📚 Year 12 OCR Biology: Core Concepts Review | Year 12 OCR 生物：核心知识点梳理
Year 12 OCR Biology builds the essential foundation for the full A Level, covering Modules 1 to 4 of the OCR A specification. This resource distils the key concepts for effective revision, linking molecular detail to whole-organism biology.
Year 12 OCR 生物为完整 A Level 课程打下关键基础，涵盖 OCR A 规格的模块 1 至模块 4。本文提炼核心知识点，帮助大家高效复习，将分子细节与整体生物功能联系起来。
1. Cell Structure and Microscopy | 细胞结构与显微镜技术
Eukaryotic cells contain a membrane-bound nucleus and organelles such as mitochondria (site of aerobic respiration), rough endoplasmic reticulum (protein synthesis and transport), and Golgi apparatus (modifying and packaging proteins).
真核细胞含有膜包裹的细胞核和细胞器，如线粒体（有氧呼吸场所）、粗面内质网（蛋白质合成与运输）和高尔基体（修饰与包装蛋白质）。
Prokaryotic cells lack a nucleus and membrane-bound organelles; their circular DNA lies free in the nucleoid region, and they possess 70S ribosomes. Bacterial cell walls contain peptidoglycan.
原核细胞没有细胞核和膜结合细胞器；环状 DNA 游离于拟核区，并具有 70S 核糖体。细菌细胞壁含有肽聚糖。
Viruses are acellular and consist of genetic material (DNA or RNA) surrounded by a protein capsid; they rely on host cells for replication.
病毒是非细胞结构，由遗传物质（DNA 或 RNA）和蛋白质衣壳组成，依赖宿主细胞进行复制。
Light microscopes use visible light and lenses to magnify up to about ×1500; electron microscopes (TEM and SEM) use beams of electrons, offering much higher resolution and revealing ultrastructure.
光学显微镜利用可见光和透镜放大，最高约 1500 倍；电子显微镜（透射电镜和扫描电镜）用电子束获得更高分辨率，揭示超微结构。
2. Biological Molecules | 生物分子
Carbohydrates include monosaccharides such as α-glucose and β-glucose, fructose and ribose. Disaccharides (maltose, sucrose, lactose) form via condensation reactions, and polysaccharides (starch, glycogen, cellulose) serve as energy stores or structural components.
碳水化合物包括单糖，如 α–葡萄糖与 β–葡萄糖、果糖和核糖。二糖（麦芽糖、蔗糖、乳糖）通过缩合反应形成，多糖（淀粉、糖原、纤维素）作为能量储存或结构组分。
Lipids: triglycerides consist of one glycerol and three fatty acids joined by ester bonds; phospholipids have two fatty acids and a phosphate group, making them amphipathic – essential for membrane bilayer formation. Cholesterol regulates membrane fluidity.
脂质：甘油三酯由一分子甘油和三分子脂肪酸通过酯键连接；磷脂含两条脂肪酸和一个磷酸基团，具有两亲性，是膜双分子层的基础。胆固醇调节膜的流动性。
Proteins are polymers of amino acids linked by peptide bonds. The primary structure is the sequence of amino acids; secondary structure includes α-helices and β-pleated sheets; tertiary structure is the overall 3D folding held by hydrogen, ionic, disulfide bonds and hydrophobic interactions; quaternary structure involves multiple polypeptide chains, as in haemoglobin.
蛋白质是由氨基酸经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；三级结构是整体三维折叠，由氢键、离子键、二硫键和疏水作用维持；四级结构涉及多条多肽链，如血红蛋白。
Nucleic acids: DNA is a double helix of nucleotides (deoxyribose, phosphate, and bases A, T, C, G) held by hydrogen bonds between complementary base pairs. RNA contains ribose and uracil instead of thymine, and is single-stranded. ATP is an energy currency with three phosphate groups.
核酸：DNA 是脱氧核苷酸的双螺旋（脱氧核糖、磷酸和碱基 A、T、C、G），互补碱基对之间以氢键连接。RNA 含核糖和尿嘧啶而非胸腺嘧啶，通常为单链。ATP 是三磷酸腺苷，是细胞的能量通货。
3. Enzyme Action and Regulation | 酶的作用与调控
Enzymes are globular proteins that act as biological catalysts, lowering activation energy. The induced-fit model states that the active site changes shape slightly as the substrate binds, putting strain on bonds and facilitating reaction.
酶是球状蛋白质，作为生物催化剂降低活化能。诱导契合模型认为，底物结合时活性部位形状发生轻微改变，使化学键产生应力，从而促进反应。
Temperature and pH affect enzyme activity by altering the protein’s tertiary structure. Each enzyme has an optimal temperature and pH; denaturation occurs at extremes. Substrate concentration increases rate up to a maximum (Vmax) when active sites become saturated.
温度和 pH 通过改变蛋白质的三级结构影响酶活性。每种酶都有最适温度和 pH；极端条件下酶会变性。底物浓度增加可使反应速率上升，直到活性部位饱和达到最大速率（Vmax）。
Competitive inhibitors resemble the substrate and bind to the active site, which can be overcome by increasing substrate concentration. Non-competitive inhibitors bind elsewhere, changing the active site’s shape and reducing Vmax permanently.
竞争性抑制剂与底物结构相似，结合于活性部位，可通过增加底物浓度来克服。非竞争性抑制剂结合于其他部位，使活性部位变形，从而永久降低 Vmax。
Q₁₀ = rate at (T+10)°C / rate at T°C
温度系数 Q₁₀ 用于衡量温度每升高 10 °C 反应速率的变化。
4. Cell Membranes and Transport | 细胞膜与运输
The fluid mosaic model describes the membrane as a phospholipid bilayer with embedded proteins, cholesterol and glycoproteins. Phospholipids can move laterally, providing fluidity, while proteins are scattered like a mosaic.
流动镶嵌模型将膜描述为磷脂双分子层，其中嵌入蛋白质、胆固醇和糖蛋白。磷脂可以横向移动，赋予膜流动性；蛋白质则像马赛克一样分布。
Passive transport includes simple diffusion (movement of small, non-polar molecules down a concentration gradient), facilitated diffusion via channel or carrier proteins, and osmosis – the net movement of water from a higher to a lower water potential across a partially permeable membrane. Water potential (ψ) = solute potential (ψₛ) + pressure potential (ψₚ).
被动运输包括简单扩散（小分子、非极性物质顺浓度梯度移动）、通过通道蛋白或载体蛋白的易化扩散，以及渗透——水通过半透膜从高水势向低水势移动。水势 ψ = 溶质势 ψₛ + 压力势 ψₚ。
Active transport uses carrier proteins and ATP to move substances against their concentration gradient. Bulk transport involves endocytosis (engulfing materials) and exocytosis (secreting materials via vesicles).
主动运输利用载体蛋白和 ATP 将物质逆浓度梯度转运。批量运输包括胞吞（吞噬物质）和胞吐（通过囊泡分泌物质）。
5. Cell Division and DNA Replication | 细胞分裂与 DNA 复制
The cell cycle consists of interphase (G₁, S, G₂) and mitosis. During interphase, protein synthesis and DNA replication occur. DNA replication is semi-conservative: the double helix unwinds, each strand acts as a template, and DNA polymerase adds complementary nucleotides in the 5′ to 3′ direction. Meselson and Stahl’s experiment proved this model.
细胞周期包括间期（G₁、S、G₂）和有丝分裂。间期进行蛋白质合成和 DNA 复制。DNA 复制是半保留的：双螺旋解旋，每条链作为模板，DNA 聚合酶沿 5′ 到 3′ 方向添加互补核苷酸。Meselson 和 Stahl 的实验证实了这一模型。
Mitosis is divided into prophase (chromosomes condense, spindle forms), metaphase (chromosomes align at equator), anaphase (sister chromatids separate) and telophase (nuclear envelopes reform). Cytokinesis follows, splitting the cytoplasm. Mitosis produces two genetically identical diploid cells, used for growth, repair and asexual reproduction.
有丝分裂分为前期（染色体凝集、纺锤体形成）、中期（染色体排列在赤道板）、后期（姐妹染色单体分离）和末期（核膜重新形成）。随后胞质分裂将细胞质分开。有丝分裂产生两个遗传相同的二倍体细胞，用于生长、修复和无性生殖。
6. Exchange Surfaces and Gas Exchange | 交换表面与气体交换
Organisms need specialised exchange surfaces to maximise diffusion efficiency. Features include a large surface area, thin surface, good blood supply (or ventilation) and a steep concentration gradient – summarised by Fick’s law: rate of diffusion ∝ (surface area × concentration difference) / diffusion distance.
生物体需要特化的交换表面以最大化扩散效率。特征包括大面积、薄表面、良好的血液供应（或通风）以及陡峭的浓度梯度，可用菲克定律概括：扩散速率 ∝ (表面积 × 浓度差) / 扩散距离。
In mammals, ventilation brings air into the alveoli. The ribcage and diaphragm change thoracic volume; inhalation is active, exhalation largely passive. Alveoli provide a large surface area, are one cell thick and are surrounded by capillaries, ensuring rapid O₂ and CO₂ exchange.
哺乳动物通过通气将空气带入肺泡。肋骨和膈肌改变胸腔容积；吸气是主动过程，呼气大多是被动的。肺泡提供巨大的表面积，仅一个细胞厚，并被毛细血管包绕，确保快速的 O₂ 和 CO₂ 交换。
Fish gills use a countercurrent flow system where blood and water flow in opposite directions, maintaining a diffusion gradient along the entire lamella. Insects have a tracheal system delivering air directly to tissues via spiracles and tracheoles, using diffusion and abdominal ventilation.
鱼鳃采用逆流交换系统，血液与水流动方向相反，沿整个鳃板维持扩散梯度。昆虫的气管系统通过气门和微气管将空气直接输送至组织，依靠扩散和腹部通风实现气体交换。
7. Animal Transport Systems | 动物运输系统
Mammals have a closed, double circulatory system: the right side pumps deoxygenated blood to the lungs (pulmonary circulation), the left side pumps oxygenated blood to the body (systemic circulation).
哺乳动物具有封闭式双循环系统：右心将脱氧血泵至肺部（肺循环），左心将含氧血泵向全身（体循环）。
The heart consists of four chambers: atria receive blood, ventricles pump blood out. Atrioventricular valves (tricuspid and bicuspid) and semilunar valves prevent backflow. The cardiac cycle involves coordinated contraction (systole) and relaxation (diastole), initiated by the sinoatrial node acting as the pacemaker.
心脏由四个腔室组成：心房接收血液，心室泵出血液。房室瓣（三尖瓣和二尖瓣）以及半月瓣防止血液回流。心动周期包括协调的收缩（心缩期）和舒张（心舒期），由起搏点窦房结启动。
Haemoglobin in red blood cells binds oxygen cooperatively, producing an S-shaped oxygen dissociation curve. The Bohr effect shifts the curve to the right when CO₂ is high (e.g. in respiring tissues), promoting O₂ release. Foetal haemoglobin has a higher affinity for O₂, enabling oxygen transfer across the placenta.
红细胞中的血红蛋白协同结合氧，产生 S 形氧解离曲线。波尔效应指高 CO₂ 时曲线右移（例如在呼吸组织中），促进 O₂ 释放。胎儿血红蛋白对氧的亲和力更高，有利于氧气通过胎盘转移。
8. Transport in Plants | 植物运输
Xylem tissue transports water and mineral ions from roots to leaves. The cohesion-tension theory explains this: evaporation from mesophyll cells lowers water potential, pulling water into the leaf apoplast; cohesion between water molecules and adhesion to xylem walls maintain an unbroken transpiration stream under tension.
木质部组织将水和矿物质离子从根部运输到叶片。内聚力-张力理论解释了蒸腾流：叶肉细胞蒸发降低水势，将水拉进叶片质外体；水分子之间的内聚力以及对木质部壁的黏附力维持了在张力下连续的水柱。
Phloem translocates sucrose and amino acids from sources (e.g. leaves) to sinks (e.g. roots, developing fruits) via the pressure-flow hypothesis. At the source, sucrose is actively loaded into sieve tubes, decreasing water potential; water enters by osmosis, creating high hydrostatic pressure that pushes sap toward areas of lower pressure at the sink, where sucrose is unloaded.
韧皮部通过压力流动假说将蔗糖和氨基酸从源（如叶片）运输到库（如根、发育中的果实）。在源端，蔗糖被主动装载进入筛管，降低水势；水通过渗透进入，产生高静水压，推动汁液流向库端低压区，蔗糖随即被卸载。
Water and solutes move through the apoplast and symplast pathways. The Casparian strip in the endodermis blocks apoplastic flow, forcing water and minerals to cross the plasma membrane, enabling selective uptake.
水和溶质沿质外体和共质体途径移动。内皮层的凯氏带阻断质外体通道，迫使水和矿物质穿过细胞膜，实现选择性吸收。
9. Pathogens and the Immune System | 病原体与免疫系统
Pathogens include bacteria (e.g. TB), viruses (e.g. HIV, influenza), fungi (e.g. athlete’s foot) and protists (e.g. malaria). They can be transmitted via direct contact, airborne droplets, vectors, contaminated food or water.
病原体包括细菌（如结核杆菌）、病毒（如 HIV、流感病毒）、真菌（如脚癣真菌）和原生生物（如疟原虫）。传播途径有直接接触、飞沫、媒介生物、污染的食物或水等。
Non-specific defences include physical barriers (skin, mucus), phagocytosis (neutrophils and macrophages engulf and digest pathogens) and inflammation. Specific immunity involves B lymphocytes producing antibodies (humoral response) and T lymphocytes that destroy infected cells (cell-mediated response). Memory cells enable a faster, stronger secondary response, which is the basis of vaccination.
非特异性防御包括物理屏障（皮肤、粘液）、吞噬作用（中性粒细胞和巨噬细胞吞噬并消化病原体）和炎症反应。特异性免疫涉及 B 淋巴细胞产生抗体（体液免疫）和 T 淋巴细胞杀伤感染细胞（细胞免疫）。记忆细胞使得二次应答更快更强，是疫苗接种的基础。
Antibodies are Y-shaped proteins with variable regions that bind specific antigens, neutralising toxins or marking pathogens for destruction. Antibiotic resistance evolves when a random mutation confers resistance, and natural selection increases the frequency of resistant bacteria.
抗体是 Y 形蛋白质，其可变区能结合特定抗原，从而中和毒素或标记病原体供摧毁。当随机突变赋予细菌耐药性时，自然选择便会增加耐药菌的频率。
10. Biodiversity and Classification | 生物多样性与分类
Biodiversity can be measured in terms of species richness (number of different species) and species evenness (relative abundance). Simpson’s index of diversity (D) accounts for both:
生物多样性可通过物种丰富度（不同物种数）和物种均匀度（相对丰度）衡量。辛普森多样性指数（D）兼顾了两者：
D = 1 − ( Σ n(n−1) ) / ( N(N−1) )
where N = total number of organisms, n = number of individuals of each species. A high value indicates high diversity.
其中 N 为个体总数，n 为某一物种的个体数。数值越高表示多样性越高。
Classification organises living things into hierarchical groups: domain, kingdom, phylum, class, order, family, genus, species. The three-domain system (Archaea, Bacteria, Eukarya) reflects evolutionary relationships based on ribosomal RNA analysis. Binomial nomenclature gives each species a two-part Latin name (e.g. Homo sapiens).
分类将生物按等级排列：域、界、门、纲、目、科、属、种。三域系统（古菌域、细菌域、真核域）根据核糖体 RNA 分析反映了进化关系。双名法赋予每个物种一个两部分的拉丁学名（如 Homo sapiens）。
11. Evolution by Natural Selection | 自然选择与进化
Natural selection requires genetic variation, overproduction of offspring and a struggle for existence. Individuals with advantageous alleles are more likely to survive and reproduce, passing these alleles on. Over time, the frequency of these beneficial alleles increases in the population.
自然选择需要遗传变异、过度繁殖和生存斗争。带有有利等位基因的个体更可能生存并繁殖，将这些等位基因传下。随时间推移，种群中这些有利等位基因的频率上升。
Speciation occurs when populations become reproductively isolated. Allopatric speciation involves geographical isolation; sympatric speciation occurs without geographical barriers, often through behavioural or temporal changes. Reproductive isolation can be prezygotic or postzygotic.
当种群间出现生殖隔离时便形成物种。异域物种形成涉及地理隔离；同域物种形成则无地理障碍，常由行为或时间变化引起。生殖隔离可以是合子前的或合子后的。
Antibiotic resistance in bacteria is a clear example of evolution by natural selection. Random mutations confer resistance; when antibiotics are used, susceptible bacteria die while
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