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Key ideas in A-level biology

Each of these 579 concise statements sets out one idea to get right. They are grouped by unit and link to the lesson that explains each point.

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 and muscles · 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

579 ideas

Scientific method and quantitative biologyUnit B0024 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.

Averages, spread and the null hypothesis

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.
A p-value is calculated assuming the null hypothesis is true, so it cannot say how likely that assumption was, and it says nothing about how big the effect is.

Choosing, carrying out and concluding a statistical test

Counts against a predicted ratio go to chi-squared, two means to a t-test, two different quantities measured on each individual to Spearman's rank. The same quantity measured before and after on each individual is a difference question, and belongs to the paired t-test.
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.
Pearson's r and Spearman's rank both run from −1 to +1 and both test for association. Pearson uses the measurements and assumes a normal distribution and a linear relationship; Spearman uses the ranks and assumes neither.
Parametric tests assume a distribution, and Pearson's r and the t-test are the parametric ones here. Spearman's rank and chi-squared are non-parametric.

Cells, microscopy and biological organisationUnit B0133 ideas

Eukaryotic cell structure and organelles

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.
Microfilaments change a cell's shape, intermediate filaments carry its loads, and microtubules are the tracks that motor proteins walk along, spending ATP as they go.
Amyloplast: a plastid with a double envelope, no thylakoids and a large starch grain. Middle lamella: calcium pectate cementing two walls. Pit: a gap in the secondary wall.

Microscopy, magnification and resolution

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.
Homogenising solution: cold, isotonic, buffered. Enzymes slowed, osmosis prevented, protein shape held.
Ultracentrifugation separates by size and density, largest and densest first: nuclei, chloroplasts, mitochondria, lysosomes, ER, ribosomes.

Prokaryotic cells and viruses

Bacterium: no nucleus, no membrane-bound organelles, one main circular DNA molecule not associated with histones, 70S ribosomes, murein wall.
Plasmids are small extra DNA circles, replicated separately and shared between cells.
Mitochondria and chloroplasts also contain 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, in a retrovirus such as HIV, reverse transcriptase; no ribosomes and no cytoplasm.
Lytic cycle: attach, inject, replicate, assemble, lyse. Latency: the genome stays, as a prophage in lambda, until a trigger such as DNA damage.
No metabolism to poison, so antivirals block replication: entry, genome copying, integration, assembly or release.

Cell specialisation and biological organisation

Most nucleated somatic cells contain the same genes; differentiation depends on 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.
Both come from one meristem: the cambium divides, inner daughters become xylem and outer daughters become phloem.
Cell theory: all organisms are made of cells, the cell is the basic unit of structure, function and organisation, and cells come only from cells.

Plan diagrams and tissue sections

A plan diagram shows tissue boundaries in proportion and never shows individual cells.
Root: central xylem star, phloem between the arms. Stem: ring of bundles, xylem inner, phloem outer. Leaf: veins with xylem upper.
Cut across for the map; cut lengthways and tubes become parallel lines, and a cut between bundles can miss them entirely.
Airway walls read from the lumen outwards, and the cartilage names the organ: C-ring trachea, plates bronchus, none bronchiole.
On a smear: no nucleus, erythrocyte; lobed nucleus, neutrophil; round filling nucleus, lymphocyte; kidney nucleus and largest cell, monocyte.

Biological molecules, water and inorganic ionsUnit B0220 ideas

Carbohydrate structure and function

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.
Microfibrils running along a fibre, cemented by lignin, make it strong in tension; tensile strength is breaking force divided by cross-sectional area.

Lipids: triglycerides, phospholipids and ester bonds

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 contains two fatty-acid tails and a phosphate-containing head group, and the phosphate group makes the head hydrophilic.
Emulsion test: ethanol, then water, then a white emulsion.

Water and inorganic ions

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.
In water, a hydrogen bond runs from a hydrogen of one molecule to the oxygen of another, and each 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, potassium for repolarisation and stomata, phosphate for nucleotides and ATP, calcium for muscle and synapses, nitrate for amino acids.
The rest of the roster: ammonium for the nitrogen cycle, chloride for the chloride shift and amylase, hydrogencarbonate for carrying carbon dioxide and buffering, hydroxide for setting pH alongside hydrogen.

Protein structure and function

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 B0320 ideas

Enzyme action and specificity

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.

Enzyme inhibition and metabolic control

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 enzyme catalysing the first committed step of a pathway, so no substrate and no ATP is spent making intermediates the cell does not need.

Cofactors, coenzymes and where enzymes work

A cofactor is a non-protein component an enzyme requires for activity; chloride for amylase is the named inorganic example.
An organic cofactor is a coenzyme. In the usual A-level convention it binds temporarily and carries hydrogen or chemical groups between enzymes; NAD and coenzyme A are made from B vitamins.
A prosthetic group is a cofactor tightly bound to the protein, and may be organic, like the haem of catalase, or inorganic, like the zinc of carbonic anhydrase.
Intracellular enzymes act in the cell that made them, such as catalase; extracellular enzymes are secreted to act outside it, such as amylase and trypsin, because food molecules cannot cross a membrane undigested.
Km is inversely related to apparent affinity in the simple Michaelis-Menten model: a lower Km means half Vmax is reached at a lower substrate concentration.

Membranes and transport across cellsUnit B0415 ideas

Membrane structure and the fluid mosaic model

Heads are hydrophilic and face the water; tails are hydrophobic and face each other.
Neighbouring phospholipids are not covalently bonded. The bilayer is the arrangement that shields 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

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 at atmospheric pressure is 0 kPa; a solution at atmospheric pressure 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 bulk transport

Active transport moves a substance against its gradient, using a carrier protein and ATP.
In a P-type ATPase, ATP is hydrolysed, the phosphate binds to the carrier, and the resulting conformational change moves 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 structure

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

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 and protein synthesis

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 B0640 ideas

The cell cycle and mitosis

Interphase occupies about nine tenths of a 24-hour cycle, and the cell is metabolically active throughout it.
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.
A telomere is repeated non-coding sequence capping each chromosome end; replication cannot copy the very end of the lagging strand, so the repeats are lost instead of genes.
Telomerase rebuilds the repeats and is active in germ line cells, stem cells and most cancer cells, but not in most body cells.
Loss of checkpoint control allows unrestrained division, which is how a tumour forms.

Stem cells and cell potency

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 cease to be expressed; 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, fertilisation and early development

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 organelles, maternal RNAs and nutrients, zona pellucida for binding, cortical granules for the block to polyspermy.
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.

Sexual reproduction in flowering plants

Stamen is anther plus filament; carpel is stigma, style and ovary, with ovules inside the ovary.
Pollen mother cell (2n) by meiosis to four microspores (n), then mitosis to a tube nucleus and a generative nucleus.
The generative nucleus divides by mitosis into two male nuclei, which are the male gametes.
Megaspore mother cell (2n) by meiosis to four megaspores; one survives and three mitoses give eight haploid nuclei.
The embryo sac holds an egg and two synergids at the micropyle end, two polar nuclei in the centre and three antipodals at the far end.
The tube nucleus controls growth, the enzymes digest the style, and the tube enters the ovule through the micropyle.
Male nucleus with egg gives a 2n zygote; male nucleus with two polar nuclei gives a 3n endosperm.
Ovule becomes seed, integuments become testa, ovary wall becomes fruit.

Homeobox genes, Hox genes and apoptosis

The homeobox is 180 base pairs of DNA; the homeodomain is the 60 amino acids it codes for.
Homeobox genes code for transcription factors, so they regulate other genes rather than contributing a structural product.
Hox genes lie in a cluster whose order along the chromosome is the order of the regions they specify from head to tail, which is colinearity.
Apoptosis runs in order: the cell shrinks and the chromatin condenses, the membrane blebs, the cell breaks into membrane-wrapped apoptotic bodies, and a phagocyte engulfs them.
The membrane stays intact in apoptosis and breaks in necrosis, which is why necrosis causes inflammation and apoptosis does not.
Mitosis and apoptosis act together in development: mitosis forms the digit tissue, apoptosis removes cells between the developing digits, and the digit count does not change.
Too little apoptosis alongside unchecked division gives a tumour; too much destroys healthy tissue that may not be replaceable.

Cloning in plants and animals

A clone is genetically identical at the nuclear-DNA level to the organism or cell it came from, because it is produced by mitosis.
Vegetative propagation uses runners, rhizomes, stem tubers, bulbs and leaf plantlets, and every offspring is a clone of the parent.
Micropropagation runs explant, sterilise, callus, shoots then roots, subdivide, acclimatise, with the auxin-to-cytokinin ratio influencing whether roots or shoots form.
Clones are uniform and show no genetic variation: an advantage for commercial uniformity, and a disadvantage for disease resistance in a crop.
Identical twins and split embryos are clones of each other and of neither parent.
In nuclear transfer the nuclear DNA is the body-cell donor's and the mitochondrial DNA is the egg donor's.
Reproductive cloning implants the embryo; therapeutic cloning takes stem cells from it and never implants it.

Exchange surfaces and gas exchangeUnit B0718 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.

Gas exchange in mammals: lungs and ventilation

Cartilage in rings means trachea; in blocks means bronchus; absent with smooth muscle means bronchiole; a single squamous layer means alveolus.
Of the order of 300 million alveoli, a surface area commonly quoted as about 70 m², and a barrier under one micrometre thick.
Inspiration is always active; quiet expiration is largely elastic recoil.
The causal order is muscles, volume, pressure, air.
Pulmonary ventilation rate = tidal volume × ventilation rate, in dm³ min⁻¹.
FEV₁ is the volume forced out in the first second, FVC the whole forced breath; FEV₁ ÷ FVC below about 0.7 means narrowed airways.
Fibrosis thickens the exchange surface, emphysema and tuberculosis reduce its area, and asthma narrows the airways.

Gas exchange in insects, fish and plants

Insects pipe air to their cells through spiracles, tracheae and tracheoles; their blood plays little or no part in carrying 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; each lamella is covered by an epithelium one cell thick, with blood flowing just beneath 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 B0821 ideas

Xylem and the transpiration stream

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.
Fibre cap outside, phloem, cambium, xylem inside: fibres and vessels are dead and lignified, sieve tubes are alive and are not.

Transpiration and stomatal control

Transpiration is an unavoidable consequence of opening stomata for carbon-dioxide uptake. It also contributes to water and mineral transport, turgor and leaf cooling.
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.
Stomatal opening is a circadian rhythm: it keeps its roughly daily cycle in constant conditions, so an internal clock sets it; light resets the clock and also opens stomata directly.
Abscisic acid closes stomata under water stress, independently of light intensity.
Abscisic acid binds a receptor, cytosolic calcium rises, and the calcium is the second messenger that opens the anion channels.
Xerophyte adaptations reduce the gradient or the number of stomata; a floating hydrophyte leaf has stomata on its upper surface.

Phloem translocation and the mass-flow hypothesis

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 B0923 ideas

The heart and cardiac cycle

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.
Myogenic means nothing outside starts the beat. The medulla still sets the rate, through sympathetic (faster) and parasympathetic (slower) nerves.
Chemoreceptors watch pH, pressure receptors watch pressure, and both loops answer a rise with the response that produces a fall.
P wave, atria depolarise; QRS complex, ventricles depolarise; T wave, ventricles repolarise. The QRS is biggest because the ventricles hold the most muscle.
Heart rate from a trace = 60 ÷ the interval between two R peaks in seconds.
Tachycardia is fast and regular, bradycardia is slow and regular, atrial fibrillation is irregular with no P waves, and an ectopic beat is one early complex in an otherwise regular rhythm.

Blood vessels, tissue fluid and lymph

Open circulation: haemolymph leaves the dorsal vessel and bathes the organs in the haemocoel, so pressure is low, delivery is slow and flow cannot be directed.
Closed circulation: blood stays in vessels, so pressure is maintained, delivery is fast and arterioles can direct flow to particular tissues.
Elastic arteries near the heart stretch and recoil to smooth the pulse; muscular arteries further out constrict and dilate to distribute blood.
Wall thickness increases with the pressure a vessel carries; the lumen occupies the remainder of the cross-section.
Mean pressure falls steeply across the arterioles, and the pulse is lost there.
Tissue fluid forms where hydrostatic pressure exceeds oncotic pressure, at the arteriole end, and returns where the balance reverses, at the venule end.
In the classical A-level model, hydrostatic pressure falls along a capillary while oncotic pressure is treated as approximately constant, because the plasma proteins remain inside the capillary.
In that model about a tenth of the fluid leaving does not return directly to the capillary; it is returned through the lymphatic system.

Haemoglobin and oxygen dissociation curves

Haemoglobin is a conjugated globular protein: four chains, four haem groups, four oxygen molecules.
The curve is sigmoid because binding is cooperative: each oxygen bound makes the next bind more readily.
Association occurs on the flat upper part of the curve and dissociation on the steep middle section, so saturation at the lungs is consistently high while dissociation in the tissues is sensitive to partial pressure.
A right shift means oxygen dissociates more readily; a left shift means a higher affinity. Fetal haemoglobin lies to the left of adult.
Most carbon dioxide is carried as hydrogencarbonate, and the chloride shift maintains electrical neutrality in the red blood cell.

Nutrition, digestion and health evidenceUnit B1015 ideas

Digestion and absorption in the human gut

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 B1135 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.
Exotoxins are secreted proteins (Staphylococcus); endotoxins are wall lipopolysaccharide (Salmonella); M. tuberculosis damages by invasion.
Interferon from an infected cell puts its neighbours into an antiviral state, against any virus.
Stem rust: wind-borne spores, entry through stomata, haustoria taking sugars from living cells, shrivelled grain.

The specific immune response

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 B cells secrete nothing for years, then divide into plasma cells; memory T cells give a faster cellular response.
The secondary response is faster, larger and longer-lasting because it starts with thousands of the right cells instead of a few.
T-helper cells activate by releasing cytokines: B cells in the humoral response, T-killer cells in the cell-mediated 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.
A monoclonal antibody comes from a hybridoma: an antibody-producing B lymphocyte from the spleen of an immunised mouse, which makes the right antibody, fused with a myeloma cell, which divides indefinitely.
Immunise, isolate spleen B lymphocytes, fuse, select the fused cells, screen for the clone making the antibody wanted, culture it, harvest.
Withering found digitalis by trial with no control or blinding; a modern drug passes phases I, II and III, with a placebo and double-blinding.
An untested drug in an epidemic sets one patient's hope against unknown harm and lost evidence; trials run inside the epidemic.

Infectious disease: transmission and control

Four cases, four routes: cholera in water and food, malaria in a vector, tuberculosis on shared air and in unpasteurised milk, HIV in body fluids.
Control acts on the route. Break the journey between hosts and the epidemic ends without a cure.
Malaria is the only one of the four with a vector: the female Anopheles carries it, and four Plasmodium species cause it.
Tuberculosis is two organisms: M. tuberculosis in people, M. bovis from cattle, which is why pasteurisation is a TB control.
Control has three axes, biological, social and economic, and a programme standing on one of them falls over.
Evasion mechanisms are the evidence for an evolutionary race: each one was selected by a host defence it now defeats.
A malaria control method is trusted after field trials, peer review, repetition and review of all the evidence, and is then monitored for resistance.

Plant disease and plant defences

Ring rot is a bacterium and TMV is a virus, and neither can be cured: control is hygiene, destruction and resistant varieties.
Callose, a beta-1,3 glucan, seals plasmodesmata and sieve plates within minutes; lignin makes the seal permanent over days.
A plant defends in place: no phagocytes, no antibodies, no memory, so containment and chemistry do all the work.
Alkaloids poison herbivores, tannins bind their digestive enzymes, and volatile signals recruit the herbivore's own parasites.
The flytrap counts: two touches within about thirty seconds to close, around five to digest, because false alarms cost energy.

Respiration and cellular energyUnit B1225 ideas

Glycolysis, ATP 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).
Yeast plus substrate plus methylene blue in a stoppered tube: time the loss of blue, take the rate as 1 ÷ time, and vary temperature, substrate concentration or the substrate itself.
Boiled yeast is the control, because it differs from the experimental tube only in having no working dehydrogenases, and it should stay blue.
Aerobic yeast swaps oxygen for carbon dioxide volume for volume, so compare the two conditions with a respirometer run with and without soda lime, not with a gas syringe alone.

Exercise, sports medicine and performance-enhancing drugs

Too little exercise: more risk of obesity, CVD and type 2 diabetes. Too much: worn joints and a spell of weaker immune defence.
Articular cartilage has no blood supply, so once worn it repairs poorly, and osteoarthritis follows.
The J-shaped curve hypothesis: infection risk is lowest with moderate exercise and rises after very heavy exercise.
Observational data show correlation. A dose-response, controlled confounders, repeated results and a mechanism build the case for a cause.
Keyhole surgery: small incisions, so less tissue damage, less blood loss, lower infection risk and faster rehabilitation.
Anabolic steroids act as transcription factors for muscle protein genes; EPO makes the marrow produce more red blood cells.
Doping debates set duty, harm and fair play against autonomy and the arbitrariness of where the line is drawn.

Photosynthesis and primary productivityUnit B1322 ideas

Chloroplast structure and the light-dependent reactions

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

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 lie on either side of fixation, so removing light or carbon dioxide changes their concentrations in opposite directions.
Temperature acts on every enzyme in the cycle. Cooling: GP, TP and RuBP usually all fall; if fixation slows most, RuBP rises instead.
Most of the sugar is respired; the rest becomes polysaccharides, lipids, amino acids and nucleic acids.

Limiting factors and primary productivity

The limiting factor is the one in shortest supply, and increasing anything else changes nothing.
Where the curve rises, the factor on the x-axis is limiting; where it levels off, something else is.
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.
Biomass is dry mass, dried to constant mass; its energy comes from a calorimeter, and neither step leaves the sample usable afterwards.
N = I − (F + R) for a consumer. Keep the bracket: F and R are both losses from the same intake.
Light intensity from a lamp is proportional to 1/d²: double the distance, a quarter of the light.
Pyramids of numbers and biomass can be inverted; a pyramid of energy never can.
Farming raises transfer by cutting out competing food chains and by cutting livestock respiration.

Nervous coordination, receptors and musclesUnit B1430 ideas

Organisation of the nervous system

CNS is brain plus spinal cord; the PNS is every neurone outside them.
Somatic serves skeletal muscle and is voluntary; autonomic serves glands, smooth muscle and cardiac muscle and is not.
Sympathetic readies the body and parasympathetic rests it, acting antagonistically on the same organs.
The cerebrum handles conscious function, the cerebellum coordinates movement, the medulla controls heart rate and breathing, the hypothalamus monitors the blood, and the pituitary releases hormones under its control.
Sensory neurones carry a cell body on a side branch and a long dendron; motor neurones keep the cell body in the CNS and run one long axon out.

Receptors: transduction and the reflex arc

Stimulus, receptor, coordinator, effector, response: in that order, every time.
A withdrawal 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 on shared bipolar neurones, giving high sensitivity and low acuity; cones converge little at the fovea, so they need more light and resolve more detail.

The action potential and its propagation

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 given axon under fixed conditions are the same size; intensity is coded by frequency and by the number of neurones firing.
The refractory period keeps impulses discrete, keeps them one-way, and caps their frequency.
Conduction is faster in myelinated, wide and warm axons and slower in bare, narrow and cold ones.

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.

Muscles and movement: the sliding-filament model

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.
Stored ATP, phosphocreatine, anaerobic glycolysis and aerobic respiration supply ATP at decreasing rates and for increasing lengths of time.

Brain imaging, development and learning

CT and MRI show structure; PET and fMRI show function.
CT uses X-rays and PET a radioactive tracer; MRI and fMRI use a magnet and radio waves, with no ionising radiation.
In the critical period, active synapses from the eyes are kept and inactive ones are lost, permanently.
Habituation: repeated harmless stimulus, fewer calcium channels open, less transmitter, weaker response.
Nature and nurture are studied through new-born babies, animal experiments, damaged brain areas, twins and other cultures, and each has its own weakness.

Hormonal communication, plant responses and homeostasisUnit B1538 ideas

Hormonal communication and negative feedback

Exocrine glands secrete into a duct; endocrine glands secrete into the blood.
A hormone is carried to tissues throughout the body; 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, G protein, 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.

Regulation of blood glucose and diabetes mellitus

About 5 mmol dm⁻³, 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 caused by destruction of the beta cells; type 2 by a poor response of target cells to insulin. Distinguish them by cause rather than by age.
A test strip is glucose oxidase, then peroxidase, then a chromogen changing colour; a biosensor turns the same reaction into a current and reads a number off it.

Osmoregulation and kidney function

Fibrous capsule outside, then cortex, then medulla, with the renal pelvis in the middle draining into the ureter.
Small molecules are filtered under pressure, then useful substances are selectively reabsorbed.
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 establishes the medullary gradient; the regulated water reabsorption that concentrates urine happens at the collecting duct.
ADH inserts aquaporins and changes permeability. Longer loops mean steeper gradients mean more concentrated urine.

The liver: detoxification, deamination and the making of urea

Excretion removes the waste products of metabolism, mainly carbon dioxide at the lungs and urea at the kidney; faeces are egested.
Two vessels supply the liver: the hepatic artery with oxygen and the hepatic portal vein with everything absorbed from the gut.
In a lobule, blood runs inwards along the sinusoids to the central vein, and bile runs outwards along the canaliculi.
Detoxification changes a toxin into something less harmful and more easily excreted; alcohol goes to ethanal, then to ethanoate.
Deamination gives ammonia and a keto acid; in the ornithine cycle one nitrogen of urea comes from ammonia, the second from aspartate and the carbon from bicarbonate.
Making urea costs ATP and conserves water, which is why land mammals excrete urea and freshwater fish excrete ammonia.

Thermoregulation in endotherms and ectotherms

Core temperature is regulated; skin temperature is allowed to vary, and reducing blood flow to the surface conserves core heat.
Skin receptors detect the surroundings and predict a change; hypothalamic receptors detect the temperature of the blood itself.
Vasoconstriction and vasodilation are changes in the arterioles, with the shunt vessels doing the opposite. Capillaries never move.
Sweat cools by evaporation, taking latent heat from the skin, so it is much less effective in humid air.
Shivering and raised metabolic rate in the liver make heat; hairs standing up trap air, which insulates.
An ectotherm regulates by behaviour and posture, needs roughly a tenth of the food, and pays for it with periods when it cannot be active.

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.
Auxin from the apical bud contributes to keeping lateral buds dormant; remove the tip and they grow, unless auxin is applied in its place. Cytokinins and strigolactones are also involved.
Red light makes Pfr, far-red light and darkness remove it, and Pfr is the active form.
A plant times the night: Pfr inhibits flowering in a short-day plant and promotes it in a long-day plant.
IAA and Pfr each cause a regulatory protein in the nucleus to be broken down, which changes the genes transcribed.

Inheritance and population geneticsUnit B1627 ideas

Meiosis and the sources of genetic variation

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.
Eight stages: know what the spindle, the nuclear envelope and the cell surface membrane are doing in each.
An n + 1 gamete gives polysomy (trisomy 21, Down's syndrome); an n − 1 gamete gives monosomy (45,X, Turner's syndrome).

Monohybrid inheritance and genetic diagrams

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.
Name the gene, name the protein, then say what the faulty allele does to it: TYR, HBB, F8, HTT, and Le for tall and dwarf peas.
In a pedigree, unaffected parents with an affected child mean recessive; affected parents with an unaffected child mean dominant.
An affected daughter of an unaffected father rules out X-linked recessive inheritance; a son who can only have had the allele from his father rules out the X altogether.

Dihybrid inheritance, linkage, epistasis 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 appears as an excess of the two parental classes; recombination frequency is recombinants over total, and does not exceed 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 is evidence against the null hypothesis, not evidence for the genetic hypothesis being tested.

Population genetics and Hardy-Weinberg equilibrium

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 all five specifications: AQA, OCR A, Cambridge International and both Pearson Edexcel specifications.

Gene regulation, genomics and biotechnologyUnit B1751 ideas

Control of gene expression

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 allolactose, made from 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 the base sequence.
siRNA destroys an mRNA that has already been made, so the gene is transcribed but the protein is not.

Mutation, gene expression and cancer

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.
An oncogene is a permanently active proto-oncogene; a tumour suppressor gene is one whose product restrains division, and in most cases both alleles must be lost.
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.
A chromosome mutation changes the structure or number of chromosomes; a balanced translocation carrier is usually healthy but can produce unbalanced gametes.
In cystic fibrosis too little chloride and water are secreted, so mucus thickens and blocks the bronchioles, the pancreatic duct and the vas deferens.

Recombinant DNA technology

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, a terminator behind the gene, 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.
In vivo amplification is replication of a vector in dividing cells; in vitro amplification is PCR. Cells are used for bulk protein production and PCR for rapid amplification of small amounts of DNA.
An evaluation gives both sides and a stated judgement, and states the mechanism behind each objection.
Viral vectors carry genes into bacterial and animal cells; the gene gun fires DNA-coated metal particles through plant cell walls.
A knockout mouse lacks a working copy of one gene, and what goes wrong shows what the gene does.
High-oleic soya has FAD2 silenced, so its oil holds more oleic acid, fewer double bonds, and resists oxidation.

Genome analysis, genetic screening and gene therapy

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 that the person is not the source, and reversing it is the prosecutor's fallacy.
Somatic gene therapy is not inherited; germ line therapy would be, which is why the two are regulated separately.
A bacterial genome can often be annotated to a predicted list of proteins, which is what makes computer screening for vaccine antigens possible. Viral genomes may use overlapping genes, RNA processing or polyproteins, so the same route is less direct there.
Profiling is not only forensic: medical diagnosis, pedigree and allele selection in breeding, and genetic variability across a population.
Personalised medicine sorts patients with the same diagnosis by genotype, then chooses the drug, as with HER2 and trastuzumab, or the dose, as with TPMT and azathioprine.
Its limits are cost, reference data drawn mostly from people of European ancestry, and the polygenic nature of most common disease.

Industrial biotechnology and fermentation

Industry cultures microorganisms because they grow fast on low-cost substrates under conditions set inside the vessel, without animal-welfare objections, and are readily modified.
Mycoprotein: fungal hyphae grown continuously, protein-rich and meat-textured. Argue the food case on both sides, with mechanisms.
A fermenter controls temperature, pH, oxygen and nutrients at an organism's optima, under aseptic conditions, with probes feeding back to each control.
Batch culture runs through the whole growth curve and harvests at the stationary phase; continuous culture holds the exponential phase by matching inflow to draw-off, with growth set by the dilution rate and a limiting nutrient.
Primary metabolites are made during growth and suit continuous culture; secondary metabolites, including penicillin, are made as growth slows and are produced in batch.
Broth grows cells in bulk, agar separates and counts them, and a selective medium lets only some kinds grow.
Dilution plating gives a viable count; haemocytometer counts, dry mass and turbidity include dead cells.
k = (log₁₀ N − log₁₀ N₀) ÷ 0.301t, both values from the log phase; the mean generation time is 1 ÷ k.

Pharming, recombinant medicines and synthetic biology

Factor VIII for haemophilia and adenosine deaminase for severe combined immunodeficiency are the named recombinant medicines; recombinant factor VIII replaced extraction from donated human plasma.
Proteins requiring chaperoned folding, disulfide bridges and glycosylation are produced in eukaryotic cells; bacteria are used for proteins needing little post-translational modification, such as insulin.
Pharming: human gene behind a milk-protein promoter, introduced into a fertilised egg; founders are screened for mosaicism and a confirmed line established, and the transgenic female secretes the protein in her milk.
Argue pharming on both sides: inefficient founder production and the treatment of animals as production systems against safe supply and a well-kept herd; regulators require batch consistency, pathogen-free certification and containment.
Synthetic biology comprises standard parts, synthesised genomes and engineered pathways; yeast producing artemisinic acid is the pathway example.
Biosecurity and ownership are argued rather than settled: sequence screening against synthesis misuse, which is voluntary and unevenly applied, and patents that fund development while also affecting price and displacing existing producers.
GM plants make medicines by stable transgenic lines or transient expression in leaves; taliglucerase alfa from carrot cells is the approved example.

Classification, biodiversity and conservationUnit B1827 ideas

Classification, phylogeny and molecular evidence

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.

The five kingdoms, species concepts and viruses

Prokaryotae have no nucleus and no membrane-bound organelles, and a bacterial wall is made of peptidoglycan.
Protoctista are the eukaryotes left over: mostly unicellular, varied in nutrition, and not fungi, plants or animals.
Fungi have walls of chitin, grow as hyphae forming a mycelium, feed saprotrophically and store glycogen.
Plantae have walls of cellulose and chloroplasts, photosynthesise and store starch.
Animalia have no cell wall, ingest their food and digest it internally, and are the only kingdom with nervous coordination.
The biological species concept turns on fertile offspring; the morphological concept on shared structure; the ecological concept on a shared niche.
Viruses are acellular and belong to no kingdom, and are classified as DNA or RNA and as double-stranded or single-stranded.

Measuring biodiversity

Richness counts species; evenness describes how the individuals are shared; diversity needs both.
Σ(n ÷ N)² measures dominance and falls as diversity rises, so it is not an index of diversity.
OCR A and CAIE: D = 1 − Σ(n ÷ N)², between 0 and 1. AQA (d) and Edexcel (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.
Non-random sampling comes in three named kinds: opportunistic, stratified and systematic. State which was used and what inference it supports.
Sweep nets and pooters collect invertebrates from vegetation, pitfall traps collect what walks, and each method is biased in its own way.
A keystone species has an effect on community structure out of proportion to its abundance, which a diversity index does not register.

Conservation in situ and ex situ

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 pollution or climate change that start outside a reserve, or restore 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.
Assisted reproduction in captive breeding means artificial insemination, in vitro fertilisation, embryo transfer and surrogacy.
A frozen zoo stores gametes, embryos and tissue in liquid nitrogen. It preserves alleles, but it needs living animals to put them back into.

Evolution and speciationUnit B1933 ideas

Natural selection: variation, selection and allele frequency

Mutation creates new alleles. Gene flow can introduce alleles that already exist in another population, while meiosis and fertilisation produce new combinations of existing alleles.
Mutation is random with respect to fitness. An environment selects among existing variation and does not induce a specific mutation.
Fitness is relative reproductive contribution, counted in surviving fertile offspring.
Stabilising narrows the distribution, directional moves it, disruptive splits it. AQA A-level, Cambridge International and Edexcel B name disruptive selection; OCR A names only stabilising and directional.
Selection acts on the phenotype; evolution is measured as the change in allele frequency it leaves behind.
Selection against a recessive allele becomes less effective as the allele becomes rare because most copies are in heterozygotes. In a finite population, chance and selection can both affect the remaining copies.

Evidence for evolution, resistance and genetic drift

Resistance variants precede the antibiotic or arise independently of it. The drug selects among variation; it does not induce the mutation that defeats it.
Vertical transfer spreads resistant cells; horizontal transfer on plasmids spreads the gene itself, between species as well as within one.
Take antibiotics exactly as prescribed. The least susceptible cells are the last to be removed, and unnecessary extra exposure also applies selection.
Fossils, homologous structures, conserved molecules and biogeography are independent lines of evidence, and their agreement is the argument for common ancestry.
Homologous means same structure, different function, common ancestry. Analogous means same function, different structure, convergent evolution.
Drift changes allele frequencies by chance, with the largest effect in small populations. A bottleneck and a founder event are both forms of drift.
Penicillin is bactericidal: it blocks cross-linking of the wall and growing cells burst. Tetracycline is bacteriostatic: it binds the 70S ribosome and stops protein synthesis.
Hospitals concentrate antibiotic use, vulnerable patients and routes of transmission; their codes of practice cut the selection pressure and break the chain of transmission.

Speciation and reproductive isolation

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 prevent fertilisation; 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.

Continuous and discontinuous variation

Discontinuous variation puts a population into a few distinct classes with no intermediates; continuous variation spreads it over an unbroken range.
Discontinuous characteristics are controlled by one gene or very few and are usually little affected by the environment, though Himalayan rabbit fur is an exception; ABO blood group is the example to quote.
Continuous characteristics are controlled by polygenes and are modified by the environment; height and mass are the examples to quote.
More loci means more phenotype classes, closer together, until the classes stop being visible at all.
The environment changes the phenotype but not the alleles, so environmental variation is not inherited.
Intraspecific variation is within one species, interspecific variation is between different species.

Artificial selection and selective breeding

Artificial selection depends on heritable variation and differential reproduction, as natural selection does, but the breeder rather than the environment determines which individuals reproduce.
The variation is already there. Breeding changes allele frequencies; it does not create alleles.
Crop resistance is bred in by crossing to a resistant wild relative, screening with the pathogen, then backcrossing to the commercial variety.
Selfing maize halves heterozygosity every generation and gives low-yielding inbred lines; crossing two such lines gives hybrid vigour.
Bulls are chosen by progeny testing and used through artificial insemination, because milk yield cannot be measured in the bull.
Inbreeding depression follows from homozygosity exposing harmful recessive alleles, and wild relatives and old varieties are maintained as a reserve of alleles.

Ecology, populations and environmental changeUnit B2042 ideas

Populations and their limits

Population is one species; community is all of them; ecosystem adds the abiotic surroundings.
A growth curve levels off because birth rate falls and death rate rises until the two are equal.
Density-dependent factors regulate a population around a carrying capacity; density-independent factors change its size without regulating it.
Intraspecific competition is usually the most intense, because individuals of one species have almost identical requirements.
The predator peak follows the prey peak in the classic cycles, because reproduction, survival and immigration all take time to change predator numbers.
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 and environmental change

In the facilitation model, succession proceeds because each community changes the abiotic environment in ways that suit other species better than itself. Inhibition and tolerance are alternative models.
Pioneers stabilise the surface, hold water and add the first organic matter when they die.
Each replacement in the model results from competition, usually for light.
Secondary succession is faster because soil, nutrients, decomposers and a seed bank are already present.
Soil depth and biomass rise to the climax; species richness often peaks at the stage before it.
A plagioclimax is a community held below its climax by repeated management, and much British conservation maintains one.
A long dataset can measure a change and falsify a prediction; only a control or a manipulation can establish its cause.
A point quadrat gives cover as hits per pin; a gridded quadrat gives cover or frequency from counted squares; ACFOR is quick and subjective.
Time of death: body temperature for the first day, rigor for hours to days, then decomposition, insect development and succession.
Rigor mortis sets in when ATP runs out and myosin heads cannot detach from actin.

The carbon cycle and climate change

Stores hold carbon; transfers move it. Photosynthesis, respiration, decomposition and combustion are transfers.
Decomposition is respiration carried out by bacteria and fungi on dead material, and it is what stops carbon accumulating in dead bodies.
Fossil fuels formed where dead organisms were buried in low-oxygen conditions that slowed decomposition, so most of their carbon was not returned to the air.
Before industrialisation the cycle was in approximate long-term dynamic balance. Combustion greatly accelerated the transfer of geological carbon to the air, deforestation added more, and about half of what is added stays there.
A changing climate moves the places where a species can survive, so ranges shift polewards and uphill, and species with nowhere to go are the ones at risk.
Temperature and carbon dioxide records are measured directly; pollen in peat and tree rings are proxies from which past climate is inferred.
Climate models give ranges for each emissions scenario, because clouds, feedbacks and future emissions are uncertain.
Rainfall sets where many species can live and when they germinate, breed and migrate.
Peer review filters a claim before publication; independent groups repeating the work validate it afterwards.

Managing ecosystems sustainably

Sustainable yield is what can be taken without reducing the population, so the same amount can be taken again.
In the simple logistic model the maximum sustainable yield comes at about half the carrying capacity, where net growth is fastest, and exceeding it has no stable outcome.
Coppicing cuts to a stool near the ground and pollarding cuts higher up, out of reach of grazing animals.
Cutting one compartment of a wood each year gives a steady yield and a patchwork of habitats.
Selective felling removes individual mature trees and leaves the forest structure, unlike clear felling.
Quotas cap the catch, mesh sizes spare immature fish, closed seasons protect spawning, and no-take zones let stocks recover and spill over.
Aquaculture reduces pressure on wild stocks but brings fishmeal demand, disease, pollution and escapes with it.
A method lowers carbon dioxide only by shrinking a transfer into the air or moving carbon into a store that keeps it.
Reforestation stores carbon while the forest grows and lasts; mixed native planting gives more biodiversity than a monoculture.
Biofuels return recently fixed carbon, but farming, processing and land clearance mean none is fully carbon neutral.