amazon.co.ukA-Level Biology revision on biological molecules and respiration: carbohydrates, lipids and proteins, enzyme kinetics (Km, Vmax, inhibitors), glycolysis, the Krebs cycle, chemiosmosis and anaerobic pathways, with multi-board exam practice.
Monosaccharides (glucose, galactose, fructose) are monomers joined by condensation reactions releasing water to form glycosidic bonds; hydrolysis reverses this. α-glucose monomers join into starch (amylose: unbranched 1,4-bonds, coiled helix; amylopectin: branched 1,4- and 1,6-bonds) and glycogen (more highly branched, animals). The storage-function link is a guaranteed four-marker: insoluble so it does not affect water potential or draw water into cells; compact so lots of glucose fits into a small volume; branched so many terminal ends allow rapid simultaneous enzymatic hydrolysis when glucose demand is high; α-glycosidic bonds are easily hydrolysed by amylase/maltase. Cellulose contrasts: β-glucose in unbranched straight chains with hydrogen bonds between chains forming microfibrils - structural, not storage.
Triglycerides are one glycerol + three fatty acids joined by three ester bonds in condensation reactions. Function links: hydrophobic energy store releasing more energy per gram than carbohydrate (more C-H bonds to oxidise); insoluble storage; metabolic water source (e.g. camel hump, desert eggs); low mass. Phospholipids have one fatty acid replaced by a hydrophilic phosphate group - hydrophilic head, hydrophobic tails - explaining bilayer formation in membranes. Saturated fatty acids have no C=C double bonds (straight chains, pack tightly, solid at room temperature); unsaturated have double bonds (kinked chains, lower melting point, oils).
Tests are free marks - quote the sequence and positive result: Benedict's: heat with Benedict's reagent, brick-red precipitate = reducing sugar (green→yellow→red grades concentration with a colorimeter); non-reducing sugar: boil with dilute HCl to hydrolyse glycosidic bonds, neutralise with sodium hydrogencarbonate, re-test with Benedict's - positive means non-reducing sugar was present; iodine/potassium iodide: blue-black = starch; Biuret: NaOH then copper sulfate, purple = protein (peptide bonds); emulsion test: dissolve in ethanol, add to water, white emulsion = lipid. AQA RP6 and OCR PAG6 test the colorimetric calibration-curve technique.
Amino acids have an amino group, carboxyl group, hydrogen and variable R-group on a central carbon; peptide bonds join amino acids in condensation reactions. Know the four structure levels with the bonds that stabilise each: primary - unique amino-acid sequence (peptide bonds); secondary - hydrogen bonding into α-helix or β-pleated sheet; tertiary - 3D folding via hydrogen, ionic and disulfide bonds between R-groups, determining active-site shape; quaternary - multiple polypeptide chains (haemoglobin: 4 chains + haem prosthetic groups). Tertiary structure is what enzymes and inhibitors attack - link any denaturation question to disruption of hydrogen/ionic bonds changing active-site shape.
Induced fit beats lock-and-key in mark schemes: the active site is close to complementary but changes shape slightly as the substrate binds, putting the enzyme-substrate complex under stress that lowers activation energy. Enzymes are biological catalysts, globular proteins, specific to substrates. Rate factors: temperature - more kinetic energy, more successful collisions, rate rises to an optimum (~37 °C in humans), then hydrogen/ionic bonds vibrate too much and break, denaturing the active site so rate falls sharply; pH - deviation from the optimum changes H⁺/OH⁻ concentrations, disrupting ionic and hydrogen bonds, denaturing the enzyme (often irreversibly at extremes); substrate concentration - rate rises to Vmax when all active sites are saturated; enzyme concentration - proportional rise if substrate is non-limiting.
Km and Vmax interpretation: Km is the substrate concentration at half-Vmax; lower Km = higher affinity. Competitive inhibitors bind the active site reversibly, competing with substrate: raising substrate concentration overcomes inhibition, so Vmax unchanged, Km increased. Non-competitive inhibitors bind an allosteric site, changing tertiary conformation so the active site is no longer complementary: substrate concentration cannot help, so Vmax decreased, Km effectively unchanged (for the fraction of uninhibited enzyme). Statins (competitive, HMG-CoA reductase) and cyanide (non-competitive, cytochrome c oxidase) are the stock examples. Metabolic pathways are regulated by feedback inhibition: the end product inhibits the first enzyme of the pathway when it accumulates (e.g. isoleucine inhibiting threonine deaminase), and by allosteric effectors that switch enzymes between active/inactive conformations - link these to homeostatic control of respiration rate.
ATP is a nucleotide derivative: adenine + ribose + three phosphates. Energy is released in small, manageable packets when the terminal phosphate bond is hydrolysed by ATPase to ADP + Pi (30.6 kJ mol⁻¹), and ATP is resynthesised from ADP + Pi using energy from respiration. Properties that suit its role: small (moves easily), water-soluble, releases energy in small amounts, quickly resynthesised, and couples energy-releasing to energy-requiring reactions via transfer of the terminal phosphate. Phosphorylation of substrates (e.g. glucose → glucose phosphate in glycolysis) also makes them more reactive.
Glycolysis occurs in the cytoplasm and is anaerobic-compatible: (1) glucose is phosphorylated using 2 ATP to fructose bisphosphate (6C); (2) it splits into two triose phosphate (3C) molecules; (3) each TP is oxidised (losing H to NAD) and converted to pyruvate, producing 4 ATP by substrate-level phosphorylation. Net yield per glucose: 2 ATP, 2 NADH, 2 pyruvate. Substrate-level phosphorylation = direct transfer of a phosphate from an intermediate to ADP (contrast with oxidative phosphorylation via the proton gradient).
The link reaction occurs in the mitochondrial matrix: pyruvate is actively transported into the matrix, oxidatively decarboxylated - each pyruvate loses CO₂, loses H to NAD (forming NADH) and combines with coenzyme A to form acetyl CoA (2C). Per glucose: 2 CO₂ + 2 NADH. The 2C acetyl group then feeds the Krebs cycle, also in the matrix: acetyl CoA (2C) + oxaloacetate (4C) → citrate (6C); citrate is decarboxylated and dehydrogenated in a series of redox reactions regenerating oxaloacetate. Per turn: 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP (by substrate-level phosphorylation); per glucose (two turns): 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP. Coenzymes NAD and FAD carry H atoms (electrons) to the electron transport chain; remember that dehydrogenation is oxidation. When asked to state where each stage occurs and its products, a quick table of stage/location/yields is the fastest full-mark format.
Oxidative phosphorylation happens on the folded inner mitochondrial membrane: NADH and FADH₂ deliver electrons (from their H atoms) to the electron transport chain of protein complexes I-IV. As electrons pass along the carriers of decreasing energy, energy released pumps H⁺ from the matrix into the intermembrane space, building a steep electrochemical gradient. Protons then flow back into the matrix through ATP synthase channels, and this flow (chemiosmosis) drives phosphorylation of ADP to ATP. Oxygen is the terminal electron acceptor: it combines with electrons and protons to form water (½O₂ + 2H⁺ + 2e⁻ → H₂O). Without oxygen, electrons stop flowing, the proton gradient dissipates, NADH cannot be reoxidised, and oxidative phosphorylation ceases - explaining both cyanide poisoning (inhibits complex IV) and the far lower anaerobic yield.
ATP yield per glucose: glycolysis 2 ATP + 2 NADH; link 2 NADH; Krebs 2 ATP + 6 NADH + 2 FADH₂. Each NADH yields ~2.5 ATP and each FADH₂ ~1.5 ATP via chemiosmosis, giving a theoretical ~32 ATP. Actual yields (30-32) are lower because: some protons leak back across the inner membrane without making ATP; energy is spent shuttling NADH from the cytoplasm into the mitochondrion (glycerol-3-phosphate vs malate-aspartate shuttles yield different returns); and ATP is used to actively transport pyruvate and ADP across the mitochondrial membranes. Exam questions may ask you to calculate yield from NADH/FADH₂ numbers or to explain why the theoretical maximum is not reached - learn both directions.
Respirometers measure oxygen consumption (a manometer fluid moves as O₂ is consumed and CO₂ is absorbed by soda lime): link rate of oxygen uptake to rate of respiration, and control temperature with a water bath. RQ calculations: for tripalmitin C₅₁H₉₈O₆ + 72.5 O₂ → 51 CO₂ + 49 H₂O, RQ = 51 ÷ 72.5 = 0.70. RQ near 1.0 = carbohydrate substrate; below 1 = lipid/protein; above 1 = some anaerobic respiration occurring. These interpretation questions appear regularly on OCR (5.2.1) and WJEC C1 papers.
When oxygen is limiting, glycolysis continues but the link reaction, Krebs cycle and oxidative phosphorylation stop because NADH cannot unload its electrons. Anaerobic pathways regenerate NAD⁺ so glycolysis can keep producing its net 2 ATP. In animals: pyruvate + NADH → lactate + NAD⁺, catalysed by lactate dehydrogenase in muscle during intense exercise. Lactate accumulates, lowering pH; it is transported in the blood to the liver and oxidised back to pyruvate (or converted to glucose via gluconeogenesis) - the oxygen debt. Blood lactate assays and muscle fatigue questions expect this. In yeast (fermentation): pyruvate → acetaldehyde + CO₂ (pyruvate decarboxylase), then acetaldehyde + NADH → ethanol + NAD⁺ (ethanol dehydrogenase) - the basis of brewing and of AQA RP-style investigations into yeast respiration rate under different variables.
Why anaerobic yields far less ATP: glucose is only partially oxidised - the energy-rich hydrogen in pyruvate/lactate is never transferred to the ETC, so only the 2 ATP from substrate-level phosphorylation in glycolysis are made, versus ~32 aerobically; the large chemiosmotic yield from oxidising 10 NADH + 2 FADH₂ is lost. In animals the lactate pathway also costs ATP later (liver conversion), and lactate acidosis impairs muscle contraction - an exam staple linking respiration to muscle physiology (Edexcel T7, WJEC).
Regulation of respiration completes the topic: phosphofructokinase (an early glycolytic enzyme) is allosterically inhibited by high ATP and citrate, and activated by high AMP/ADP, so glycolysis slows when energy charge is high - a concrete feedback-inhibition example worth quoting. Mitochondrial diseases that impair the ETC reduce ATP yield and raise dependence on anaerobic pathways; uncouplers (e.g. DNP) dissipate the proton gradient, so respiration continues without ATP production, releasing energy as heat - questions use these to test whether you really understand chemiosmosis rather than recite stages.