📚 A-Level CIE Biology: End-of-Term Revision Guide | A-Level CIE 生物：期末复习提纲
This revision guide provides a comprehensive overview of the core topics in the CIE A-Level Biology (9700) syllabus. Use it to consolidate your knowledge, identify key concepts, and prepare effectively for your end-of-term examinations. Each section presents the essential points in English followed by a matching Chinese explanation, so that bilingual learners can move between both languages without losing the scientific precision required by the mark scheme.
本复习提纲全面概述了 CIE A-Level 生物学（9700）教学大纲的核心主题。用它来巩固知识、识别关键概念，并为期末考试高效备考。每一节先给出英文要点，随后附上对应的中文讲解，方便双语学习者在两种语言之间自由切换，同时不丢失评分标准所要求的科学性。
1. Cell Structure and Function | 细胞结构与功能
Cells are the basic structural and functional units of life. Prokaryotic cells (e.g., bacteria) lack a nucleus and membrane-bound organelles, while eukaryotic cells (plants, animals, fungi) have a true nucleus and compartmentalised organelles. Key organelles include mitochondria (ATP production), chloroplasts (photosynthesis), rough endoplasmic reticulum (RER; protein synthesis and transport), smooth endoplasmic reticulum (SER; lipid synthesis), Golgi apparatus (modification and packaging), lysosomes (digestion), ribosomes (protein synthesis), and vacuoles (storage). The cell surface membrane controls the passage of substances via the fluid mosaic model.
细胞是生命的基本结构和功能单位。原核细胞（如细菌）没有细胞核和膜结合细胞器，而真核细胞（植物、动物、真菌）具有真正的细胞核和分隔化的细胞器。关键细胞器包括线粒体（ATP 产生）、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（修饰与包装）、溶酶体（消化）、核糖体（蛋白质合成）和液泡（储存）。细胞表面膜通过流动镶嵌模型控制物质进出。
For the exam, always link structure to function. The many folds of the inner mitochondrial membrane (cristae) increase the surface area for the electron transport chain; the flattened sacs of the Golgi stack allow sequential modification of proteins; the double membrane of the chloroplast separates the stroma from the cytosol. Remember also the difference between magnification and resolution: magnification enlarges an image, while resolution is the minimum distance at which two points can still be distinguished.
考试时一定要把"结构"与"功能"对应起来写。线粒体内膜折叠形成嵴（cristae），增大了电子传递链的附着面积；高尔基体的扁平囊泡使蛋白质能够被逐步修饰；叶绿体的双层膜把基质与细胞质基质分隔开。还要记住放大倍数与分辨率的区别：放大倍数只是把图像放大，而分辨率是仍能区分两个点的最小距离。
| Organelle | Main function |
|---|---|
| Nucleus | Contains genetic material (DNA); controls cell activities |
| Mitochondrion | Site of aerobic respiration; produces ATP |
| Chloroplast | Site of photosynthesis; contains chlorophyll |
| Rough ER | Protein synthesis and transport |
| Smooth ER | Lipid and steroid synthesis |
| Golgi apparatus | Modification and packaging of proteins |
| Lysosome | Digestion of molecules using hydrolytic enzymes |
| Ribosome | Protein synthesis |
| Vacuole | Storage; maintains turgor in plant cells |
2. Biological Molecules | 生物分子
All living organisms are built from four main classes of biological molecules: carbohydrates, lipids, proteins and nucleic acids. Water is also essential because of its polarity, high specific heat capacity and role as a solvent. You must be able to describe the condensation and hydrolysis reactions that build and break these polymers, and to carry out the standard biochemical tests with their expected colours.
所有生物都由四大类生物分子构成：碳水化合物、脂质、蛋白质和核酸。水同样必不可少，因为它具有极性、高比热容，并且是良好的溶剂。你必须能够描述构建和分解这些聚合物的缩合反应与水解反应，并掌握标准生化检测方法及其预期颜色变化。
Benedict's test for reducing sugars: blue → brick-red precipitate on heating.
本尼迪克特试剂检测还原糖：蓝色 → 加热后出现砖红色沉淀。
Iodine solution for starch: yellow-brown → blue-black.
碘液检测淀粉：黄棕色 → 蓝黑色。
Biuret test for protein: blue → purple/lilac.
双缩脲试剂检测蛋白质：蓝色 → 紫色/淡紫色。
Emulsion test for lipids: white emulsion forms above the water layer.
乳化检测脂质：在水分层上方形成白色乳浊液。
The structure of a triglyceride (one glycerol + three fatty acids) explains its energy storage role, while the phospholipid bilayer explains membrane behaviour. In proteins, the sequence of amino acids determines the primary structure; hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions produce the secondary, tertiary and quaternary levels.
甘油三酯（1 分子甘油 + 3 分子脂肪酸）的结构解释了其储能作用，而磷脂双分子层则解释了细胞膜的行为。在蛋白质中，氨基酸序列决定一级结构；氢键、离子键、二硫键和疏水相互作用共同形成二级、三级和四级结构。
3. Enzymes | 酶
Enzymes are globular proteins that act as biological catalysts by lowering the activation energy of a reaction. They are specific because the active site is complementary in shape and charge to the substrate, forming an enzyme-substrate complex. The lock-and-key model has been refined into the induced-fit model, in which the active site moulds slightly around the substrate.
酶是球状蛋白质，通过降低反应的活化能充当生物催化剂。酶具有专一性，因为活性位点在形状和电荷上与底物互补，形成酶-底物复合物。锁钥模型已被诱导契合模型修正：在诱导契合中，活性位点会稍微围绕底物发生形变。
Four factors affect the rate of enzyme-catalysed reactions: temperature, pH, substrate concentration and enzyme concentration. Above the optimum temperature, hydrogen and ionic bonds break, the tertiary structure is lost and the enzyme is denatured - a permanent change. A non-competitive inhibitor binds away from the active site and changes its shape; a competitive inhibitor binds at the active site and is overcome by increasing substrate concentration.
影响酶促反应速率的因素有四个：温度、pH、底物浓度和酶浓度。超过最适温度后，氢键和离子键断裂，三级结构被破坏，酶发生变性——这是永久性的改变。非竞争性抑制剂结合在活性位点之外并改变其形状；竞争性抑制剂结合在活性位点上，可通过提高底物浓度来克服其抑制作用。
rate of reaction = 1 ÷ time taken for the reaction to complete
Q₁₀ = rate at (T + 10 °C) ÷ rate at T
4. Cell Membranes and Transport | 细胞膜与物质运输
The fluid mosaic model describes the cell surface membrane as a phospholipid bilayer with embedded proteins, glycoproteins, glycolipids and cholesterol. The hydrophobic tails face inwards, forming a barrier to water-soluble substances, while the hydrophilic heads face the aqueous environment. Cholesterol restricts the movement of phospholipids and so controls membrane fluidity and permeability.
流动镶嵌模型把细胞表面膜描述为磷脂双分子层，其中镶嵌着蛋白质、糖蛋白、糖脂和胆固醇。疏水的尾部朝向内侧，形成对水溶性物质的屏障；亲水的头部朝向水环境。胆固醇限制磷脂的运动，从而控制膜的流动性和通透性。
Substances cross membranes by diffusion (passive, down the concentration gradient), facilitated diffusion (through channel or carrier proteins, also passive), osmosis (the movement of water from a higher to a lower water potential through a partially permeable membrane), active transport (using ATP and carrier proteins against the gradient), and bulk transport by endocytosis and exocytosis.
物质跨膜的方式包括：扩散（被动，顺浓度梯度）、易化扩散（通过通道蛋白或载体蛋白，也是被动的）、渗透（水通过部分透性膜从高水势一侧移向低水势一侧）、主动运输（消耗 ATP，借助载体蛋白逆浓度梯度运输），以及通过胞吞和胞吐进行的大批量运输。
When plotting water potential data, remember that water potential is measured in kPa and is always zero or negative; pure water has a water potential of 0 kPa at standard pressure.
在处理水势数据时要注意：水势的单位是 kPa，其数值总是零或负值；在标准压力下纯水的水势为 0 kPa。
5. DNA, RNA and Protein Synthesis | DNA、RNA 与蛋白质合成
DNA is a double helix of two antiparallel polynucleotide strands held together by hydrogen bonds between complementary base pairs: adenine pairs with thymine, and guanine pairs with cytosine. The sugar-phosphate backbone is joined by phosphodiester bonds. RNA is single-stranded, contains ribose instead of deoxyribose and uses uracil in place of thymine.
DNA 是由两条反向平行的多核苷酸链构成的双螺旋，链间通过互补碱基对之间的氢键相连：腺嘌呤与胸腺嘧啶配对，鸟嘌呤与胞嘧啶配对。糖-磷酸骨架之间由磷酸二酯键连接。RNA 是单链的，含核糖而非脱氧核糖，并用尿嘧啶代替胸腺嘧啶。
Semi-conservative DNA replication involves unwinding by DNA helicase, complementary base pairing of free nucleotides, and joining by DNA polymerase. Protein synthesis has two stages: transcription in the nucleus, where DNA is copied into messenger RNA, and translation at the ribosome, where transfer RNA molecules bring specific amino acids according to the codon sequence. The genetic code is degenerate, non-overlapping and universal.
半保留复制的过程包括：DNA 解旋酶使双链解开，游离核苷酸按互补碱基配对，再由 DNA 聚合酶连接成新链。蛋白质合成分为两个阶段：在细胞核中进行的转录，把 DNA 上的信息拷贝到信使 RNA；以及在核糖体上进行的翻译，转运 RNA 按照密码子序列携带特定氨基酸。遗传密码具有简并性、不重叠且具有通用性。
DNA → mRNA → polypeptide (transcription then translation)
6. Cell Division and the Cell Cycle | 细胞分裂与细胞周期
The cell cycle consists of interphase (G₁, S and G₂ phases) followed by nuclear division and cytokinesis. DNA replication occurs during the S phase. Mitosis produces two genetically identical diploid daughter cells and is used for growth, repair and asexual reproduction; its stages are prophase, metaphase, anaphase and telophase.
细胞周期由间期（G₁、S、G₂ 期）以及随后的核分裂和胞质分裂组成。DNA 复制发生在 S 期。有丝分裂产生两个遗传上完全相同的二倍体子细胞，用于生长、修复和无性繁殖；其各阶段为前期、中期、后期和末期。
Meiosis produces four genetically different haploid cells through two divisions. Crossing over in prophase I and independent assortment in metaphase I generate genetic variation; in meiosis II, sister chromatids of homologous chromosomes are separated. A key distinction: mitosis separates sister chromatids, while the first meiotic division separates homologous chromosomes, halving the chromosome number.
减数分裂通过两次分裂产生四个遗传上各不相同的单倍体细胞。前期 I 的交叉互换与中期 I 的独立分配产生遗传变异；减数分裂 II 则分离姐妹染色单体。一个关键区别是：有丝分裂分离的是姐妹染色单体，而减数第一次分裂分离的是同源染色体，使染色体数目减半。
7. Exchange Surfaces and Gas Exchange | 交换表面与气体交换
Single-celled organisms exchange gases directly across their surface, but larger organisms need specialised surfaces with a large surface area to volume ratio, thin diffusion distance and a maintained concentration gradient. Human lungs achieve this with millions of alveoli, a thin squamous epithelium, a rich capillary network and ventilation by the diaphragm and intercostal muscles.
单细胞生物可直接通过体表进行气体交换，但较大的生物需要特化的交换表面：表面积与体积之比大、扩散距离薄，并且能维持浓度梯度。人的肺通过数百万个肺泡、薄的鳞状上皮、丰富的毛细血管网，以及膈肌和肋间肌完成的通气来实现这些条件。
Insects use tracheae and spiracles, and may ventilate by abdominal pumping; fish use countercurrent flow across gill lamellae, so water and blood flow in opposite directions to maintain a diffusion gradient along the whole gill. Plants exchange gases through stomata, which open and close to balance carbon dioxide uptake against water loss.
昆虫利用气管和气门，并可通过腹部泵压进行通气；鱼类通过鳃小片实现逆流交换，水和血液沿相反方向流动，从而在整个鳃片上维持扩散梯度。植物通过气孔进行气体交换，气孔开闭在摄取二氧化碳与减少水分散失之间取得平衡。
8. Transport in Plants and Mammals | 植物与哺乳动物的运输
In mammals, the heart pumps blood through arteries, arterioles, capillaries and veins. The cardiac cycle includes atrial systole, ventricular systole and diastole, and the heartbeat is initiated by the sinoatrial node and coordinated by the atrioventricular node and Purkyne tissue. The left ventricle has a thicker muscular wall because it pumps blood around the whole body.
在哺乳动物中，心脏把血液泵入动脉、小动脉、毛细血管和静脉。心动周期包括心房收缩期、心室收缩期和舒张期；心跳由窦房结发起，并由房室结和浦肯野组织协调。左心室的肌肉壁更厚，因为它要把血液泵向全身。
In plants, water and mineral ions move up the xylem in the transpiration stream, driven by cohesion-tension, root pressure, capillary action and adhesion to the walls. Transpiration rate rises with higher temperature, wind speed and light intensity, but falls with higher humidity. Sucrose and amino acids are translocated in the phloem from source to sink according to the mass flow hypothesis, in which water enters the sieve tube at the source and leaves at the sink, creating hydrostatic pressure.
在植物中，水和矿质离子通过蒸腾流沿木质部向上运输，其动力来自内聚力-张力、根压、毛细作用以及水分子与管壁的附着力。蒸腾速率随温度、风速和光照强度升高而升高，随湿度升高而下降。蔗糖和氨基酸则按压力流动假说在韧皮部中从源运输到库：水在源端进入筛管、在库端离开，从而产生静水压力。
9. Energy for Biological Processes | 生物过程的能量
Photosynthesis converts light energy into chemical energy. The light-dependent stage occurs in the thylakoid membranes, where light excites electrons in chlorophyll, photolysis of water releases oxygen and provides hydrogen, and the products are reduced NADP and ATP. The light-independent stage takes place in the stroma, where carbon dioxide is fixed to ribulose bisphosphate and reduced to triose phosphate using ATP and reduced NADP.
光合作用把光能转化为化学能。光依赖阶段发生在类囊体膜上：光激发叶绿素中的电子，水的光解释放氧气并提供氢，产物是还原型 NADP 和 ATP。光不依赖阶段发生在基质中：二氧化碳被固定到核酮糖二磷酸上，再利用 ATP 和还原型 NADP 还原为三碳糖磷酸酯。
Aerobic respiration has four stages: glycolysis in the cytoplasm, the link reaction, the Krebs cycle and oxidative phosphorylation in the mitochondria. Anaerobic respiration in mammals produces lactate; in yeast it produces ethanol and carbon dioxide, with a much lower ATP yield.
有氧呼吸包括四个阶段：细胞质中的糖酵解、连接反应、克雷布斯循环，以及线粒体中的氧化磷酸化。哺乳动物的无氧呼吸产生乳酸；酵母则产生乙醇和二氧化碳，ATP 产量远低于有氧呼吸。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (photosynthesis)
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (aerobic respiration)
10. Homeostasis and Coordination | 稳态与协调
Homeostasis is the maintenance of a stable internal environment despite external change. Negative feedback loops detect a deviation, respond to correct it and restore the optimum value; examples include control of blood glucose by insulin and glucagon, thermoregulation, and the control of blood water potential by ADH acting on the collecting ducts of the kidney.
稳态是指在外界环境变化时仍维持内环境稳定。负反馈回路检测偏差、作出反应加以纠正，并恢复最适值；典型例子包括胰岛素与胰高血糖素对血糖的调节、体温调节，以及 ADH 作用于肾集合管对血液水势的调节。
In the nervous system, a resting potential of about -70 mV is maintained by the sodium-potassium pump and the differential permeability of the axon membrane. A stimulus opens voltage-gated sodium channels, causing depolarisation; repolarisation follows as potassium channels open. Myelination and saltatory conduction increase the speed of transmission, and a synapse uses neurotransmitters to transmit the impulse across a synaptic cleft.
在神经系统中，约 -70 mV 的静息电位由钠钾泵和轴突膜的选择通透性维持。刺激使电压门控钠通道开放，引起去极化；随后钾通道开放导致复极化。髓鞘化和跳跃式传导提高传导速度，突触则通过神经递质把冲动跨越突触间隙传递。
11. Genetics and Inheritance | 遗传与变异
Genes are sections of DNA that code for polypeptides. Alleles are different forms of the same gene; an organism's genotype describes the alleles present, while the phenotype is the observable characteristic. Monohybrid crosses between two heterozygotes give a 3:1 ratio in the F₂ generation. Multiple alleles, codominance and sex linkage explain deviations from simple Mendelian ratios.
基因是编码多肽的 DNA 片段。等位基因是同一基因的不同形式；基因型描述个体携带的等位基因，表现型则是可观察的性状。两个杂合子之间的单基因杂交在 F₂ 代中产生 3:1 的比例。复等位基因、共显性和伴性遗传解释了与简单孟德尔比例不符的现象。
Genetic variation arises from meiosis, random fertilisation and mutation. Gene mutations involve changes in the base sequence (substitution, deletion or insertion) and may be silent, missense or nonsense; chromosome mutations include changes in chromosome number, such as the trisomy that causes Down syndrome.
遗传变异来自减数分裂、随机受精和突变。基因突变涉及碱基序列的改变（替换、缺失或插入），可能是同义突变、错义突变或无义突变；染色体突变包括染色体数目的改变，例如导致唐氏综合征的三体。
monohybrid F₁ cross: Aa × Aa → 1 AA : 2 Aa : 1 aa (3 : 1 phenotype)
12. Ecology, Evolution and Experimental Skills | 生态、进化与实验技能
Ecology studies the interactions between organisms and their environment. Energy flows through ecosystems from producers to consumers, and only about 10 % is transferred between trophic levels because of respiration, excretion and incomplete consumption. Nutrients, by contrast, are recycled, as shown in the carbon and nitrogen cycles.
生态学研究生物与其环境之间的相互作用。能量在生态系统中从生产者流向消费者，由于呼吸消耗、排泄和未被完全取食，营养级之间只有约 10 % 的能量被传递。与之不同，营养物质是循环利用的，如碳循环和氮循环所示。
Natural selection explains evolution: individuals with alleles that increase fitness are more likely to survive and reproduce, passing those alleles to the next generation. Over many generations, allele frequencies change - this is evolution.
自然选择解释进化：拥有提高适应度的等位基因的个体更容易存活并繁殖，从而把这些等位基因传给下一代。经过许多世代，等位基因频率发生改变——这就是进化。
Finally, do not neglect the practical skills assessed in Paper 3 and Paper 5. You should be able to identify independent and dependent variables, describe how to control confounding variables, choose suitable apparatus, plot data with correct axes and units, calculate means and standard deviations, and evaluate sources of error. Microscopy, sampling with quadrats and transects, and colorimetry are frequently examined techniques.
最后，不要忽视 Paper 3 和 Paper 5 考查的实验技能。你应当能够识别自变量与因变量、描述如何控制干扰变量、选择合适的仪器、按正确的坐标轴和单位作图、计算平均值与标准差，并评估误差来源。显微镜观察、样方法与样带取样以及比色法都是经常考查的技术。
13. Exam Technique and Revision Timetable | 应试技巧与复习计划
Aim to write answers using the exact command words in the question: 'state' needs a short factual answer, 'describe' needs a sequence of observations, 'explain' needs a cause-and-effect link, and 'suggest' requires applying knowledge to a novel context. Marks are awarded for key scientific terms, so always write 'complementary' rather than 'fitting', and 'partially permeable' rather than 'semi-permeable'.
答题时要紧扣题目中的命令词：'state' 需要简短的事实性答案，'describe' 需要按顺序描述观察到的现象，'explain' 需要建立因果关系，而 'suggest' 则需要把知识应用到新情境中。得分点往往落在关键科学术语上，因此要写 'complementary' 而不是 'fitting'，写 'partially permeable' 而不是 'semi-permeable'。
Plan your final two weeks carefully: spend the first week reviewing cell biology, biochemistry, enzymes and transport, and the second week on energy, homeostasis, genetics and ecology. Finish each topic with past-paper questions under timed conditions, then mark them yourself using the official mark scheme and note every term you missed.
合理安排最后两周：第一周复习细胞生物学、生物化学、酶与物质运输；第二周复习能量、稳态、遗传与生态。每个专题结束后，在限时条件下完成历年真题，然后用官方评分标准自评，并记录下每一处遗漏的术语。
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