能量、环境、微生物学和免疫学

The overall reaction: $6\mathrm{CO_2} + 6\mathrm{H_2O} \rightarrow \mathrm{C_6H_{12}O_6} + 6\mathrm{O_2}$, driven by light energy. In the thylakoid 类囊体 membranes, the light-dependent reactions 光反应 photolyse water (releasing O₂ and H⁺), photophosphorylate 光合磷酸化 ADP and reduce NADP. In the stroma 基质, the light-independent reactions 暗反应 (Calvin cycle) fix carbon dioxide onto ribulose bisphosphate with rubisco, reduce the products using the ATP and reduced NADP from stage one, and make GALP — some to glucose, most to regenerate RuBP.

Structure matches function: thylakoids stacked for absorbing surface; stroma packed with enzymes for the cycle.

Pigments and spectra

Chlorophyll a and b absorb red and blue light; carotenoids widen the range and protect. The absorption spectrum 吸收光谱 shows what each pigment takes up; the action spectrum 作用光谱 shows what rate of photosynthesis each wavelength drives — their agreement is evidence that these pigments do the work. Chromatography separates pigments by solubility; $\mathrm{Rf} = \text{distance moved by pigment} / \text{distance moved by solvent}$.

Limiting factors 限制因素: light intensity, CO₂ concentration, temperature. A greenhouse raises the weakest of the three — and exam answers say which and why.

Gross primary productivity 总初级生产力 (GPP) is the energy fixed by photosynthesis per area per time (kJ m⁻² yr⁻¹). Plants spend much of it on their own respiration; what remains is net primary productivity 净初级生产力:

Only about 10 % of the energy at each trophic level passes upward — lost to respiration as heat, egestion, excretion and parts never eaten. That is why chains rarely exceed four or five levels and why top predators are rare.

Worked check. A grassland fixes 45 000 kJ m⁻² yr⁻¹ and respirers 19 000. NPP $= 26\,000$ kJ m⁻² yr⁻¹. Cattle eating that grass keep about 10 %: 2 600 kJ m⁻² yr⁻¹. Humans eating the cattle keep 260 — the energy arithmetic behind "eating lower on the chain feeds more people".

Four definitions, precisely: habitat 栖息地 (where), population 种群 (one species' individuals), community 群落 (all populations), ecosystem 生态系统 (community + environment). Abiotic factors (light, water, temperature, pH) and biotic factors (competition, predation, disease) set who lives where and how abundantly — together, the niche 生态位.

Predator and prey cycle with a delay: hares rise, lynx follow, hares fall, lynx starve — a lag of about a quarter cycle.

Succession 演替 begins when pioneer species colonise bare ground; each stage changes the soil and microclimate, enabling the next, until the climax community 顶极群落 stabilises. Measuring abundance uses quadrats and transects (random sampling for estimates, systematic along a gradient) and capture-mark-recapture for animals ($N \approx \text{first catch} \times \text{second catch} / \text{marked recaptures}$).

Evidence comes from direct temperature records, ice cores (trapped air giving ancient CO₂), tree rings (dendrochronology 树轮年代学) and pollen layers in peat. The enhanced greenhouse effect 温室效应: outgoing long-wave radiation is absorbed by CO₂, methane and nitrous oxide and re-emitted downward. The gases differ — methane is potent but short-lived; CO₂ lasts centuries; nitrous oxide combines both — so policy weighs global warming potential 全球增温潜势 against lifetime.

Extrapolating a trend beyond its data is a judgement, not a fact: state the assumption (the trend continues) and its weakness. Effects already examined: shifting rainfall and species ranges, and — close to this course — temperature's effect on enzyme rates in cold-blooded organisms and whole ecosystems.

Worked check. Methane's warming potential is about 28× CO₂'s per kg, but it lasts ~12 years against centuries. A one-tonne methane leak matters 28× a one-tonne CO₂ leak this decade, but fades while the CO₂ persists — the arithmetic of "warming potential vs cumulative load".

Evolution is a change in allele frequency 等位基因频率. It needs variation (mutation, meiosis) and a filter: directional selection 定向选择 shifts the mean when the environment moves; stabilising selection 稳定选择 trims the extremes in a stable one. Drift changes frequencies by chance in small populations; gene flow (migration) mixes them back.

Speciation 物种形成 needs isolation: allopatric 异域的 (a physical barrier — allopatric = different homeland) or sympatric 同域的 (reproductive isolation in the same place — behavioural, temporal or genetic). Isolated populations diverge until they can no longer interbreed.

Aseptic technique protects you and the culture: flame the loop to red heat, flame bottle necks, lift lids briefly, seal plates with tape, incubate at 25 °C (never body temperature, so human pathogens cannot multiply). Measure growth by direct cell count (haemocytometer), turbidity 浊度 in broth, or viable counts by dilution plating 稀释涂布 (colonies counted × dilution factor).

The growth curve: lag 延迟期 (enzymes synthesising), log 对数期 (exponential doubling), stationary 稳定期 (deaths = divisions), death 衰亡期. The exponential constant:

Worked check. From $2\times10^4$ to $1.6\times10^7$ cells in 5 hours: log difference $= 2.903$; $k = 2.903/(0.301\times5) = 1.93$ generations per hour — a doubling time of about 31 minutes.

Bacteria: cell wall, plasmids, ribosomes, circular DNA, some with capsule and flagellum. Viruses: nucleic acid (DNA or RNA) in a capsid 衣壳, some with an envelope — no organelles, no metabolism of their own; they hijack host cells. Mycobacterium tuberculosis 结核分枝杆菌 infects lungs, surviving inside macrophages; HIV 人类免疫缺陷病毒 destroys T helper cells, so the immune system collapses and TB reactivates — the classic AIDS-defining illness.

Barriers come first: skin, mucus and cilia, stomach acid, lysozyme in tears, and the clotting cascade sealing wounds.

Non-specific: inflammation (vasodilation and fluid leak bringing phagocytes), phagocytosis, interferon. Specific — antigens 抗原 trigger it:

• Humoral 体液: B effector cells → plasma cells → antibodies 抗体; B memory cells B记忆细胞 remain for the faster, stronger secondary response.
• Cellular 细胞: T helper cells coordinate, T killer cells T杀伤细胞 destroy infected cells, T memory cells persist.

An antibody is four polypeptide chains (two heavy, two light) with a variable region 可变区 forming one specific binding site — the shape that fits one antigen. Immunity splits two ways: active 主动 (your own antibodies — natural infection or vaccination) vs passive 被动 (given antibodies — mother to baby, antiserum); natural vs artificial in each. Vaccination works at the scale of populations too (herd immunity 群体免疫) — but pathogens evolve back: the evolutionary race.

Worked check. A vaccine's second dose produces antibody levels ten times the first and within days rather than weeks: clonal selection finds the memory B cells, which divide rapidly into plasma cells — the secondary response the primary response built.

Bacteriostatic 抑菌的 antibiotics stop bacterial growth; bactericidal 杀菌的 ones kill. Resistance arises by mutation and spreads by selection whenever exposure kills the susceptible and spares the resistant — finishing a course and avoiding unnecessary use both slow it. Hospital-acquired infections thrive where vulnerable patients, invasive devices and resistant strains meet; hand hygiene, sterile procedure and isolating carriers break the chain.

Decomposers recycle carbon and nitrogen — the ecosystem's waste system. The polymerase chain reaction 聚合酶链式反应 cycles denaturation (95 °C), primer annealing (50–60 °C) and extension (72 °C), doubling the target DNA each cycle: $n$ cycles give $2^n$ copies. Gel electrophoresis 凝胶电泳 drags negatively charged DNA fragments through gel — short fragments run furthest. The profile 电泳图谱 of band positions identifies individuals (enough loci differ between unrelated people) and measures relatedness (shared bands).

Time of death narrows by four clocks: body temperature (falls ~1 °C per hour, modified by size, clothing, air), rigor mortis 尸僵 (sets then passes), decomposition degree, and insect succession on the body — each stage's arrivals time-stamp the interval.

Worked check. A body found at 22 °C core temperature in a 15 °C room: $(37-22)/(1\ ^\circ\text{C per hour}) \approx 15$ hours — but state the assumptions (still air, average build) before trusting it.