highest in arteries and lowest in veins; the pulse is the pressure wave from ventricular contraction.

Coronary arteries supply the heart muscle; blockage causes angina or myocardial infarction (heart attack).

Risk factors for coronary heart disease: smoking, high saturated fat intake, obesity, lack of exercise, high blood pressure, family history.

Haemoglobin binds oxygen in the lungs (high oxygen concentration) and releases it in tissues (low oxygen concentration and higher temperature).

Xylem vessels are dead, hollow tubes strengthened with lignin; phloem sieve tubes are living and have companion cells.

Practical: measure pulse rate before and after exercise; plot a bar chart of class results and identify the trend.

The kidney has three roles: removal of urea, adjustment of water content, and adjustment of ion concentration of the blood.

Ultrafiltration occurs at high pressure in the glomerulus; the filtrate contains glucose, ions, water and urea but not proteins or cells.

Selective reabsorption returns all glucose and some ions and water to the blood; remaining filtrate becomes urine.

Negative feedback loop components: receptor, control centre, effector, and response that reverses the original change.

Blood glucose is monitored by the pancreas; insulin is secreted when glucose is high and glucagon when it is low.

Thermoregulation is controlled by the hypothalamus, which triggers sweating, vasodilation, shivering or vasoconstriction.

Kidney failure is treated by dialysis, which removes urea by diffusion across a partially permeable membrane, or by transplant.

Practical: test urine samples with Benedict's or a dipstick to detect glucose, which indicates diabetes.

Neurone structure: dendrites receive impulses, the cell body contains the nucleus, and the axon carries impulses away; the myelin sheath speeds conduction.

Sensory neurones carry impulses to the CNS; motor neurones carry impulses to effectors; relay neurones connect sensory and motor neurones.

The reflex arc sequence: receptor → sensory neurone → relay neurone → motor neurone → effector.

Pupil reflex: bright light → circular muscles contract, radial muscles relax → pupil constricts to protect the retina.

Accommodation: ciliary muscles contract and suspensory ligaments slacken to thicken the lens for near objects; the reverse occurs for distant objects.

Hormone action is slower and longer-lasting than nervous action; hormones travel in blood and act only on target cells with matching receptors.

Phototropism: auxin moves to the shaded side, cells elongate more there, and the shoot bends towards light.

Practical: investigate reaction time using a dropped ruler and calculate the distance fallen; repeat trials and average the results.

Meiosis produces four haploid gametes; fertilisation restores the diploid number; mitosis then produces growth of the embryo.

Crossing over in prophase I and independent assortment in metaphase I generate genetic variation in gametes.

In humans, fertilisation occurs in the oviduct; the zygote divides by mitosis to form a ball of cells that implants in the uterus lining.

Placental transfer: oxygen, glucose, amino acids and antibodies pass to the fetus; carbon dioxide and urea pass to the mother.

The placenta also acts as a barrier to some pathogens but not to alcohol, nicotine or some viruses.

Birth involves oxytocin-stimulated uterine contractions and the breaking of the amniotic sac; the placenta is delivered after the baby.

Plant fertilisation: pollen tube grows down the style to the ovule, where the male nucleus fuses with the egg nucleus.

Seed dispersal by wind, water, animals or explosion reduces competition between offspring and the parent plant.

Genetic diagrams should show parental genotypes, gametes, a Punnett square, and the offspring genotype and phenotype ratios.

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

Sex-linked characteristics are carried on the X chromosome; males are more often affected because they have only one X chromosome.

Pedigree analysis: a trait appearing in offspring of unaffected parents indicates a recessive allele.

DNA replication is semi-conservative; each new molecule contains one original strand and one new strand.

The genetic code is a triplet code; a change in a single base (substitution) may change one amino acid or have no effect.

Genetic engineering involves isolating a gene, inserting it into a vector, and transferring it to a host organism to produce a desired protein.

Selective breeding crosses organisms with desired traits over generations; it reduces genetic variation and can increase inherited disorders.

Sampling: quadrats estimate population size or distribution; the capture-recapture method estimates animal population size using marked individuals.

Capture-recapture formula: population = (number marked in first sample × total in second sample) ÷ number of marked recaptures in second sample.

Decomposition releases carbon dioxide and mineral ions into the soil; it is carried out by bacteria and fungi and is faster in warm, moist conditions.

Eutrophication: fertiliser run-off raises nitrate levels, algae bloom, then decompose, depleting oxygen and killing fish.

Global warming is linked to greenhouse gases (carbon dioxide, methane); consequences include rising sea levels and altered species distribution.

Conservation strategies include protected areas, seed banks, captive breeding, and sustainable resource use.

Indicator species reveal pollution levels: lichens indicate air quality; mayfly larvae indicate clean, oxygen-rich water.

Energy and nutrient cycles differ: energy flows one way and is lost as heat, while nutrients are recycled.

Species definition: a group of organisms with similar characteristics that can interbreed to produce fertile offspring.

Evolution evidence includes fossils, comparative anatomy (homologous structures), embryology, and molecular (DNA and protein) comparisons.

Natural selection requires heritable variation, competition for resources, differential survival, and reproduction of favourable alleles.

Speciation occurs when populations become isolated (geographically or reproductively) and accumulate different alleles over time.

Extinction occurs when a species cannot adapt to environmental change, competition or habitat loss.

Classification is now informed by DNA sequence comparisons; molecular data can reveal relationships not obvious from appearance.

Adaptations to extreme environments: xerophytes reduce water loss with thick cuticles and sunken stomata; halophytes tolerate high salt.

Practical: use a dichotomous key, following paired statements step by step, to identify an unknown organism.

Immune response: antigens on a pathogen trigger lymphocytes to produce specific antibodies; memory cells give a faster response on re-exposure.

Vaccines contain antigens from weakened or killed pathogens; they provide active immunity without causing disease.

Antibiotic resistance arises because resistant bacteria survive treatment and reproduce, passing resistance alleles to offspring.

Non-communicable diseases (e.g. coronary heart disease, diabetes, cancer) are influenced by lifestyle and genetics, not transmitted between people.

Cancer results from uncontrolled cell division caused by mutations in genes controlling the cell cycle.

Practical design: state a hypothesis, control variables, use a suitable sample size, and repeat to improve reliability.

Data presentation: line graphs for continuous variables, bar charts for categories; label axes with quantities and units.

Evaluate conclusions by considering anomalies, sample size, control of variables, and whether the data support the stated hypothesis.

Using a light microscope: start with the lowest power objective, focus with the coarse knob, then use the fine knob at higher power.

Total magnification = eyepiece magnification × objective lens magnification.

Stains improve contrast: iodine for starch in plant cells, methylene blue for animal cells, eosin for cytoplasm.

Onion epidermis cells show a cell wall, nucleus and vacuole; cheek cells show a membrane, cytoplasm and nucleus but no wall.

Plasmolysis occurs when a plant cell loses water in a concentrated solution; the vacuole shrinks and the membrane pulls away from the wall.

Osmosis investigation: place potato cylinders in different sucrose concentrations, measure mass change, and find the concentration giving zero change.

Diffusion investigation: use agar cubes containing indicator to show that smaller cubes decolourise faster due to a larger surface area to volume ratio.

Record observations in tables with units; calculate percentage change in mass as (final mass − initial mass) ÷ initial mass × 100.

Enzyme practical: use amylase and starch, test samples with iodine at timed intervals, and record the time for starch to disappear.

Control variables in enzyme experiments include pH, temperature, substrate concentration and enzyme concentration; change only one at a time.

Use a water bath to maintain constant temperature; use buffer solutions to maintain constant pH.

Plot rate of reaction against temperature or pH; the peak of the curve gives the optimum condition.

Qualitative food tests should include a positive control (known substance) and a negative control (water).

Benedict's test: heat the sample in a water bath for several minutes; a brick-red precipitate indicates a reducing sugar.

Emulsion test: add ethanol then water; a cloudy white emulsion indicates lipid.

Calculate rates as amount of product ÷ time, and report units clearly, e.g. cm³ min⁻¹.

Energy in food can be estimated by burning a food sample under a boiling tube of water and measuring the temperature rise.

Energy released (J) = mass of water (g) × 4.2 × temperature rise (°C).

Control variables in food energy experiments include the volume of water, starting temperature, and distance of the flame from the tube.

Sources of error in burning food: heat lost to surroundings, incomplete combustion, and variable sample mass.

Test a range of foods (e.g. peanut, bread, biscuit) and compare energy values per gram.

Use a thermometer with a suitable range and read it at eye level to avoid parallax error.

Record results in a table and calculate a mean for repeated trials; identify and exclude anomalous values.

Vitamin C content can be estimated by timing the decolourisation of DCPIP solution.

Use limewater to test for carbon dioxide: it turns cloudy or milky when carbon dioxide is bubbled through it.

Use a respirometer to measure oxygen uptake by organisms such as germinating seeds or woodlice.

Control experiments in respiration practicals use dead or boiled organisms to show that the change is due to living processes.

Measure breathing rate by counting chest movements per minute; measure pulse rate at the wrist or neck.

Exercise investigations should control duration and intensity of exercise, and allow rest periods between trials.

Record results before and after exercise and calculate the increase in rate.

Vital capacity can be estimated with a peak flow meter or spirometer; report results in dm³ or litres.

Plot a line graph of breathing rate against exercise time and describe the trend.

Measure pulse rate by counting beats for 30 seconds and multiplying by 2; repeat for reliability.

Investigate the effect of exercise on pulse rate: record resting rate, exercise for a fixed time, then record recovery rate.

Control variables include the type and duration of exercise, the subject's fitness, and the time of measurement.

Dissect a heart or use a model to identify atria, ventricles, valves and major vessels.

Investigate transpiration using a potometer; measure the distance moved by an air bubble in a set time.

Control variables in potometer experiments include light intensity, temperature, humidity and wind speed.

Calculate transpiration rate as distance moved ÷ time, and compare rates under different conditions.

Use a microscope to compare artery, vein and capillary cross-sections; note wall thickness and lumen size.

Test urine with a dipstick or Benedict's reagent to detect glucose, which indicates diabetes mellitus.

Compare urine samples before and after exercise or fluid intake to show changes in concentration.

Investigate temperature regulation by measuring skin temperature or sweat production before and after exercise.

Control variables in homeostasis practicals include room temperature, clothing and duration of activity.

Use a data logger to record skin temperature continuously and plot a graph against time.

Investigate osmoregulation by measuring urine volume after drinking different volumes of water.

Record results in tables with units and calculate means from repeated trials.

Interpret data by identifying the set point and the direction of the response that restores it.

Investigate reaction time using a dropped ruler: reaction time = √(2 × distance ÷ acceleration due to gravity).

Repeat reaction time trials and calculate a mean; compare results with and without a distraction.

Investigate tropisms by growing seedlings in light from one direction or in a clinostat.

Control variables in tropism experiments include light intensity, temperature, moisture and the age of seedlings.

Record the direction and angle of growth and present results as a labelled diagram or table.

Investigate the effect of caffeine or exercise on reaction time while controlling the subject and time of day.

Use a stopwatch or light gate for accurate timing; take repeat readings to reduce random error.

Interpret results by linking the observed response to the stimulus and the receptor involved.

Examine flower structure by dissecting a flower and identifying sepals, petals, stamens, anther, stigma, style and ovary.

Use a microscope to view pollen grains and compare their shape between species.

Investigate seed germination by varying water, oxygen, temperature or light, with a control for each condition.

Control variables in germination experiments include seed type, number of seeds and volume of water.

Record the number of seeds germinated each day and plot a graph of germination percentage against time.

Investigate the effect of temperature on germination rate by comparing days to first germination.

Interpret results by identifying which conditions are required for germination and which are not.

Use a hand lens or microscope to observe the developing embryo in a soaked bean seed.

Use a Punnett square to predict offspring ratios and compare predictions with observed class data.

Investigate inherited characteristics in a class survey (e.g. tongue rolling, earlobe attachment) and calculate phenotype frequencies.

Control variables in genetic surveys include the population sampled and the definition of the characteristic.

Use a chi-squared test to compare observed and expected ratios and decide whether differences are significant.

Extract DNA from plant tissue using detergent, salt and ethanol; DNA appears as a white precipitate.

Interpret pedigree diagrams to determine the mode of inheritance of a characteristic.

Record genetic data in tables and present ratios as fractions or percentages.

Evaluate genetic data by considering sample size and possible misclassification of phenotypes.

Use a quadrat to sample plant distribution; place it randomly to avoid bias and record percentage cover or frequency.

Use a transect to investigate how species distribution changes along an environmental gradient.

Capture-recapture: mark animals, release them, then sample again and calculate population size using the formula.

Control variables in ecology practicals include quadrat size, sampling time and the area sampled.

Measure abiotic factors with a light meter, thermometer, pH meter and moisture meter.

Record results in a table and calculate means or percentages for each site.

Present results as a bar chart or kite diagram to compare distribution between sites.

Interpret results by linking distribution to abiotic and biotic factors.

Use a dichotomous key by starting at the first pair of statements and following the instruction for the observed feature.

Construct a dichotomous key with paired, mutually exclusive statements based on observable features.

Compare homologous structures in different organisms to infer common ancestry.

Use a microscope to compare cell types and classify organisms into kingdoms.

Record classification data in a table and justify each grouping with observed features.

Interpret fossil data by comparing rock strata and identifying changes over time.

Evaluate classification by considering that molecular data may revise traditional groupings.

Present evolutionary relationships as a simple branching diagram based on shared features.

Investigate the effect of antibiotics on bacterial growth using agar plates and paper discs; measure the zone of inhibition.

Aseptic technique: sterilise equipment, work near a flame, and seal plates to prevent contamination.

Control variables in antibiotic experiments include the type of agar, incubation temperature and disc size.

Record zone diameters in a table and compare the effectiveness of different antibiotics.

Investigate the effect of hand washing on microbial growth by comparing agar plates inoculated before and after washing.

Interpret data by identifying the most effective treatment and considering the reliability of the results.

Evaluate practical work by identifying sources of error and suggesting improvements.

Report results with units and use appropriate significant figures.