A-level and IB · Sciences

A-level Biology: core concepts

A-level Biology flashcards on molecules, cells, genetics, exchange, respiration, photosynthesis, ecology and nerves, with worked calculations and examples.

42 cards6 topicsAbout 15 minutes a roundUK A-level Biology, comparable to Grade 12 biology plus parts of first-year university cell biology and physiology

Independent study material written by StudyBrix, not produced or reviewed by any exam board or by the International Baccalaureate Organization. Check your own specification or subject guide.

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About this deck

This deck gathers the ideas that appear across most A-level Biology courses in England: biological molecules and enzymes, cell structure and transport, the cell cycle and immunity, DNA, protein synthesis and inheritance, gas exchange and mass transport, photosynthesis and respiration, ecology and evolution, and nervous and hormonal control. It is meant for students revising for the written papers and for Canadian students heading into first-year biology who want a compact review.

Exam boards (AQA, Edexcel, OCR) share a national core but differ in topic order, depth, the required practicals they name and some formulae, such as the version of the diversity index. Check your own specification for what you must know and how your board phrases its practical skills.

You will find it easier if you remember GCSE combined or separate science biology and basic chemistry such as bonds and pH. When you study, explain each process out loud in sequence, then check the card; for calculation cards, cover the example and work it first. Link each structure to its function, because long answers are marked on that link.

Common mistakes: saying enzymes are killed rather than denatured; describing osmosis as movement of solute instead of water; mixing up genes and alleles; and stating that ATP is energy rather than a molecule whose hydrolysis releases energy. In calculations, watch units such as micrometres versus millimetres.

Before you start: GCSE biology and chemistry (or Grade 11 biology and chemistry), including cells, DNA basics and simple chemical reactions.

All 42 cards

Cover the definitions with a sheet of paper and test yourself, or use the study modes above.

Biological molecules

Condensation reaction: Two molecules join by a new covalent bond and a molecule of water is released.Example: Two alpha glucose molecules join by a 1,4 glycosidic bond to form maltose plus water.
Hydrolysis: Breaking a covalent bond between two units by adding a molecule of water, often catalysed by an enzyme.Example: Amylase breaks starch down into maltose; peptidases break peptide bonds in proteins.
Starch, glycogen and cellulose: Glucose polymers: amylose is coiled, amylopectin and glycogen are branched for fast release, and cellulose has straight beta glucose chains held in microfibrils.Example: Glycogen in liver and muscle is more branched than amylopectin, so many ends can be broken down at once.
Triglyceride: A lipid made of one glycerol joined to three fatty acids by ester bonds; a compact, insoluble energy store.Example: Three condensation reactions form one of these lipids and release three water molecules.
Tertiary structure of a protein: The overall 3D folding of one polypeptide, held by hydrogen bonds, ionic bonds and disulfide bridges between R groups.Example: High temperature breaks hydrogen and ionic bonds, changing the active site shape of an enzyme.
Induced fit model: The active site changes shape slightly as the substrate binds, forming a closer enzyme-substrate complex that lowers activation energy.Example: Hexokinase closes around glucose as it binds, putting strain on the bonds to be changed.
Competitive inhibition: An inhibitor with a shape similar to the substrate binds the active site; raising substrate concentration reduces its effect.Example: Malonate competes with succinate for succinate dehydrogenase.
Complementary base pairing: In DNA, adenine pairs with thymine (two hydrogen bonds) and cytosine with guanine (three); the two strands are antiparallel.Example: If a DNA sample is 30% adenine, it is 30% thymine and 20% each of cytosine and guanine.
Semi-conservative replication: Each new DNA molecule keeps one original strand and gains one newly built strand, made by DNA polymerase.Example: After one round in light nitrogen, DNA that began fully heavy appears at an intermediate density.

Cells, transport and immunity

Magnification: How many times bigger an image is than the real object: image size divided by actual size, in the same units.Example: Image 25 mm long, cell 50 micrometres: 25 mm = 25000 micrometres, so the image is 25000/50 = 500 times larger.
Fluid mosaic model: A membrane is a phospholipid bilayer with proteins, cholesterol and glycoproteins scattered through it, all able to move sideways.Example: Channel proteins let ions cross; cholesterol reduces fluidity at higher temperatures.
Water potential: The tendency of water to move out of a solution, in kPa. Pure water is 0; adding solute makes it negative. Water moves from higher to lower values.Example: A cell at -400 kPa in a solution at -700 kPa loses water by osmosis.
Active transport: Movement of a substance against its concentration gradient using carrier proteins and energy from ATP hydrolysis.Example: Root hair cells take up nitrate ions from soil where the concentration is lower than inside the cell.
Mitosis: Nuclear division producing two genetically identical nuclei: prophase, metaphase, anaphase and telophase.Example: In anaphase, spindle fibres pull sister chromatids to opposite poles.
Meiosis and genetic variation: Two divisions make four haploid cells; crossing over and independent assortment give each one a new combination of alleles.Example: With 23 chromosome pairs, independent assortment alone gives 2^23 = 8,388,608 combinations.
Clonal selection: A B or T lymphocyte with a receptor matching an antigen is activated and divides to form many identical cells.Example: Plasma cells from a selected B cell secrete antibodies; memory cells allow a faster secondary response.

Genetics and evolution

Genetic code: Triplets of bases each code for one amino acid; the code is degenerate, non-overlapping and nearly universal.Example: There are 64 triplets but only 20 amino acids, so several triplets can code for the same amino acid.
Transcription: RNA polymerase builds mRNA complementary to the template strand of a gene; in eukaryotes introns are then spliced out.Example: Template strand TAC gives the mRNA codon AUG.
Translation: At a ribosome, tRNA anticodons pair with mRNA codons and the amino acids they carry are joined by peptide bonds.Example: Codon AUG pairs with anticodon UAC, which carries methionine.
Frameshift mutation: An insertion or deletion of a base that shifts the reading frame, changing every codon after it.Example: Deleting one base early in a gene usually changes many amino acids and often creates an early stop codon.
Sex linkage: A gene on the X chromosome; males have one X, so a single recessive allele shows in the phenotype.Example: Carrier mother and unaffected father: half the sons are expected to be affected and half the daughters carriers.
Chi-squared test: Compares observed and expected counts: sum of (O - E)^2/E. A value below the critical value means the difference could be due to chance.Example: Expected 90:30, observed 84:36: 36/90 + 36/30 = 1.6, below 3.84 (1 degree of freedom, p = 0.05), so the 3:1 ratio fits.
Hardy-Weinberg equations: p + q = 1 and p^2 + 2pq + q^2 = 1, where p and q are allele frequencies in a large, randomly mating population with no selection.Example: If 16% show the recessive trait, q^2 = 0.16, q = 0.4, p = 0.6 and carriers are 2pq = 0.48.
Natural selection: Individuals with alleles that improve survival and reproduction pass them on more often, so those allele frequencies rise over generations.Example: Bacteria with a resistance allele survive an antibiotic and the resistant population grows.
Allopatric speciation: A population is split by a physical barrier; separate mutations and selection make the groups too different to interbreed.Example: Squirrels on two sides of a canyon diverge until offspring are no longer fertile.

Exchange and mass transport

Surface area to volume ratio: As an organism gets bigger its surface grows more slowly than its volume, so large organisms need specialized exchange surfaces.Example: A 1 cm cube: 6 cm^2 to 1 cm^3 (6:1). A 3 cm cube: 54 to 27 (2:1).
Oxygen dissociation curve: An S-shaped graph of haemoglobin saturation against partial pressure of oxygen; cooperative binding makes it steep in the middle.Example: Near the lungs haemoglobin is about 95% saturated; in active muscle it unloads much more.
Bohr effect: Higher carbon dioxide concentration lowers the affinity of haemoglobin for oxygen, shifting the curve right so more oxygen is released.Example: Working muscles produce CO2, so blood gives up more oxygen exactly where it is needed.
Cardiac output: The volume of blood the heart pumps per minute: stroke volume times heart rate.Example: Stroke volume 70 mL and heart rate 72 beats/min give 5040 mL/min, about 5 L/min.
Cohesion-tension theory: Water evaporating from leaves pulls a continuous column of water up the xylem, held together by hydrogen bonds between molecules.Example: On a hot, windy day the xylem column is under more tension and stems can shrink slightly in diameter.
Mass flow in phloem: Sucrose loaded at a source lowers water potential, water enters, and pressure pushes sap toward sinks where sucrose is removed.Example: Sap flows from mature leaves (source) to growing roots or fruit (sink).

Energy and ecosystems

Light-dependent reaction: In the thylakoids, light excites chlorophyll electrons; photolysis of water, ATP synthesis and reduced NADP follow.Example: Oxygen released by plants comes from the splitting of water in this stage.
Calvin cycle: In the stroma, rubisco fixes CO2 to RuBP; ATP and reduced NADP turn GP into triose phosphate, and most is used to rebuild RuBP.Example: If light stops, GP builds up and RuBP falls because GP can no longer be reduced.
Oxidative phosphorylation: Electrons pass along carriers in the inner mitochondrial membrane, protons are pumped, and their flow back through ATP synthase makes ATP. Oxygen is the final acceptor.Example: Without oxygen the chain stops, so cells rely on glycolysis and anaerobic pathways.
Net primary production: The chemical energy left in plant biomass after respiration: NPP = GPP - R.Example: GPP 20000 kJ/m^2/year and R 12000 give NPP = 8000 kJ/m^2/year.
Nitrogen fixation: Conversion of nitrogen gas into ammonia by bacteria such as Rhizobium in root nodules or free-living soil bacteria.Example: Legumes like clover enrich soil with nitrogen compounds, which is why they are used in crop rotation.
Succession: Gradual change in a community over time as each stage alters conditions, from pioneer species to a climax community.Example: Bare rock: lichens, then mosses, grasses, shrubs and finally woodland.

Control and coordination

Resting potential: The potential difference across a neurone membrane at rest, about -70 mV, kept by the sodium-potassium pump and leakier potassium channels.Example: 3 Na+ are pumped out for every 2 K+ pumped in.
Action potential: A rapid reversal of membrane potential: sodium channels open (depolarization), then potassium channels open (repolarization).Example: Once threshold (about -55 mV) is passed, the response is all or nothing.
Cholinergic synapse: Acetylcholine released from the presynaptic neurone binds receptors on the next neurone and is then broken down by acetylcholinesterase.Example: Some insecticides block acetylcholinesterase, so signals keep firing.
Negative feedback: A change from the normal level triggers a response that reverses the change, keeping conditions near a set point.Example: High blood glucose leads to insulin release, glucose uptake by cells and a return toward normal.
ADH and osmoregulation: When blood water potential falls, ADH from the posterior pituitary makes collecting duct walls more permeable to water, so more is reabsorbed.Example: After heavy sweating, urine becomes smaller in volume and more concentrated.