Cameroon GCE O-Level Human Biology cheat sheet

Cell theory: all living things are made of cells; the cell is the basic functional unit; new cells arise only from pre-existing cells by division.

Prokaryotic cells lack a true nucleus and membrane-bound organelles; human cells are eukaryotic, with a nucleus, mitochondria, ribosomes and an endomembrane system.

Organelle functions: nucleus holds DNA and controls activity; mitochondrion releases energy by aerobic respiration; ribosome builds protein; lysosome digests; rough endoplasmic reticulum transports protein.

Plant-only structures are the cellulose cell wall, large permanent vacuole and chloroplast; human cells lack all three, so human cells are never described as having a cell wall.

Mitosis produces two genetically identical diploid daughter cells and serves growth, repair and replacement; it maintains the chromosome number (46 in humans).

Meiosis halves the chromosome number to 23 per gamete and creates variation by crossing over and independent assortment of homologous chromosomes.

Levels of organisation in order: cell, tissue, organ, organ system, organism; epithelial, connective, muscle and nervous tissue are the four main human tissue types.

Diffusion moves particles down a concentration gradient without energy; osmosis is water movement across a partially permeable membrane from dilute to concentrated solution; active transport uses energy and can move against the gradient.

Surface area to volume ratio falls as an organism grows, which is why large organisms need specialised exchange surfaces and transport systems rather than relying on diffusion alone.

Balanced diet components: carbohydrates and fats for energy, proteins for growth and repair, vitamins and minerals for specific functions, water and dietary fibre (roughage) for gut health.

Deficiency diseases: vitamin C gives scurvy, vitamin D or calcium gives rickets, iron gives anaemia, vitamin A gives night blindness, protein-energy malnutrition gives kwashiorkor and marasmus.

Enzymes are biological catalysts made of protein; they are specific to a substrate and are denatured (shape destroyed, activity lost) by high temperature or extremes of pH.

Key enzymes: salivary amylase digests starch to maltose; pepsin (acid pH) digests protein to peptides; lipase digests fats to fatty acids and glycerol; maltase digests maltose to glucose.

Digestion sequence: mouth (chewing, amylase) then oesophagus by peristalsis then stomach (HCl, pepsin) then duodenum (bile, pancreatic enzymes) then ileum (absorption by villi) then colon (water reabsorption).

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

Villi of the ileum are adapted by a large surface area, thin walls, dense capillary network and lacteals for absorbing glucose and amino acids into blood and fats into lymph.

Food tests: iodine for starch (blue-black), Benedict's for reducing sugar (brick-red on heating), Biuret for protein (purple), ethanol emulsion for fat (milky white).

Blood components: plasma carries dissolved substances, red cells carry oxygen using haemoglobin, white cells defend against infection, platelets clot blood.

Red blood cells are biconcave, have no nucleus in humans and contain haemoglobin, which binds oxygen in the lungs and releases it in respiring tissues.

Arteries carry blood away from the heart under high pressure and have thick elastic muscular walls; veins carry blood towards the heart under low pressure and have valves to prevent backflow.

Capillaries are one cell thick, giving a short diffusion distance for exchange of gases, nutrients and waste between blood and tissue fluid.

Human circulation is double (pulmonary and systemic circuits) and the heart has four chambers; the left ventricle wall is thickest because it pumps blood at high pressure around the whole body.

Blood flow path: vena cava to right atrium to right ventricle to pulmonary artery to lungs to pulmonary vein to left atrium to left ventricle to aorta to body.

The cardiac cycle is controlled by the pacemaker (sinoatrial node) in the right atrium; valves close to prevent backflow, producing heart sounds.

Coronary arteries supply the heart muscle itself; blockage causes myocardial infarction (heart attack), and risk factors include smoking, high saturated fat intake, obesity and inactivity.

Aerobic respiration: glucose + oxygen gives carbon dioxide + water + energy; it releases more energy per glucose molecule than anaerobic respiration.

Anaerobic respiration in human muscle: glucose gives lactic acid + small energy; in yeast it gives ethanol + carbon dioxide + energy.

Breathing (ventilation) is a mechanical process, distinct from respiration, which is a chemical reaction releasing energy in cells.

Inhalation: intercostal muscles and diaphragm contract, rib cage moves up and out, diaphragm flattens, volume increases and pressure falls, so air enters.

Alveoli are adapted for gas exchange by a huge surface area, one-cell-thick walls, a moist surface and a rich capillary network; oxygen diffuses into blood and carbon dioxide diffuses out.

Lung features: trachea supported by cartilage rings, bronchi divide into bronchioles ending in alveoli; cilia and mucus trap dust and pathogens.

Smoking effects: tar damages cilia and causes cancer, nicotine is addictive and raises heart rate, carbon monoxide reduces haemoglobin oxygen carriage, and emphysema reduces alveolar surface area.

Exhaled air compared with inhaled air: more carbon dioxide, more water vapour, warmer, and less oxygen.

Excretion is removal of metabolic waste (carbon dioxide, urea, water, salts, bile pigments); egestion is removal of undigested food, which is not excretion.

Nephron sequence: glomerulus filtration by pressure, then selective reabsorption of glucose, some salts and most water in the proximal tubule, then urine formation, then collection in the bladder.

Urine normally contains water, urea, salts and excess hormones or drugs, but not glucose or protein; glucose in urine indicates diabetes mellitus.

Negative feedback keeps conditions steady: a change is detected, a corrective response is triggered, and the change is reversed back towards the set point.

Osmoregulation: low water in blood is detected and ADH from the pituitary increases water reabsorption in kidney tubules, producing concentrated urine.

Thermoregulation: sweating and vasodilation cool the body; shivering, vasoconstriction and hair erection conserve heat; the hypothalamus acts as the thermostat.

Skin structure and function: epidermis forms a waterproof barrier, dermis holds sweat glands, blood vessels, nerve endings and hair follicles, and fat insulates.

Urea is formed in the liver from excess amino acids by deamination and is carried in blood plasma to the kidneys for removal.

Neurone parts: dendrites receive impulses, cell body contains the nucleus, axon carries the impulse, myelin sheath insulates and speeds conduction, and synapses transmit with a chemical transmitter.

Reflex arc order: receptor, sensory neurone, relay neurone in the spinal cord, motor neurone, effector; it is rapid and involuntary and protects the body from harm.

Central nervous system is the brain and spinal cord; the peripheral nervous system links receptors and effectors; the autonomic system controls involuntary functions.

Eye: cornea refracts, iris controls pupil size, lens focuses by changing shape (accommodation), retina holds rods (dim light) and cones (colour), and the optic nerve carries impulses to the brain.

Ear: pinna collects sound, eardrum vibrates, ossicles amplify, cochlea converts sound to impulses, and semicircular canals control balance.

Hormones are chemical messengers secreted by endocrine glands into blood and act on target organs; they act more slowly and for longer than nerve impulses.

Key hormones: insulin from the pancreas lowers blood glucose, glucagon raises it, adrenaline prepares the body for action, thyroxine controls metabolic rate, testosterone and oestrogen control sexual development.

Diabetes mellitus results from insufficient insulin or loss of insulin response; type 1 is treated with insulin injections, and control involves diet, exercise and monitoring blood glucose.

Skeleton functions: support, protection of organs, muscle attachment, movement, mineral storage and blood cell formation in bone marrow.

The human skeleton has axial parts (skull, vertebral column, ribs, sternum) and appendicular parts (limbs, shoulder and pelvic girdles).

Bone types: long bones act as levers, flat bones protect, and cartilage at joints reduces friction and cushions shock.

Joints: a synovial joint has cartilage, synovial fluid and a capsule; hinge joints (elbow, knee) allow movement in one plane, ball-and-socket joints (shoulder, hip) allow movement in all planes.

Antagonistic muscle pairs work in opposition: the biceps flexes the elbow and the triceps extends it; muscles can only pull, never push, so pairs are needed.

Bone and muscle connect by tendons (muscle to bone) and ligaments (bone to bone); a muscle that contracts becomes shorter and thicker.

Vertebral column regions in order: cervical, thoracic, lumbar, sacral, caudal; discs between vertebrae absorb shock.

Disorders: osteoporosis is loss of bone density, arthritis is joint inflammation, and a sprain is a stretched or torn ligament.

Male system: testes produce sperm and testosterone, sperm travel through sperm ducts, urethra and penis; semen contains sperm plus gland secretions.

Female system: ovaries release one egg per cycle, fallopian tubes carry the egg, the uterus holds the embryo, and the vagina is the birth canal.

Menstrual cycle: menstruation (loss of lining), follicle growth and oestrogen rise, ovulation (egg release) around day 14, then progesterone maintains the lining; if no fertilisation, the lining breaks down.

Fertilisation occurs in the fallopian tube; the zygote divides by mitosis to form a ball of cells that implants in the uterine lining.

Placenta functions: exchange of nutrients, oxygen and waste between mother and fetus, and secretion of hormones; the umbilical cord connects fetus to placenta.

Growth is measured by height and mass plotted on percentile charts; development means increased complexity and function, not just size.

Puberty changes: in males voice deepens, body hair grows, testes enlarge; in females breasts develop, hips widen, menstruation begins.

Contraception methods include the condom (also prevents sexually transmitted infection), the pill (hormonal), and intrauterine devices; these differ in effectiveness and mode of action.

Pathogen types: bacteria (e.g. tuberculosis, cholera), viruses (e.g. HIV, measles), protozoa (e.g. malaria), and fungi (e.g. athlete's foot).

Transmission routes: airborne droplets, contaminated food and water, vectors such as mosquitoes, sexual contact, and blood contact.

Body defences: skin and mucus as barriers, stomach acid, phagocytes engulfing pathogens, and lymphocytes producing antibodies specific to antigens.

Immunity: antibodies lock onto antigens and destroy pathogens; memory cells give long-lasting immunity after infection or vaccination.

Vaccination introduces weakened or inactive antigens so antibodies and memory cells form without disease; it also gives herd protection.

Hygiene measures: handwashing, boiling or treating drinking water, safe food storage, mosquito nets, and safe disposal of sewage.

HIV attacks lymphocytes and weakens immunity, leading to AIDS; it is transmitted by unprotected sex, contaminated blood and from mother to child.

Non-communicable diseases such as cancer, diabetes and heart disease are not transmitted between people but are influenced by lifestyle and environment.

Genes are sections of DNA on chromosomes and code for proteins; humans have 46 chromosomes in 23 pairs, one pair being sex chromosomes (XX female, XY male).

Alleles are alternative forms of a gene; dominant alleles are expressed in the heterozygote, recessive alleles only in the homozygote.

Genotype is the allele combination and phenotype is the observable characteristic; homozygous means identical alleles and heterozygous means different alleles.

Monohybrid cross: a 3:1 phenotype ratio appears in the offspring of two heterozygous parents; a test cross with a homozygous recessive reveals an unknown genotype.

Sex determination: the father's sperm determines sex, since the mother contributes only an X chromosome.

Continuous variation (height, skin colour) is controlled by many genes plus environment; discontinuous variation (blood group, tongue rolling) is controlled by one or a few genes.

Mutation is a change in DNA or chromosome structure and is a source of new alleles; it may be harmful, neutral or occasionally beneficial.

Genetic disorders: sickle-cell anaemia and cystic fibrosis are recessive; Huntington's disease is dominant; haemophilia is sex-linked and more common in males.

Ecosystem terms: habitat is where an organism lives, population is one species in an area, community is all species, and ecosystem includes the physical environment.

Food chains show energy flow from producer to consumer; arrows point in the direction of energy flow, and only about 10% of energy passes to the next trophic level.

Producers are photosynthetic organisms; decomposers break down dead material and recycle nutrients such as carbon and nitrogen.

Carbon cycle: photosynthesis removes carbon dioxide, respiration and combustion release it, and fossil fuels store carbon over long periods.

Human impacts: deforestation reduces carbon dioxide absorption and causes soil erosion; burning fossil fuels adds carbon dioxide and sulphur dioxide, causing acid rain and climate change.

Eutrophication: fertiliser runoff adds nitrates, algae bloom, then decay depletes oxygen and kills fish.

Conservation measures: protected areas, reforestation, recycling, reduced fossil fuel use, and sustainable fishing and farming.

Population growth increases demand for food, water, housing and energy, intensifying pressure on ecosystems and biodiversity.