ALevel AQA生物 生物分子 细胞膜 酶 DNA 备考
A-Level AQA Biology is one of the most content-rich science qualifications, covering everything from the molecular machinery inside cells to whole-organism physiology and ecosystems. This study plan breaks down the key topics into manageable chunks, with bilingual explanations to help EAL learners master both the concepts and the terminology. A-Level AQA生物是内容最丰富的科学资格考试之一，涵盖从细胞内的分子机器到整体生理学和生态系统。这份备考计划将关键知识点拆解为可消化的模块，中英双语讲解帮助EAL学生同时掌握概念和术语。
1. Biological Molecules 生物大分子
A-Level biology starts with the chemistry of life. Carbohydrates come as monosaccharides (glucose, fructose, galactose), disaccharides (maltose, sucrose, lactose formed by condensation), and polysaccharides (starch, glycogen, cellulose). The key test is Benedict's reagent for reducing sugars: blue solution turns brick-red precipitate on heating. Lipids are triglycerides (glycerol + 3 fatty acids joined by ester bonds) and phospholipids (phosphate head replaces one fatty acid tail). The emulsion test gives a cloudy white layer for lipids.
A-Level生物学从生命的化学基础开始。碳水化合物包括单糖（葡萄糖、果糖、半乳糖）、二糖（麦芽糖、蔗糖、乳糖，由缩合反应形成）和多糖（淀粉、糖原、纤维素）。关键实验是Benedict's试剂检测还原糖：蓝色溶液加热后变为砖红色沉淀。脂质包括甘油三酯（甘油+3个脂肪酸通过酯键连接）和磷脂（磷酸头替换一个脂肪酸尾）。乳液试验检测脂质产生乳白色层。
Proteins are polymers of amino acids joined by peptide bonds. The four structural levels are essential: primary (amino acid sequence), secondary (alpha helix and beta pleated sheet via hydrogen bonds), tertiary (3D folding via hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions), and quaternary (multiple polypeptide chains, e.g. haemoglobin with 4 subunits). Enzymes lower activation energy through the induced fit model: the active site changes shape around the substrate to form enzyme-substrate complexes.
蛋白质是氨基酸通过肽键连接而成的聚合物。四个结构层次至关重要：一级结构（氨基酸序列），二级结构（alpha螺旋和beta折叠片，通过氢键维持），三级结构（三维折叠，通过氢键、离子键、二硫键、疏水相互作用维持），四级结构（多条多肽链，如血红蛋白有4个亚基）。酶通过诱导契合模型降低活化能：活性位点围绕底物改变形状形成酶底物复合物。
DNA and RNA are polynucleotides. DNA has deoxyribose, bases A/T/C/G, and a double helix with antiparallel strands and complementary base pairing (A=T via 2 H-bonds, C≡G via 3 H-bonds). RNA has ribose, uracil instead of thymine, and is usually single-stranded. ATP (adenosine triphosphate) is the universal energy currency: hydrolysis of ATP to ADP + Pi releases 30.6 kJ/mol and is catalysed by ATP hydrolase.
DNA和RNA是多核苷酸。DNA含有脱氧核糖、碱基A/T/C/G，双螺旋结构具有反平行链和互补碱基配对（A=T通过2个氢键，C≡G通过3个氢键）。RNA含有核糖、尿嘧啶代替胸腺嘧啶，通常为单链。ATP（三磷酸腺苷）是通用能量货币：ATP水解为AD+Pi释放30.6 kJ/mol，由ATP水解酶催化。
2. Cell Membranes and Transport 细胞膜与物质运输
The fluid mosaic model describes the cell membrane: a phospholipid bilayer (hydrophilic heads facing out, hydrophobic tails inward) with proteins embedded throughout. Intrinsic proteins span the entire bilayer (channel proteins for facilitated diffusion, carrier proteins for active transport and co-transport). Extrinsic proteins are on the surface only (receptors, antigens). Cholesterol sits between phospholipids, reducing membrane fluidity and increasing stability.
流动镶嵌模型描述细胞膜：磷脂双分子层（亲水头朝外，疏水尾朝内），蛋白质嵌入其中。内在蛋白贯穿整个双分子层（通道蛋白用于协助扩散，载体蛋白用于主动运输和协同转运）。外在蛋白仅存在于表面（受体、抗原）。胆固醇位于磷脂之间，降低膜流动性，增加稳定性。
Four transport mechanisms: Diffusion (passive, down concentration gradient, no protein needed for small non-polar molecules). Osmosis (passive, water through partially permeable membrane from high to low water potential). Facilitated diffusion (passive, through channel or carrier proteins for polar/charged molecules and ions). Active transport (requires ATP, against concentration gradient via carrier proteins, e.g. sodium-potassium pump: 3 Na+ out, 2 K+ in). Co-transport: sodium ions moving down their electrochemical gradient pull glucose or amino acids into cells (e.g. in the ileum).
四种运输机制：扩散（被动，沿浓度梯度，小非极性分子不需蛋白质）。渗透（被动，水通过半透膜从高水势到低水势）。协助扩散（被动，通过通道蛋白或载体蛋白运输极性或带电分子和离子）。主动运输（需要ATP，逆浓度梯度通过载体蛋白，如钠钾泵：3个Na+出，2个K+入）。协同转运：钠离子沿电化学梯度移动，将葡萄糖或氨基酸拉入细胞（如在回肠中）。
3. Enzymes in Depth 酶深度解析
The induced fit model refined the older lock-and-key model: the active site is not a rigid shape but moulds around the substrate, stressing bonds and lowering activation energy more effectively. V max is the maximum rate of reaction when all active sites are occupied. Km (Michaelis constant) is the substrate concentration at half V max: a low Km means high affinity.
诱导契合模型改进了旧的锁钥模型：活性位点不是刚性形状，而是围绕底物塑形，施加应力使化学键变形，更有效地降低活化能。V max是所有活性位点被占据时的最大反应速率。Km（米氏常数）是达到一半V max时的底物浓度：低Km意味着高亲和力。
Competitive inhibitors resemble the substrate and bind the active site. They do not affect V max (more substrate overcomes inhibition) but raise Km. Non-competitive inhibitors bind an allosteric site, altering enzyme shape. V max drops permanently; Km unchanged.
竞争性抑制剂形状与底物相似，与活性位点结合。它们不影响V max（加入更多底物可克服抑制作用）但增加Km。非竞争性抑制剂与别构位点结合（非活性位点），改变酶的构象。V max降低且加入更多底物无法恢复；Km不变。
4. DNA Replication and Protein Synthesis DNA复制与蛋白质合成
Semi-conservative replication: DNA helicase unwinds the double helix and breaks hydrogen bonds between bases. Each strand acts as a template. Free DNA nucleotides pair with exposed bases via complementary base pairing. DNA polymerase joins nucleotides in the 5' to 3' direction via phosphodiester bonds. The leading strand is synthesised continuously; the lagging strand in Okazaki fragments that are later joined by DNA ligase.
半保留复制：DNA解旋酶解开双螺旋并断裂碱基间的氢键。每条链作为模板。游离DNA核苷酸通过互补碱基配对与暴露的碱基结合。DNA聚合酶在5'至3'方向通过磷酸二酯键连接核苷酸。前导链连续合成；后随链以冈崎片段合成，后续由DNA连接酶连接。
Transcription: RNA polymerase binds to the promoter region. It unzips DNA and uses one strand as a template to synthesise pre-mRNA (complementary bases, uracil replacing thymine). In eukaryotes, splicing removes introns (non-coding) and joins exons (coding) to form mature mRNA. Translation: mRNA attaches to a ribosome. tRNA molecules carry specific amino acids; the anticodon on tRNA binds to the complementary codon on mRNA. Peptide bonds form between adjacent amino acids. The process continues until a stop codon is reached.
转录：RNA聚合酶与启动子区域结合。它解链DNA并以一条链为模板合成前体mRNA（互补碱基，尿嘧啶代替胸腺嘧啶）。在真核生物中，剪接去除内含子（非编码），连接外显子（编码）形成成熟mRNA。翻译：mRNA附着于核糖体。tRNA分子携带特定氨基酸；tRNA上的反密码子与mRNA上的互补密码子结合。相邻氨基酸间形成肽键。此过程持续直至到达终止密码子。
5. Photosynthesis 光合作用
The light-dependent reaction occurs in the thylakoid membranes. Light energy excites electrons in chlorophyll (photosystem II). Photolysis of water produces electrons, protons (H+), and oxygen: 2H2O = 4H+ + 4e- + O2. Electrons pass along an electron transport chain, releasing energy to pump H+ into the thylakoid space. A proton gradient forms, and H+ flows back through ATP synthase (chemiosmosis), producing ATP. Electrons are re-energised in photosystem I and combine with NADP+ and H+ to form reduced NADP.
光反应发生在类囊体膜中。光能激发叶绿素中的电子（光系统II）。水的光解产生电子、质子(H+)和氧气：2H2O = 4H+ + 4e- + O2。电子沿电子传递链传递，释放能量将H+泵入类囊体空间。形成质子梯度，H+通过ATP合酶回流（化学渗透），产生ATP。电子在光系统I中被重新激发，与NADP+和H+结合形成还原型NADP。
The light-independent reaction (Calvin cycle) occurs in the stroma, using ATP and reduced NADP from the light-dependent reaction. Three stages: (1) Carbon fixation: CO2 combines with RuBP (5C), catalysed by rubisco, forming an unstable 6C intermediate that splits into two GP (3C). (2) Reduction: GP is reduced to TP using ATP and reduced NADP. (3) Regeneration: 5 out of every 6 TP molecules regenerate RuBP using ATP. The remaining TP produces glucose, starch, cellulose, lipids or amino acids.
暗反应（卡尔文循环）发生在基质中。它使用光反应产生的ATP和还原型NADP。三个阶段：(1)碳固定：CO2与RuBP（核酮糖二磷酸，5C）结合，由rubisco催化，形成不稳定的6C中间体，分裂为两个GP（甘油酸3磷酸，3C）。(2)还原：GP利用ATP和还原型NADP还原为TP（磷酸丙糖）。(3)再生：每6个TP分子中有5个利用ATP再生RuBP。剩余的1个TP分子用于生成葡萄糖、淀粉、纤维素、脂质或氨基酸。
6. Respiration 呼吸作用
Glycolysis (cytoplasm): glucose (6C) is phosphorylated using 2 ATP, split into two triose phosphate (3C), oxidised to two pyruvate (3C). Net yield: 2 ATP, 2 reduced NAD, 2 pyruvate per glucose.
糖酵解（细胞质）：葡萄糖(6C)利用2个ATP磷酸化，分裂成两个磷酸丙糖(3C)，氧化为两个丙酮酸(3C)。净产出：2个ATP、2个还原型NAD、每分子葡萄糖产生2个丙酮酸。
Link reaction and Krebs cycle: pyruvate enters the mitochondrial matrix, decarboxylated (CO2 released) and dehydrogenated, combined with CoA to form acetyl CoA (2C). In the Krebs cycle, acetyl CoA joins oxaloacetate (4C) to form citrate (6C), which is decarboxylated and dehydrogenated to regenerate oxaloacetate. Per cycle: 2 CO2, 1 ATP, 3 reduced NAD, 1 reduced FAD.
连接反应与克雷布斯循环：丙酮酸进入线粒体基质，脱羧（释放CO2）和脱氢，与辅酶A结合形成乙酰辅酶A(2C)。在克雷布斯循环中，乙酰辅酶A与草酰乙酸(4C)结合形成柠檬酸(6C)，经脱羧和脱氢再生草酰乙酸。每次循环：2个CO2、1个ATP、3个还原型NAD、1个还原型FAD。
Oxidative phosphorylation: reduced NAD and reduced FAD donate electrons to the electron transport chain on the inner mitochondrial membrane (cristae). Energy released pumps H+ into the intermembrane space. The proton gradient drives H+ back through ATP synthase (chemiosmosis), producing ~34 ATP. Oxygen is the final electron acceptor, forming water. Total ATP per glucose: ~38 (glycolysis 2, link 0, Krebs 2, oxidative 34). In anaerobic respiration in animals, pyruvate is reduced to lactate (lactic acid fermentation), regenerating NAD so glycolysis can continue, yielding only 2 ATP per glucose.
氧化磷酸化：还原型NAD和还原型FAD将电子捐赠给线粒体内膜（嵴）上的电子传递链。释放的能量将H+泵入膜间隙。质子梯度驱动H+通过ATP合酶回流（化学渗透），产生约34个ATP。氧气是最终电子受体，形成水。每分子葡萄糖总ATP：约38个（糖酵解2个，连接反应0个，克雷布斯循环2个，氧化磷酸化34个）。在无氧呼吸（动物）中，丙酮酸被还原为乳酸（乳酸发酵），再生NAD使糖酵解得以继续，每分子葡萄糖仅产生2个ATP。
7. Exchange, Transport and Homeostasis 交换运输与稳态
Gas exchange in humans: alveoli provide a large surface area, thin epithelium, rich capillary network, and steep concentration gradient maintained by ventilation and blood flow. Haemoglobin has quaternary structure (4 polypeptide chains, each with a haem group containing Fe2+). It binds oxygen cooperatively (sigmoidal dissociation curve). The Bohr effect: higher CO2 shifts the curve right, reducing affinity for O2 and releasing more oxygen to respiring tissues.
人体气体交换：肺泡提供大表面积、薄上皮（单细胞厚）、丰富的毛细血管网，以及由通气和血流维持的陡峭浓度梯度。血红蛋白具有四级结构（4条多肽链，每条含有一个含Fe2+的血红素基团）。它以协同方式结合氧气：第一个O2分子的结合改变构象，使后续O2更容易结合（S形解离曲线）。Bohr效应：较高的CO2浓度（更酸性）使曲线右移，降低血红蛋白对O2的亲和力，向呼吸组织释放更多氧气。
Homeostasis maintains a stable internal environment. Blood glucose regulation: when glucose rises, pancreatic beta cells release insulin. Insulin increases membrane permeability to glucose in muscle and liver cells and activates enzymes that convert glucose to glycogen (glycogenesis). When glucose falls, alpha cells release glucagon, which activates enzymes converting glycogen back to glucose (glycogenolysis) and promotes gluconeogenesis. Osmoregulation: the hypothalamus detects changes in blood water potential. ADH is released from the posterior pituitary, increasing the permeability of collecting duct walls to water, so more water is reabsorbed.
稳态维持稳定的内部环境。血糖调节：血糖升高时，胰岛beta细胞释放胰岛素。胰岛素增加肌肉和肝细胞膜对葡萄糖的通透性，并激活将葡萄糖转化为糖原的酶（糖原生成）。血糖下降时，alpha细胞释放胰高血糖素，激活将糖原转化回葡萄糖的酶（糖原分解）并促进糖异生。渗透调节：下丘脑检测血液水势的变化。ADH从垂体后叶释放，增加集合管壁对水的通透性，使更多水被重吸收。
8. Nervous Coordination and Muscles 神经协调与肌肉
Resting potential (-70mV): the axon membrane is more permeable to K+ than Na+. The sodium-potassium pump actively transports 3 Na+ out and 2 K+ in, maintaining the electrochemical gradient. Action potential: (1) stimulus causes Na+ channels to open, Na+ rushes in (depolarisation to +40mV). (2) Na+ channels close, K+ channels open, K+ rushes out (repolarisation). (3) Hyperpolarisation (too many K+ leave), then the pump restores the resting potential. Saltatory conduction: in myelinated neurones, action potentials jump between nodes of Ranvier, speeding up transmission.
静息电位(-70mV)：轴突膜对K+的通透性大于Na+。钠钾泵主动运输3个Na+出、2个K+入，维持电化学梯度。动作电位：(1)刺激导致Na+通道打开，Na+涌入（去极化至+40mV）。(2)Na+通道关闭，K+通道打开，K+涌出（复极化）。(3)超极化（过多K+离开），然后泵恢复静息电位。跳跃传导：在有髓鞘神经元中，动作电位在郎飞氏结之间跳跃，加快传导速度。
Muscle contraction follows the sliding filament theory. An action potential depolarises the sarcolemma, spreads through T-tubules, and triggers Ca2+ release from the sarcoplasmic reticulum. Ca2+ binds troponin, tropomyosin moves, exposing myosin-binding sites on actin. Myosin heads form cross-bridges, power stroke pulls actin inward (ATP hydrolysis), new ATP detaches heads, cycle repeats. When stimulation stops, Ca2+ is pumped back and tropomyosin re-blocks binding sites.
肌肉收缩遵循滑动丝理论。动作电位到达神经肌肉接头，去极化肌膜经T小管传播，触发肌质网释放Ca2+。Ca2+与肌钙蛋白结合，使原肌球蛋白移动，暴露肌动蛋白上肌球蛋白结合位点。肌球蛋白头结合形成横桥，动力冲程将肌动蛋白向内拉动（ATP水解），新ATP使肌球蛋白头脱离，循环重复。停止刺激时Ca2+被泵回肌质网，原肌球蛋白重新阻断结合位点。
9. Genetics, Populations and Evolution 遗传种群与进化
Monohybrid inheritance: a cross involving one gene. Using Punnett squares, predict genotypic and phenotypic ratios. Dihybrid inheritance: two genes on different chromosomes (9:3:3:1 ratio in F2 generation). Sex linkage: genes on the X chromosome (e.g. haemophilia, colour blindness) affect males more frequently because males have only one X chromosome (XY). Codominance: both alleles expressed equally in the phenotype (e.g. AB blood type). Multiple alleles: more than two alleles in the gene pool (e.g. ABO blood system: IA, IB, IO).
单基因遗传：涉及一个基因的杂交。使用庞尼特方格，预测基因型和表型比例。双基因遗传：两个基因位于不同染色体上（F2代9:3:3:1比例）。伴性遗传：X染色体上的基因（如血友病、色盲）对男性的影响更频繁，因为男性只有一条X染色体(XY)。共显性：两个等位基因在表型中同等表达（如AB血型）。复等位基因：基因库中有两个以上的等位基因（如ABO血型系统：IA、IB、IO）。
Hardy-Weinberg principle: p + q = 1 and p2 + 2pq + q2 = 1, where p is the frequency of the dominant allele and q is the recessive. This holds under five conditions: no mutations, random mating, no natural selection, large population, no gene flow. Natural selection: individuals with advantageous alleles survive and reproduce more, shifting allele frequencies over generations (e.g. antibiotic resistance). Allopatric speciation: geographical isolation separates populations, different selection pressures cause divergence, and reproductive isolation eventually prevents interbreeding.
Hardy-Weinberg原理：p + q = 1（等位基因频率）和p2 + 2pq + q2 = 1（基因型频率），其中p为显性等位基因频率，q为隐性。仅在五个条件下成立：无突变、随机交配、无自然选择、大种群规模、无基因流动（迁移）。自然选择：种群中存在变异，具有有利等位基因的个体更可能生存和繁殖，将这些等位基因传递下去。经过数代，等位基因频率发生改变（如细菌的抗生素耐药性）。异域物种形成：地理隔离分隔种群，不同的选择压力导致分化，最终生殖隔离阻止了杂交。
10. Six Week Study Plan 六周备考计划
Week 1 第一周
Biological Molecules + Nucleic Acids
生物大分子 + 核酸
2 hours/day 每天2小时
Week 2 第二周
Cells + Membranes + Transport
细胞 + 细胞膜 + 物质运输
2 hours/day 每天2小时
Week 3 第三周
DNA Replication + Protein Synthesis + Genetics
DNA复制 + 蛋白质合成 + 遗传
2 hours/day 每天2小时
Week 4 第四周
Photosynthesis + Respiration
光合作用 + 呼吸作用
2.5 hours/day 每天2.5小时
Week 5 第五周
Exchange + Transport + Homeostasis
交换 + 运输 + 稳态
2 hours/day 每天2小时
Week 6 第六周
Nerves + Muscles + Past Papers
神经 + 肌肉 + 真题演练
3 hours/day 每天3小时
11. Exam Tips and Key Strategies 考试技巧与核心策略
Command Words 指令词
Describe: say what you see, no reasons. Explain: give reasons why. Suggest: apply knowledge to an unfamiliar context. Evaluate: give pros/cons, reach a conclusion. 描述：说你所见，不给原因。解释：给出原因。建议：将知识应用于不熟悉的背景。评估：给出利弊，得出结论。
Application Qs 应用题
AQA Papers 2 and 3 include a 25-mark essay and an extended response. Use specific terminology: read the stem carefully, identify the topic area, and link concepts across modules. AQA Paper 2和3包含25分Essay和长篇回答。使用专业术语：仔细阅读题干，识别知识领域，跨模块链接概念。
Maths in Biology 生物数学
10% of marks involve maths: magnification, chi-squared, Hardy-Weinberg, rates, standard deviation, gradients. Always show working and include units. 10%的分数涉及数学：放大倍数、卡方检验、Hardy-Weinberg、速率、标准差、梯度。始终展示计算步骤并包含单位。
Key Bilingual Terms 关键双语术语
Phospholipid bilayer 磷脂双分子层 | Fluid mosaic model 流动镶嵌模型 | Induced fit 诱导契合 | Co-transport 协同转运 | Chemiosmosis 化学渗透 | Calvin cycle 卡尔文循环 | Glycolysis 糖酵解 | Krebs cycle 克雷布斯循环 | Oxidative phosphorylation 氧化磷酸化 | Sliding filament theory 滑动丝理论 | Hardy-Weinberg principle Hardy-Weinberg原理 | Allopatric speciation 异域物种形成 | Saltatory conduction 跳跃传导 | Bohr effect Bohr效应 | Semi-conservative replication 半保留复制
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