Pearson Edexcel · International A-Level · Biology

A red blood cell leaving your lung carries oxygen on haemoglobin, rides an artery, squeezes through a capillary narrower than itself, releases the oxygen to a respiring cell, and returns through a vein. Every structure in that journey is built from the molecules this unit starts with, and every exam question about it asks you to connect structure to function.

WBI11 Molecules, Diet, Transport and Health is the first IAS unit: a written paper of 1 hour 30 minutes, 80 marks, all compulsory. It assesses Topic 1 (Molecules, Transport and Health) and Topic 2 (Membranes, Proteins, DNA and Gene Expression). Expect data questions on health studies, calculations with standard form, and extended answers that name structures and explain mechanisms.

Water is the transport medium because it is a polar solvent 极性溶剂. In a water molecule, oxygen pulls the shared electrons more strongly than hydrogen: each O–H bond has a slight negative end at O and a slight positive end at H. This is the dipole 偶极 nature of water. Opposite charges between neighbouring molecules form hydrogen bonds 氢键, and polar or charged particles (glucose, sodium ions, amino acids) dissolve because water clusters around them.

Carbohydrates are joined and split by two reactions:

• Condensación 缩合 removes a water molecule as a glycosidic bond 糖苷键 forms between two sugars.
• Hidrólisis 水解 adds water back and breaks that bond.

Two glucose molecules condense to maltose; glucose plus fructose give sucrose; glucose plus galactose give lactose. Long chains make polysaccharides: amylose 直链淀粉 (1,4-linked, coils into a helix, compact), amylopectin 支链淀粉 y glycogen 糖原 (both 1,4-chains with 1,6-branches; glycogen is the animal store in liver and muscle). Branching means many chain ends, so glucose can be released quickly; the helix means a lot of energy fits in a small space.

Core Practical 1: estimating sugar and starch concentration

Reducing sugars reduce blue Benedict's reagent on heating, giving a brick-red precipitate. To estimate an unknown concentration, compare the colour against standards of known concentration, or read the solution in a colourimeter.

Worked check. Standards of 0, 0.5, 1, 2 and 4 % glucose give absorbances 0.02, 0.09, 0.18, 0.35, 0.64. An unknown gives 0.58. On the straight section, each 1 % adds about 0.16 absorbance, so the unknown is near $0.58/0.16 \approx 3.6\%$. Reading beyond the standards (extrapolating past 4 %) is less reliable than measuring within them — a point the examiners like you to make. Iodine turns blue-black with starch; intensity of the blue-black gives the same kind of estimate for starch.

A triglyceride 甘油三酯 is one glycerol plus three fatty acids, joined by three condensation reactions that form ester bonds 酯键. Saturated 饱和 fatty acids have no C=C bonds: straight chains pack closely, so animal fats are solid at room temperature. Unsaturated 不饱和 acids have one or more double bonds that kink the chain, packing is looser, and oils are liquid. Lipids store more energy per gram than carbohydrates because they are more reduced.

Diffusion supplies cells only a few tenths of a millimetre thick. A big organism needs mass transport 大规模运输: a pump and pipes that carry every cell's supply and waste. The three vessel types match three jobs:

The mammalian heart is two pumps in one organ. The right side sends blood to the lungs; the left side, with the thicker ventricle wall, sends blood to the body. Atria receive; ventricles deliver. The coronary arteries feed the heart muscle itself — blockage there is a heart attack.

El ciclo cardíaco

One beat has three phases. Sístole auricular 心房收缩期 squeezes the last blood into the ventricles. Sístole ventricular 心室收缩期 raises ventricular pressure above arterial pressure, the semilunar valves open, and blood is driven out. Diástole 舒张期 relaxes the muscle, pressure falls, the semilunar valves slam shut (the second heart sound), and the chambers refill from the veins.

Read the figure the way an examiner wants: valve state is decided by pressure difference, never by an instruction. Atrio-ventricular valves open when atrial pressure exceeds ventricular pressure; semilunar valves open when ventricular pressure exceeds aortic pressure.

Haemoglobin is a protein of four polypeptide chains, each folded around a haem group carrying one Fe²⁺ ion that binds one O₂ molecule — four in total. Binding the first oxygen changes the shape of the whole molecule and makes the next bindings easier: cooperative binding 协同结合 gives the oxygen dissociation curve its shallow-then-steep S shape.

In the lungs (high pO₂) the curve flattens near 100 % saturation. In resting tissue near 1–2 kPa the curve is steep, so a small fall in pO₂ unloads a large amount of oxygen — exactly where it is needed. When tissue respires hard it releases more carbon dioxide; hydrogen ions attach to haemoglobin and shift the whole curve to the right (the Bohr effect 波尔效应), forcing even more oxygen to unload in the busiest tissue.

Worked check. At pO₂ 2 kPa the normal curve reads about 30 % saturation and the shifted curve about 15 % — haemoglobin holds only about a third as much oxygen at that partial pressure, so most of its four sites are released to the tissue sooner.

Atherosclerosis 动脉粥样硬化 begins with damage to the endothelium 内皮 lining an artery. The inflammatory response grows, low-density lipoproteins invade, smooth muscle and connective tissue build a plaque 斑块, the artery narrows and hardens, and a clot can finally block it or break loose.

Clotting itself is a cascade. Damage releases thromboplastin 凝血酶原激酶, which (with calcium ions) converts the soluble plasma protein prothrombin 凝血酶原 into the enzyme thrombin 凝血酶; thrombin converts soluble fibrinogen 纤维蛋白原 into the insoluble mesh of fibrin 纤维蛋白 that traps platelets and cells.

Blood pressure 血压 is reported as systolic over diastolic. Hypertension, smoking, high blood LDL-cholesterol, diet, inactivity, age, gender and genetics are risk factors 危险因素 — and they multiply, not add: two moderate risks can matter more than one severe risk.

Reading health-risk data

Questions 8 and 9 of every paper live here. Four habits cover almost every mark:

• Correlation is not causation. 相关不等于因果 Two variables moving together may share a hidden cause, or the link may be chance.
• Judge the study: How large was the sample? How was it selected? What was controlled? Was it blind? Are there confounders 混杂变量 such as age or smoking?
• Do the arithmetic carefully: percentage change, ratios in standard form, per-capita rates.
• Ask who or what is missing: non-responders, unpublished studies, extrapolated lines.

Worked check. A study finds gum disease correlates with endothelial damage. Before claiming one causes the other you would want a mechanism, a dose–response pattern, and confounders (diet, smoking, age) excluded — and the question "were the groups matched for blood pressure?" is exactly a confounder question.

Energy balance decides body mass. Overweight and obesity are screened by BMI (mass in kg divided by height in metres squared; 18.5–24.9 is the healthy band) and waist-to-hip ratio (WHR). Dietary saturated fat raises blood LDL-cholesterol; unsaturated fat and soluble fibre lower it.

Worked check. A woman of mass 74 kg and height 1.66 m: BMI $= 74 / 1.66^2 = 74/2.76 = 26.8$ — just into the overweight band. Waist 84 cm and hips 98 cm give WHR $= 84/98 = 0.86$.

Treatments all carry risks alongside benefits:

• Antihypertensives 抗高血压药 (beta blockers, ACE inhibitors, diuretics) lower blood pressure.
• Statins 他汀类 (for example simvastatin) block a liver enzyme in cholesterol synthesis, so cells take up LDL from blood.
• Anticoagulants 抗凝剂 (warfarin) and antiplatelet drugs (aspirin) reduce clotting, so bleeding risk rises.

A question asking you to "discuss" wants both sides plus a judgement anchored in the data given.

Every exchange surface is a membrane doing controlled business. The fluid mosaic model 流动镶嵌模型 is the accepted picture: a phospholipid bilayer (fluid because the tails can slide), studded with intrinsic proteins that span the bilayer (channels, carriers), extrinsic proteins, cholesterol that controls fluidity, and glycoproteins and glycolipids for recognition.

Four transport routes:

• Difusión 扩散 — net movement down a concentration gradient, no ATP.
• Facilitated diffusion 易化扩散 — down the gradient through a channel or carrier protein.
• Transporte activo 主动运输 — against the gradient, powered by ATP, through carrier proteins that change shape.
• Ósmosis 渗透 — water moving from higher to lower water potential 水势 across a partially permeable membrane. Endocytosis and exocytosis move bulk material in vesicles.

For diffusion, Fick's law: rate is proportional to (surface area × concentration difference) ÷ diffusion distance. Gas exchange surfaces answer all three: huge area, maintained gradient, and a wall one or two cells thick. This is why alveoli, capillaries and fish gills all look the way they do.

An amino acid carries an amino group, a carboxyl group, a hydrogen and an R group on one central carbon; two join by a peptide bond 肽键 in a condensation reaction. The primary structure 一级结构 is the sequence; hydrogen bonds coil it into the secundario 二级结构; further hydrogen, ionic and disulfide bonds fold the tertiary 三级结构; separate chains assemble into the quaternary 四级结构 — haemoglobin is the classic example.

An enzyme 酶 is a protein whose active site 活性部位 fits one substrate shape. Binding is not rigid: the induced-fit 诱导契合 model has both molecule and site flexing until complementary. This is why temperature and pH change rate — they act on the tertiary structure — and why the active site is specific.

Competitive inhibitors 竞争性抑制剂 bind the active site and are outnumbered as substrate concentration rises; non-competitive inhibitors 非竞争性抑制剂 bind elsewhere and distort the site, so rate falls whatever the concentration.

Worked check. Q10 is quoted at 2.4: between 10 °C and 20 °C the rate is 2.4 times faster; between 45 °C and 55 °C the same factor means the denatured enzyme is collapsing at an increasing rate. Q10 only describes; it does not explain.

A mononucleotide 单核苷酸 is a pentose (deoxyribose or ribose), a phosphate and a nitrogenous base. Condensation between phosphate and sugar of neighbours makes a strand held by phosphodiester bonds 磷酸二酯键. DNA is two antiparallel strands: A pairs with T (two hydrogen bonds), G with C (three), twisted into a double helix.

Replication is semi-conservative 半保留: helicase unzips the two strands, each is a template, and DNA polymerase ADN聚合酶 joins free nucleotides in the 5' to 3' direction. Meselson and Stahl grew bacteria first on heavy ¹⁵N then light ¹⁴N nitrogen; after one round of replication all the DNA was one intermediate density, after two rounds half was light — exactly what semi-conservative predicts and the alternatives forbid.

A gene is a sequence of bases coding for a sequence of amino acids. The code is a triplete 三联体 code: three bases per amino acid. It is non-overlapping 非重叠 (read in successive groups), degenerate 简并 (most amino acids have several codons, so some mutations are silent), and carries start and stop codons.

Worked check. A gene of 300 base pairs coding a protein of 99 amino acids plus a stop codon: $99 \times 3 + 3 = 300$. ✓

Transcription: RNA polymerase binds the gene, uses one strand as template and builds ARNm 信使RNA. Translation: in the cytoplasm, a ribosome 核糖体 reads the mRNA codons, and tRNA 转运RNA molecules whose anticodons 反密码子 pair with those codons deliver amino acids in order; peptide bonds join them.

Mutaciones 突变 arise from replication errors. A substitution 替换 swaps one base: one amino acid changes, or nothing changes (degeneracy). An insertion 插入 or deletion 缺失 adds or removes a base and shifts every reading frame after it — a frameshift 移码 — usually destroying the protein.

Cystic fibrosis is the course's model: deletion of three bases removes one phenylalanine from the CFTR protein, so chloride transport fails, mucus stays thick, and airways clog. The allele is recessive: carriers are healthy. Genetic screening 基因筛查 can identify carriers, test embryos (pre-implantation genetic diagnosis) or foetuses (amniocentesis, chorionic villus sampling). Screening questions always carry ethics: who is tested, who is told, what happens to a positive result.

1. Explain why amylopectin releases glucose faster than amylose, referring to bonds and ends of chains.
2. Calculate the BMI of a 91 kg man 1.83 m tall and classify him.
3. During ventricular systole, which valves are open and which are shut, and why?
4. The oxygen saturation at 4 kPa falls from about 60 % to about 34 % when CO₂ rises. Explain the molecular cause.
5. Describe the clotting cascade in order, from endothelial damage to fibrin.
6. A study reports that people who eat more yoghurt have fewer heart attacks. List three checks you would make before believing a causal claim.
7. Explain why a competitive inhibitor lowers the rate at low substrate concentration but not at very high concentration.
8. Meselson and Stahl ruled out conservative replication. Which observation, after one round, did the job?
9. A frameshift mutation is usually worse than a substitution. Explain why, using the words triplet and degenerate.
10. Name two benefits and two risks of genetic screening for cystic fibrosis carriers.

Answers: 1 many 1,6-branch points give many chain ends for enzymes to attack simultaneously; 2 $91/1.83^2 = 27.2$, overweight; 3 atrio-ventricular shut, semilunar open, because ventricular pressure exceeds aortic but not atrial-from-above pressure; 4 hydrogen ions from CO₂ bind haemoglobin, changing its shape and lowering oxygen affinity (Bohr effect); 5 damage exposes collagen, platelets and damaged cells release thromboplastin, thromboplastin + Ca²⁺ converts prothrombin to thrombin, thrombin converts fibrinogen to fibrin, fibrin mesh traps cells; 6 sample size and selection, confounders such as age/exercise/diet, blinded comparison, mechanism or dose–response; 7 the inhibitor competes for the active site, so at very high substrate concentration nearly every collision with the site is substrate; 8 after one round all DNA was a single intermediate band — conservative predicts one heavy and one light band; 9 a frameshift changes every triplet after the mutation, so the whole protein from that point is a different amino-acid sequence, while degeneracy lets many substitutions change nothing; 10 benefits — informed family planning, earlier treatment or lifestyle change; risks — anxiety, discrimination by insurers or employers, false positives or negatives.

You began as one fertilised egg cell. This unit explains how that cell divided, how the copies were shuffled and halved to make gametes, how identical genes became different tissues, and how the variety that produces is counted, classified and — increasingly — defended. It is the unit of microscopes, calculations and definitions done precisely.

WBI12 Cells, Development, Biodiversity and Conservation is the second IAS unit: 1 hour 30 minutes, 80 marks, all compulsory. It assesses Topic 3 (Cell Structure, Reproduction and Development) and Topic 4 (Plant Structure and Function, Biodiversity and Conservation). Expect labelling diagrams, microscope and magnification calculations, the index of diversity and Hardy–Weinberg questions, and long answers on conservation evaluation.

A eukaryotic 真核 cell is a set of specialised compartments:

Cells organise into tissues 组织 (similar cells), organs 器官 (cooperating tissues) and organ systems 器官系统. A prokaryotic 原核 cell has none of the membrane-bound organelles: a cell wall, a slime capsule 荚膜, one circular DNA molecule plus plasmids 质粒, 70S ribosomes, and sometimes a flagellum 鞭毛.

Ampliación 放大倍数 is how much bigger the image is; resolution 分辨率 is the smallest separation still seen as two. Light microscopes resolve about 200 nm; electron beams, far shorter in wavelength, resolve to a few nanometres — but need thin stained sections (TEM) or gold-coated surfaces (SEM). $M = \text{image size} / \text{actual size}$, keeping units consistent.

Worked check. A mitochondrion measures 14 mm on a photograph labelled ×20 000. Actual length $= 14\,000\ \mu\text{m} / 20\,000 = 0.7\ \mu$m. A nucleus that must be seen with its double membrane needs an electron microscope: the two membranes lie closer than a light microscope's resolution.

Meiosis 减数分裂 is two successive divisions after one DNA replication, producing four genetically different haploid 单倍体 cells from one diploid 二倍体 cell. Two processes create the variation:

• Asumiento independiente 独立分配 — each homologous pair lines up and separates at random in meiosis I; each gamete gets one of each pair, from either parent at random ($2^n$ combinations).
• Entrecruzamiento 交叉互换 — in prophase I, homologous chromatids swap sections, mixing alleles within chromosome pairs.

Mammalian gametes are specialised for delivery and provisioning: the sperm 精子 carries an acrosome 顶体 (digestive enzymes), a midpiece of mitochondria and a flagellum; the egg 卵子 contributes nearly all cytoplasm and a zona pellucida 透明带 that locks out extra sperm after the cortical reaction 皮质反应. In flowering plants the pollen tube delivers two male nuclei: one fertilises the egg, the other the polar nuclei — double fertilisation 双受精.

Mitosis 有丝分裂 produces two genetically identical diploid cells: growth, repair, asexual reproduction. Interphase ($G_1$, S — DNA replication, $G_2$) fills most of the cycle; mitosis itself is prophase (chromosomes condense, spindle forms), metaphase (chromosomes line at the equator), anaphase (spindle fibres pull chromatids to poles), telophase (nuclear membranes reform), then cytokinesis 胞质分裂.

El mitotic index 有丝分裂指数 $=$ cells in mitosis $\div$ total cells counted. High values in a tissue biopsy mean rapid division — a cancer signature.

Worked check. A root-tip squash shows 36 of 450 cells in mitosis: index $= 36/450 = 0.08 = 8\%$. If the cycle takes 20 hours, mitosis occupies $0.08 \times 20 = 1.6$ hours.

A totipotent 全能性 stem cell can form a whole organism (the first divisions of the zygote); a pluripotent 多能性 cell (inner blastocyst 胚泡) can form any tissue but not the whole organism. Differentiation happens by differential gene expression 差异基因表达: every cell keeps the whole genome, but each type switches on only its own subset. One gene can yield several proteins by post-transcriptional modification 转录后修饰 — RNA splicing choices of the pre-mRNA. The final phenotype also reflects the environment (height needs both genes and nutrition) and, for many traits, multiple genes acting together (polygenic 多基因 inheritance giving continuous variation).

Plant cells add a cellulose cell wall 细胞壁 joined to neighbours by plasmodesmata 胞间连丝. Cellulose chains bundle into microfibrils 微纤维 whose criss-cross laying gives tensile strength; secondary thickening with lignin strengthens and waterproofs.

Water and ions matter beyond drinking: nitrate 硝酸盐 builds amino acids, calcio 钙离子 cross-links pectin in walls and signals inside cells, magnesio 镁离子 sits at the heart of chlorophyll — a shortage yellows the leaves.

Classification arranges life by shared characteristics; the three-domain system 三域系统 (Bacteria, Archaea, Eukarya) rests on molecular evidence — ribosomal RNA sequences — that revealed Archaea are closer to us than to bacteria. Within a habitat, diversity has two measures:

• Species richness 物种丰富度 — how many species.
• Index of diversity 多样性指数 — $D = \dfrac{N(N-1)}{\sum n(n-1)}$, where $N$ is all individuals and $n$ each species' count. Higher $D$, more diverse.

Worked check. Habitat A: 3 species with counts 40, 35, 25 ($N = 100$). $D = 100 \times 99 / (40 \times 39 + 35 \times 34 + 25 \times 24) = 9900 / (1560 + 1190 + 600) = 9900/3350 = 2.96$. Habitat B: 3 species, counts 90, 6, 4: $D = 9900/(8010 + 30 + 12) = 1.23$. Same richness, very different balance — $D$ sees what richness cannot.

Within one species, the heterozygosity index 杂合度指数 $H =$ (number of heterozygotes) $\div$ (number of individuals) measures genetic variety. A species found nowhere else is endemic 特有种 — and its island home makes it both precious and fragile.

A niche 生态位 is a species' way of life: its role, its requirements, its tolerances. Adaptations come in three registers — anatomical (thick fur), physiological (enzyme variants), behavioural (migration timing).

El Hardy–Weinberg equation 哈迪–温伯格方程 tracks allele frequencies in a large, randomly breeding population with no selection, migration or mutation:

$q^2$ = frequency of the homozygous recessive genotype. If 4 % of a population shows the recessive phenotype, $q^2 = 0.04$, so $q = 0.2$, $p = 0.8$, carriers $2pq = 0.32$.

Worked check. Of 500 foxes, 45 are homozygous recessive: $q^2 = 0.09$, $q = 0.3$, $p = 0.7$. Expected carriers $= 2pq \times 500 = 0.42 \times 500 = 210$.

Conservation questions want balance. Zoos and seed banks 种子库 keep species against extinction and run captive breeding, but cost space, lose natural behaviour and serve few individuals; habitat protection and corridors 走廊 preserve ecology at scale but need land, money and political will. Credit the specific argument, not the sentiment.

For the microbiology here (and Unit 4's): grow bacteria on nutrient agar or in broth, using aseptic technique 无菌技术 — sterilise loops by flaming, flame bottle necks, work near an updraught, seal plates, incubate below body temperature so human pathogens cannot grow. Count colonies on spread or pour plates (each colony from one cell or a clump: colony-forming units), or count cells with a haemocytometer, or follow turbidity 浊度 in broth.

Worked check. In the exponential phase, the growth-rate constant $k = (\log_{10} N_t - \log_{10} N_0)/(0.301 \times t)$. From $1 \times 10^4$ hasta $4 \times 10^7$ cells in 4 hours: $\log$ difference $= 3.602$, so $k = 3.602/(0.301 \times 4) \approx 3.0$ divisions per hour — about a 20-minute generation time.

1. Calculate the actual width of a nucleus that appears 24 mm wide at ×4 000. Give your answer in µm.
2. Name the two processes in meiosis that create genetic variation, and state when each happens.
3. A cell has 2 a.u. of DNA in G1. Sketch how DNA content changes through S phase, meiosis I and meiosis II.
4. Explain why all cells of a blastocyst can become any tissue, but a mature muscle cell cannot.
5. Explain two ways xylem is adapted to its function, one structural and one chemical.
6. Two habitats each contain 5 plant species. Explain why the index of diversity can still differ between them.
7. In a population of 1 000, 16 people show a recessive condition. Calculate q, p and the expected number of carriers.
8. A species of snail is endemic to one island. Explain why endemic species face higher extinction risk.
9. State three conditions a population must meet for the Hardy–Weinberg equation to hold.
10. Give one benefit and one limitation of seed banks compared with protecting habitat.

Answers: 1 $24\,000/4\,000 = 6\ \mu$m; 2 crossing over in prophase I, independent assortment in metaphase/anaphase I (and II); 3 rises to 4 in S, halves to 2 after meiosis I, halves to 1 after meiosis II, restored to 2 at fertilisation; 4 blastocyst cells are pluripotent — all genes still switchable; a muscle cell has switched off other pathways permanently in ordinary conditions; 5 hollow, lignified dead cells form continuous water columns with no cytoplasm to resist flow; lignin waterproofs and strengthens walls against collapse; 6 D weighs the evenness of abundance, not just the species count; 7 $q^2 = 0.016$, $q \approx 0.126$, $p \approx 0.874$, carriers $2pq \approx 0.22$, about 220 people; 8 a single local event (storm, disease, introduced predator, habitat loss) removes the whole species at once; 9 large population, random mating, no selection/mutation/migration; 10 seed banks store genetic diversity cheaply and safely against habitat loss (benefit) but species exist out of their ecology — no interactions, no evolution in place, and recolonisation is uncertain (limitation).

Sprint for a bus and your body performs a symphony: muscles burn glucose faster, the heart doubles its output, ventilation deepens, sweat starts, and the kidney re-tunes its water recovery — all controlled without a conscious thought. This unit follows that machinery: respiration at molecular scale, muscle and heart at tissue scale, nerves and hormones as the controllers, and gene technology as the modern window onto it.

WBI15 Respiration, Internal Environment, Coordination and Gene Technology is the second IA2 unit: 1 hour 45 minutes, 90 marks. It assesses Topic 7 (Respiration, Muscles and the Internal Environment) and Topic 8 (Coordination, Response and Gene Technology), and one 20-mark question is built on a pre-released scientific article — you will have studied it in advance.

Respiration releases energy in steps, each catalysed by a specific enzyme. Aerobic respiration's summary:

Anaerobic conditions stop the chain — reduced NAD cannot be recycled — so pyruvate accepts its hydrogen and becomes lactate 乳酸 in muscle. After exercise, lactate is carried to the liver and rebuilt into glucose (needing oxygen: the oxygen debt 氧债).

Respiratory quotient 呼吸商: $\mathrm{RQ} = \mathrm{CO_2}\ \text{produced} / \mathrm{O_2}\ \text{consumed}$. Carbohydrate 1.0, protein 0.9, lipid ≈ 0.7 — an organism's RQ reveals what it is burning.

Worked check. A respirometer shows 60 cm³ CO₂ produced and 60 cm³ O₂ consumed in the same interval: RQ = 1.0 — carbohydrate. A germinating seed with RQ 0.7 is burning lipid reserves.

A tendon 肌腱 joins muscle to bone; a ligament 韧带 joins bone to bone across a joint; muscles work in antagonistic pairs. A skeletal muscle fibre is a fused, multinucleate cell packed with myofibrils 肌原纤维 of actin 肌动蛋白 y myosin 肌球蛋白.

Contraction is the sliding filament cycle: an action potential releases Ca²⁺; calcium exposes actin's binding sites; the energised myosin head (its ATP split) attaches, pulls the actin, then detaches when new ATP binds — thousands of heads per second, each stroke a few nanometres.

Fast-twitch fibres: thick, few mitochondria, quick and powerful, fatigue fast (sprinting). Slow-twitch: many mitochondria, rich blood supply, myoglobin — endurance.

Cardiac muscle is myogenic 自主性的: it beats without nerve input. The sino-atrial node 窦房结 sets the pace; the wave spreads over the atria to the atrio-ventricular node, then along Purkyne fibres to the ventricles — atria contract first, ventricles a fraction later.

The cardiovascular control centre and the ventilation centre in the medulla oblongata 延髓 adjust both systems to demand: chemical receptors sense CO₂ and pH, baroreceptors pressure.

Worked check. At rest: 70 cm³ × 72 min⁻¹ ≈ 5.0 dm³ min⁻¹. During exercise: 120 cm³ × 150 min⁻¹ = 18 dm³ min⁻¹ — the table the exam loves. Ventilation rises in step, read from spirometer traces (tidal volume × breathing rate = minute ventilation 每分钟通气量).

Homeostasis 稳态 holds the internal environment in dynamic equilibrium — not stillness, but variables oscillating inside narrow limits. Retroalimentación negativa 负反馈 corrects deviations in either direction (the thermostat pattern); positive feedback 正反馈 amplifies them (a spiral, not a loop — as in an action potential or fever runaway).

Thermoregulation runs through the hypothalamus 下丘脑: heat loss by vasodilation and sweating; heat conservation by vasoconstriction, shivering and hair erection. Adrenaline 肾上腺素 prepares the fight-or-flight response — heart rate, blood glucose and airways all respond together.

Blood enters the nephron 肾单位's glomerulus at high pressure — the afferent arteriole is wider than the efferent — and small molecules are forced through podocytes and the basement membrane into Bowman's capsule 肾小囊 (ultrafiltration: everything except cells and large proteins). The proximal convoluted tubule 近曲小管 reabsorbs all glucose and amino acids plus most salts and water (active transport then osmosis). The loop of Henle 亨勒环 builds a water-potential gradient down the medulla by countercurrent multiplication. The collecting duct 集合管 makes the final decision: ADH 抗利尿激素 from the pituitary inserts aquaporins so more water is reabsorbed — concentrated urine when dehydrated, dilute when not.

Excess amino acids are deaminated 脱氨 in the liver; the toxic ammonia becomes urea 尿素 for excretion.

Worked check. ADH present → collecting duct walls permeable → water leaves down the gradient into the medullary blood → small volume, concentrated urine. Alcohol suppresses ADH — large volume, dilute.

A transcription factor 转录因子 binds a gene's promoter and switches transcription on or off. Steroid hormones diffuse through the membrane and act as transcription factors themselves — one gene can serve several tissues. Epigenetics 表观遗传学 silences genes without changing the base sequence: methylation 甲基化 of DNA and modification of histones (acetylation) pack the DNA out of the transcription machinery's reach. Failures matter: a methylated tumour-suppressor gene cannot hold cell division in check; imprinting errors explain Prader–Willi syndrome.

The resting neurone pumps Na⁺ out and K⁺ in (the sodium-potassium pump 钠钾泵), holding an interior at about −70 mV. A stimulus past umbral 阈值 (about −55 mV) opens sodium channels: an all-or-nothing 全或无 action potential spikes to +30 mV, then potassium channels repolarise; the período refractario 不应期 prevents backward travel and sets a maximum frequency.

In myelinated 有髓 neurones the impulse jumps node to node (saltatory conduction 跳跃传导) — far faster. At the synapse 突触, calcium entry triggers vesicles of neurotransmitter to fuse with the presynaptic membrane; the transmitter diffuses and binds postsynaptic receptors; enzymes then clear the gap. Drugs act here: SSRIs block serotonin re-uptake, so the signal persists; lidocaine blocks sodium channels; ecstasy (MDMA) floods the cleft with serotonin then depletes it.

The nervous system divides: CNS (brain, spinal cord) and PNS — somatic (voluntary) and autonomic (involuntary; sympathetic accelerates, parasympathetic calms). Receptors are transducers: rods (dim light, many per bipolar cell — sensitivity), cones (colour, one-to-one — acuity), plus thermoreceptors, chemoreceptors, baroreceptors, nociceptors 伤害感受器 (pain). The spinal reflex arc 反射弧 passes sensory → relay → motor neurone in the cord's grey matter — fast, involuntary, protective. Habituation 习惯化 is a decreasing response to a repeated harmless stimulus.

Phytochrome 光敏色素 flips between Pr (absorbs red) and Pfr (absorbs far-red); sunlight's red bias leaves Pfr dominant, controlling germination and flowering by day length. Auxin 生长素 elongates cells by acidifying walls; gibberellins 赤霉素 induce enzymes (amylase in the aleurone layer) that mobilise starch in germination.

Landmarks and jobs: cerebral hemispheres 大脑半球 (voluntary, thought), cerebellum 小脑 (coordination, balance), hypothalamus y pituitary 垂体 below them (homeostasis, hormones), medulla oblongata (heart, breathing). MRI shows structure; fMRI shows activity via oxygenated-blood flow; CT fast structural scans; PET metabolism with radioactive tracers.

Chemistry first: Parkinson's is a dopamine 多巴胺 deficit — treated with L-Dopa, which crosses the blood-brain barrier and is converted to dopamine in the brain. Low serotonin links to depression — the target of SSRIs.

Recombinant DNA 重组DNA: a restriction enzyme 限制性内切酶 cuts DNA at its recognition sequence, leaving sticky ends 黏性末端; the human gene (made without introns from mRNA) is spliced into a plasmid 质粒 with DNA ligase 连接酶; the plasmid transforms host bacteria; marker genes (antibiotic resistance or fluorescence) select the cells that took it up. Scale up in a fermenter and the bacteria secrete human insulin 胰岛素 — cleaner and safer than extracting from pigs. GM animals make human proteins in milk; risks and benefits of GM crops (resistance, yields versus gene flow to wild relatives) are the standard evaluation.

Microarrays 微阵列 identify active genes: sample mRNA hybridises to complementary probes on the chip; spots that fluoresce mark genes being transcribed. Bioinformatics 生物信息学 compares sequences across species and individuals — how relatedness is now measured.

1. Name the stage of respiration occurring in the mitochondrial matrix and one of its inputs.
2. A runner's muscles produce lactate. Explain why, and what happens to the lactate afterwards.
3. Myosin heads cannot detach without ATP. Explain what this means for muscles after death.
4. Calculate cardiac output for stroke volume 85 cm³ and heart rate 140 min⁻¹.
5. Explain why the afferent arteriole is wider than the efferent arteriole.
6. State two differences between negative and positive feedback, with one example of each.
7. Why does myelination increase conduction speed?
8. An axon's threshold is −55 mV. A stimulus moves it to −60 mV. What happens, and why?
9. Red light converts phytochrome to which form?
10. Explain why L-Dopa is given instead of dopamine itself.
11. A microarray spot fluoresces strongly. What does that show?
12. Why must the human insulin gene be made from mRNA rather than from genomic DNA when inserting into bacteria?

Answers: 1 the link reaction (or Krebs cycle) — input pyruvate (or acetyl); 2 oxygen is short so reduced NAD is recycled by reducing pyruvate to lactate; afterwards lactate goes to the liver and is converted back to glucose, using oxygen; 3 without ATP the myosin stays bound to actin — rigor mortis; 4 $85 \times 140 = 11\,900\ \text{cm}^3\,\text{min}^{-1} = 11.9\ \text{dm}^3\,\text{min}^{-1}$; 5 the pressure difference drives ultrafiltration of plasma into Bowman's capsule; 6 negative returns a variable toward its set point (thermoregulation); positive pushes it further away (fever rising); 7 the impulse jumps between nodes of Ranvier instead of depolarising the whole membrane — fewer, bigger steps; 8 nothing — threshold is not reached, so no action potential (all-or-nothing); 9 Pfr; 10 dopamine cannot cross the blood-brain barrier but L-Dopa can, and is converted to dopamine inside the brain; 11 that gene was being actively transcribed — its mRNA was present and hybridised; 12 bacteria cannot splice introns, so the gene must come from mature mRNA that already lacks them.