BGCSE Biology 0572 cheat sheet

Cell theory: all living organisms are made of cells, the cell is the basic unit of life, and new cells arise from pre-existing cells by division.

Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles; eukaryotic cells have a true nucleus and organelles such as mitochondria and chloroplasts.

Plant cells have a cellulose cell wall, a large permanent vacuole and chloroplasts; animal cells lack these but may have small temporary vacuoles and centrioles.

Magnification = image size ÷ actual size; convert all lengths to the same unit before dividing.

Diffusion moves particles from high to low concentration down a 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 using energy from respiration and carrier proteins in the membrane.

Enzymes are protein catalysts with a specific active site; rate rises with temperature or substrate concentration up to an optimum, then falls as the enzyme denatures.

Photosynthesis: carbon dioxide + water → glucose + oxygen, requiring light and chlorophyll; it stores energy, while respiration releases it.

Balanced diet components: carbohydrates (energy), proteins (growth and repair), lipids (energy and insulation), vitamins, minerals, water and dietary fibre.

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

Digestion is the breakdown of large insoluble molecules into small soluble ones by mechanical action and enzymes, so they can be absorbed.

Key enzymes: amylase converts starch to maltose; protease converts proteins to amino acids; lipase converts lipids to fatty acids and glycerol.

Bile is made in the liver, stored in the gall bladder, emulsifies fats and neutralises stomach acid to give alkaline conditions for pancreatic enzymes.

Absorption occurs mainly in the small intestine, whose villi increase surface area; each villus has a thin wall and a rich blood supply.

The alimentary canal order: mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus.

Teeth: incisors cut, canines tear, premolars and molars crush; dental decay results from acid produced by bacteria acting on sugar.

Aerobic respiration: glucose + oxygen → carbon dioxide + water + energy; it occurs mainly in mitochondria.

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

Breathing mechanism: intercostal muscles and diaphragm contract → thorax volume increases → pressure falls → air enters the lungs.

Exhaled air contains less oxygen, more carbon dioxide and more water vapour than inhaled air; nitrogen content is essentially unchanged.

Alveoli are efficient gas exchange surfaces because they are numerous, thin-walled, moist and surrounded by capillaries, giving a steep diffusion gradient.

Smoking damages cilia and alveoli, causing smoker's cough, bronchitis, emphysema and increasing cancer risk; nicotine is addictive.

Plants exchange gases through stomata, which open and close via guard cells; lenticels serve woody stems.

Respiration is a chemical process releasing energy in cells; breathing is the mechanical movement of air - the two must not be confused.

Blood components: red cells (haemoglobin, oxygen transport), white cells (defence), platelets (clotting) and plasma (carries dissolved substances and heat).

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

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

Arteries carry blood away from the heart under high pressure with thick elastic walls; veins carry blood back and have valves; capillaries are one cell thick.

The left ventricle wall is the thickest because it pumps blood at high pressure around the whole body.

Transpiration is water loss as vapour from leaves; it pulls water up the xylem, and rate increases with light, temperature, wind and low humidity.

Translocation moves dissolved sugars through phloem from leaves to growing and storage regions.

Root hair cells absorb water by osmosis and mineral ions by active transport; xylem vessels are dead, hollow tubes.

Excretion is removal of metabolic waste such as carbon dioxide, urea and excess water and salts; egestion removes undigested food, which is not excretion.

The kidney removes urea by filtration and reabsorption; useful substances such as glucose and most water are reabsorbed into the blood.

Urine composition varies with water intake: low intake gives small volumes of concentrated urine; high intake gives large dilute urine.

Homeostasis maintains a constant internal environment around a set point using negative feedback.

Thermoregulation: shivering and vasoconstriction conserve heat; sweating and vasodilation lose heat.

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

Diabetes: type 1 involves insufficient insulin secretion; type 2 involves reduced cell sensitivity to insulin.

The skin is an excretory and homeostatic organ, containing sweat glands, hair erector muscles, capillaries and receptors.

A stimulus is a change in the environment detected by a receptor; the response is the resulting reaction of an effector.

The reflex arc pathway: receptor → sensory neurone → relay neurone in the spinal cord → motor neurone → effector.

Reflex actions are rapid and automatic and protect the body because they bypass conscious decision-making in the brain.

Synapses are junctions between neurones; the impulse crosses as a chemical transmitter, so transmission is one-way.

The eye: the iris controls pupil size, the lens focuses by changing shape, and the retina contains light receptor cells.

In bright light the pupil constricts via the circular iris muscles; in dim light it dilates; the blind spot has no receptors.

Tropisms are directional growth responses: shoots are positively phototropic and negatively geotropic; roots are the reverse.

Auxins are plant hormones that accumulate on the shaded side, causing cell elongation and bending toward light.

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

Nerve impulses are electrical along neurones but chemical across synapses; myelin speeds conduction.

Hormones are chemicals secreted by glands into the blood, acting on target organs more slowly and for longer than nervous impulses.

Adrenaline from the adrenal glands raises heart rate, breathing rate and blood glucose in readiness for action.

The pituitary gland is controlled by the hypothalamus and releases hormones that regulate other glands, including growth hormone.

Thyroxine from the thyroid gland controls metabolic rate; iodine is required for its synthesis.

Insulin and glucagon from the pancreas act antagonistically to control blood glucose concentration.

Comparing control systems: nervous - electrical, fast, brief, along neurones; endocrine - chemical, slower, longer lasting, via the blood.

Sexual reproduction fuses gametes to form genetically varied offspring; asexual reproduction produces genetically identical clones from one parent.

Male reproductive parts: testes produce sperm and testosterone; sperm travel through sperm duct, urethra and penis.

Female parts: ovaries release eggs and secrete oestrogen; fertilisation normally occurs in the oviduct, and the zygote implants in the uterus lining.

The menstrual cycle: oestrogen builds the uterus lining, ovulation occurs around day 14, and progesterone maintains the lining.

Pregnancy: the placenta exchanges nutrients, oxygen and waste between mother and fetus; the umbilical cord connects them; the amniotic fluid cushions the fetus.

Growth is a permanent increase in size and mass; development is the acquisition of new structures and functions.

Growth can be measured by height, mass or dry mass; growth curves show sigmoid patterns with a rapid adolescent phase.

Flower structure: sepals protect, petals attract pollinators, stamens produce pollen and carpels contain ovules; pollination precedes fertilisation.

A gene is a section of DNA coding for a protein; alleles are alternative forms of a gene; the genotype is the allele combination and the phenotype the visible characteristic.

Dominant alleles are expressed when present; recessive alleles only show in the homozygous state; homozygous means identical alleles and heterozygous means different alleles.

Monohybrid cross of two heterozygotes (Aa × Aa) gives a 3:1 phenotype ratio and a 1:2:1 genotype ratio.

A test cross with a homozygous recessive individual reveals whether a dominant-phenotype organism is homozygous or heterozygous.

Human sex determination: XX is female and XY is male; the father's sperm determines the sex of the offspring.

Continuous variation (height, mass) is controlled by many genes and environment; discontinuous variation (blood group) falls into distinct categories.

Mutation is a random change in DNA that creates new alleles and is a source of genetic variation.

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

Food chain order: producer → primary consumer → secondary consumer; arrows show the direction of energy transfer.

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

Pyramids of number, biomass and energy represent trophic levels; energy pyramids are always widest at the base.

Decomposers such as bacteria and fungi break down dead material and recycle carbon and nitrogen into the soil.

The carbon cycle: photosynthesis removes carbon dioxide; respiration, combustion and decomposition return it.

Population size increases with food, space and lack of disease or predators, and is limited by predation, competition and disease.

Adaptations: xerophytes reduce water loss; hydrophytes have air spaces; animals show behavioural, physiological and structural adaptations.

Human impacts include deforestation, pollution, eutrophication from fertiliser runoff, and conservation measures such as protected areas and recycling.

The binomial system gives each species a two-part Latin name: genus (capitalised) then species (lower case), both italicised.

The hierarchy of classification: kingdom → phylum → class → order → family → genus → species.

The five kingdoms are animals, plants, fungi, protoctists and prokaryotes (bacteria).

Vertebrate groups: fish (scales, gills), amphibians (moist skin, metamorphosis), reptiles (dry scaly skin), birds (feathers), mammals (fur, mammary glands).

Arthropods have jointed legs and an exoskeleton; insects have three body parts and six legs; arachnids have eight legs.

Plants are classified as ferns (no flowers, spores), conifers (cones, needle leaves) and flowering plants (monocots and dicots).

Viruses are not classified in the five kingdoms because they are not cellular and cannot reproduce independently.

Dichotomous keys use paired contrasting statements to identify organisms; each choice must be unambiguous.

A hypothesis is a testable prediction; the independent variable is changed, the dependent variable is measured, and all other variables are controlled.

A fair test changes only one variable; a control group is untreated and provides a baseline for comparison.

Reliability improves by repeating measurements and calculating a mean; accuracy improves with better instruments and technique.

Variables must be identified and stated explicitly; random errors scatter results, while systematic errors shift all results in one direction.

When drawing tables, put the independent variable in the first column and include units in headings; plot the independent variable on the x-axis.

A line graph suits continuous data; a bar chart suits categories; anomalous results should be identified and excluded with justification.

Biological drawings use a sharp pencil, continuous unbroken lines, no shading, and labels with straight lines that do not cross.

Magnification of a drawing = drawing length ÷ actual length; safety precautions and ethical treatment of living organisms must be stated.