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Key ideas in A-level biology

The 375 sentences worth carrying into an exam, in the words the lessons use. Each one is there because it is the thing students get wrong, not because it is the thing that is easiest to say.

These are corrections rather than summaries. A summary of a topic tells you what it was about; a correction tells you which of the two plausible things is the true one, which is what a mark scheme is testing when it will not accept the other. Read a unit before an exam, then open the lesson behind any line that does not already feel obvious.

Jump to: Scientific method and quantitative biology · Cells, microscopy and biological organisation · Biological molecules, water and inorganic ions · Enzymes and metabolic control · Membranes and transport across cells · Nucleic acids, genomes and protein synthesis · Cell cycles, reproduction and development · Exchange surfaces and gas exchange · Plant transport and mineral nutrition · Animal transport and cardiovascular biology · Nutrition, digestion and health evidence · Pathogens, disease and immunity · Respiration and cellular energy · Photosynthesis and primary productivity · Nervous coordination, receptors, muscles and behaviour · Hormonal communication, plant responses and homeostasis · Inheritance and population genetics · Gene regulation, genomics and biotechnology · Classification, biodiversity and conservation · Evolution and speciation · Ecology, populations and environmental change

375 ideas

Scientific method and quantitative biologyUnit B0021 ideas

Variables and controls: what an experiment can show

One variable changes, one is measured, everything else is pinned and you say how.
A control variable is a quantity held constant; a control experiment is a whole extra run.
A negative control rules out a false positive; a positive control rules out a false negative.
A confounding variable changes with your independent variable, and more repeats will never separate them.
Precision is agreement between readings; accuracy is agreement with the truth; you can have either without the other.
A conclusion holds for the range tested, the material used and the sample taken, and no further.

Handling data: units, scales, rates and uncertainty

Carry the unit through every line; the mismatch is the error message.
Significant figures come from the instrument, and the answer takes the fewest that went into it.
A ratio has no units, so magnification never has one.
Percentage change divides by where you started, and keeps its sign.
A rate is a gradient; on a curve it is the gradient of a tangent, and the initial rate is the tangent at t = 0.
Equal distances on a log axis mean equal multiplications, and one pH unit is a factor of ten.
Absolute uncertainties add when you add or subtract; percentage uncertainties add when you multiply or divide.

Statistics for biologists: spread, error bars and the three tests

The mean uses every reading, the median ignores extremes, the mode is the only one that works on categories.
The range is fixed by two readings and grows with sample size; the standard deviation uses them all.
Error bars can suggest a difference and can never establish one, and what they suggest depends on whether they show SD, standard error or a confidence interval.
The null hypothesis says there is no significant difference, association or correlation, and is written before the data are seen.
Counts against a predicted ratio go to chi-squared, two means to a t-test, two measurements per individual to Spearman's rank.
The A-level t-test formula assumes two samples of roughly equal spread, and its n₁ + n₂ − 2 degrees of freedom belong to that assumption.
Calculated value at or above the critical value means reject the null hypothesis; below it means the data give no reason to.
Significant means unlikely to have arisen by chance under the null hypothesis. It does not mean large, and it does not mean proved.

Cells, microscopy and biological organisationUnit B0120 ideas

Cell structure: what each organelle is shaped for

Two membranes: nucleus, mitochondrion, chloroplast. One: ER, Golgi, lysosome, vesicle, vacuole. None: ribosome.
Rough ER makes protein for export; smooth ER makes lipids.
Rough ER, transport vesicle, Golgi, secretory vesicle, membrane, out.
80S in eukaryotic cytoplasm; 70S in prokaryotes and inside mitochondria and chloroplasts.
Wall, chloroplasts, permanent vacuole and plasmodesmata are the plant additions; centrioles are the animal one.

Microscopy: magnification, resolution and what you can trust

Magnification = image ÷ actual. No units on the answer.
Resolution is a distance, and smaller is better: 0.1 nm beats 0.2 µm.
Shorter wavelength, better resolution, which is why electrons beat light.
TEM: thin section, electrons through, flat image. SEM: whole surface, electrons off, three-dimensional image.
Recalibrate the eyepiece graticule every time you change objective lens.

Prokaryotic cells and viruses: less machinery, same problems

Prokaryote: no nucleus, no membrane-bound organelles, one circular DNA molecule with no histones, 70S ribosomes, murein wall.
Plasmids are small extra DNA circles, replicated separately and shared between cells.
Mitochondria and chloroplasts also hold circular DNA and 70S ribosomes.
Binary fission doubles the population each time: N = N₀ × 2ⁿ.
A virus is acellular: capsid, genetic material, attachment proteins, sometimes an envelope, and nothing else.

Specialisation: how one genome becomes two hundred kinds of cell

Every cell has the same genes; differentiation is about which are expressed.
Totipotent makes anything including the placenta; pluripotent anything in the body; multipotent a limited range.
Cell, tissue, organ, organ system: each built from the one before it.
Squamous epithelium is thin for a short diffusion path; ciliated epithelium sweeps mucus.
Xylem is dead, hollow and lignified; phloem is alive, has sieve plates and needs a companion cell.

Biological molecules, water and inorganic ionsUnit B0218 ideas

Carbohydrates: from one sugar to a store of thousands

α-glucose has the hydroxyl on carbon 1 below the ring; β-glucose has it above.
One glycosidic bond made, one water released; n monosaccharides give n − 1 of each.
Amylose is unbranched and coiled, amylopectin is branched, glycogen is more branched still, cellulose is straight.
Benedict's solution finds reducing sugars and needs heating; iodine in potassium iodide solution finds starch and does not.

Lipids: triglycerides, phospholipids and the ester bond

One glycerol plus three fatty acids gives three ester bonds and three molecules of water.
Saturated means no carbon to carbon double bond, so the tail is straight and packs closely; unsaturated means at least one, so it bends.
Lipids are macromolecules but not polymers, because the units are not all the same.
A phospholipid is a triglyceride with one fatty acid replaced by a phosphate group, and that group is what makes the head hydrophilic.
Emulsion test: ethanol, then water, then a white emulsion.

Water and inorganic ions: the properties life depends on

The oxygen end of a water molecule is slightly negative and the hydrogen ends slightly positive: that is polarity, and hydrogen bonds follow from it.
A hydrogen bond runs between molecules, not within one, and each water molecule can hold four.
High specific heat capacity resists a change in temperature; high latent heat of vaporisation removes heat when water evaporates.
Ice is less dense than liquid water, so it floats and insulates what is below it.
Every ion needs a named role: hydrogen for pH, iron for haemoglobin, sodium for co-transport and impulses, phosphate for nucleotides and ATP, calcium for muscle and synapses, nitrate for amino acids.

Proteins: from one amino acid to a working shape

Amino acids share one skeleton and differ only in the side chain.
One peptide bond made, one water released; n amino acids give n − 1 bonds.
Primary is peptide bonds, secondary is hydrogen bonds along the backbone, tertiary is four bond types between side chains, quaternary is several chains together.
Denaturation loses the tertiary structure and keeps the primary one.

Enzymes and metabolic controlUnit B0315 ideas

Enzymes: what a catalyst can and cannot do

An enzyme lowers activation energy. It does not change the energy of the reactants or of the products, and it does not move an equilibrium.
The active site is built by tertiary structure, so anything that changes the fold changes the site.
Substrate and active site are complementary, not identical.
Induced fit: the site moulds around the substrate as it binds, and that moulding is where the catalysis comes from.
A catalyst finishes the reaction in the state it started it, ready to go again.

Factors affecting enzyme rate: temperature, pH and concentration

Compare initial rates, never rates read part-way through a run.
The temperature optimum is a balance between rising kinetic energy and rising denaturation, so the curve is lopsided: gentle up, steep down.
Q10 is roughly 2 below the optimum, and meaningless above it.
pH acts by changing charge on R groups, which breaks ionic and hydrogen bonds and alters the shape of the active site.
Substrate concentration gives a curve that plateaus at saturation; enzyme concentration gives a straight line, for as long as substrate stays in excess.

Inhibition: how cells, poisons and medicines all turn enzymes down

Competitive inhibitors bind the active site; non-competitive inhibitors bind elsewhere and change its shape.
More substrate overcomes competitive inhibition and does nothing for non-competitive inhibition.
Competitive: Vmax unchanged, apparent Km raised. Non-competitive: Vmax lowered, Km unchanged.
Reversible inhibitors are held by weak interactions; irreversible ones form a covalent bond, as penicillin does with the transpeptidase that builds a bacterial wall.
End-product inhibition acts on the first enzyme of a pathway, so no substrate and no ATP is spent making intermediates the cell does not need.

Membranes and transport across cellsUnit B0415 ideas

The fluid mosaic model: what a membrane is made of

Heads are hydrophilic and face the water; tails are hydrophobic and face each other.
Nothing bonds the bilayer together. It is the arrangement that hides the most tails from water.
Intrinsic proteins are embedded in the bilayer; extrinsic proteins rest on one surface.
Cholesterol reduces fluidity at high temperature and prevents tight packing at low temperature.
Above about 45 °C, permeability rises sharply because the proteins denature.

Diffusion and osmosis: movement that costs nothing

Rate of diffusion rises with surface area and concentration difference, and falls as the exchange surface gets thicker.
Facilitated diffusion uses proteins but no ATP, and its rate plateaus when they are all occupied.
Pure water is 0 kPa; every solution is negative; water moves from the less negative to the more negative.
ψ = ψs + ψp, with ψs always negative and ψp zero or positive in a living plant cell.
At incipient plasmolysis ψp = 0, so ψ = ψs.

Active transport, co-transport and moving things in bulk

Active transport moves a substance against its gradient, using a carrier protein and ATP.
ATP is hydrolysed, the phosphate binds to the carrier, and the shape change is what carries the substance across.
The sodium-potassium pump moves three sodium ions out and two potassium ions in per ATP, leaving the inside more negative.
In co-transport the ATP is spent at the pump, not at the co-transporter.
Endocytosis and exocytosis move material in bulk by folding or fusing the membrane, and both need ATP.

Nucleic acids, genomes and protein synthesisUnit B0520 ideas

DNA and RNA: four letters and a rule about pairing

One nucleotide is a phosphate, a pentose sugar and a base.
The backbone alternates sugar and phosphate, joined by phosphodiester bonds made by condensation.
A pairs with T through two hydrogen bonds; G pairs with C through three.
The strands are antiparallel: 5′ on one side lies against 3′ on the other.
RNA has ribose instead of deoxyribose and uracil instead of thymine, and it is single-stranded.

DNA replication: one old strand in every new molecule

Semi-conservative: every daughter molecule is one old strand and one new one.
Helicase breaks hydrogen bonds; DNA polymerase makes phosphodiester bonds; ligase seals the fragments.
DNA polymerase adds nucleotides only to a free 3′ end, so synthesis is always 5′ to 3′.
Antiparallel templates plus a one-way enzyme give one leading strand and one lagging strand.
One intermediate band after one generation rules out the conservative model; two bands after two rules out the dispersive one.

The genetic code and transcription

Triplet, degenerate, non-overlapping, universal, and a consequence for each.
4³ = 64 triplets, 61 coding and 3 stop, for 20 amino acids.
RNA polymerase transcribes; only the template strand is read.
The mRNA matches the coding strand with U in place of T.
Splicing removes introns and joins exons; alternative splicing lets one gene give several polypeptides.

Translation: turning a message into a working protein

Codons are on mRNA; anticodons are on tRNA; the two pair by hydrogen bonding.
AUG starts translation and codes for methionine; UAA, UAG and UGA stop it and code for nothing.
The P site holds the growing chain, the A site accepts the next tRNA, and a peptide bond forms between them by condensation.
ATP loads each tRNA with its amino acid, and more is spent moving the ribosome along.
Cutting and the finishing of sugar chains happen after translation, in the endoplasmic reticulum and Golgi apparatus; folding begins on the ribosome.

Cell cycles, reproduction and developmentUnit B0616 ideas

The cell cycle: growth, copying, and one division that changes nothing

Interphase is about nine tenths of a 24-hour cycle, and it is not a rest.
S phase doubles the DNA and leaves the chromosome number alone.
Prophase, metaphase, anaphase, telophase: the spindle forms in the first, and anaphase starts when the centromeres divide.
Cytokinesis divides the cytoplasm and is separate from mitosis.
Losing a checkpoint means division without a brake, which is how a tumour starts.

Stem cells: what a cell can still become, and what fixes it

A stem cell is unspecialised, renews itself by mitosis, and can differentiate.
Totipotent makes the placenta as well; pluripotent makes any body cell; multipotent makes one family of related types; unipotent makes one type.
Potency narrows because genes are switched off, and the genome is unchanged throughout.
Embryonic and induced pluripotent cells are pluripotent; adult tissue and cord blood give multipotent cells.
An evaluation needs both sides and a judgement that follows from them.

Gametes and fertilisation: from two cells to a blastocyst

Four gametes come from one primary spermatocyte and one from one primary oocyte, because oogenesis divides its cytoplasm unequally.
Acrosome for enzymes, mid-piece for ATP, tail for movement; on the egg, cytoplasm for food, zona pellucida for binding, cortical granules for the block.
Binding, then the acrosome reaction, then the path through the zona, then fusion of the membranes, then fusion of the nuclei.
The cortical reaction changes the zona pellucida, and that is what prevents polyspermy.
Cleavage raises the number of cells and does not raise the total size.
Fertilisation in the oviduct, implantation in the endometrium about six or seven days later.

Exchange surfaces and gas exchangeUnit B0716 ideas

Surface area to volume ratio: why size limits diffusion

Surface grows as the square of length and volume as the cube, so the ratio falls as size rises: 6 ÷ side for a cube, 3 ÷ radius for a sphere.
Shape changes the ratio at fixed volume. A sphere is the worst case; a sheet is much better.
Rate of diffusion goes up with area and with the difference in concentration, and down with the thickness of the surface.
Diffusion time rises with the square of the distance, so it is excellent over micrometres and hopeless over millimetres.
Every specialised exchange surface is large, thin, permeable, and kept supplied on both sides so the gradient does not flatten.

Lungs: seventy square metres, folded into a chest

Cartilage in rings means trachea; in blocks means bronchus; absent with smooth muscle means bronchiole; a single squamous layer means alveolus.
Roughly 300 million alveoli, about 70 m², and a barrier under one micrometre thick.
Inspiration is always active; quiet expiration is largely elastic recoil.
The order is muscles, volume, pressure, air, and questions are marked on that order.
Pulmonary ventilation rate = tidal volume × ventilation rate, in dm³ min⁻¹.

Three other answers: tracheae, gills and stomata

Insects pipe air to their cells through spiracles, tracheae and tracheoles; their blood carries no oxygen.
Tracheal fluid withdraws during activity because lactate lowers the water potential of the muscle cells, shortening the liquid part of the pathway.
A fish gill is arch, filament, lamella, and the lamella is one cell thick with a capillary network inside it.
Counter-current flow keeps water richer than blood along the whole lamella; parallel flow equalises part-way along and stops.
A leaf exchanges gases through stomata into air spaces, using the wet walls of mesophyll cells as the surface, and pays in water vapour.
Every xerophyte adaptation cuts the area, lengthens the pathway, or traps humid air to flatten the gradient.

Plant transport and mineral nutritionUnit B0818 ideas

Xylem and the transpiration stream: the pull comes from the top

Root hairs give surface area; active uptake of ions keeps the water potential gradient pointing inwards.
Apoplast through the walls, symplast through the cytoplasm, and the Casparian strip ends the apoplast route at the endodermis.
A xylem vessel is dead, hollow, lignified and open end to end, and every one of those is an adaptation.
Evaporation at the leaf creates the pull; cohesion holds the column together; the sap is under tension, not pressure.
Root pressure is small, needs oxygen, and is not the mechanism.
A potometer measures uptake, and uptake is slightly more than transpiration.

Transpiration: the price of keeping the stomata open

Transpiration is the cost of opening stomata for carbon dioxide, not a purpose.
Evaporation from the mesophyll walls, then diffusion out of the stomata: two steps, and factors act on one or the other.
Light and temperature raise the rate; humidity lowers it; wind raises it by removing the boundary layer.
Protons out, potassium in, water in, turgid, thicker inner wall bows the cell and the pore opens.
Abscisic acid shuts stomata under water stress, whatever the light is doing.
Xerophyte adaptations make the gradient shallower or the exits fewer; a hydrophyte puts its stomata on top.

Phloem and translocation: pumped at the ends, flowing in the middle

Sieve tube elements lose their nucleus and most organelles; companion cells keep both and do the work.
Sucrose travels because it is soluble, non-reducing and not a respiratory intermediate.
Loading is a hydrogen ion pump plus a co-transporter; the flow along the tube is not pumped.
Solute in, water potential down, water in, pressure up: that chain at the source and its reverse at the sink is the whole of mass flow.
Source and sink are roles, and an organ can swap between them with the season.
Ringing shows the route, aphid stylets show the pressure, tracers show the tissue; none of them proves the mechanism.

Animal transport and cardiovascular biologyUnit B0915 ideas

The heart and the cardiac cycle: pressure decides everything

Every valve movement in the heart is caused by a pressure difference, never by muscle pulling on the valve.
A valve is open when the chamber behind it holds the higher pressure, and shut when it does not.
Artery means away from the heart, vein means towards it; the pulmonary pair are the exception to the oxygenation you expect.
Cardiac output = stroke volume × heart rate; at rest about 70 cm³ × 75 min⁻¹ ≈ 5.25 dm³ min⁻¹.
SAN → atria → AVN (delay ≈ 0.13 s) → bundle of His → apex → Purkyne fibres → ventricles contract from the bottom up.

Blood vessels and tissue fluid: what leaks out and what comes back

Wall thickness follows the pressure a vessel carries; lumen width follows what is left over.
The arterioles are where mean pressure collapses and where the pulse disappears.
Tissue fluid forms where hydrostatic pressure exceeds oncotic pressure, at the arteriole end, and returns where the balance reverses, at the venule end.
Only the hydrostatic pressure changes along a capillary. The oncotic pressure stays put because the proteins do.
About a tenth of the fluid leaving never returns directly; the lymphatic system takes it back the long way.

Haemoglobin: an S-shaped curve and everything it explains

Haemoglobin is a conjugated globular protein: four chains, four haem groups, four oxygen molecules.
The curve is sigmoid because binding is cooperative: each oxygen makes the next one easier.
Loading happens on the flat top of the curve, unloading on the steep middle, which is why one is reliable and the other is sensitive.
Right shift means unloads more readily; left shift means higher affinity. Foetal haemoglobin is to the left of adult.
Most carbon dioxide travels as hydrogencarbonate, and the chloride shift is what keeps the red blood cell electrically neutral while it does.

Nutrition, digestion and health evidenceUnit B1015 ideas

Digestion and absorption: one tube, four chemistries

Digestion is hydrolysis, and physical digestion provides surface area for it rather than doing any of it.
Endopeptidases first, exopeptidases second, dipeptidases at the membrane: cutting the middle multiplies the ends.
Bile salts emulsify and form micelles; pancreatic lipase does the hydrolysis.
Micelles deliver, they do not enter. Chylomicrons leave in the lymph.
In co-transport the ATP is spent by the sodium-potassium pump on the far side of the cell, never at the carrier the glucose uses.

Diet, energy and how to read a health claim

Basal metabolism is roughly two thirds of a day's energy expenditure, and lean mass is what mostly sets it.
A daily surplus does not extrapolate, because expenditure rises as mass rises until the two match again.
Cohort starts with healthy people and looks forward; case-control starts with the disease and looks back; only randomisation balances the confounders nobody measured.
A relative risk without an absolute risk is unreadable, and an odds ratio is not a risk at all.
Sample size fixes precision, not bias. A bigger study of a badly measured exposure is a more precise wrong answer.

Cholesterol, atheroma and what a risk factor means

Cholesterol is a sterol, the liver makes most of it, and membranes, steroid hormones, bile salts and vitamin D all depend on it.
HDL and LDL are particles, not kinds of cholesterol: LDL delivers, HDL collects, and the density comes from the protein-to-lipid ratio.
Atheroma begins with damage to the endothelium and ends with a fibrous cap; what proves fatal is usually a clot on a torn cap, not the narrowing.
A risk factor licenses prediction in a population, not a mechanism and not a prophecy about a person.
LDL is regarded as causal because genetics, mechanism and drug trials agree; HDL is not, because only the observational association holds.

Pathogens, disease and immunityUnit B1115 ideas

Pathogens and non-specific defences

Four groups of pathogen: bacteria, viruses, fungi, protoctista. Learn one named disease for each.
Pathogens damage the host mainly by releasing toxins or by destroying the cells they reproduce inside.
Physical barriers block; chemical barriers destroy. Both act on any pathogen at all.
Phagocytosis in order: chemotaxis, attachment, engulfing into a phagosome, lysosome fusion, hydrolysis, antigen presentation.
Antigen presentation is the handover from the non-specific response to the specific one.

The specific response: selected, cloned, remembered

Lymphocytes are selected by an antigen, not instructed by it: the shapes exist before the infection does.
Clonal selection is the binding; clonal expansion is the mitosis that follows.
T cells respond to antigen presented on a cell surface; B cells can bind free antigen, but still need a T-helper cell to be fully activated.
Plasma cells secrete antibody for days. Memory cells secrete nothing for years, then divide into plasma cells.
The secondary response is faster, larger and longer-lasting because it starts with thousands of the right cells instead of one.

Antibodies, vaccination and the four kinds of immunity

An antibody is four chains, two identical binding sites, variable regions at the tips and a constant region below.
Antibodies agglutinate, neutralise and mark for phagocytosis. Phagocytes do the destroying.
Active immunity makes memory cells and lasts; passive immunity supplies antibody and fades.
A vaccine supplies antigen, provokes a primary response, and leaves memory cells so that the real infection meets a secondary one.
Herd immunity has a threshold of 1 − 1/R₀, which for measles is 93 to 94% under the formula's idealised assumptions. The 95% coverage target is the operational figure that allows for real vaccines and real mixing, not the same figure.

Respiration and cellular energyUnit B1215 ideas

Spending ATP to make ATP: glycolysis and the link reaction

ATP is hydrolysed to ADP and phosphate, releasing about 30.5 kJ per mole in one step, and a cell holds only seconds' worth of it.
Glycolysis: cytoplasm, no oxygen, net 2 ATP, 2 reduced NAD, 2 pyruvate.
Phosphorylation, lysis, oxidation: 2 ATP in, 4 ATP out.
The link reaction takes place in the matrix: decarboxylation, dehydrogenation, acetyl CoA. Twice per glucose.
After glycolysis and the link reaction: 2 ATP, 4 reduced NAD, 2 CO₂ per glucose.

The Krebs cycle and oxidative phosphorylation

One turn of the Krebs cycle: 2 CO₂, 3 reduced NAD, 1 reduced FAD, 1 ATP. Two turns per glucose.
The acceptor is four carbons, citrate is six, and the acceptor is regenerated, since otherwise the cycle could not turn again.
Electrons pass down the carriers; the energy pumps protons out of the matrix; the protons return through ATP synthase and ATP is made.
Oxygen's only role is as the final electron acceptor, combining with electrons and protons to form water.
38 ATP is a theoretical ceiling. Around 30 is realistic, and the shortfall has named causes.

Anaerobic respiration and the respiratory quotient

Glycolysis needs oxidised NAD, not oxygen. Anaerobic routes exist to hand that NAD back.
Animals: pyruvate to lactate, three carbons kept, reversible. Yeast: pyruvate to ethanal plus CO₂, then ethanal to ethanol, irreversible.
Anaerobic yield is 2 ATP per glucose against about 30 aerobically.
Lipid gives roughly 39 kJ per gram because it is highly reduced: more hydrogen per gram means more reduced coenzyme.
RQ = CO₂ out ÷ O₂ in. About 1.0 for carbohydrate, 0.9 for protein, 0.7 for lipid; above 1.0 usually indicates some anaerobic respiration (or carbohydrate being converted to lipid).

Photosynthesis and primary productivityUnit B1315 ideas

Chloroplasts: catching light and turning it into ATP

Thylakoid membranes do the light-dependent stage; the stroma does the light-independent stage.
Chlorophyll absorbs blue and red, reflects green, and that is why leaves look green.
Photolysis splits water into electrons, protons and oxygen.
Protons are pumped into the thylakoid space and flow back through ATP synthase: that is chemiosmosis.
Non-cyclic gives ATP, reduced NADP and oxygen; cyclic gives ATP alone.

The Calvin cycle: three steps, and the arithmetic behind them

Rubisco fixes one CO₂ onto one RuBP and the product splits immediately into two GP.
Reduced NADP gives the hydrogen, ATP gives the energy, and GP becomes TP.
Six turns, twelve TP, two out and ten back into RuBP, eighteen ATP spent.
Whatever comes before a blocked step accumulates; whatever comes after it falls.
GP and RuBP always move in opposite directions.

Limiting factors and primary productivity

The limiting factor is the one in shortest supply, and increasing anything else changes nothing.
The steep part of the curve shows the current limiting factor; the height of the plateau shows what took over.
At the compensation point photosynthesis exactly equals respiration, and plants respire all day and all night.
NPP = GPP − R, and productivity is measured in kJ m⁻² year⁻¹.
Roughly a tenth of the energy gets through each trophic level, which is why food chains are short.

Nervous coordination, receptors, muscles and behaviourUnit B1420 ideas

Receptors: turning a stimulus into something a neurone can carry

Stimulus, receptor, coordinator, effector, response: in that order, every time.
A reflex arc has three neurones and two synapses, and the response does not wait for the brain.
A receptor is a transducer: it converts stimulus energy into a change in membrane potential.
The generator potential is graded; the action potential it may trigger is not.
Rods converge, so they are sensitive and blurry; cones do not, so they need light and see detail.

The action potential: the same size, however hard you push

Resting potential is about −70 mV, held by a 3:2 pump and a membrane far more permeable to potassium than to sodium.
Sodium in for the rise, potassium out for the fall, potassium slow to shut for the undershoot.
All action potentials in a neurone are the same size; intensity is frequency and neurone number.
The refractory period keeps impulses discrete, keeps them one-way, and caps their frequency.
Myelinated and wide and warm is fast; bare and thin and cold is slow.

The synapse: transmission, and why it runs one way

The impulse stops at the presynaptic membrane; a chemical crosses the gap and a new impulse starts.
Calcium in, vesicles fuse, acetylcholine diffuses across, sodium in, threshold, enzyme clears the cleft.
Vesicles on one side and receptors on the other is the whole reason transmission is one-way.
Spatial summation is several neurones at once; temporal summation is one neurone in quick succession.
Inhibitory synapses hyperpolarise the postsynaptic membrane, so more excitation is needed to fire it.

Sliding filaments: shortening a muscle without shortening anything

Muscle, fibre, myofibril, sarcomere, and the sarcomere runs from one Z line to the next.
Calcium binds troponin, troponin moves tropomyosin, and the binding site is exposed.
Attach, power stroke, ATP binds and the head detaches, hydrolysis re-cocks it. Repeat while calcium lasts.
The A band never changes; the I band and H zone shorten by the same amount as the sarcomere.
ATP, phosphocreatine, anaerobic glycolysis, aerobic respiration: seconds, seconds, a minute or two, and indefinitely.

Hormonal communication, plant responses and homeostasisUnit B1520 ideas

Hormones: a message broadcast in the blood that only some cells can read

Exocrine into a duct, endocrine into the blood. That is the whole distinction.
A hormone reaches every cell; only cells with a complementary receptor respond.
Steroid hormones cross the membrane and act on genes; peptide hormones cannot cross and use a second messenger.
Adrenaline, receptor, adenylate cyclase, cyclic AMP, kinase, phosphorylase, glucose: in that order.
Negative feedback reverses a change; positive feedback amplifies one, and something outside the loop has to stop it.

Blood glucose: two hormones, one liver, and two different diabetes

About 5 mmol per dm3, held there by insulin from beta cells and glucagon from alpha cells.
Insulin is a signal, not an enzyme: receptors, transporters, and the enzymes of glycogenesis.
Glucagon and adrenaline act on the liver through cyclic AMP; the liver is the organ that puts glucose back.
In a beta cell: glucose in, ATP up, potassium channel shut, depolarised, calcium in, vesicles fuse.
Type 1 is destroyed beta cells; type 2 is cells that no longer respond. Cause, not age.

The kidney: filter everything, then take almost all of it back

Filter everything small under pressure, then reclaim what is worth keeping.
The basement membrane is the sieve; the efferent arteriole being narrower is why there is any pressure to filter with.
The proximal tubule takes back all the glucose by co-transport, and the ATP is spent on the sodium pump.
The loop builds the medulla's gradient; the collecting duct is where the water actually leaves.
ADH inserts aquaporins and changes permeability. Longer loops mean steeper gradients mean more concentrated urine.

Plant responses: tropisms, auxins and the classic experiments

A tropism is unequal growth whose direction is set by the direction of the stimulus.
Tip detects, chemical diffuses, chemical alone bends in the dark: one experiment each, and no more.
Auxin is redistributed to the shaded side; the total does not fall.
Elongation is proton pumps, an acid wall, expansins, and water entering by osmosis.
The same concentration stimulates a shoot and inhibits a root, which is why they bend opposite ways.

Inheritance and population geneticsUnit B1622 ideas

Meiosis: two divisions, and why no two gametes come out the same

One replication, two divisions, four haploid cells.
Meiosis I separates homologues and halves the chromosome number; meiosis II separates chromatids and does not.
Crossing over happens in prophase I, between non-sister chromatids, and makes sister chromatids no longer identical.
Independent assortment happens at metaphase I and gives 2 to the power n combinations, where n is the number of pairs.
Non-disjunction at anaphase I affects all four products; at anaphase II it affects two.

Monohybrid inheritance: genetic diagrams that score

Parental phenotypes, parental genotypes, gametes, offspring, ratio: in that order, every time.
3 : 1 from two heterozygotes, 1 : 1 from a test cross, all dominant from a homozygous dominant parent.
Codominance and incomplete dominance both give 1 : 2 : 1 and three phenotypes; the genotype ratio and the phenotype ratio are the same.
Three alleles in a population, two in any one individual: I^A and I^B codominant, both dominant to I^O.
A male has one allele of a sex-linked gene and expresses it; a father gives his X to every daughter and to no son.

Two genes at once: 9 : 3 : 3 : 1, the ways it breaks, and the chi-squared test

RrYy makes four gametes in equal numbers; a dihybrid selfing gives 9 : 3 : 3 : 1 and a dihybrid test cross gives 1 : 1 : 1 : 1.
Linkage shows as an excess of the two parental classes; recombination frequency is recombinants over total, and never exceeds 50%.
Epistasis merges classes and keeps the total in sixteenths: 9 : 3 : 4, 12 : 3 : 1 and 9 : 7 are the ones to recognise.
χ² = Σ (O − E)² ÷ E, on raw counts, with expected values built from the ratio.
Degrees of freedom = categories − 1; at p = 0.05 the critical values run 3.84, 5.99, 7.81, 9.49, 11.07 for 1 to 5 degrees of freedom.
A large χ² argues against your hypothesis, not for it.

Hardy-Weinberg: what alleles do in a population when nothing acts on them

A gene pool holds 2N alleles of an autosomal gene in N diploid individuals.
p + q = 1 for two alleles; p² + 2pq + q² = 1 for the three genotypes they make.
q² is the frequency of the recessive phenotype, and the square root of it is the allele frequency.
The five conditions: large population, random mating, no selection, no mutation, no migration.
A significant departure from the predicted proportions means a condition is being broken, and the pattern of the departure says which.
Examined by AQA, OCR A and Cambridge International; not listed by Edexcel's Salters-Nuffield specification.

Gene regulation, genomics and biotechnologyUnit B1725 ideas

Control of gene expression: which genes a cell reads

Every cell has the whole genome; what differs between cell types is which genes are transcribed.
A transcription factor binds a specific base sequence near a gene and raises or lowers the rate of transcription.
In the lac operon the repressor sits on the operator until lactose changes its shape; then the three structural genes are transcribed together.
Oestrogen crosses the membrane because it is lipid-soluble, binds a receptor, and the complex acts as a transcription factor.
More methylation of DNA and less acetylation of histones both mean less transcription, and neither changes a base.
siRNA destroys an mRNA that has already been made, so the gene is transcribed but the protein is not.

Mutation and cancer: what goes wrong, and where it goes wrong

Substitutions affect one triplet; insertions and deletions that are not multiples of three shift the whole reading frame.
Silent, missense and nonsense are the three outcomes of a substitution, and each has a distinct effect on the polypeptide.
A mutation in a control sequence changes how much protein is made without changing what the protein is.
Oncogenes are proto-oncogenes stuck on; tumour suppressor genes are brakes that have been lost, and both alleles must go.
Methylating the promoter of a tumour suppressor gene silences it without altering a single base.
Bone marrow and limbal stem cell treatments are established; most other stem cell therapies are still in trials.

Recombinant DNA technology: isolating, inserting and expressing a gene

Three routes to a gene: reverse transcriptase from mRNA, restriction enzymes from the chromosome, or synthesis by the gene machine.
Restriction enzymes cut at palindromic recognition sequences; an offset cut leaves sticky ends.
Sticky ends pair by hydrogen bonds; ligase makes the phosphodiester bonds.
A vector needs an origin of replication, a promoter the host recognises, and a marker gene.
PCR: 95 °C to separate the strands, 50 to 65 °C for the primers, 72 °C for Taq polymerase, and the number of copies doubles each cycle.
In a gel, DNA runs towards the positive electrode and the shortest fragments travel furthest.

Genomes, screening and gene therapy: what a sequence is good for

Sequencing gives you bases; annotation, done afterwards, gives you genes.
In eukaryotes the genome does not give the proteome directly, because of introns, alternative splicing and post-translational modification.
A probe is a short labelled single strand that binds a complementary sequence by hydrogen bonding; a microarray is thousands of probes at once.
Screening is the test; counselling is the non-directive conversation about what the result means.
DNA profiles use variable number tandem repeats in non-coding DNA, amplified by PCR and separated by electrophoresis.
A match probability is the chance of a match given innocence, and reversing it is the prosecutor's fallacy.
Somatic gene therapy is not inherited; germ line therapy would be, and that is the whole reason they are treated as separate questions.

Classification, biodiversity and conservationUnit B1815 ideas

Classification and phylogeny: sorting by ancestry, not by looks

Domain, kingdom, phylum, class, order, family, genus, species: nested, with no overlap at any rank.
Binomial: genus capitalised, specific name not, both italicised or underlined.
Homologous means same plan, different job, and is evidence of ancestry. Analogous means same job, different plan, and is evidence of convergence.
The three domains came from comparing small-subunit ribosomal RNA, and Archaea are closer to Eukarya than to Bacteria.
A cladogram claims branching order and common ancestry. It does not claim time, amount of change, or progress.

Measuring biodiversity: richness, evenness and the index that catches both

Richness counts species; evenness describes how the individuals are shared; diversity needs both.
D = Σ(n ÷ N)² measures dominance and falls as diversity rises. Never quote it as a diversity score.
OCR A and CAIE: D = 1 − Σ(n ÷ N)², between 0 and 1. AQA: d = N(N − 1) ÷ Σn(n − 1), minimum 1, no maximum. Both rise with diversity.
Genetic diversity is measured as the proportion of polymorphic gene loci, using DNA, mRNA or amino acid sequences rather than what the organism looks like.
Monoculture and hedgerow removal cut habitat and species diversity; selective breeding cuts genetic diversity within the crop.

Conservation: keeping a species where it lives, or somewhere else

In situ means in the natural habitat; ex situ means outside it. Most programmes use both.
In situ conserves the community and a big enough population to keep alleles; it cannot stop poaching, invasives or a habitat that has already gone.
Ex situ gives control and can hold a species that is gone from the wild; it cannot hold much genetic diversity, learned behaviour or the habitat.
Seed banks: dry to about 5 per cent moisture, freeze at −20 °C, germinate a sample periodically. Recalcitrant seeds cannot be stored this way.
The economic, ecological, ethical and aesthetic cases are four positions with four objections, not four ticks on a list.

Evolution and speciationUnit B1919 ideas

Natural selection: variation, selection and allele frequency

Mutation is the only source of new alleles; meiosis and random fertilisation make new combinations of alleles that already exist.
Mutation is random with respect to fitness. Environments select; they do not instruct.
Fitness is relative reproductive contribution, counted in surviving fertile offspring.
Stabilising narrows the distribution, directional moves it, disruptive splits it. AQA names the first two; OCR A and Cambridge International expect all three.
Selection acts on the phenotype; evolution is measured as the change in allele frequency it leaves behind.
Recessive alleles hide in heterozygotes, which is why selection removes them ever more slowly as they get rarer; drift, not selection, removes the last copies if anything does.

Resistance, drift and the evidence: selection where you can watch it happen

The mutation comes first and the antibiotic second. The drug selects; it does not induce.
Vertical transfer spreads resistant cells; horizontal transfer on plasmids spreads the gene itself, between species as well as within one.
Follow the prescribed, evidence-based course length: the least susceptible cells are the last to die, and unnecessary extra exposure selects too.
Fossils, homologous structures, conserved molecules and biogeography are independent lines of evidence, and their agreement is the argument.
Homologous means same structure, different function, common ancestry. Analogous means same function, different structure, convergent evolution.
Drift changes allele frequencies by chance, hardest in small populations. A bottleneck and a founder event are both drift.

Speciation: how one gene pool becomes two, and why the line is genuinely fuzzy

A species: similar characteristics, interbreeds, fertile offspring, reproductively isolated from other groups.
Speciation needs gene flow to stop, or fall sharply, first, and the isolated gene pools to diverge second.
Divergence has three sources, in proportions that vary by case: different mutations, different selection pressures, and genetic drift acting separately in each population.
Allopatric speciation starts with a geographical barrier; sympatric speciation starts with ecological, behavioural or temporal separation, or with polyploidy.
Prezygotic mechanisms stop gametes meeting; postzygotic mechanisms act on the zygote or the hybrid.
Polyploidy is the one route that makes a new species in a single generation, and it is overwhelmingly a plant phenomenon.
The boundary between two species is often genuinely gradual, which is what a gradual process should produce.

Ecology, populations and environmental changeUnit B2020 ideas

Populations and their limits: what stops the numbers rising

Population is one species; community is all of them; ecosystem adds the abiotic surroundings.
A growth curve bends because birth rate falls and death rate rises, not because growth simply 'stops'.
Density-dependent factors regulate a population; density-independent ones only reset it.
Intraspecific competition is usually the fiercest kind, because the niches are so nearly identical.
The predator peak follows the prey peak in the classic cycles, because predators respond by breeding.
In the Lincoln index, fewer marks recaptured always means a larger estimate.

Nutrient cycles: nitrogen, phosphorus and eutrophication

Fixation is the only way nitrogen enters from the air; denitrification is the only way it leaves.
Nitrification is two oxidations in the soil: ammonium to nitrite by Nitrosomonas, nitrite to nitrate by Nitrobacter.
Ammonification returns nitrogen from dead matter and waste as ammonium.
Nitrification needs oxygen; denitrification needs it absent.
Plants take up nitrogen as nitrate, by active transport.
The phosphorus cycle has no gaseous stage, and its rock reservoir turns over on a geological timescale.
In eutrophication the oxygen is taken by bacteria respiring as they decompose, not by the algae.

Succession: how bare rock becomes woodland, and why we stop it

In the examined model, succession happens because each community changes the abiotic environment and makes it suit something else better.
Pioneers stabilise the surface, hold water and add the first organic matter when they die.
Each handover in the model is competition, and usually competition for light.
Secondary succession is faster because soil, nutrients, decomposers and a seed bank are already present.
Soil depth and biomass rise all the way to the climax; species richness often peaks just before it.
A plagioclimax is succession held back on purpose, and much of British conservation is exactly that.
A long dataset can measure a change and falsify a prediction; only a control or a manipulation can establish its cause.