📚 Year 12 Cambridge Biology: Core Concepts Review | 剑桥AS生物核心知识点梳理
This article summarises the key topics covered in the Cambridge AS Biology syllabus. It is designed to help Year 12 students consolidate their understanding of cell biology, biochemistry, physiology, and immunity. Each section pairs essential concepts in English and Chinese, making it an ideal bilingual revision resource.
本文梳理了剑桥AS生物课程的核心知识点，覆盖细胞生物学、生物化学、生理学和免疫学等模块。每个小节均以中英双语呈现关键概念，帮助Year 12学生构建清晰的知识框架，是理想的复习指南。
1. Cell Structure and Function | 细胞结构与功能
All living organisms are made of cells. Prokaryotic cells, such as bacteria, lack a true nucleus and membrane-bound organelles; their DNA lies free in the cytoplasm. Eukaryotic cells possess a distinct nucleus enclosed by a nuclear envelope and contain various organelles like mitochondria, endoplasmic reticulum (ER), Golgi apparatus, and lysosomes. Plant cells additionally have a cellulose cell wall, chloroplasts, and a large permanent vacuole.
所有生物都由细胞构成。原核细胞（如细菌）没有真正的细胞核和膜包被的细胞器，其DNA游离在细胞质中。真核细胞具有由核膜包被的细胞核，并含有线粒体、内质网、高尔基体和溶酶体等多种细胞器。植物细胞还拥有纤维素细胞壁、叶绿体和一个大型中央液泡。
Key organelles perform specialised functions: mitochondria are the site of aerobic respiration, ribosomes carry out protein synthesis, the rough ER transports proteins synthesised by attached ribosomes, and the Golgi apparatus modifies and packages proteins for secretion. The nucleus stores genetic material and controls cellular activities.
重要细胞器的功能高度特化：线粒体是有氧呼吸的场所，核糖体负责蛋白质合成，粗面内质网转运核糖体合成的蛋白质，高尔基体负责修饰和分拣蛋白质。细胞核储存遗传信息并调控细胞活动。
2. Biological Molecules | 生物分子
Living organisms are built from carbon-based macromolecules: carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates include monosaccharides (e.g., glucose, fructose), disaccharides (maltose, sucrose), and polysaccharides (starch, glycogen, cellulose). They serve as energy sources and structural components. Lipids, mainly triglycerides, are formed from glycerol and three fatty acids; they function as energy stores, insulation, and components of cell membranes (phospholipids).
生物体由碳基大分子构建而成：糖类、脂质、蛋白质和核酸。糖类包括单糖（如葡萄糖、果糖）、二糖（麦芽糖、蔗糖）和多糖（淀粉、糖原、纤维素），既提供能量又参与结构构建。脂质主要是甘油三酯，由甘油和三个脂肪酸组成，具有储能、隔热和构成细胞膜（磷脂）的作用。
Proteins are polymers of amino acids linked by peptide bonds. Their primary structure is the sequence of amino acids; secondary structure includes α-helices and β-pleated sheets; tertiary structure is the overall 3D folding; and quaternary structure involves multiple polypeptide chains. Proteins function as enzymes, transporters, hormones, and structural components.
蛋白质是由氨基酸通过肽键连接而成的多聚体。一级结构为氨基酸序列，二级结构包括α螺旋和β折叠，三级结构是三维空间的折叠方式，四级结构则由多条多肽链组装而成。蛋白质可充当酶、运输载体、激素和结构物质。
Nucleic acids, DNA and RNA, are polymers of nucleotides. Each nucleotide consists of a pentose sugar (deoxyribose or ribose), a phosphate group, and a nitrogenous base. DNA is a double helix with complementary base pairing (A–T, C–G); RNA is single-stranded and contains uracil instead of thymine.
核酸包括DNA和RNA，由核苷酸聚合而成。每个核苷酸由一个戊糖（脱氧核糖或核糖）、一个磷酸基团和一个含氮碱基组成。DNA呈双螺旋结构，遵循碱基互补配对（A–T，C–G）；RNA为单链，含有尿嘧啶而非胸腺嘧啶。
3. Enzymes | 酶
Enzymes are globular proteins that act as biological catalysts, lowering the activation energy of biochemical reactions without being consumed. The substrate binds to the enzyme's active site. The lock-and-key model proposes a rigid complementary fit, while the induced-fit model suggests that the active site moulds around the substrate to enhance catalysis.
酶是球状蛋白质，作为生物催化剂降低化学反应的活化能，自身不被消耗。底物与酶的活性部位结合。锁钥模型认为活性部位与底物形状严格互补，诱导契合模型则提出活性部位在底物诱导下发生构象变化，使结合更加紧密。
Enzyme activity is influenced by temperature, pH, enzyme concentration, and substrate concentration. At low temperatures, kinetic energy is reduced; at high temperatures, enzymes denature. Each enzyme has an optimum pH. As substrate concentration increases, the rate of reaction rises until the active sites become saturated, reaching Vmax.
酶活性受温度、pH、酶浓度和底物浓度的影响。低温使动能降低，高温会导致酶变性失活。每种酶有其最适pH。随着底物浓度增加，反应速率上升直至活性部位被饱和，达到最大反应速率Vmax。
Inhibitors reduce enzyme activity. Competitive inhibitors resemble the substrate and compete for the active site; their effect can be overcome by increasing substrate concentration. Non-competitive inhibitors bind to an allosteric site, altering the shape of the active site, and cannot be overcome by adding more substrate.
抑制剂可降低酶活性。竞争性抑制剂结构类似底物，占据活性部位，可通过提高底物浓度克服；非竞争性抑制剂结合于酶的别构部位，改变活性部位构象，增加底物浓度无法解除抑制。
4. Cell Membranes and Transport | 细胞膜与运输
Cell membranes are described by the fluid mosaic model. They consist of a phospholipid bilayer with embedded proteins, cholesterol (in animal cells), and glycoproteins. The phospholipid molecules have hydrophilic heads and hydrophobic tails, creating a selectively permeable barrier. Membrane proteins function as channels, carriers, receptors, and enzymes.
细胞膜的结构可用流动镶嵌模型解释。它由磷脂双分子层构成基本骨架，其中镶嵌着蛋白质、胆固醇（动物细胞）和糖蛋白。磷脂分子头部亲水、尾部疏水，形成选择透过性屏障。膜蛋白可作为通道、载体、受体和酶。
Substances cross membranes by passive or active processes. Diffusion is the net movement of particles down a concentration gradient. Facilitated diffusion uses channel or carrier proteins, still moving down the gradient. Osmosis is the diffusion of water through a partially permeable membrane from a region of higher water potential to lower water potential.
物质通过被动或主动过程跨越细胞膜。简单扩散是粒子沿浓度梯度的净运动。协助扩散借助通道蛋白或载体蛋白，仍然顺浓度梯度进行。渗透是水分子通过部分透性膜从高水势区域向低水势区域的扩散。
Active transport moves substances against the concentration gradient, requiring energy from ATP and specific carrier proteins. Endocytosis and exocytosis are bulk transport processes that involve the folding of the membrane to form vesicles.
主动运输逆浓度梯度转运物质，需要ATP提供能量和特异载体蛋白。胞吞和胞吐是借助膜凹陷形成囊泡来运输大分子物质的过程。
5. Cell Cycle and Mitosis | 细胞周期与有丝分裂
The cell cycle consists of interphase (G1, S, G2) and mitotic phase. During interphase, the cell grows, carries out normal metabolic activities, and replicates its DNA. In G1, proteins and organelles are synthesised; in S phase, DNA is replicated; in G2, the cell prepares for division. Mitosis produces two genetically identical daughter nuclei and is followed by cytokinesis.
细胞周期包括间期（G1、S、G2期）和分裂期。间期内细胞生长，执行代谢功能并复制DNA。G1期合成蛋白质和细胞器，S期进行DNA复制，G2期为分裂做准备。有丝分裂产生两个遗传物质相同的子细胞核，随后进行胞质分裂。
The stages of mitosis are prophase, metaphase, anaphase, and telophase. In prophase, chromosomes condense, the nuclear envelope breaks down, and the spindle apparatus forms. During metaphase, chromosomes align at the metaphase plate. In anaphase, sister chromatids are pulled to opposite poles. Telophase sees the reformation of nuclear envelopes and decondensation of chromosomes.
有丝分裂分为前期、中期、后期和末期。前期染色体凝缩，核膜解体，纺锤体形成。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，染色体解旋。
Mitosis is vital for growth, repair, and asexual reproduction. Uncontrolled mitosis can lead to cancer. The cell cycle is regulated by checkpoints ensuring accurate DNA replication and chromosome segregation.
有丝分裂对生长、组织修复和无性生殖至关重要。细胞周期受检查点调控，确保DNA正确复制和染色体正常分离，失控可导致癌症。
6. Nucleic Acids and Protein Synthesis | 核酸与蛋白质合成
DNA (deoxyribonucleic acid) is a double-stranded polynucleotide held together by hydrogen bonds between complementary bases. Its structure allows it to store genetic information and replicate accurately. RNA (ribonucleic acid) is usually single-stranded and exists as mRNA, tRNA, and rRNA, each playing a distinct role in protein synthesis.
DNA为双链多核苷酸，通过互补碱基间的氢键维持双螺旋结构，可储存遗传信息并精确复制。RNA通常为单链，分为mRNA、tRNA和rRNA，在蛋白质合成中各司其职。
DNA replication is semi-conservative: each new DNA molecule consists of one original strand and one newly synthesised strand. The enzyme DNA helicase unwinds the double helix, and DNA polymerase adds free nucleotides according to base-pairing rules, extending the new strand in the 5' to 3' direction.
DNA复制是半保留的：每条新DNA分子含有一条旧链和一条新合成链。解旋酶解开双螺旋，DNA聚合酶按碱基互补原则从5'端向3'端延伸新链。
Protein synthesis involves transcription and translation. During transcription, mRNA is synthesised from a DNA template. The mRNA then moves to a ribosome, where translation occurs: tRNA molecules carry specific amino acids, and anticodons pair with mRNA codons, allowing the ribosome to assemble a polypeptide chain.
蛋白质合成包括转录和翻译。转录时以DNA为模板合成mRNA。mRNA转运至核糖体进行翻译：tRNA携带特定氨基酸，其反密码子与mRNA上的密码子配对，核糖体将氨基酸连接成多肽链。
7. Transport in Plants | 植物运输
Plants possess two transport systems: xylem and phloem. Xylem vessels transport water and mineral ions from roots to leaves. They are composed of dead, hollow cells with lignified walls, providing structural support. The movement of water is mainly driven by transpiration pull, explained by the cohesion-tension theory.
植物具有两套运输系统：木质部和韧皮部。木质部导管将水和无机盐从根部运往叶片。导管由死细胞组成，细胞壁木质化加厚，既输导水分又起支撑作用。水分运输主要依靠蒸腾拉力，用内聚力-张力理论解释。
Cohesion between water molecules due to hydrogen bonding and adhesion of water to xylem walls maintain a continuous water column. As water evaporates from stomata during transpiration, tension is created, pulling water upward. Factors affecting transpiration rate include light intensity, temperature, humidity, and wind speed.
水分子之间的内聚力（氢键）以及水与导管壁的附着力维持连续水柱。气孔蒸腾失水时产生张力，将水柱向上拉。影响蒸腾速率的因素有光照强度、温度、湿度和风速。
Phloem transports assimilates, mainly sucrose, from sources (e.g., leaves) to sinks (e.g., roots, fruits) through translocation. The mass flow hypothesis proposes that active loading of sucrose into phloem at the source lowers water potential, causing water to enter by osmosis and generating hydrostatic pressure that drives sap flow toward sinks.
韧皮部通过转运作用将同化物（主要是蔗糖）从源（如叶片）运往库（如根、果实）。压力流动假说认为，在源端韧皮部主动装载蔗糖降低了水势，水分通过渗透进入，产生静水压力推动汁液流向库端。
8. Transport in Mammals | 哺乳动物运输
Mammals have a closed, double circulatory system. The heart is a muscular organ with four chambers: right atrium, right ventricle, left atrium, and left ventricle. Deoxygenated blood is pumped to the lungs via the pulmonary artery; oxygenated blood returns via the pulmonary vein and is then pumped to the body through the aorta.
哺乳动物具有闭合式双循环系统。心脏为四腔肌肉器官：右心房、右心室、左心房和左心室。脱氧血经肺动脉泵入肺部，氧合血经肺静脉返回心脏，再通过主动脉泵往全身。
The cardiac cycle consists of atrial systole, ventricular systole, and diastole. Valves (atrioventricular and semilunar) prevent backflow. The heart's rhythmic contraction is initiated by the sinoatrial node (SAN), the natural pacemaker, and coordinated by the atrioventricular node (AVN) and Purkyne fibres.
心动周期包括心房收缩期、心室收缩期和舒张期。房室瓣和半月瓣防止血液倒流。心脏节律由窦房结发起，经房室结和浦肯野纤维传导协调。
Blood vessels include arteries (thick, elastic walls to withstand high pressure), veins (thinner walls, valves to prevent backflow), and capillaries (one-cell-thick walls for exchange). Blood consists of plasma, red blood cells (erythrocytes), white blood cells, and platelets. Red blood cells contain haemoglobin, which binds oxygen reversibly.
血管分为动脉（壁厚有弹性，耐高压）、静脉（壁较薄，有瓣膜）和毛细血管（单层细胞壁，利于物质交换）。血液由血浆、红细胞、白细胞和血小板组成。红细胞中的血红蛋白可逆性结合氧气。
The oxygen dissociation curve shows the relationship between oxygen partial pressure and haemoglobin saturation. The sigmoid shape reflects cooperative binding. The Bohr effect describes how increased CO2 concentration (or decreased pH) reduces haemoglobin's affinity for oxygen, enhancing unloading in respiring tissues.
氧解离曲线表示氧分压与血红蛋白氧饱和度的关系，呈S形，反映协同结合。波尔效应指出，二氧化碳浓度升高（或pH降低）会降低血红蛋白对氧的亲和力，促进在呼吸组织处释放氧气。
9. Gas Exchange | 气体交换
In humans, the gas exchange system includes the nasal passages, trachea, bronchi, bronchioles, and alveoli. The alveoli are the primary site of gas exchange, surrounded by an extensive capillary network. Their walls are extremely thin, moist, and have a large total surface area, enabling rapid diffusion of O2 and CO2.
人类气体交换系统包括鼻腔、气管、支气管、细支气管和肺泡。肺泡是气体交换的主要场所，由丰富的毛细血管网包绕。肺泡壁极薄、湿润且总表面积巨大，有利于氧气和二氧化碳的快速扩散。
Ventilation is the process of moving air in and out of the lungs. Inspiration is an active process: the diaphragm contracts and flattens, and the external intercostal muscles raise the ribcage, increasing thoracic volume and lowering pressure. Expiration at rest is passive, relying on elastic recoil.
通气指空气进出肺的过程。吸气为主动过程：膈肌收缩变平，外肋间肌上提肋骨，胸腔体积增大，压力降低。静息呼气为被动过程，依赖于弹性回缩。
Tidal volume, vital capacity, and breathing rate are measured using a spirometer. Gas exchange in other organisms, such as fish gills (countercurrent flow), also demonstrates adaptations maximizing diffusion gradients.
潮气量、肺活量和呼吸频率可用肺量计测定。其他生物如鱼鳃的逆流交换系统，也体现了最大化扩散梯度的适应性结构。
10. Infectious Diseases and Immunity | 传染病与免疫
Infectious diseases are caused by pathogens, including bacteria, viruses, fungi, and protoctists. Examples covered in Cambridge AS are cholera (Vibrio cholerae), tuberculosis (Mycobacterium tuberculosis), malaria (Plasmodium spp.), and HIV/AIDS. Transmission can occur via contaminated water, airborne droplets, vectors, or bodily fluids.
传染病由病原体引起，包括细菌、病毒、真菌和原生生物。AS课程涉及的典型疾病有霍乱、结核病、疟疾和艾滋病。传播途径包括污染水源、飞沫、媒介生物和体液等。
The immune system defends the body through non-specific and specific mechanisms. Phagocytes, such as neutrophils and macrophages, engulf pathogens by phagocytosis. The specific immune response involves lymphocytes: B cells produce antibodies, while T cells assist or directly kill infected cells.
免疫系统通过非特异性和特异性机制保护机体。吞噬细胞（中性粒细胞和巨噬细胞）通过吞噬作用消灭病原体。特异性免疫应答涉及淋巴细胞：B细胞产生抗体，T细胞辅助或直接杀伤受感染细胞。
Antibodies are Y-shaped proteins that bind to specific antigens, causing agglutination and marking pathogens for destruction. Memory cells generated after primary infection ensure a rapid, stronger secondary response. Vaccination exploits this principle by introducing harmless antigens, conferring active artificial immunity.
抗体呈Y形，与特定抗原结合导致凝集，并标记病原体以便消灭。初次感染后产生的记忆细胞保证了二次应答更快速、更强烈。疫苗接种正是利用这一原理，通过引入无害抗原使人获得人工主动免疫。
Antibiotics, such as penicillin, target bacterial cell wall synthesis, but resistance can develop through natural selection. Universal precautions and public health measures help control the spread of disease.
青霉素等抗生素通过破坏细菌细胞壁合成发挥作用，但自然选择会导致耐药性产生。普遍预防措施和公共卫生干预有助于控制疾病传播。
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