📚 A-Level CCEA Biology: Pre-exam Revision Notes | A-Level CCEA 生物：考前冲刺笔记
As the CCEA A-Level Biology exams approach, effective revision requires more than just reading through notes—it demands active recall, clear understanding of core concepts, and the ability to apply knowledge to unfamiliar contexts. These pre‑exam revision notes distil the essential topics across AS and A2, highlighting key facts, common pitfalls, and exam‑focused tips to boost your confidence.
随着 CCEA A‑Level 生物考试临近，高效的复习不仅仅是通读笔记，更需要主动回忆、清晰理解核心概念，并能在陌生情境中应用知识。这份考前冲刺笔记浓缩了 AS 和 A2 的核心主题，突出关键事实、常见易错点以及聚焦考试的技巧，帮助增强信心。
1. Cell Structure and Organelles | 细胞结构与细胞器
Eukaryotic cells possess a true nucleus and membrane‑bound organelles, unlike prokaryotic cells which lack a nuclear envelope and have 70S ribosomes. Key organelles include mitochondria (site of aerobic respiration), chloroplasts (photosynthesis), rough endoplasmic reticulum (protein synthesis and transport), smooth ER (lipid synthesis), Golgi apparatus (modifying and packaging proteins), and lysosomes (digestion).
真核细胞具有真正的细胞核和膜包围的细胞器，而原核细胞没有核膜且含有 70S 核糖体。主要细胞器包括线粒体（有氧呼吸场所）、叶绿体（光合作用）、粗面内质网（蛋白质合成与运输）、滑面内质网（脂质合成）、高尔基体（蛋白质修饰与包装）和溶酶体（消化作用）。
Be able to interpret and draw electron micrographs, recognising organelles by their characteristic features such as cristae in mitochondria and stacked thylakoids (grana) in chloroplasts. In CCEA papers, you may be asked to label diagrams or calculate actual sizes using the formula: Magnification = Image size ÷ Actual size.
要能够识别并绘制电子显微镜图像，根据典型特征辨认细胞器，如线粒体的嵴和叶绿体的类囊体堆叠（基粒）。在 CCEA 考试中，可能要求为示意图标注或使用公式：放大倍数 = 图像尺寸 ÷ 实际尺寸计算实际大小。
Prokaryotic cells may contain plasmids, a capsule, and flagella; their cell wall is composed of peptidoglycan (murein). Viruses are acellular and rely on host cells for replication—they are not considered living.
原核细胞可能含有质粒、荚膜和鞭毛；其细胞壁由肽聚糖（胞壁质）组成。病毒是非细胞结构，依赖宿主细胞繁殖——它们不被视为生物。
2. Biological Molecules: Carbohydrates, Lipids and Proteins | 生物分子：糖类、脂质和蛋白质
Monosaccharides such as glucose (α‑ and β‑glucose differ in the orientation of the –OH group on carbon‑1) are the monomers of carbohydrates. Glucose + glucose forms maltose; glucose + galactose makes lactose; many α‑glucose units polymerise to form starch (amylose and amylopectin) and glycogen, while β‑glucose forms cellulose with its straight chains and hydrogen bonds between adjacent chains giving high tensile strength.
单糖如葡萄糖（α‑葡萄糖和 β‑葡萄糖在碳‑1 上羟基方向不同）是糖类的单体。葡萄糖+葡萄糖形成麦芽糖；葡萄糖+半乳糖形成乳糖；多个 α‑葡萄糖聚合形成淀粉（直链淀粉和支链淀粉）和糖原，而 β‑葡萄糖形成纤维素，其直链和链间氢键赋予高抗拉强度。
Triglycerides are formed from one glycerol and three fatty acids joined by ester bonds; they are hydrophobic, energy‑dense, and provide insulation. Phospholipids have two fatty acids and a phosphate group, making them amphipathic—essential for membrane bilayers. The emulsion test for lipids adds ethanol and then water; a milky white emulsion indicates a positive result.
甘油三酯由一个甘油和三个脂肪酸通过酯键连接而成；它们疏水、能量密度高，并具有绝缘作用。磷脂有两个脂肪酸和一个磷酸基团，使其具两亲性——对形成膜双分子层至关重要。脂质的乳剂测试加入乙醇后加水；乳白色乳剂表示阳性结果。
Proteins are polymers of amino acids linked by peptide bonds. The primary structure is the sequence; secondary structure includes α‑helices and β‑pleated sheets stabilised by hydrogen bonds; tertiary structure is the overall 3D folding maintained by hydrogen bonds, disulfide bridges, ionic bonds and hydrophobic interactions; quaternary structure involves more than one polypeptide chain (e.g. haemoglobin). The biuret test detects peptide bonds, turning purple in the presence of protein.
蛋白质是由肽键连接的氨基酸聚合物。一级结构是序列；二级结构包括 α‑螺旋和 β‑折叠，由氢键稳定；三级结构是通过氢键、二硫键、离子键和疏水相互作用维持的整体三维折叠；四级结构涉及不止一条多肽链（如血红蛋白）。双缩脲试验检测肽键，遇蛋白质变紫色。
3. Cell Membranes and Transport | 细胞膜与物质运输
The fluid‑mosaic model describes the cell membrane as a phospholipid bilayer with embedded proteins, cholesterol (in animal cells), and glycolipids/glycoproteins. The ‘fluid’ nature allows lateral movement of components; ‘mosaic’ refers to the patchwork of proteins. Cholesterol maintains membrane fluidity at different temperatures.
流动镶嵌模型将细胞膜描述为磷脂双分子层，其中镶嵌着蛋白质、胆固醇（动物细胞中）以及糖脂/糖蛋白。“流动性”允许组分侧向移动；“镶嵌”指蛋白质的拼凑图案。胆固醇在不同温度下维持膜流动性。
Passive transport includes simple diffusion (small, non‑polar molecules down a concentration gradient), facilitated diffusion via channel or carrier proteins (no energy required), and osmosis—the net movement of water through a partially permeable membrane from a region of higher water potential (Ψ) to lower water potential. Water potential Ψ = Ψₛ (solute potential) + Ψₚ (pressure potential); pure water has Ψ = 0 kPa; adding solutes makes Ψ more negative.
被动运输包括简单扩散（小分子、非极性物质顺浓度梯度）、通过通道蛋白或载体蛋白的协助扩散（无需能量），以及渗透——水通过半透膜从水势（Ψ）较高区域向水势较低区域的净移动。水势 Ψ = Ψₛ（溶质势）+ Ψₚ（压力势）；纯水 Ψ = 0 kPa；加入溶质使 Ψ 变得更负。
Active transport uses carrier proteins and ATP to move substances against their concentration gradient (e.g. sodium‑potassium pump). Bulk transport includes endocytosis (phagocytosis for solids, pinocytosis for liquids) and exocytosis. CCEA exam questions often present experiments with Visking tubing or plant tissue to analyse osmotic changes.
主动运输利用载体蛋白和 ATP 将物质逆浓度梯度移动（如钠钾泵）。批量运输包括内吞作用（吞噬固体、胞饮液体）和胞吐作用。CCEA 考题常给出透析管或植物组织实验来剖析渗透变化。
4. Enzymes: Kinetics and Inhibition | 酶：动力学与抑制
Enzymes are globular proteins that act as biological catalysts, lowering activation energy without being consumed. The induced‑fit model proposes that the active site changes shape slightly to accommodate the substrate, forming enzyme‑substrate complexes. Enzyme specificity depends on the complementary shape and chemical properties of the active site.
酶是球状蛋白质，充当生物催化剂，降低活化能而自身不被消耗。诱导契合模型认为活性部位轻微改变形状以容纳底物，形成酶‑底物复合物。酶的专一性取决于活性部位与底物在形状和化学性质上的互补。
Temperature and pH affect enzyme activity by altering the bonds that maintain tertiary structure. As temperature rises, kinetic energy increases and more successful collisions occur until the optimum is reached; beyond this, the enzyme denatures (heat breaks hydrogen and other bonds). pH deviations from the optimum affect charges on amino acids, also causing denaturation. Substrate concentration: at low [S], rate increases linearly; at high [S], rate plateaus as active sites become saturated (Vmax).
温度和 pH 通过改变维持三级结构的键来影响酶活性。温度升高，动能增加，成功碰撞增多，直至达到最适温度；超过此温度，酶变性（热破坏氢键等）。偏离最适 pH 会影响氨基酸所带电荷，同样导致变性。底物浓度：在低 [S] 时，反应速率线性增加；高 [S] 时，因活性部位饱和，速率达到平台（Vmax）。
Competitive inhibitors resemble the substrate and bind reversibly to the active site; increasing substrate concentration can overcome the inhibition. Non‑competitive inhibitors bind to an allosteric site, changing the active site’s shape so the substrate cannot bind; this cannot be overcome by adding more substrate. End‑product inhibition is a form of feedback regulation (e.g. ATP inhibiting phosphofructokinase in respiration).
竞争性抑制剂与底物相似，可逆地与活性部位结合；增加底物浓度可克服抑制。非竞争性抑制剂结合于变构部位，改变活性部位形状，使底物无法结合；增加底物无法克服该抑制。终产物抑制是一种反馈调节（例如 ATP 抑制呼吸过程中的磷酸果糖激酶）。
5. Nucleic Acids and DNA Replication | 核酸与 DNA 复制
DNA is a double helix composed of nucleotides (deoxyribose sugar, phosphate, and nitrogenous base: adenine, thymine, cytosine, guanine). Two polynucleotide strands run antiparallel (5’→3′ and 3’→5′), held together by hydrogen bonds between complementary base pairs—A=T (two H‑bonds), C≡G (three H‑bonds). RNA is single‑stranded, contains ribose, and uses uracil instead of thymine.
DNA 是双螺旋，由核苷酸组成（脱氧核糖、磷酸和含氮碱基：腺嘌呤 A、胸腺嘧啶 T、胞嘧啶 C、鸟嘌呤 G）。两条多核苷酸链反向平行（5’→3′ 和 3’→5’），由互补碱基对之间的氢键连接——A=T（两个氢键），C≡G（三个氢键）。RNA 是单链，含核糖，以尿嘧啶 U 代替胸腺嘧啶。
DNA replication is semi‑conservative, each new molecule containing one original strand and one new strand. Key enzymes: DNA helicase unwinds the double helix by breaking hydrogen bonds; DNA polymerase adds free nucleotides to the 3′ end of the growing strand, requiring a primer; the leading strand is synthesised continuously, the lagging strand in Okazaki fragments, later joined by DNA ligase. Meselson and Stahl confirmed semi‑conservative replication using ¹⁵N/¹⁴N isotopes.
DNA 复制是半保留的，每个新分子含一条原始链和一条新链。关键酶：DNA 解旋酶通过断裂氢键解开双螺旋；DNA 聚合酶将游离核苷酸加到生长链的 3′ 端，需要引物；前导链连续合成，后随链以冈崎片段合成，之后由 DNA 连接酶连接。Meselson 和 Stahl 使用 ¹⁵N/¹⁴N 同位素证实了半保留复制。
PCR (polymerase chain reaction) amplifies DNA in vitro: denaturation (95°C), annealing (50‑65°C), extension (72°C, Taq polymerase). CCEA may ask for applications of PCR or ethical issues around DNA technology.
PCR（聚合酶链反应）在体外扩增 DNA：变性（95°C）、退火（50‑65°C）、延伸（72°C，Taq 聚合酶）。CCEA 可能要求 PCR 的应用或围绕 DNA 技术的伦理议题。
6. Cell Division: Mitosis and Meiosis | 细胞分裂：有丝分裂与减数分裂
The cell cycle consists of interphase (G₁, S, G₂) and mitosis. During S phase, DNA is replicated; chromosomes then consist of two sister chromatids joined at the centromere. Mitosis produces two genetically identical diploid daughter cells for growth, repair, and asexual reproduction. Phases: prophase (chromosomes condense, spindle forms, nuclear envelope breaks down), metaphase (chromosomes align at equator), anaphase (sister chromatids pulled to poles), telophase (nuclear envelopes re‑form, cytokinesis).
细胞周期包括间期（G₁、S、G₂）和有丝分裂。S 期 DNA 复制；染色体此时由着丝粒连接的两个姐妹染色单体组成。有丝分裂产生两个遗传上相同的二倍体子细胞，用于生长、修复和无性生殖。分期：前期（染色质凝集，纺锤体形成，核膜解体）、中期（染色体排列在赤道板）、后期（姐妹染色单体被拉向两极）、末期（核膜重新形成，胞质分裂）。
Meiosis involves two divisions producing four genetically varied haploid gametes. Homologous chromosomes pair (synapsis) and crossing over occurs at chiasmata in prophase I, exchanging alleles. Independent assortment of homologous pairs at metaphase I and of chromatids at metaphase II creates further variation. Non‑disjunction can lead to aneuploidy (e.g. Down syndrome).
减数分裂经历两次分裂，产生四个遗传变异的单倍体配子。同源染色体在前期 I 配对（联会），并在交叉处发生交换，互换等位基因。中期 I 同源染色体对的独立分配和中期 II 染色单体的独立分配产生更多变异。不分离可导致非整倍体（如唐氏综合征）。
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | 1 | 2 |
| Daughter cells | 2, diploid, identical | 4, haploid, varied |
| Pairing of homologues | No | Yes (prophase I) |
| Crossing over | No | Yes |
| Function | Growth, repair | Gamete production |
7. Genetics and Inheritance Patterns | 遗传与遗传模式
Monohybrid crosses follow Mendel’s laws. The law of segregation states that allele pairs separate during gamete formation; the law of independent assortment applies to genes on different chromosomes. Use Punnett squares to predict phenotypic ratios, e.g. 3:1 for a heterozygote cross in complete dominance, 1:2:1 for codominant alleles (both expressed, as in human MN blood group), and 9:3:3:1 for a dihybrid cross with independent assortment.
单基因杂交遵循孟德尔定律。分离定律指出等位基因在配子形成时分开；自由组合定律适用于不同染色体上的基因。使用旁氏表预测表型比率，如完全显性下杂合杂交得 3:1，共显性等位基因（均表达，如人类 MN 血型）得 1:2:1，独立分配的双基因杂交得 9:3:3:1。
Sex‑linkage: genes located on the sex chromosomes (usually X) show different inheritance patterns. For example, haemophilia A is an X‑linked recessive disorder; a carrier female (XᴴXʰ) and normal male (XᴴY) can produce affected sons. In pedigree charts, circles are females, squares are males; use shading to indicate phenotype.
性连锁：位于性染色体（通常是 X 染色体）上的基因表现出不同的遗传模式。例如，血友病 A 是一种 X 连锁隐性遗传病；携带者女性 (XᴴXʰ) 与正常男性 (XᴴY) 可能生出患病儿子。在家系图中，圆圈表示女性，方框表示男性；用阴影表示表型。
Codominance and multiple alleles are exemplified by the ABO blood group system: alleles Iᴬ and Iᴮ are codominant, both dominant to Iᴼ. Genotypes IᴬIᴬ or IᴬIᴼ produce group A; IᴮIᴮ or IᴮIᴼ produce group B; IᴬIᴮ gives group AB; IᴼIᴼ gives group O. Be able to solve genetic crosses and interpret chi‑squared tests for goodness of fit.
共显性与复等位基因以 ABO 血型系统为例：等位基因 Iᴬ 和 Iᴮ 为共显性，均对 Iᴼ 为显性。基因型 IᴬIᴬ 或 IᴬIᴼ 产生 A 型血；IᴮIᴮ 或 IᴮIᴼ 产生 B 型；IᴬIᴮ 产生 AB 型；IᴼIᴼ 产生 O 型。需能处置遗传杂交题并解释卡方适合度检验。
8. Evolution and Natural Selection | 进化与自然选择
Darwin’s theory of evolution by natural selection states that individuals with advantageous alleles are more likely to survive, reproduce, and pass on those alleles. Over time, allele frequencies in the gene pool shift, leading to adaptation. Key requirements: variation within population, environmental selection pressure, differential reproductive success, and heritability of traits.
达尔文自然选择进化论指出，具有有利等位基因的个体更易存活、繁殖并将等位基因传递下去。随时间推移，基因库中等位基因频率改变，导致适应。关键要素：种群内变异、环境选择压、繁殖成功率差异，以及性状的可遗传性。
Antibiotic resistance in bacteria is a classic example: a mutation confers resistance; when antibiotics are used, sensitive bacteria die and resistant ones thrive, passing resistance via horizontal gene transfer (conjugation). CCEA often links this to the importance of completing antibiotic courses and reducing misuse.
细菌的抗生素耐药性是经典例子：突变产生耐药；使用抗生素时，敏感菌死亡，耐药菌繁殖，并通过水平基因转移（接合）传播耐药性。CCEA 常将此与完成抗生素疗程和减少滥用联系起来。
Speciation occurs when populations become reproductively isolated. Allopatric speciation involves geographical barriers (e.g. mountain ranges, rivers) preventing gene flow; sympatric speciation occurs within the same area due to behavioural, temporal, or mechanical isolation. Polyploidy in plants can cause instant speciation.
物种形成发生在种群生殖隔离时。异域物种形成涉及地理障碍（如山脉、河流）阻断基因流；同域物种形成发生在同一区域，由于行为、时间或机械隔离引起。植物中的多倍体可以导致瞬时物种形成。
9. Ecology: Energy Flow and Pyramids | 生态学：能量流动与金字塔
In an ecosystem, energy enters through photosynthesis and is transferred along food chains: producer → primary consumer → secondary consumer → tertiary consumer. Only about 10% of energy (variable) is passed to the next trophic level; the rest is lost as heat from respiration, not digested (egested), or not assimilated. Pyramids of energy are always upright, pyramids of numbers can be inverted.
在生态系统中，能量通过光合作用进入并沿食物链传递：生产者 → 初级消费者 → 次级消费者 → 三级消费者。仅有约 10% 的能量（可变）传递到下一个营养级；其余以呼吸热、未消化（排出）或未同化的形式损失。能量金字塔总是正立的，数量金字塔可能倒置。
Net primary productivity (NPP) = gross primary productivity (GPP) − respiratory losses (R). NPP represents energy available to consumers. Measure biomass in g m⁻² or energy in kJ m⁻² yr⁻¹. Detritivores and decomposers (bacteria, fungi) recycle nutrients by breaking down dead organic matter, crucial in carbon and nitrogen cycles.
净初级生产量 (NPP) = 总初级生产量 (GPP) − 呼吸消耗 (R)。NPP 代表可供消费者利用的能量。生物量以 g m⁻² 计算，能量以 kJ m⁻² yr⁻¹ 计算。腐食者和分解者（细菌、真菌）通过分解死亡有机物循环养分，对碳循环和氮循环至关重要。
Remember to label trophic levels in ecological pyramids and be able to calculate efficiency = (energy in higher level / energy in lower level) × 100. CCEA data‑analysis questions often include energy flow diagrams.
记得在生态金字塔中标注营养级，并能计算效率 =（高营养级能量 / 低营养级能量）× 100。CCEA 数据分析题常包含能量流动图。
10. Nutrient Cycles: Carbon and Nitrogen | 物质循环：碳循环与氮循环
The carbon cycle: photosynthesis fixes atmospheric CO₂ into organic carbon. Respiration by all organisms returns CO₂. Combustion of fossil fuels and biomass also releases CO₂. Decomposers break down dead matter, releasing CO₂ through respiration. In aquatic systems, CO₂ dissolves and can form carbonates. Peat and fossil fuels are long‑term carbon sinks.
碳循环：光合作用固定大气 CO₂ 为有机碳。所有生物的呼吸作用归还 CO₂。化石燃料和生物质的燃烧也释放 CO₂。分解者分解死物质，通过呼吸释放 CO₂。在水生系统中，CO₂ 溶解并可形成碳酸盐。泥炭和化石燃料是长期碳汇。
The nitrogen cycle: nitrogen fixation converts atmospheric N₂ to ammonia (NH₃) by free‑living (Azotobacter) or mutualistic (Rhizobium in legume root nodules) bacteria. NH₃ is converted to ammonium ions (NH₄⁺). Nitrification: Nitrosomonas oxidises NH₄⁺ to nitrite (NO₂⁻); Nitrobacter oxidises NO₂⁻ to nitrate (NO₃⁻), which plants absorb. Denitrification by Pseudomonas returns N₂ to the atmosphere under anaerobic conditions. Ammonification by decomposers releases NH₄⁺ from organic nitrogen compounds.
氮循环：固氮作用通过自由生活的细菌（如固氮菌）或共生的根瘤菌（在豆科根瘤中）将大气 N₂ 转化为氨 (NH₃)。NH₃ 转为铵离子 (NH₄⁺)。硝化作用：亚硝化单胞菌将 NH₄⁺ 氧化为亚硝酸盐 (NO₂⁻)；硝化杆菌将 NO₂⁻ 氧化为硝酸盐 (NO₃⁻)，供植物吸收。反硝化作用由假单胞菌在厌氧条件下将 NO₃⁻ 还原为 N₂ 返回大气。氨化作用由分解者从有机氮化合物中释放 NH₄⁺。
Leaching and eutrophication: excess nitrate from fertilisers runs off into water bodies, causing algal bloom. Algae die and are decomposed by aerobic bacteria, which deplete dissolved oxygen, killing fish. This is a common CCEA essay context.
淋溶与富营养化：化肥中过量的硝酸盐流入水体，引起藻华。藻类死亡后被需氧细菌分解，消耗溶解氧，导致鱼类死亡。这是 CCEA 常见的论述题情景。
11. Homeostasis and Excretion | 稳态与排泄
Homeostasis maintains a stable internal environment via negative feedback, where a change triggers a corrective mechanism to restore the set point. Key examples: thermoregulation (vasodilation/vasoconstriction, shivering, sweating) and blood glucose regulation (insulin and glucagon from pancreatic islets).
稳态通过负反馈维持稳定的内环境，即某一变化触发纠正机制以恢复设定值。关键实例：体温调节（血管舒张/血管收缩、颤抖、出汗）和血糖调节（胰岛分泌的胰岛素和胰高血糖素）。
The kidney plays a central role in osmoregulation and excretion. Ultrafiltration occurs in the Bowman’s capsule: blood enters the glomerulus under high pressure; water, glucose, salts, urea, and small molecules pass into the renal capsule, forming glomerular filtrate; large proteins and blood cells remain. Selective reabsorption in the proximal convoluted tubule reabsorbs all glucose (via active transport), most salts, and some water. The loop of Henle creates a concentration gradient in the medulla, enabling water reabsorption; ADH adjusts the permeability of the distal tubule and collecting duct.
肾脏在渗透调节和排泄中起核心作用。超滤发生在鲍曼囊：血液在高压下进入肾小球；水、葡萄糖、盐、尿素和小分子物质进入肾小囊形成肾小球滤液；大分子蛋白质和血细胞留在血液中。近曲小管的选择性重吸收通过主动运输回收全部葡萄糖、大部分盐和一些水。亨勒袢在髓质建立浓度梯度以便水分重吸收；抗利尿激素 (ADH) 调节远端小管和集合管的通透性。
Diabetes mellitus: Type 1 is an autoimmune disease destroying β‑cells, leading to insufficient insulin; Type 2 involves insulin resistance. Monitoring and treatment may be required in exam application questions.
糖尿病：1 型为自身免疫疾病，破坏 β 细胞，导致胰岛素不足；2 型涉及胰岛素抵抗。考试应用题可能涉及监测和治疗。
12. Photosynthesis and Plant Transport | 光合作用与植物运输
Photosynthesis occurs in chloroplasts. The light‑dependent reaction (thylakoid membranes) photolysis water, producing O₂, ATP and reduced NADP; electrons pass through an electron transport chain, generating a proton gradient for chemiosmosis. The light‑independent reaction (Calvin cycle, stroma) uses ATP and reduced NADP to fix CO₂; RuBP combines with CO₂ (catalysed by rubisco) to form GP, which is reduced to tri
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