📚 AS OCR Biology: Essential Topics Overview | AS OCR 生物：核心知识点梳理
Revision for AS OCR Biology requires a firm grasp of the foundational topics, from cell structure and biological molecules to exchange, transport and disease. This article summarises the core concepts across Modules 2–4, supporting your exam preparation and reinforcing key links between topics.
AS OCR 生物学复习需要牢牢掌握从细胞结构和生物分子到物质交换、运输和疾病的基础内容。本文梳理模块二至模块四的核心知识点，帮助你备考并加强对各主题之间关键联系的理解。
1. Cell Structure and Microscopy | 细胞结构与显微镜
The cell theory states that all living organisms are composed of cells, cells are the basic unit of life, and all cells arise from pre-existing cells. Eukaryotic cells possess a true nucleus and membrane-bound organelles, whereas prokaryotic cells lack a nucleus and contain smaller 70S ribosomes, with DNA free in the cytoplasm.
细胞学说指出，所有生物体都由细胞构成，细胞是生命的基本单位，并且所有细胞都来自已存在的细胞。真核细胞具有真正的细胞核和膜结合的细胞器，而原核细胞没有核，DNA 游离在细胞质中，且含有更小的 70S 核糖体。
Key organelles include the nucleus (stores genetic information), mitochondria (site of ATP synthesis), rough and smooth endoplasmic reticulum (protein synthesis and lipid metabolism), Golgi apparatus (modifies and packages proteins), ribosomes (translation), lysosomes (digestion) and, in plants, chloroplasts (photosynthesis), a large permanent vacuole and a cellulose cell wall.
关键细胞器包括：细胞核（储存遗传信息）、线粒体（ATP 合成场所）、粗面和滑面内质网（蛋白质合成与脂质代谢）、高尔基体（修饰和分装蛋白质）、核糖体（翻译）、溶酶体（消化作用）；植物细胞还含有叶绿体（光合作用）、大型中央液泡和纤维素细胞壁。
Microscopy techniques differ in magnification and resolution. Light microscopes have a resolution of about 200 nm and can show stained whole cells, whereas electron microscopes offer resolutions as fine as 0.2 nm, revealing ultrastructure but requiring dead, fixed specimens.
显微技术因放大率和分辨率而不同。光学显微镜的分辨率约为 200 nm，可观察染色的完整细胞；电子显微镜的分辨率可达 0.2 nm，能显示超微结构，但需要死亡、固定的样品。
| Feature | Light Microscope | Transmission Electron Microscope (TEM) |
|---|---|---|
| Resolution | ~200 nm | ~0.2 nm |
| Magnification | Up to ×1500 | Over ×500 000 |
| Specimen | Living or dead | Dead, fixed, ultrathin |
2. Biological Molecules | 生物分子
Carbohydrates are composed of monosaccharides such as glucose (C₆H₁₂O₆) and exist as monomers or polymers. Disaccharides like maltose, sucrose and lactose are formed by glycosidic bonds. Polysaccharides serve as energy stores (starch in plants, glycogen in animals) or structural components (cellulose in plant cell walls).
碳水化合物由单糖如葡萄糖（C₆H₁₂O₆）组成，以单体或聚合物的形式存在。麦芽糖、蔗糖和乳糖等二糖通过糖苷键形成。多糖既可作能量储存（植物的淀粉、动物的糖原），也可作结构成分（植物细胞壁的纤维素）。
Lipids, mainly triglycerides, consist of one glycerol molecule bonded to three fatty acids by ester bonds. Saturated fatty acids have no double bonds and pack closely, making fats solid at room temperature; unsaturated fatty acids contain double bonds, introducing kinks that lower melting points.
脂质主要是甘油三酯，由一个甘油分子通过酯键与三个脂肪酸相连。饱和脂肪酸无双键，分子排列紧密，常温为固体；不饱和脂肪酸含有双键，导致碳链弯曲，熔点较低。
Proteins are polymers of amino acids linked by peptide bonds. The sequence of amino acids forms the primary structure. Secondary structures (α‑helix, β‑pleated sheet) are stabilised by hydrogen bonds, tertiary structure is the overall 3D fold maintained by hydrogen, ionic, hydrophobic interactions and disulfide bridges, and quaternary structure involves multiple polypeptide subunits.
蛋白质是由氨基酸通过肽键连接而成的聚合物。氨基酸序列形成一级结构；二级结构（α‑螺旋、β‑折叠）由氢键维持；三级结构是整体的三维折叠，由氢键、离子键、疏水作用和二硫键稳定；四级结构包含多条多肽链的组合。
Key biochemical tests: reducing sugars (Benedict’s reagent turns brick‑red upon heating), non‑reducing sugars (hydrolyse then test with Benedict’s), starch (iodine solution turns blue‑black), proteins (Biuret test – violet colour) and lipids (ethanol emulsion test – white emulsion).
重要生化检测：还原糖（本尼迪克特试剂加热后变砖红色）、非还原糖（先水解再用本尼迪克特检测）、淀粉（碘液变蓝黑色）、蛋白质（双缩脲试验呈紫色）和脂质（乙醇乳化试验产生白色乳浊液）。
3. Nucleotides and Nucleic Acids | 核苷酸与核酸
A nucleotide consists of a pentose sugar, a phosphate group and a nitrogenous base. DNA contains deoxyribose and the bases adenine (A), thymine (T), cytosine (C), guanine (G); RNA contains ribose and uracil (U) instead of thymine. ATP (adenosine triphosphate) is a nucleotide derivative that acts as the energy currency of cells.
核苷酸由戊糖、磷酸基团和含氮碱基组成。DNA 含脱氧核糖与碱基腺嘌呤（A）、胸腺嘧啶（T）、胞嘧啶（C）、鸟嘌呤（G）；RNA 含核糖，尿嘧啶（U）取代胸腺嘧啶。ATP（腺苷三磷酸）是核苷酸衍生物，充当细胞的能量通货。
DNA forms a double helix with two antiparallel strands held by hydrogen bonds between complementary base pairs (A‑T and C‑G). RNA is typically single‑stranded. DNA replication is semi‑conservative: helicase unwinds the helix, DNA polymerase adds free nucleotides in the 5′ to 3′ direction, producing a continuous leading strand and a lagging strand built in Okazaki fragments, later joined by ligase.
DNA 形成双螺旋，两条反平行链由互补碱基对（A‑T、C‑G）之间的氢键维持。RNA 通常是单链。DNA 复制是半保留的：解旋酶解开双螺旋，DNA 聚合酶沿 5′→3′ 方向添加游离核苷酸，产生连续的前导链和不连续的后随链（冈崎片段），最后由连接酶连接。
4. Enzymes | 酶
Enzymes are globular proteins that act as biological catalysts, lowering activation energy. The induced‑fit model describes how the active site changes shape slightly to accommodate the substrate, forming an enzyme‑substrate complex and destabilising bonds.
酶是球状蛋白质，作为生物催化剂能降低活化能。诱导契合模型描述活性部位如何略微改变形状以契合底物，形成酶‑底物复合物并使化学键变得不稳定。
The rate of an enzyme‑controlled reaction is affected by temperature (increase until an optimum, then denaturation), pH (each enzyme has an optimal pH), substrate concentration (rate plateaus at Vmax) and enzyme concentration. Competitive inhibitors resemble the substrate and bind to the active site; non‑competitive inhibitors bind elsewhere, altering the active site’s shape.
酶促反应速率受温度（速率上升至最适温度后因变性而下降）、pH（每种酶有其最适 pH）、底物浓度（速率在 Vmax 时达到平台）和酶浓度的影响。竞争性抑制剂与底物相似，结合在活性部位；非竞争性抑制剂结合在其他部位，改变活性部位的形状。
5. Cell Membranes and Transport | 细胞膜与运输
The fluid‑mosaic model describes the plasma membrane as a phospholipid bilayer with embedded proteins, cholesterol (in animal cells) and glycoproteins. Phospholipids have hydrophilic heads facing outward and hydrophobic tails inward, providing a selectively permeable barrier.
流动镶嵌模型描述质膜为磷脂双分子层，嵌有蛋白质、胆固醇（动物细胞）和糖蛋白。磷脂的亲水头部朝外，疏水尾部朝内，形成选择透过性屏障。
Passive transport includes simple diffusion (small, non‑polar molecules move down a concentration gradient), facilitated diffusion via channel or carrier proteins, and osmosis – the net movement of water from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
被动运输包括简单扩散（小的非极性分子顺浓度梯度移动）、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区域净移动到低水势区域。
Active transport uses ATP to move substances against their concentration gradient via carrier proteins. Bulk transport encompasses endocytosis (taking in material by membrane infolding) and exocytosis (secreting substances via vesicle fusion).
主动运输利用 ATP 通过载体蛋白逆浓度梯度运输物质。批量运输包括内吞（质膜内陷包裹物质）和外排（囊泡与膜融合分泌物质）。
6. Cell Division: Mitosis | 细胞分裂：有丝分裂
The cell cycle consists of interphase (G₁, S, G₂) and the mitotic phase. During S phase DNA is replicated, resulting in two identical sister chromatids held together at the centromere. Mitosis ensures genetic stability by producing two genetically identical daughter cells.
细胞周期包括间期（G₁ 期、S 期、G₂ 期）和分裂期。S 期进行 DNA 复制，产生两条由着丝粒连接在一起的相同姐妹染色单体。有丝分裂产生两个遗传上完全相同的子细胞，保证遗传稳定性。
Stages of mitosis: prophase (chromosomes condense, spindle fibres form, nuclear envelope breaks down), metaphase (chromosomes align at the metaphase plate), anaphase (sister chromatids are pulled to opposite poles) and telophase (nuclear envelopes reform, chromosomes decondense). Cytokinesis then divides the cytoplasm.
有丝分裂各阶段：前期（染色体凝缩，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）和末期（核膜重新形成，染色质去凝缩）。随后胞质分裂将细胞质分开。
7. Exchange Surfaces and Gas Exchange | 交换表面与气体交换
Effective exchange surfaces have a large surface area, thin diffusion barrier, good blood supply and a steep concentration gradient maintained by ventilation or circulation. Fick’s law summarises the rate of diffusion:
高效的交换表面具备大面积、薄的扩散屏障、良好的血液供应以及通过通风或循环维持的陡峭浓度梯度。菲克定律总结扩散速率：
Rate of diffusion ∝ (Surface Area × Concentration Gradient) / Diffusion Distance
The mammalian respiratory system uses alveoli —tiny air sacs with walls one cell thick, surrounded by capillaries— for gas exchange. Inhalation contracts the diaphragm and external intercostal muscles, increasing thoracic volume; exhalation is largely passive at rest.
哺乳动物呼吸系统利用肺泡——仅一个细胞厚的薄壁微囊，被毛细血管包绕——进行气体交换。吸气时膈肌和外肋间肌收缩，胸腔容积增大；静息呼气主要是被动过程。
In fish, gills provide a countercurrent exchange system where water flows over the gill lamellae in the opposite direction to blood flow, maintaining a concentration gradient for oxygen uptake along the entire length. Insects use a tracheal system of air‑filled tubes that deliver oxygen directly to tissues.
鱼类通过鳃实现逆流交换：水流经鳃小片的方向与血液流动方向相反，沿整个长度维持氧浓度梯度。昆虫利用充满空气的气管系统将氧气直接输送到组织。
8. Transport in Animals | 动物体内运输
Mammals have a closed, double circulatory system. The heart contains four chambers: the right atrium and ventricle pump blood to the lungs (pulmonary circuit) and the left side pumps oxygenated blood to the body (systemic circuit). Atrioventricular and semilunar valves prevent backflow.
哺乳动物具有封闭的双循环系统。心脏有四个腔室：右心房和右心室将血液泵入肺部（肺循环），左侧则将含氧血泵到全身（体循环）。房室瓣和半月瓣防止血液倒流。
The cardiac cycle is coordinated by the sinoatrial node (SAN), which initiates a wave of electrical activity, causing atrial contraction; the impulse then passes to the atrioventricular node (AVN) and through the bundle of His and Purkyne fibres, triggering ventricular contraction from the apex upwards.
心动周期由窦房结（SAN）协调，它发出电信号引起心房收缩；随后冲动传至房室结（AVN），经由希氏束和浦肯野纤维传导，从心尖向上引发心室收缩。
Arteries carry blood away from the heart under high pressure and have thick muscular walls with elastic fibres. Veins return blood at low pressure and contain valves. Capillaries are one‑cell‑thick and allow exchange of substances. Tissue fluid forms when hydrostatic pressure forces fluid out of capillaries; the remaining fluid returns via lymph vessels.
动脉在高压下将血液带离心脏，管壁厚，富含弹性纤维。静脉在低压下回流血液，含有瓣膜。毛细血管仅一个细胞厚，允许物质交换。组织液在毛细血管的静水压下滤出形成，多余液体通过淋巴管回流。
Oxygen dissociation curves show how haemoglobin binds oxygen cooperatively. The Bohr effect describes a shift to the right in the presence of increased CO₂ or decreased pH, promoting oxygen release to respiring tissues. Carbon dioxide is transported as hydrogencarbonate ions, dissolved CO₂ and carbamino‑hemoglobin.
氧解离曲线显示血红蛋白协同结合氧。波尔效应描述在 CO₂ 升高或 pH 降低时曲线右移，促进氧气向呼吸组织释放。二氧化碳以碳酸氢根离子、溶解的 CO₂ 和氨基甲酸血红蛋白的形式运输。
9. Transport in Plants | 植物体内运输
Xylem vessels are dead, hollow tubes strengthened by lignin, transporting water and mineral ions from roots to leaves. The cohesion‑tension theory explains the upward pull: transpiration from leaves creates tension, and cohesive hydrogen bonds between water molecules maintain the continuous water column.
木质部导管是死的中空管，木质素加厚，将水和无机盐从根部运输到叶片。内聚力‑张力假说解释向上拉力：叶片蒸腾作用产生张力，水分子间的氢键内聚力维持连续水柱。
Transpiration rate is influenced by light intensity, temperature, humidity and wind speed. Water moves from the soil into root hairs, then travels via the apoplast (cell wall) and symplast (cytoplasm) pathways until it reaches the xylem.
蒸腾速率受光照强度、温度、湿度和风速的影响。水从土壤进入根毛，经质外体途径（细胞壁）和共质体途径（细胞质）到达木质部。
Phloem transports organic solutes (mainly sucrose) from sources to sinks. The pressure‑flow hypothesis states that active loading of sucrose at the source lowers water potential, causing water to enter; the resulting high hydrostatic pressure drives flow towards sinks where sucrose is unloaded.
韧皮部将有机溶质（主要是蔗糖）从源运输到库。压力流假说认为，源端主动装载蔗糖使水势降低，水进入筛管，产生高静水压，推动溶质流向库端并卸载。
10. Biodiversity and Classification | 生物多样性与分类
Biodiversity encompasses species diversity (number and abundance of species in a habitat), genetic diversity (variation of alleles within a species) and ecosystem diversity. Simpson’s Index of Biodiversity can quantify species diversity.
生物多样性包括物种多样性（栖息地内物种的数量和丰度）、遗传多样性（物种内等位基因的变异）和生态系统多样性。辛普森多样性指数可量化物种多样性。
Classification organises organisms into hierarchical groups: domain, kingdom, phylum, class, order, family, genus, species. The three‑domain system divides life into Archaea, Bacteria and Eukarya, reflecting fundamental differences in ribosomal RNA and cell structure.
分类将生物按等级分组：域、界、门、纲、目、科、属、种。三域系统将生命分为古菌域、细菌域和真核域，反映核糖体 RNA 和细胞结构的基本差异。
A species is defined as a group of organisms that can interbreed to produce fertile offspring. Binomial nomenclature assigns each species a two‑part Latin name (genus and species).
物种定义为可以相互交配并产生可育后代的一群生物。双名法赋予每个物种一个由属名和种名组成的拉丁学名。
11. Evolution by Natural Selection | 自然选择与进化
Evolution is the change in allele frequencies in a population over time. Darwin’s theory of natural selection states that individuals with advantageous alleles are more likely to survive, reproduce and pass on those alleles, increasing their frequency in subsequent generations.
进化是种群等位基因频率随时间的变化。达尔文的自然选择学说指出，拥有有利等位基因的个体更可能生存、繁殖并将这些等位基因传递给后代，从而在后代中增加其频率。
Selection pressures such as predation, disease and competition drive natural selection. Evidence for evolution includes fossil records, comparative anatomy, molecular phylogenetics (DNA and protein sequences, like cytochrome c) and observable examples such as antibiotic resistance in bacteria.
捕食、疾病和竞争等选择压力驱动自然选择。进化证据包括化石记录、比较解剖学、分子系统发生学（DNA 和蛋白质序列，如细胞色素 c）以及可观察的实例，如细菌的抗生素抗性。
12. Communicable Diseases and Immunity | 传染病与免疫
Communicable diseases are caused by pathogens: bacteria (e.g. tuberculosis), viruses (e.g. influenza), fungi and protoctists. Transmission can be direct (contact, droplet, sexual) or indirect (vectors, contaminated surfaces, water).
传染病由病原体引起：细菌（如结核病）、病毒（如流感）、真菌和原生生物。传播方式可以是直接传播（接触、飞沫、性传播）或间接传播（媒介、污染表面、水源）。
The body’s first line of defence includes physical barriers (skin, mucous membranes) and chemical secretions. The second line comprises phagocytosis and the inflammatory response. The specific immune response involves T‑lymphocytes (cell‑mediated immunity) and B‑lymphocytes (humoral immunity), producing memory cells that enable a faster secondary response.
机体的第一道防线包括物理屏障（皮肤、黏膜）和化学分泌物。第二道防线包括吞噬作用和炎症反应。特异性免疫应答涉及 T 淋巴细胞（细胞免疫）和 B 淋巴细胞（体液免疫），产生记忆细胞以形成更快的二次应答。
Vaccination introduces antigens in a harmless form, stimulating the production of memory cells without causing disease. Herd immunity protects vulnerable individuals when a high proportion of the population is immune. Antibiotics inhibit bacterial growth but are ineffective against viruses.
疫苗接种以无害形式引入抗原，刺激记忆细胞产生而不致病。当大部分人口免疫时，群体免疫可保护易感个体。抗生素抑制细菌生长但对病毒无效。
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