O Level / IGCSE Biology key facts

Every chapter of O Level / IGCSE Biology on one page: the 344 key facts, definitions and facts to remember, in syllabus order. Use it for a last look before a test, then check yourself.

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Cells

Cell structure and function

Nucleus: contains the DNA (genes) and controls the activities of the cell.
Cell membrane: controls which substances enter and leave the cell. Cytoplasm: where most chemical reactions take place.
Mitochondria: site of aerobic respiration, which releases energy. Ribosomes: where proteins are made.
Chloroplasts: contain chlorophyll and carry out photosynthesis. Sap vacuole: stores cell sap and helps keep the cell firm.
Cellulose cell wall: strong and fully permeable; it supports the cell and stops it bursting.
Bacterial cell: circular DNA and plasmids, ribosomes, cytoplasm, cell membrane, cell wall; no nucleus.
Stains make cell parts easier to see: iodine solution for plant cells, methylene blue for animal cells such as cheek cells.

Specialised cells, tissues and organs

Order of size: cell → tissue → organ → organ system → organism.
Tissue: a group of similar cells that work together to carry out the same function.
Organ: several different tissues working together, e.g. stomach, heart, leaf.
magnification = image size ÷ actual size
actual size = image size ÷ magnification; image size = actual size × magnification
1 mm = 1000 µm. Put both sizes in the same unit before you divide.
Examples: red blood cell (no nucleus, more room for haemoglobin), ciliated cell (cilia sweep mucus), xylem vessel (hollow dead tube, carries water), root hair cell (large surface area).

Classification

Concept and use of a classification system

Species: a group of organisms that can reproduce to produce fertile offspring.
Binomial name = genus + species, e.g. Homo sapiens.
The genus starts with a capital letter and the species with a small letter. The name is in italics when printed and underlined when handwritten.
Same first name means same genus, so those species are closely related.
Each step of a dichotomous key has exactly two choices, based on features you can see clearly.
A key for n organisms needs at least n − 1 pairs of statements.
The more similar the DNA base sequences of two species, the more closely related they are.

Features of organisms

Animals: many cells, nucleus, no cell wall, feed on other organisms. Plants: many cells, nucleus, cellulose cell wall, chloroplasts, photosynthesise.
Fungi: nucleus, cell wall not made of cellulose, no chlorophyll, body of thread-like hyphae, feed by digesting food outside the cells. Prokaryotes: one cell, no nucleus, circular DNA, cell wall. Protoctists: mostly one cell, with a nucleus; some have chloroplasts.
Vertebrates: mammals (fur or hair, milk from mammary glands), birds (feathers, beak, hard-shelled eggs), reptiles (dry scaly skin, eggs with leathery shells laid on land), amphibians (moist skin without scales, eggs laid in water), fish (wet scales, gills, fins).
Arthropods: insects (3 pairs of legs, 3 body parts, 1 pair of antennae), arachnids (4 pairs of legs, 2 body parts, no antennae), crustaceans (more than 4 pairs of legs, 2 pairs of antennae), myriapods (many segments, each with legs, 1 pair of antennae).
Ferns: roots, stems and leaves (fronds), reproduce by spores, no flowers or seeds. Flowering plants: reproduce by seeds made in flowers.
Monocotyledons: one cotyledon, parallel leaf veins, flower parts in threes. Dicotyledons: two cotyledons, branching network of veins, flower parts in fours or fives.
Virus: a protein coat around genetic material; no cytoplasm, no cell membrane, no ribosomes.

Movement into and out of cells

Diffusion and osmosis

Diffusion: net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient, by random movement.
Diffusion is faster with a larger surface area, a higher temperature, a steeper concentration gradient and a shorter distance.
Osmosis: net movement of water molecules from higher water potential to lower water potential through a partially permeable membrane.
Pure water has the highest water potential. Dissolving more solute lowers the water potential.
Plant cell in water: takes in water, becomes turgid; turgor pressure on the cell wall supports the plant. In concentrated solution: loses water, becomes flaccid, then plasmolysed (membrane pulls away from the wall).
Animal cell in water: swells and bursts. In concentrated solution: shrinks.
percentage change in mass = (change in mass ÷ starting mass) × 100

Active transport

Active transport: movement of molecules or ions through the cell membrane from lower to higher concentration (against a concentration gradient), using energy released during respiration.
Diffusion and osmosis need no energy from the cell; active transport does.
Carrier proteins in the cell membrane carry the particles across.
Root hair cells take up mineral ions such as nitrate ions from the soil by active transport.
Anything that slows respiration slows active transport: lack of oxygen (waterlogged soil), a respiratory poison, low temperature.
Cells that carry out a lot of active transport have many mitochondria.

Biological molecules

Elements: carbohydrates C, H, O; lipids C, H, O; proteins C, H, O, N (some also S); DNA C, H, O, N, P.
Starch, cellulose and glycogen are made from glucose; proteins from amino acids; lipids from fatty acids and glycerol; DNA from nucleotides.
Starch: add iodine solution. Orange-brown turns blue-black.
Reducing sugars (glucose, maltose): add Benedict's solution and heat in a water bath. Blue turns green, yellow, orange, then brick red as the amount of sugar increases.
Protein: biuret test. Blue turns purple.
Lipids: shake with ethanol, then pour into water. A cloudy white emulsion forms.

Enzymes

Enzyme action

Catalyst: a substance that increases the rate of a chemical reaction and is not changed by the reaction.
Enzymes are proteins that act as biological catalysts in all metabolic reactions.
Sequence: enzyme + substrate → enzyme-substrate complex → enzyme + products.
Lock and key: the enzyme is the lock, the substrate is the key.
Enzymes are specific: the active site has a shape complementary to one substrate only.
Examples: amylase breaks down starch to maltose; protease breaks down protein to amino acids; lipase breaks down lipids to fatty acids and glycerol.

Effects of temperature and pH

Below the optimum: higher temperature → more kinetic energy → more frequent effective collisions → faster reaction.
Above the optimum: the active site changes shape, the substrate no longer fits, the enzyme is denatured and the rate falls quickly.
Denaturation is permanent. Low temperature makes an enzyme inactive, but it works again when warmed.
Each enzyme has its own optimum pH: about pH 2 for pepsin in the stomach, about pH 7 for salivary amylase.
A shorter time to finish the reaction means a faster rate.
Fair test: change one factor only; keep enzyme concentration, substrate concentration and volumes the same.

Plant nutrition

Photosynthesis

carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Uses of glucose: respiration (energy), starch (energy store), cellulose (cell walls), sucrose (transport in the phloem).
Starch test on a leaf: iodine solution turns blue-black where photosynthesis has taken place.
Destarch a plant first by keeping it in the dark for 48 hours, so any starch found afterwards was made during the experiment.
If the rate rises when a factor is increased, that factor was limiting. Where the graph is level, a different factor is limiting.
Temperature affects the rate because photosynthesis is controlled by enzymes.

Leaf structure

Cuticle: waxy and waterproof, reduces water loss; transparent, so light passes through.
Upper epidermis: transparent cells without chloroplasts, letting light through to the palisade layer.
Palisade mesophyll: tall, tightly packed cells with many chloroplasts; the main site of photosynthesis.
Spongy mesophyll: loosely packed cells with large air spaces, so gases diffuse quickly to and from the cells.
Guard cells turgid: stoma open. Guard cells flaccid: stoma closed.
Xylem brings water and mineral ions to the leaf; phloem carries sucrose and amino acids away.
Most stomata are on the lower surface, which is cooler and shaded, so less water is lost.

Mineral nutrition

Nitrate ions → amino acids → proteins → growth.
Lack of nitrate: stunted growth, with weak stems and yellowing older leaves.
Magnesium ions → chlorophyll.
Lack of magnesium: yellow leaves (chlorosis), less photosynthesis, poor growth.
Mineral ions are usually taken up by root hair cells by active transport.
Fertilisers containing nitrates are added to soil to increase the growth and yield of crops.

Transport in flowering plants

Uptake and transport of water and ions

Pathway of water: root hair cell → root cortex cells → xylem → mesophyll cells of the leaf.
Water enters root hair cells by osmosis, from higher water potential (soil water) to lower water potential (cell sap).
Mineral ions enter root hair cells by active transport, using energy from respiration.
Root hair cells have a large surface area and no chloroplasts.
Xylem carries water and mineral ions upwards. Phloem carries sucrose and amino acids.
A stem left in coloured water shows the stain only in the xylem, which lies in the vascular bundles.
In a dicotyledonous stem the vascular bundles are arranged in a ring.

Transpiration and translocation

Transpiration = loss of water vapour from leaves: evaporation from mesophyll cell surfaces, then diffusion out through the stomata
Higher temperature → faster transpiration (water molecules have more kinetic energy)
More wind → faster transpiration (moist air is blown away from the leaf, so the gradient stays steep)
Higher humidity → slower transpiration (smaller water vapour gradient between leaf and air)
Brighter light → faster transpiration (stomata open wider)
A potometer measures water uptake by a shoot: volume taken up = distance the bubble moves × cross-sectional area of the tube
Translocation = movement of sucrose and amino acids in the phloem, from sources (e.g. leaves) to sinks (e.g. roots, fruits, growing tips)

Human nutrition

Diet

Carbohydrates (rice, bread, potatoes, sugar): the main source of energy
Lipids (butter, oils, nuts, fatty meat): energy store, insulation, making cell membranes
Proteins (meat, fish, eggs, milk, pulses): growth and repair; making enzymes and antibodies
Vitamin C (citrus fruits, fresh vegetables): lack causes scurvy (bleeding gums, wounds heal slowly)
Vitamin D (oily fish, eggs; also made in skin in sunlight) and calcium (milk, cheese): strong bones and teeth; lack of either causes rickets (soft, bent bones)
Iron (red meat, liver, green leafy vegetables): making haemoglobin; lack causes anaemia (tiredness, pale skin)
Fibre (vegetables, fruit, wholegrain cereals) adds bulk and prevents constipation; water is the solvent for transport and for reactions in cells

Human digestive system

Amylase (salivary glands, pancreas): starch → maltose. Maltase (lining of small intestine): maltose → glucose
Protease: protein → amino acids. Pepsin acts in the stomach (acid); trypsin from the pancreas acts in the duodenum (slightly alkaline)
Lipase (pancreas): lipids → fatty acids + glycerol
Hydrochloric acid in the stomach kills bacteria in food and gives the acid pH that pepsin needs
Bile: made in the liver, stored in the gall bladder; emulsifies fats into small droplets (physical, no enzyme) and neutralises stomach acid
Teeth: incisors cut, canines tear, premolars and molars crush and grind. Enamel outside, dentine under it, pulp with nerves and blood vessels in the centre, cement holding the root in place
Order: mouth → oesophagus → stomach → duodenum → ileum → colon → rectum → anus

Absorption and assimilation

Absorption: nutrients move from the gut into the lining cells and then into the blood (small intestine)
Assimilation: uptake and use of nutrients from the blood by body cells
Capillaries in a villus absorb glucose and amino acids; the lacteal absorbs fatty acids and glycerol
Villi and microvilli increase the surface area; the one-cell-thick wall gives a short diffusion distance; blood flow keeps the concentration gradient steep
Active transport (needs energy from respiration) takes up glucose when its concentration in the gut is lower than in the cells
Water is absorbed by osmosis in the small intestine and the colon; most is absorbed in the small intestine
Hepatic portal vein: carries absorbed nutrients from the ileum to the liver

Human gas exchange

Atmospheric (inspired) air: about 78% nitrogen, 21% oxygen, 0.04% carbon dioxide
Expired air: about 16% oxygen, 4% carbon dioxide, 78% nitrogen; warmer and with more water vapour
Breathing in: external intercostal muscles contract, ribs move up and out, diaphragm contracts and flattens → volume of thorax increases, pressure falls, air enters
Breathing out: these muscles relax, ribs move down and in, diaphragm rises into a dome → volume decreases, pressure rises, air leaves
Forced breathing out: internal intercostal muscles contract to pull the ribs down and in
Exercise: muscles make more carbon dioxide; the brain detects the higher concentration in the blood and increases the rate and depth of breathing
Volume breathed per minute = breaths per minute × volume of each breath

Respiration

Respiration = chemical reactions in all living cells that release energy from glucose
Energy is used for: muscle contraction, protein synthesis, cell division, active transport, growth, passing nerve impulses, keeping body temperature constant
Respiration releases energy; never write that it makes or produces energy
Yeast experiment: independent variable = temperature; dependent variable = volume of carbon dioxide made (or bubbles counted) in a set time
Rate of respiration = volume of carbon dioxide ÷ time
Control: boiled (dead) yeast gives no carbon dioxide, showing the gas comes from living cells
Diffusion and osmosis do not need energy from respiration

Aerobic respiration

glucose + oxygen → carbon dioxide + water
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Aerobic respiration releases much more energy per glucose molecule than anaerobic respiration
Energy is released by the reaction; it is not a substance, so it is not written as a product in the chemical equation
Carbon dioxide turns limewater cloudy and turns hydrogencarbonate indicator from red to yellow
Soda lime absorbs carbon dioxide, so in a respirometer the liquid moves because oxygen is used up

Anaerobic respiration

Human muscle: glucose → lactic acid
Yeast: glucose → ethanol + carbon dioxide
Anaerobic respiration: no oxygen used, relatively small amount of energy released
No carbon dioxide is made by anaerobic respiration in human muscle
Oxygen debt (EPOC) = extra oxygen taken in after exercise to break down the lactic acid
After exercise the heart rate stays fast, to carry lactic acid in the blood from the muscles to the liver
After exercise breathing stays deep and fast, to supply oxygen for breaking down lactic acid in the liver

Transport in humans

Circulatory system

Circulatory system = blood vessels + a pump + valves, giving one-way flow
Double circulation: blood passes through the heart twice for each complete circuit
Lung circuit: right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium (low pressure)
Body circuit: left ventricle → aorta → body tissues → vena cava → right atrium (high pressure)
Right side of the heart carries deoxygenated blood; left side carries oxygenated blood
Blood loses pressure in the lung capillaries, so it returns to the heart to be pumped again before going to the body

Heart

Arteries carry blood away from the heart; veins carry blood back to the heart
Left ventricle wall is thicker than the right: it pumps blood to the whole body at high pressure, the right only to the lungs
Atria walls are thinner than ventricle walls: they pump blood only a short distance into the ventricles
Atrioventricular valves stop backflow from ventricles to atria; semilunar valves stop backflow from arteries to ventricles
Heart activity is monitored by ECG, pulse rate and the sounds of the valves closing
Exercise raises heart rate: muscles need more oxygen and glucose, and more carbon dioxide must be removed
Coronary heart disease risk factors: diet high in saturated fat, sedentary lifestyle, stress, smoking, genetic predisposition, age, gender (higher in males)

Blood vessels

Artery: thick wall, much muscle and elastic tissue, narrow lumen, no valves along its length, high pressure
Vein: thin wall, little muscle and elastic tissue, wide lumen, valves, low pressure
Capillary: wall one cell thick, very narrow lumen, no valves; site of exchange
Heart and body: aorta leaves the left ventricle; vena cava returns blood to the right atrium
Lungs: pulmonary artery carries deoxygenated blood to them; pulmonary vein carries oxygenated blood back
Liver: hepatic artery in (oxygenated), hepatic portal vein in (from the gut, rich in absorbed food), hepatic vein out
Kidneys: renal artery in; renal vein out (less urea)

Blood

Red blood cells: transport oxygen (contain haemoglobin, no nucleus)
Lymphocytes: produce antibodies
Phagocytes: engulf and digest pathogens
Platelets: clotting; soluble fibrinogen is converted to insoluble fibrin, which forms a mesh that traps blood cells
A clot prevents blood loss and stops pathogens entering
Plasma transports blood cells, ions, glucose, amino acids, hormones, carbon dioxide, urea, vitamins and plasma proteins
Oxygen and glucose diffuse from blood → tissue fluid → cells; carbon dioxide and urea diffuse from cells → tissue fluid → blood

Disease and immunity

Disease

Malaria: the parasite Plasmodium is passed on when a female Anopheles mosquito bites an infected person and then another person
Mosquito eggs, larvae and pupae live in still water: drain it, spray oil on it, or add fish that eat larvae; use insecticide and bed nets against adults
HIV: spread by unprotected sex, shared needles, infected blood, and from mother to baby; it may lead to AIDS
HIV control: condoms, sterile needles, screening donated blood, drugs for infected mothers, education
Cholera: bacterium in water contaminated with faeces. Its toxin makes chloride ions pass into the small intestine, water follows by osmosis → diarrhoea, dehydration, loss of ions
Cholera control: clean water supply, sewage treatment, safe waste disposal, hand washing, hygienic food preparation
Alcohol: depressant, slower reactions, less self-control, liver damage. Tobacco: nicotine is addictive; tar causes lung cancer and bronchitis; carbon monoxide lowers oxygen carried by haemoglobin; smoking in pregnancy lowers birth weight

Antibiotics

Drug: any substance taken into the body that modifies or affects chemical reactions in the body.
Antibiotics treat bacterial infections only.
Antibiotics kill bacteria but do not affect viruses, so they do not cure a cold, influenza or HIV.
MRSA is a bacterium that is resistant to many antibiotics.
To limit resistance: use antibiotics only when essential and finish the whole course.
In a disc test on agar, a wider clear zone means the antibiotic is more effective against that bacterium.

Immunity

Antibodies are proteins made by lymphocytes; each has a shape complementary to one specific antigen.
Active immunity: antibodies made in the body, after infection or vaccination; memory cells give long-term immunity.
Vaccine: weakened pathogens or their antigens, which stimulate lymphocytes to make antibodies and memory cells.
Passive immunity: ready-made antibodies from another individual (placenta, breast milk); short term; no memory cells.
If most of a population is vaccinated, the pathogen cannot spread easily, so unvaccinated people are protected too.
On a second contact with the same antigen, antibodies are made faster and in larger amounts.
HIV: fewer lymphocytes and reduced ability to produce antibodies, so the immune system is weakened.

Excretion

Excretion: removal of toxic materials and the waste products of metabolism from organisms.
Carbon dioxide: waste product of respiration; excreted through the lungs.
Urea: toxic waste product made in the liver from excess amino acids.
Urea is made in the liver but removed from the blood by the kidneys.
Egestion (passing out faeces) is not excretion, because undigested food was not made by metabolism.

Urinary system

Path of urine: kidney → ureter → bladder → urethra.
Path in a nephron: glomerulus → Bowman's capsule → tubule and loop of Henle → collecting duct.
Filtered out of the blood: water, glucose, urea, ions. Not filtered: proteins and blood cells (too large).
Reabsorbed: all of the glucose, some of the ions, most of the water.
Urine contains urea, excess water and excess ions; no glucose or protein in a healthy person.
Assimilation: the liver converts amino acids into proteins, such as plasma proteins.
Deamination: removal of the nitrogen-containing part of excess amino acids in the liver, forming urea.

Coordination and control

Mammalian nervous system

CNS = brain + spinal cord; PNS = nerves outside the brain and spinal cord.
Reflex arc: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response.
Sensory neurone: receptor to CNS. Relay neurone: inside the CNS. Motor neurone: CNS to effector.
Effectors are muscles (contract) and glands (secrete).
At a synapse: an impulse arrives, vesicles release neurotransmitter into the synaptic gap, it diffuses across and binds to receptor proteins, and an impulse starts in the next neurone.
Synapses make impulses travel in one direction only, because vesicles are on one side and receptor proteins on the other.

Mammalian sense organs

Cornea refracts light; lens focuses light onto the retina; iris controls how much light enters the pupil.
Fovea: greatest density of light receptors, so the sharpest image. Blind spot: where the optic nerve leaves; no receptors.
Bright light: circular muscles contract, radial muscles relax, pupil becomes smaller.
Dim light: radial muscles contract, circular muscles relax, pupil becomes wider.
Near object: ciliary muscles contract, suspensory ligaments slacken, lens becomes fatter and refracts light more.
Distant object: ciliary muscles relax, suspensory ligaments are pulled tight, lens becomes thinner and refracts light less.
Antagonistic muscles: when one contracts the other relaxes, and they have opposite effects.

Mammalian hormones

Hormone: a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.
Adrenal glands: adrenaline.
Pancreas: insulin and glucagon.
Pituitary gland: FSH and LH.
Testes: testosterone. Ovaries: oestrogen and progesterone.
Adrenaline increases blood glucose concentration and heart rate.
Nervous control: fast, short-lasting. Hormonal control: slower, longer-lasting.

Homeostasis

Homeostasis: the maintenance of a constant internal environment.
Set point: the normal value that a condition is kept close to (about 37 °C for human body temperature).
Negative feedback: a change away from the set point causes a response that reverses the change.
Sequence: change → detected by receptors → corrective response → return to set point → response stops.
The value is never perfectly steady; it moves a little above and below the set point.

Temperature control

Sweating: water in sweat evaporates, taking heat energy from the skin.
Vasodilation: arterioles widen, more blood flows through skin surface capillaries, more heat is lost.
Vasoconstriction: arterioles narrow, less blood flows through skin surface capillaries, less heat is lost.
Shivering: muscles contract rapidly, respiration in them increases and releases heat.
Hair erector muscles contract: hairs stand up and trap a layer of air, which insulates.
Fatty tissue under the skin insulates and reduces heat loss.
Hypothalamus: detects blood temperature and coordinates the responses.

Blood glucose control

Blood glucose too high → pancreas releases insulin → liver converts glucose to glycogen → blood glucose falls.
Blood glucose too low → pancreas releases glucagon → liver converts glycogen to glucose → blood glucose rises.
Glycogen is stored in the liver and muscles.
Both hormones are made by the pancreas; the liver is the target organ.
Type 1 diabetes signs: increased blood glucose concentration and glucose in the urine.
Type 1 diabetes treatment: administration of insulin, usually by injection.

Coordination and response in plants

Phototropism: growth towards or away from light. Gravitropism: growth towards or away from gravity.
Shoots: positive phototropism, negative gravitropism. Roots: positive gravitropism.
Auxin is made in the shoot tip and spreads through the plant from the tip.
Auxin stimulates cell elongation in shoots.
Light from one side: more auxin on the shaded side, so that side grows faster and the shoot bends towards the light.
Horizontal shoot: more auxin on the lower side, so that side grows faster and the shoot bends upwards.
No tip, or tip covered: no bending towards light. A clinostat turns the plant slowly so the stimulus acts equally on all sides.

Development of organisms and continuity of life

Nuclear division

Chromosome → contains DNA → carries genetic information as genes.
Diploid: two sets of chromosomes (human: 23 pairs = 46). Haploid: one set (human: 23).
Mitosis: chromosome number maintained; cells genetically identical.
Meiosis: chromosome number halved (diploid → haploid); cells genetically different; makes gametes.
Fertilisation: haploid + haploid → diploid zygote.
Stem cells: unspecialised cells that divide by mitosis to give cells that can become specialised.
Cancer: the result of uncontrolled cell division.

Asexual and sexual reproduction

Asexual: one parent, no gametes, offspring genetically identical.
Sexual: fusion of haploid nuclei (fertilisation) → diploid zygote; offspring genetically different.
Asexual advantages: fast, only one parent needed, good characteristics are passed on unchanged.
Asexual disadvantages: no variation, so all may be killed by the same disease or change; overcrowding and competition near the parent.
Sexual advantages: variation, so the species can adapt to a changing environment and resist new diseases.
Sexual disadvantages: slower, two parents usually needed, good characteristics may not be passed on.

Sexual reproduction in plants

Insect-pollinated flower: large coloured petals, scent, nectar; anthers and stigmas inside the flower; pollen is larger, sticky or spiky, and made in smaller amounts.
Wind-pollinated flower: small dull petals, no scent or nectar; long filaments with anthers hanging outside; large feathery stigmas; pollen is small, smooth, light and made in huge amounts.
Self-pollination: pollen goes to a stigma of the same flower or the same plant. Cross-pollination: pollen goes to a flower on a different plant of the same species.
Cross-pollination gives more variation, so the population can respond better to change, but it depends on pollinators. Self-pollination gives less variation but does not depend on pollinators.
Seed: testa (seed coat) around an embryo made of radicle (young root), plumule (young shoot) and cotyledons (food store).
Dispersal by wind (wings, hairs, light) or animals (hooks, or fleshy fruit that is eaten) lets plants colonise new areas and reduces competition.
Germination needs water, oxygen and a suitable temperature. Water activates enzymes that break down stored food into soluble molecules for the growing embryo.

Sexual reproduction in humans

Sperm: flagellum for swimming, many mitochondria to release energy, acrosome with enzymes to digest a way into the egg. Small, motile, made in millions.
Egg: large, cannot move itself, has energy stores, and a jelly coat that changes after one sperm enters so that no more can enter. Usually one is released each month.
FSH (from the pituitary gland) makes an egg mature in the ovary and makes the ovary release oestrogen.
Oestrogen repairs and thickens the lining of the uterus. LH causes ovulation, at about day 14 of a 28-day cycle.
Progesterone keeps the lining thick. If there is no pregnancy, progesterone falls and the lining breaks down (menstruation).
Amniotic sac holds amniotic fluid, which protects the fetus from knocks and supports it.
Placenta: oxygen, glucose and amino acids diffuse to the fetus; carbon dioxide and urea diffuse to the mother. The two bloods do not mix. Some viruses, such as rubella, can cross.

Inheritance

Variation

Variation: differences between individuals of the same species.
Continuous: a range between two extremes; examples are body length (height) and body mass; caused by genes and environment.
Discontinuous: a limited number of phenotypes, no intermediates; examples are ABO blood groups, seed shape and seed colour in peas; usually caused by genes only.
Continuous data are measured and shown as a histogram, often with a peak in the middle.
Discontinuous data are counted in categories and shown as a bar chart with separate bars.

DNA

DNA: two strands, each a chain of nucleotides, coiled into a double helix.
Base pairing: A pairs with T; C pairs with G.
So in any DNA molecule: amount of A = amount of T, and amount of C = amount of G.
A + T + C + G = 100% of the bases.
Gene: a length of DNA that codes for a protein.
Base sequence of the gene → sequence of amino acids → shape of the protein → its function.

Inheritance

Homozygous: two identical alleles (TT or tt). Heterozygous: two different alleles (Tt). Two identical homozygous parents are pure-breeding.
Tt × Tt gives 1 TT : 2 Tt : 1 tt, a phenotype ratio of 3 dominant : 1 recessive.
Tt × tt gives 1 Tt : 1 tt, a phenotype ratio of 1 : 1.
ABO blood groups: I^A and I^B are codominant, I^o is recessive. I^AI^B = group AB; I^oI^o = group O.
Sex: female XX, male XY. The sperm carries X or Y, so each child has a 50% chance of being a boy or a girl.
Gene mutation: a random change in the base sequence of DNA (sickle cell anaemia). Chromosome mutation: a change in chromosome number or structure (Down's syndrome, 47 instead of 46).
Genetic variation comes from mutation, meiosis, random mating and random fertilisation. Ionising radiation and some chemicals raise the mutation rate.

Selection

Natural selection in order: variation → many offspring → struggle for survival → the better adapted survive and reproduce → their alleles are passed on.
Evolution: the change in the inherited features of a population over time as a result of natural selection.
Resistance: the mutation happens by chance first; the antibiotic only selects the bacteria that are already resistant.
Artificial selection: humans select individuals with desirable features, cross them, select the best offspring, and repeat over many generations.
Artificial selection gives economically important organisms, such as cows with a high milk yield and wheat with a high grain yield.
Difference: in natural selection the environment selects; in artificial selection humans select.

Biotechnology and genetic modification

Biotechnology

Anaerobic respiration in yeast: glucose → ethanol + carbon dioxide.
Fermenter temperature: kept at the optimum by a water jacket, because respiration by the microorganisms releases heat.
Fermenter pH: monitored and adjusted, because enzymes work fast only near their optimum pH.
Oxygen: sterile air is bubbled in for aerobic respiration. Nutrients are added; waste products are removed.
Biological washing powders contain enzymes (protease, lipase) that digest stains at low temperatures.
Pectinase breaks down pectin in fruit cell walls, so more juice is released and the juice is clearer.
Lactase breaks down lactose into glucose and galactose to make lactose-free milk.

Genetic modification

Genetic modification: changing genetic material by removing, changing or inserting individual genes.
Insulin: human insulin gene → inserted into bacterial DNA (a plasmid) → bacteria multiply → insulin is produced on a large scale.
Herbicide-resistant crop: the farmer can spray the field and kill the weeds without harming the crop, so there is less competition and a higher yield.
Insect-resistant crop: the plant makes a substance that kills the pests feeding on it, so less insecticide is sprayed and less of the crop is lost.
Extra vitamins: improves the diet and reduces deficiency diseases, for example rice with added vitamin A.
Risks: inserted genes may spread to wild plants in pollen (herbicide-resistant weeds); useful insects may be harmed; seed can be expensive; long-term effects are uncertain.

Relationships of organisms with one another and with the environment

Energy flow

Producer → primary consumer (herbivore) → secondary consumer (carnivore) → tertiary consumer.
Decomposers (fungi and bacteria) get their energy from dead organisms and waste.
Energy is lost between levels by respiration (heat), in movement, in faeces and urine, and in parts that are not eaten.
Percentage transferred = (energy in the higher level ÷ energy in the level below) × 100. It is often about 10%.
Food chains usually have fewer than five trophic levels because too little energy is left to support another level.
Eating crop plants is more energy efficient than eating livestock fed on crops, because one energy-losing step is removed.
A pyramid of numbers can be any shape; a pyramid of biomass is usually a pyramid; a pyramid of energy is always a pyramid.

Nutrient cycles

Photosynthesis removes carbon dioxide from the air; respiration (in plants, animals and decomposers) and combustion add it.
Fossil fuels form over millions of years from dead organisms that did not fully decompose; burning them releases the stored carbon as carbon dioxide.
Decomposition: decomposers break down protein in dead organisms and waste into ammonium ions.
Nitrification: bacteria change ammonium ions into nitrate ions. It needs oxygen.
Nitrogen fixation: nitrogen gas is changed into nitrogen compounds by lightning and by bacteria (in soil and in root nodules).
Denitrification: bacteria change nitrate ions back into nitrogen gas, mainly in waterlogged soil with little oxygen.
Plants absorb nitrate ions → amino acids → protein → eaten and digested by animals.

Ecosystems and biodiversity

Population: one species, same area, same time.
Community: all the populations of different species in an ecosystem (living things only).
Ecosystem: the community plus its environment (soil, water, air, light), interacting together.
Biodiversity: the number of different species that live in an area.
Population growth rate is affected by food supply, competition, predation and disease.
More food, less competition, fewer predators and less disease make a population grow faster.
Human population growth increases the demand for food, water, land, fuel and other resources.

Effects of humans on ecosystems

Deforestation → loss of habitat → less biodiversity and more extinction.
Deforestation → no roots to hold soil → soil erosion; less water absorbed and more run-off → flooding.
Deforestation → less photosynthesis, and burning or decay of trees → more carbon dioxide in the atmosphere.
Eutrophication 1: nitrate and other ions enter the water → producers (algae) grow rapidly.
Eutrophication 2: algae block light, plants below die, and algae die → more decomposition.
Eutrophication 3: decomposers respire aerobically and use up dissolved oxygen → fish and other animals die.
A non-native species may have no natural predators, so its population grows and it outcompetes or eats native species.

Conservation

Sustainable resource: produced as rapidly as it is removed, so it does not run out.
Sustainable harvest: amount removed per year ≤ amount replaced per year.
Quotas: a legal limit on how many trees are cut or how many fish are caught.
Larger mesh size: young, small fish escape, grow and breed before they are caught.
Closed seasons: no fishing during the breeding season, so fish can reproduce.
Protected areas: no logging or fishing is allowed there, so habitats and breeding populations are kept.
Replanting replaces felled trees. Monitoring counts fish stocks so that quotas can be set at the right level. Education teaches people why to follow the rules.