LGCSE Biology (0180) cheat sheet

Cell theory: all living organisms are composed of cells; the cell is the basic unit of structure and function; new cells arise from pre-existing cells by division.

Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles; they have a single circular DNA molecule, 70S ribosomes, a cell wall and often plasmids and flagella.

Eukaryotic cells have a true nucleus bounded by a double membrane, linear chromosomes, 80S ribosomes and membrane-bound organelles including mitochondria, chloroplasts and the Golgi apparatus.

Magnification = image size ÷ actual size; conversion factor 1 mm = 1000 µm = 1 000 000 nm.

Diffusion: net movement of particles from high to low concentration down a gradient, passive and proportional to surface area, concentration gradient and temperature, and inversely related to distance.

Osmosis: net movement of water across a partially permeable membrane from higher to lower water potential; water potential of pure water at atmospheric pressure is zero.

Active transport moves substances against a concentration gradient using ATP and carrier proteins, e.g. mineral ion uptake by root hair cells.

Enzymes are biological catalysts that lower activation energy; each has an active site specific to its substrate, and activity peaks at optimum temperature and pH.

Rate of photosynthesis increases with light intensity, carbon dioxide concentration and temperature until another factor becomes limiting; limiting-factor graphs plateau at the limiting factor's ceiling.

Levels of organisation: cell → tissue → organ → organ system → organism; a tissue is a group of similar cells performing a shared function.

The digestive system: mechanical breakdown by teeth and peristalsis, chemical digestion by amylase (starch to maltose), protease (protein to amino acids) and lipase (fat to fatty acids and glycerol).

Bile, produced by the liver and stored in the gallbladder, emulsifies fats and neutralises acidic chyme to give an alkaline optimum pH for pancreatic enzymes.

Villi in the small intestine increase surface area; each has a thin epithelium, dense capillary network and lacteal for absorption of glucose and amino acids into blood and fats into lymph.

Human blood: plasma transports dissolved substances; red blood cells carry oxygen via haemoglobin; white blood cells defend against pathogens; platelets trigger clotting.

Double circulation: blood passes through the heart twice per circuit; the left ventricle wall is thicker because it pumps blood at higher pressure around the whole body.

Gas exchange in alveoli relies on a large surface area, thin walls, moist surface, and a steep concentration gradient maintained by ventilation and blood flow.

Transpiration is the loss of water vapour from leaves through stomata; rate rises with temperature, light, wind and low humidity, and falls when stomata close.

Homeostasis keeps internal conditions stable by negative feedback, e.g. insulin lowers blood glucose and glucagon raises it, and sweating or shivering adjusts body temperature.

DNA is a double helix of nucleotides; a nucleotide comprises a deoxyribose sugar, a phosphate group and one of four bases: adenine, thymine, cytosine, guanine.

Complementary base pairing: A pairs with T and C pairs with G; the sequence of bases forms the genetic code, with each amino acid coded by a triplet (codon).

A gene is a section of DNA that codes for a protein; alleles are different versions of the same gene at the same locus on homologous chromosomes.

Genotype is the allele combination (homozygous or heterozygous); phenotype is the observable characteristic resulting from genotype and environment.

Monohybrid cross ratio: heterozygous × heterozygous (Aa × Aa) gives 3:1 dominant to recessive phenotype and 1:2:1 genotype ratio.

A test cross with a homozygous recessive individual distinguishes a homozygous dominant from a heterozygous dominant by the offspring ratios produced.

Sex determination in humans: females are XX and males are XY; the father's sperm determines sex, giving a 1:1 expected ratio of male to female offspring.

Codominance and incomplete dominance produce intermediate or dual phenotypes, e.g. AB blood group, so the 3:1 ratio does not apply.

Mutations are random changes in DNA; they may be silent, harmful or beneficial, and are the source of new alleles for natural selection.

An ecosystem comprises a community of organisms interacting with each other and with the abiotic environment; a habitat is the place where an organism lives.

A food chain shows energy flow; arrows point in the direction of energy transfer, and each trophic level holds far less energy than the one below.

Only about 10% of energy is transferred between trophic levels; the rest is lost as heat from respiration, in movement and in undigested material.

Pyramids of biomass and numbers can be inverted, but a pyramid of energy is always broadest at the base.

The carbon cycle: photosynthesis removes carbon dioxide; respiration, combustion and decomposition release it back to the atmosphere.

The nitrogen cycle: nitrogen fixation, nitrification by nitrifying bacteria, assimilation, decomposition to ammonium compounds, and denitrification returning nitrogen to air.

Decomposers such as bacteria and fungi break down dead matter, recycling mineral ions and carbon back into the ecosystem.

Population size is affected by birth rate, death rate, immigration, emigration, competition, predation and disease; carrying capacity is the maximum the environment supports.

Bioaccumulation of non-biodegradable pollutants such as mercury or DDT increases with each trophic level, so top predators receive the highest dose.

Asexual reproduction produces genetically identical offspring from one parent by mitosis, e.g. runners, tubers, budding; sexual reproduction involves gamete fusion and genetic variation.

Meiosis halves the chromosome number from diploid to haploid and produces four genetically different gametes through independent assortment and crossing over.

Human male reproductive system: testes produce sperm and testosterone; the scrotum keeps testes cooler than body temperature for sperm production.

Human female reproductive system: ovaries produce ova and oestrogen; fertilisation normally occurs in the oviduct, and the zygote implants in the uterus lining.

The menstrual cycle: FSH matures a follicle, oestrogen thickens the uterine lining, LH triggers ovulation around day 14, and progesterone maintains the lining.

Fertilisation is the fusion of a haploid sperm nucleus and a haploid egg nucleus to form a diploid zygote, restoring the full chromosome number.

The placenta allows exchange of oxygen, nutrients and waste between mother and fetus but does not normally allow mixing of the two blood supplies.

In flowers, pollination transfers pollen from anther to stigma; fertilisation follows pollen tube growth, forming a seed from the ovule and fruit from the ovary.

Growth is an increase in size or mass; development is the change in form and function as cells differentiate into specialised tissues and organs.

A stimulus is a change in the environment detected by a receptor; the response is the resulting action by an effector such as a muscle or gland.

The nervous pathway: stimulus → receptor → sensory neurone → relay neurone in the CNS → motor neurone → effector, with synapses between neurones.

A reflex arc is an involuntary, rapid response that bypasses conscious thought, protecting the body from harm, e.g. the withdrawal reflex and pupil reflex.

A synapse is a junction where a chemical neurotransmitter diffuses across the gap to transmit the impulse from one neurone to the next.

The eye: the iris controls pupil diameter, the lens changes shape for accommodation, and the retina contains rods for dim light and cones for colour.

In bright light the pupil constricts via circular muscles; in dim light it dilates via radial muscles, and the lens becomes fatter to focus on near objects.

Hormones are chemical messengers secreted by glands into the blood, acting on target organs more slowly but for longer than nerve impulses.

Auxin in shoots promotes cell elongation on the shaded side, causing phototropism toward light; in roots it inhibits elongation, so roots grow away from light.

Tropisms are directional growth responses: phototropism to light, geotropism or gravitropism to gravity, and hydrotropism to water.

Evolution is the change in the inherited characteristics of populations over generations; it is driven by natural selection acting on variation.

Darwin's theory: individuals vary, more offspring are produced than survive, those with advantageous alleles survive and reproduce, passing alleles to offspring.

An adaptation is an inherited feature that increases an organism's chance of survival and reproduction in its environment.

Adaptations may be structural (insulation, camouflage, streamlined shape), behavioural (migration, nocturnal activity) or physiological (water retention, enzyme tolerance).

Evidence for evolution includes fossil records, homologous structures, comparative biochemistry and DNA sequence similarity, and observed antibiotic resistance.

Antibiotic resistance arises because random mutations give some bacteria resistance; antibiotic use selects these survivors, which then multiply.

A species is a group of organisms with similar characteristics that can interbreed to produce fertile offspring; isolation can lead to speciation.

Extinction occurs when a species cannot adapt to environmental change, competition, predation or habitat loss, and its last individuals die.

A hypothesis is a testable prediction with a clear reason; the independent variable is what is changed, the dependent variable is what is measured.

Control variables must be kept constant to make a fair test; a control group receives no treatment or a standard treatment for comparison.

Reliability increases with repeat readings; validity depends on measuring the intended variable and controlling relevant variables.

Accuracy is closeness to the true value; precision is the degree of consistency between repeated measurements, shown by small spread.

To calculate a mean, sum the valid readings and divide by their number; discard obvious anomalies before calculating, and state why they are anomalous.

Percentage change = (change ÷ original value) × 100; percentage yield of a reaction or efficiency of energy transfer uses the same ratio form.

Uncertainty of a single reading is usually half the smallest division of the scale; report results with units and an appropriate number of significant figures.

A line graph suits continuous data; a bar chart suits categorical or discontinuous data; draw a line of best fit rather than joining every point.

Aseptic technique in microbiology: sterilise equipment, flame the loop, work near a flame, seal plates and incubate at a safe temperature to avoid culturing pathogens.

Mitosis produces two genetically identical diploid daughter cells and is used for growth, repair and asexual reproduction.

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

Surface area to volume ratio falls as a cell increases in size, which limits diffusion and explains why large organisms need exchange surfaces and transport systems.

Respiration: aerobic respiration releases energy from glucose using oxygen, producing carbon dioxide and water, while anaerobic respiration in muscle produces lactic acid.

Photolysis in photosynthesis uses light energy to split water, releasing oxygen and providing hydrogen to reduce carbon dioxide into carbohydrate.

The heart's natural pacemaker is the sinoatrial node in the right atrium, which initiates each heartbeat; the cardiac cycle includes atrial systole, ventricular systole and diastole.

Coronary arteries supply the heart muscle itself; blockage can cause oxygen shortage, chest pain or a heart attack.

Blood vessels: arteries carry blood away from the heart at high pressure, veins return it at low pressure with valves, and capillaries are one cell thick for exchange.

The respiratory system: the trachea, bronchi and bronchioles conduct air, and the diaphragm and intercostal muscles produce ventilation by changing thorax volume.

Excretion removes metabolic waste: kidneys filter blood to form urine, removing urea and excess ions and water, while the lungs excrete carbon dioxide.

A Punnett square predicts offspring genotype probabilities; a 1:1 ratio indicates a heterozygous parent crossed with a homozygous recessive parent.

Chromosomes are found in homologous pairs in body cells; human body cells contain 46 chromosomes (23 pairs) and gametes contain 23.

Sex-linked characteristics such as red-green colour blindness are carried on the X chromosome, so males are affected more often than females.

Variation may be genetic (caused by alleles and mutation) or environmental (diet, exercise, climate); continuous variation shows a normal distribution curve.

Selective breeding by humans selects desired phenotypes over generations, reducing genetic diversity and increasing susceptibility to disease.

Quadrats estimate population size or distribution; random placement avoids bias, and percentage cover or counts are averaged over several quadrats.

The mark-release-recapture method estimates population size: total = (number marked in first sample × total in second sample) ÷ number marked in second sample.

Biotic factors include competition, predation, disease and food availability; abiotic factors include light, temperature, moisture, pH and oxygen concentration.

Sustainability means using resources at a rate that allows replacement or regeneration, e.g. replanting forests, fishing quotas and reducing pollution.

Deforestation reduces biodiversity, increases soil erosion and flooding, and raises atmospheric carbon dioxide by removing a carbon sink.

Contraception methods include hormonal pills that prevent ovulation, barrier methods that prevent sperm reaching the egg, and intrauterine devices.

In vitro fertilisation collects ova and sperm, fertilises them outside the body, and implants an embryo into the uterus.

Seed dispersal by wind, water, animals or explosion reduces competition between parent and offspring for light, water and minerals.

Germination requires water, oxygen and a suitable temperature; the seed absorbs water, enzymes activate and the radicle emerges first.

Pregnancy hormones: hCG maintains the corpus luteum early on, and progesterone maintains the uterine lining throughout pregnancy.

The central nervous system comprises the brain and spinal cord; the peripheral nervous system connects receptors and effectors to it.

The cerebrum controls conscious thought and memory; the cerebellum coordinates balance and movement; the medulla controls involuntary actions such as breathing.

Insulin is secreted by the pancreas when blood glucose is high and promotes glucose uptake and glycogen storage in the liver.

Type 1 diabetes results from insufficient insulin production and is treated with insulin injections; Type 2 involves reduced responsiveness to insulin.

Adrenaline prepares the body for action by increasing heart rate, breathing rate and blood glucose concentration.

Classification groups organisms by shared characteristics; the binomial system uses genus and species names, e.g. Homo sapiens.

The five kingdoms are animals, plants, fungi, protoctists and prokaryotes; viruses are not placed in these kingdoms because they are not cellular.

Selective breeding and genetic modification differ: selective breeding crosses existing organisms, while genetic modification transfers a gene directly.

Genetic engineering steps: identify and isolate the desired gene, insert it into a vector such as a plasmid, and transfer it into the host organism.

Antibiotic resistance in bacteria spreads rapidly because bacteria reproduce quickly and can transfer plasmids between cells.

Random sampling reduces bias; systematic sampling along a transect is used to study how a factor changes across a habitat.

When investigating enzyme activity, control temperature, pH and substrate concentration while varying only the factor under test.

A pilot study identifies suitable ranges and intervals for the independent variable before the main investigation.

Ethical and safety considerations include informed consent, avoiding harm to organisms, and correct disposal of chemicals and biological material.

Conclusions must be justified by the data and should state whether the hypothesis was supported, and suggest sources of error and improvements.

Photosynthesis equation: carbon dioxide + water → glucose + oxygen, using light energy absorbed by chlorophyll.

The leaf is adapted for photosynthesis with a broad flat shape, thin lamina, palisade mesophyll rich in chloroplasts and stomata for gas exchange.

Xylem transports water and mineral ions upward and provides support; phloem transports sucrose and amino acids in either direction.

Translocation is the movement of dissolved sugars through phloem sieve tubes, driven by pressure differences between source and sink.

Root hair cells increase surface area for absorption of water by osmosis and mineral ions by active transport.

DNA replication occurs before cell division so each daughter cell receives a complete copy of the genetic material.

The genetic code is degenerate because more than one codon can specify the same amino acid, though each codon specifies only one.

A mutation that changes a single base may alter one amino acid in the protein, changing its shape and function.

Pedigree analysis uses the appearance of a trait across generations to infer whether an allele is dominant, recessive or sex-linked.

Genetic screening and counselling use family history and genetic testing to estimate the risk of inherited disorders in offspring.

Primary succession starts on bare rock with pioneer species such as lichens, which break down rock to form soil.

A climax community is the stable final stage of succession, typically dominated by the tallest trees the climate supports.

Eutrophication: nutrient runoff causes algal blooms; decomposers then deplete oxygen, killing fish and other aerobic organisms.

Greenhouse gases such as carbon dioxide and methane trap infrared radiation, contributing to global warming and climate change.

Conservation measures include protected areas, seed banks, captive breeding and legal protection of endangered species.

The amnion encloses amniotic fluid, which cushions the fetus and maintains a constant temperature.

Pregnancy is measured in weeks; the fetus is fully formed by about 12 weeks and organs mature until birth at about 40 weeks.

Breast milk provides antibodies and balanced nutrients, while formula provides a controlled but non-immune composition.

The umbilical cord contains blood vessels connecting the fetus to the placenta; the umbilical vein carries oxygenated blood to the fetus.

Cloning produces genetically identical offspring; plant cuttings and tissue culture are common techniques.

Negative feedback reverses a change to restore the set point; positive feedback amplifies a change and is less common.

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

Osmoregulation by the kidneys maintains water balance; ADH increases water reabsorption when blood is too concentrated.

The reflex arc involves a rapid response before the brain processes the stimulus, though the brain is informed afterwards.

Plant responses to touch and gravity are mediated by growth substances such as auxin and gibberellin.

Fossils provide evidence of organisms that lived in the past and can show gradual change in form over time.

Homologous structures, such as the forelimbs of different vertebrates, suggest common ancestry.

Adaptations to extreme environments include thick fur and fat for insulation in polar regions and water storage in desert plants.

Isolation of populations by geography or behaviour can lead to separate gene pools and eventually speciation.

Human activities such as hunting, habitat destruction and pollution can accelerate extinction rates.

Data loggers improve accuracy by taking frequent readings and removing human reaction time from measurements.

A scatter graph can reveal a correlation, but correlation does not prove causation.

When comparing means, note the overlap of ranges; overlapping ranges suggest the difference may not be significant.

A risk assessment identifies hazards, evaluates the risk and states control measures before practical work begins.

Recording results in a table with clear headings, units and the same number of decimal places aids comparison and analysis.

The cell membrane is a phospholipid bilayer with embedded proteins, described by the fluid mosaic model, controlling what enters and leaves.

Chloroplasts contain chlorophyll in thylakoid membranes and are the site of photosynthesis; mitochondria are the site of aerobic respiration.

Ribosomes are the site of protein synthesis and are found free in the cytoplasm or attached to the rough endoplasmic reticulum.

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

Turgor pressure in plant cells provides support; wilting occurs when cells lose water and turgor falls.

The skin, mucous membranes and stomach acid form the first line of defence against pathogens.

White blood cells defend by phagocytosis, engulfing and digesting pathogens, or by producing antibodies specific to antigens.

Vaccination introduces a weakened or inactive pathogen to stimulate antibody production and memory cells.

Antibiotics kill bacteria but are ineffective against viruses because viruses lack the cellular structures antibiotics target.

Blood clotting at a wound prevents blood loss and reduces the entry of pathogens.

The expected ratio from a monohybrid cross is a probability, best observed with large numbers of offspring.

Gene expression can be influenced by the environment, so identical twins may show different phenotypes.

Genetic modification inserts a gene from one organism into another, producing, for example, insulin-producing bacteria.

Alleles may be dominant, recessive or codominant; in codominance both alleles are expressed in the phenotype.

Carriers are heterozygous individuals who possess a recessive allele but do not show the recessive phenotype.

Producers are photosynthetic organisms that convert light energy into chemical energy stored in organic molecules.

Consumers feed on other organisms; decomposers break down dead material and return nutrients to the soil.

Competition occurs when organisms need the same limited resource, such as light, water, food or space.

Predator-prey cycles show a lag: an increase in prey is followed by an increase in predators, which then reduces the prey.

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

Pollination is the transfer of pollen from anther to stigma; self-pollination reduces genetic variation.

The pollen tube grows down the style to the ovule, delivering the male gamete for fertilisation.

Seed structure: the testa protects, the cotyledon stores food and the embryo contains the radicle and plumule.

Fruit formation follows fertilisation; the ovary wall becomes the fruit and the ovule becomes the seed.

Vegetative propagation produces clones, so all offspring share the parent's characteristics and any disease susceptibility.

Receptors are specialised cells that detect specific stimuli, such as rods and cones in the retina and hair cells in the ear.

The iris reflex adjusts pupil size in response to light intensity and is controlled by the autonomic nervous system.

Hormones travel in the blood and act only on target cells that possess the matching receptor.

Plant hormones such as auxin are produced in the shoot tip and move to regions of elongation.

The reflex arc provides a rapid response because the pathway is short and does not require conscious processing.

Variation within a species is the raw material on which natural selection acts.

Selection pressures include predation, competition for food, disease and climate, and they determine which alleles persist.

Speciation requires isolation, genetic divergence and eventually reproductive isolation between populations.

Antibiotic resistance is an example of natural selection observable within a human lifetime.

Conservation of genetic diversity helps populations adapt to future environmental change.

A fair test changes only one variable at a time while keeping all other relevant variables constant.

Repeat measurements should be averaged to reduce the effect of random error.

Systematic errors shift all readings in the same direction and cannot be removed by averaging.

The choice of apparatus determines precision; a measuring cylinder is less precise than a burette or pipette.

Scientific claims should be evaluated for sample size, control of variables and whether the data support the conclusion.

Diffusion rate increases with a steeper concentration gradient, a larger surface area, a shorter distance and a higher temperature.

Osmosis in plant cells: in pure water the cell becomes turgid; in a concentrated solution it becomes flaccid and may plasmolyse.

Enzymes are denatured by high temperature because the active site changes shape; low temperature slows activity without denaturing.

The optimum pH for a given enzyme is the pH at which its rate of reaction is highest.

Photosynthesis stores energy in glucose, while respiration releases energy from glucose for cellular processes.

The alimentary canal includes the mouth, oesophagus, stomach, small intestine and large intestine, each with a specialised role.

Peristalsis is the wave of muscular contraction that moves food along the gut.

Absorption of water and mineral ions occurs mainly in the large intestine.

The liver processes nutrients, stores glycogen and detoxifies harmful substances.

The lungs excrete carbon dioxide, and the kidneys excrete urea, excess water and salts.

The genotype ratio from Aa × Aa is 1 AA : 2 Aa : 1 aa, giving a 3:1 phenotype ratio with complete dominance.

A back cross to the homozygous recessive parent reveals the genotype of an unknown dominant individual.

The sex chromosomes determine sex, and the other 22 pairs are autosomes.

Sex-linked recessive disorders appear more often in males because they have only one X chromosome.

Mutation rates can be increased by mutagens such as radiation and certain chemicals.

Energy enters an ecosystem through photosynthesis and is lost at each trophic level, so food chains are usually short.

Decomposition is faster in warm, moist, aerobic conditions.

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

The nitrogen cycle includes nitrogen fixation, nitrification, assimilation and denitrification.

Human activities such as burning fossil fuels and deforestation alter the carbon cycle and increase atmospheric carbon dioxide.

Meiosis produces four haploid gametes, each genetically different from the parent cell.

Fertilisation restores the diploid chromosome number in the zygote.

The placenta allows exchange of gases, nutrients and waste between mother and fetus.

Pregnancy hormones maintain the uterine lining and support fetal development.

Asexual reproduction produces clones and is rapid but reduces genetic variation.

The nervous system uses electrical impulses along neurones, while the endocrine system uses hormones in the blood.

Reflex actions are involuntary and protect the body from harm.

The eye adjusts to light intensity and distance through the iris reflex and accommodation.

Auxin causes phototropism in shoots and geotropism in roots.

Negative feedback maintains stable internal conditions such as temperature and blood glucose.

Natural selection acts on variation, and advantageous alleles become more common over generations.

Fossils, comparative anatomy and DNA evidence support the theory of evolution.

Adaptations improve survival and reproduction in a particular environment.

Antibiotic resistance demonstrates natural selection in action.

Extinction results when a species cannot adapt to changing conditions.

Identify the independent, dependent and control variables before starting an investigation.

Use repeat readings to improve reliability and discard anomalies before calculating a mean.

Present data in an appropriate table or graph with labelled axes and units.

Evaluate the method for accuracy, precision, reliability and validity.

Draw conclusions supported by the data and suggest improvements to the method.