分子、饮食、运输与健康

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:

• Condensation 缩合 removes a water molecule as a glycosidic bond 糖苷键 forms between two sugars.
• Hydrolysis 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 and 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.

The cardiac cycle

One beat has three phases. Atrial systole 心房收缩期 squeezes the last blood into the ventricles. Ventricular systole 心室收缩期 raises ventricular pressure above arterial pressure, the semilunar valves open, and blood is driven out. Diastole 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:

• Diffusion 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.
• Active transport 主动运输 — against the gradient, powered by ATP, through carrier proteins that change shape.
• Osmosis 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 secondary 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 DNA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 triplet 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 mRNA 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.

Mutations 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.