📚 AS Cambridge Biology: Core Knowledge Review | AS 剑桥生物：核心知识点梳理
Mastering the core concepts of AS Cambridge Biology builds a strong foundation for the full A Level. This article guides you through the essential topics, from cell structure to immunity, with concise explanations that link key ideas together. Use it as a checklist and quick revision aid.
掌握 AS 剑桥生物的核心概念能为完整的 A Level 课程打下坚实基础。本文带你梳理从细胞结构到免疫学的必考主题，用简明的解释串联关键思想。可当作核对清单和快速复习指南来使用。
1. Cell Structure | 细胞结构
All living organisms are made of cells, which can be studied using light microscopes and electron microscopes (TEM and SEM). Magnification tells us how much larger the image appears; resolution describes the ability to distinguish two close points as separate. Electron microscopes have much higher resolution than light microscopes, allowing ultrastructure to be seen.
所有生物都由细胞构成，可用光学显微镜和电子显微镜（透射电镜 TEM、扫描电镜 SEM）进行观察。放大率指图像比实物放大的倍数，分辨率指区分两个相邻点的能力。电子显微镜的分辨率远高于光学显微镜，因此能看清细胞的超微结构。
Magnification = Image size ÷ Actual size
放大率 = 图像尺寸 ÷ 实际尺寸
Eukaryotic cells (plants, animals, fungi) contain a true nucleus and membrane-bound organelles. Prokaryotic cells (bacteria) lack a nucleus; their DNA floats freely in the cytoplasm. Key eukaryotic organelles and their functions: nucleus (stores genetic information), rough endoplasmic reticulum (protein synthesis and transport), smooth ER (lipid synthesis), Golgi apparatus (modifies, sorts, and packages proteins), mitochondria (site of aerobic respiration), chloroplasts (photosynthesis), lysosomes (digestion of waste), and ribosomes (protein synthesis, also found in prokaryotes). Plant cells additionally have a cellulose cell wall, large central vacuole, and plasmodesmata.
真核细胞（植物、动物、真菌）含有真正的细胞核和膜包被的细胞器。原核细胞（细菌）没有细胞核，DNA 游离在细胞质中。关键的真核细胞器及其功能：细胞核（储存遗传信息）、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、分拣和包装蛋白质）、线粒体（有氧呼吸的场所）、叶绿体（光合作用）、溶酶体（废物消化）以及核糖体（合成蛋白质，原核生物也有）。植物细胞还具备纤维素细胞壁、大中央液泡和胞间连丝。
2. Biological Molecules | 生物分子
Large biological molecules (polymers) are built from smaller repeating units (monomers). Carbohydrates are made from monosaccharides (e.g. glucose, fructose). Two monosaccharides join via a glycosidic bond in a condensation reaction to form disaccharides like maltose and sucrose. Polysaccharides such as starch, glycogen, and cellulose are long chains of glucose. Starch is a plant storage polysaccharide; glycogen is the animal equivalent; cellulose is a structural component of plant cell walls.
生物大分子（聚合物）由较小的重复单元（单体）构成。碳水化合物由单糖（如葡萄糖、果糖）组成。两个单糖通过糖苷键缩合反应形成二糖，例如麦芽糖、蔗糖。淀粉、糖原和纤维素等都属于由葡萄糖长链组成的多糖。淀粉是植物的储存多糖；糖原是动物的对应物；纤维素是植物细胞壁的结构成分。
Lipids (triglycerides) are formed from one glycerol molecule and three fatty acids by ester bonds. They are hydrophobic, serve as long-term energy stores, provide thermal insulation, and protect organs. Phospholipids have a phosphate group instead of one fatty acid, making them amphipathic and ideal for forming cell membranes. Proteins are polymers of amino acids linked by peptide bonds. A protein's primary structure is the sequence of amino acids; secondary structure includes α-helices and β-pleated sheets formed by hydrogen bonds; tertiary structure is the overall 3D folding held by hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions; quaternary structure involves multiple polypeptide chains.
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而成。它们呈疏水性，作为长期能量储备、提供保温并保护器官。磷脂是用一个磷酸基团替代其中一个脂肪酸，具有两亲性，非常适合构成细胞膜。蛋白质是氨基酸通过肽键连接而成的聚合物。蛋白质的一级结构是氨基酸序列；二级结构包括由氢键形成的 α-螺旋和 β-折叠；三级结构是整条肽链的三维折叠，由氢键、离子键、二硫键和疏水相互作用维持；四级结构涉及多条多肽链的组合。
Testing for biological molecules: reducing sugars give a brick-red precipitate with Benedict's reagent upon heating; starch turns iodine solution blue-black; proteins produce a violet colour with the biuret test; lipids form a cloudy white emulsion when shaken with ethanol and water.
生物分子的检测：还原糖与本尼迪克特试剂共热产生砖红色沉淀；淀粉遇碘液变蓝黑色；蛋白质与双缩脲试剂反应呈紫色；脂质与乙醇和水振荡后形成乳白色浊液。
3. Enzymes | 酶
Enzymes are globular proteins that act as biological catalysts, lowering activation energy. The lock-and-key model states that the substrate fits exactly into the active site. The induced-fit model extends this by proposing that the active site changes shape slightly to accommodate the substrate, placing strain on bonds and facilitating the reaction.
酶是球状蛋白质，作为生物催化剂降低活化能。锁钥模型认为底物精准地嵌入活性部位。诱导契合模型进一步发展，提出活性部位会略微改变形状以包裹底物，对化学键施加张力从而促进反应。
The rate of an enzyme-catalysed reaction is affected by temperature, pH, enzyme concentration, and substrate concentration. As temperature rises, kinetic energy increases and more successful collisions occur, up to an optimum. Beyond this, the enzyme denatures: its tertiary structure is disrupted and the active site loses its shape. pH deviations from the optimum alter the charges on amino acids, breaking ionic and hydrogen bonds, also leading to denaturation. Increasing substrate concentration increases rate until all active sites are occupied; the enzyme becomes saturated and reaches Vmax. Increasing enzyme concentration increases rate proportionally, provided substrate is in excess.
酶促反应速率受温度、pH、酶浓度和底物浓度影响。温度升高时，动能增加，有效碰撞增多，直至达到最适温度。超过最适温度，酶变性：三级结构被破坏，活性部位变形。pH 偏离最适值会改变氨基酸上的电荷，破坏离子键和氢键，同样导致变性。提高底物浓度可以加快反应速率，直至所有活性部位被占据；酶达到饱和并到达最大速率 Vmax。提高酶浓度则按比例增加速率，前提是底物过量。
Inhibitors can slow reactions. Competitive inhibitors resemble the substrate and bind to the active site, but can be overcome by increasing substrate concentration. Non-competitive inhibitors bind to an allosteric site, altering the enzyme's shape regardless of how much substrate is present. This reduces Vmax without changing Km (in competitive inhibition Km increases, Vmax unchanged).
抑制剂会减慢反应。竞争性抑制剂与底物结构相似，结合到活性部位，但可通过增加底物浓度来克服。非竞争性抑制剂与别构部位结合，改变酶的形状，无论底物浓度多高都会影响活性。这降低了 Vmax（在竞争性抑制中 Km 增大而 Vmax 不变）。
4. Cell Membranes and Transport | 细胞膜与运输
The cell membrane is described by the fluid mosaic model: a phospholipid bilayer with proteins floating like icebergs. Phospholipids have hydrophilic phosphate heads and hydrophobic fatty acid tails, creating a selectively permeable barrier. Intrinsic proteins span the membrane; extrinsic proteins are on the surface. Cholesterol, present in animal membranes, regulates fluidity and stability. Glycoproteins and glycolipids serve as receptors and recognition sites.
细胞膜可用流动镶嵌模型描述：磷脂双分子层中蛋白质如冰山般漂浮。磷脂具有亲水的磷酸头与疏水的脂肪酸尾，形成选择透过性屏障。内在蛋白贯穿膜层，外在蛋白位于表面。动物细胞膜中的胆固醇调节流动性和稳定性。糖蛋白和糖脂充当受体和识别位点。
Substances cross membranes through several mechanisms. Simple diffusion is the net movement of small, non-polar molecules down a concentration gradient, through the bilayer, requiring no energy. Facilitated diffusion uses channel or carrier proteins to move polar or charged particles down their gradient, also passive. Osmosis is the diffusion of water molecules across a partially permeable membrane from a region of higher water potential (less negative) to lower water potential. Water potential is affected by solute and pressure; pure water has a water potential of zero.
物质跨膜运输有多种机制。简单扩散是小分子、非极性物质沿浓度梯度穿越双分子层的净移动，不需能量。易化扩散借助通道蛋白或载体蛋白让极性或带电粒子顺浓度梯度通过，也属被动运输。渗透是水分子经部分通透膜从水势较高（负值较小）的区域向水势较低的区域扩散。水势受溶质和压力影响；纯水的水势为零。
Active transport uses carrier proteins to pump molecules against their concentration gradient, using energy from ATP. Bulk transport includes endocytosis (taking in materials by membrane infolding) and exocytosis (secreting materials via vesicle fusion). These processes allow cells to maintain internal conditions different from the surroundings.
主动运输利用载体蛋白并消耗 ATP 提供的能量，逆浓度梯度泵出分子。批量运输包括内吞（细胞膜内陷包裹物质）和外排（囊泡与膜融合分泌物质）。这些过程使细胞能维持与外界不同的内部环境。
5. The Mitotic Cell Cycle | 有丝分裂细胞周期
The cell cycle consists of interphase (G₁, S, G₂) and mitotic phase (mitosis and cytokinesis). During S phase, DNA replicates, producing two identical sister chromatids held together at the centromere. G₁ and G₂ are periods of growth and organelle synthesis. Mitosis ensures that each daughter nucleus receives an exact copy of the genetic material, and is followed by cytoplasmic division (cytokinesis).
细胞周期包括间期（G₁ 期、S 期、G₂ 期）和分裂期（有丝分裂与胞质分裂）。S 期 DNA 复制，产生两条相同的姐妹染色单体，由着丝粒连接。G₁ 和 G₂ 期用于生长和细胞器合成。有丝分裂确保每个子细胞核获得与母细胞完全相同的遗传物质，分裂后紧接胞质分裂。
Mitosis is divided into four stages: prophase (chromosomes condense, nuclear envelope breaks down, spindle fibres form), metaphase (chromosomes align at the equator, spindle fibres attach to centromeres), anaphase (sister chromatids separate to opposite poles, pulled by shortening spindle fibres), and telophase (nuclear envelopes re-form, chromosomes decondense). In animal cells, cytokinesis involves a cleavage furrow; in plant cells, a cell plate forms.
有丝分裂分为四个阶段：前期（染色体凝集，核膜解体，纺锤体形成），中期（染色体排列在赤道板，纺锤丝连接着丝粒），后期（姐妹染色单体分开移向两极，由纺锤丝缩短牵引），末期（核膜重新形成，染色体解旋）。动物细胞的胞质分裂产生分裂沟，植物细胞则形成细胞板。
Mitosis is vital for growth, repair of tissues, and asexual reproduction. Uncontrolled mitosis can lead to cancer. Cancer cells divide rapidly and may form tumours; many treatments target rapidly dividing cells.
有丝分裂对于生长、组织修复和无性繁殖至关重要。有丝分裂失控可能导致癌症。癌细胞快速分裂并可能形成肿瘤；许多治疗方法针对快速分裂的细胞。
6. Nucleic Acids and Protein Synthesis | 核酸与蛋白质合成
DNA is a double helix composed of nucleotides, each containing deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, guanine). The two strands are held together by hydrogen bonds between complementary bases: A pairs with T, C pairs with G. DNA stores genetic information through the sequence of bases. RNA is single-stranded, contains ribose, and has uracil (U) instead of thymine. Messenger RNA (mRNA) carries genetic code from DNA to ribosomes; transfer RNA (tRNA) delivers specific amino acids; ribosomal RNA (rRNA) forms part of ribosomes.
DNA 是双螺旋结构，由核苷酸组成，每个核苷酸含脱氧核糖、磷酸基团和一个含氮碱基（腺嘌呤 A、胸腺嘧啶 T、胞嘧啶 C、鸟嘌呤 G）。两条链通过互补碱基间的氢键维系：A 与 T 配对，C 与 G 配对。DNA 通过碱基序列储存遗传信息。RNA 是单链，含核糖，以尿嘧啶 U 代替胸腺嘧啶。信使 RNA (mRNA) 将遗传密码从 DNA 运至核糖体；转运 RNA (tRNA) 携带特定氨基酸；核糖体 RNA (rRNA) 构成核糖体的一部分。
Protein synthesis involves transcription and translation. In the nucleus, the DNA double helix unzips and one strand serves as a template to synthesise a complementary mRNA molecule (transcription). The mRNA then leaves through nuclear pores and attaches to a ribosome. During translation, tRNA anticodons recognize mRNA codons; each tRNA brings a specific amino acid, and peptide bonds form to build the polypeptide chain. The process continues until a stop codon is reached. The genetic code is triplet-based, degenerate (more than one codon per amino acid), and universal.
蛋白质合成包括转录和翻译。在细胞核中，DNA 双螺旋解开，一条链作为模板合成互补的 mRNA 分子（转录）。随后 mRNA 通过核孔离开细胞核，附在核糖体上。翻译过程中，tRNA 的反密码子识别 mRNA 上的密码子；每种 tRNA 携带特定氨基酸，氨基酸之间形成肽键，构建多肽链。该过程持续进行，直至遇到终止密码子。遗传密码是三联体编码，具有简并性（每个氨基酸对应多个密码子）和通用性。
7. Transport in Plants | 植物的运输
Xylem vessels transport water and dissolved mineral ions from roots to shoots. Xylem tissue is composed of dead, hollow cells with lignin-thickened walls, allowing water to move under tension. Phloem transports organic solutes (mainly sucrose) from sources (e.g. leaves) to sinks (e.g. roots, fruits). Phloem consists of sieve tube elements and companion cells that provide metabolic support.
木质部导管将水和溶解的矿物质离子从根部运输到地上部分。木质部由死细胞中空的管状结构组成，细胞壁加厚木质化，使水份能在张力下流动。韧皮部负责将有机物（主要是蔗糖）从源（如叶片）运输到库（如根、果实）。韧皮部由筛管分子和提供代谢支持的伴胞构成。
The cohesion-tension theory explains water movement in xylem. Transpiration (evaporation from leaves) generates a negative pressure (tension) at the top of the plant. Water molecules are cohesive (they stick together by hydrogen bonds) and adhesive to xylem walls. This adhesion-cohesion allows the entire water column to be pulled up as one. Factors affecting transpiration rate: light intensity (stomata open), temperature, humidity, and air movement.
内聚力-张力理论解释了木质部中水分的运输。蒸腾作用（叶片蒸发水分）在植物顶端产生负压（张力）。水分子具有内聚力（通过氢键彼此黏附）并对木质部管壁有附着力。这种内聚力-附着力使整个水柱能够一体化地被拉上去。影响蒸腾速率的因素包括：光照强度（气孔张开）、温度、湿度和空气流动。
Translocation in phloem is explained by the mass flow hypothesis. At the source, sucrose is actively loaded into sieve tubes, lowering the water potential. Water enters by osmosis from xylem, raising hydrostatic pressure. At the sink, sucrose is removed, water potential rises and water leaves, creating a pressure gradient that drives bulk flow of phloem sap.
韧皮部中的运输可用压力流动假说解释。在源端，蔗糖被主动装载到筛管中，水势降低。水分从木质部渗透进入，提高静水压力。在库端，蔗糖被卸出，水势升高，水分外流，由此产生的压力差推动韧皮部汁液的集流。
8. Transport in Mammals | 哺乳动物的运输
Mammals have a closed, double circulatory system. The heart is a muscular pump with four chambers: right and left atria, right and left ventricles. Deoxygenated blood from the body enters the right atrium, passes to the right ventricle, and is pumped to the lungs (pulmonary circulation). Oxygenated blood returns to the left atrium, flows to the left ventricle, and is pumped to the body (systemic circulation). The left ventricle has a thicker wall to generate higher pressure.
哺乳动物具有封闭式双循环系统。心脏是一个肌肉泵，有四个腔室：左右心房、左右心室。来自全身的缺氧血进入右心房，流至右心室，泵到肺部（肺循环）。含氧血返回左心房，流入左心室，泵送到全身（体循环）。左心室壁较厚，以产生较高压力。
Blood vessels: arteries carry blood away from the heart under high pressure; their walls are thick and elastic with a narrow lumen. Veins return blood to the heart at lower pressure; they have thinner walls, wider lumen, and valves to prevent backflow. Capillaries are narrow, one-cell-thick vessels that allow exchange of materials between blood and tissues. Blood consists of plasma, red blood cells (erythrocytes, containing haemoglobin), white blood cells (leucocytes, part of immune defence), and platelets (involved in clotting).
血管：动脉在高压下将血液运离心脏，管壁厚而富有弹性，管腔狭窄。静脉在低压下将血液送回心脏，管壁较薄，管腔宽大，并具有防止回流的瓣膜。毛细血管是狭窄的单细胞层管道，实现血液与组织间的物质交换。血液由血浆、红细胞（含血红蛋白）、白细胞（免疫防御）和血小板（参与凝血）组成。
Haemoglobin binds oxygen cooperatively, shown by a sigmoid-shaped oxygen dissociation curve. In the lungs, where partial pressure of oxygen is high, haemoglobin becomes saturated. In respiring tissues, lower pO₂ and higher CO₂ (Bohr effect) cause haemoglobin to release more oxygen. Carbon dioxide is transported in blood as bicarbonate ions (HCO₃⁻), dissolved in plasma, or bound to haemoglobin as carbaminohaemoglobin.
血红蛋白协同结合氧气，呈现 S 形氧解离曲线。在肺部，氧分压高，血红蛋白被饱和。在呼吸组织中，pO₂ 较低且 CO₂ 较高（波尔效应），使血红蛋白释放更多的氧。二氧化碳在血液中以碳酸氢根离子 (HCO₃⁻)、溶解在血浆中或与血红蛋白结合形成氨基甲酰血红蛋白等形式运输。
9. Gas Exchange and Smoking | 气体交换与吸烟
In humans, gas exchange occurs in the alveoli of the lungs. Alveoli are tiny air sacs with walls made of a single layer of squamous epithelial cells, surrounded by an extensive capillary network. This provides a large surface area, short diffusion distance, and steep concentration gradients maintained by ventilation and blood flow. Oxygen diffuses from alveoli into blood; carbon dioxide diffuses in the opposite direction.
人体气体交换发生在肺部的肺泡中。肺泡是微小的气囊，壁由单层扁平上皮细胞构成，周围包绕着丰富的毛细血管网。这提供了巨大的表面积、较短的扩散距离，并通过换气和血流维持陡峭的浓度梯度。氧气由肺泡扩散入血，二氧化碳则反向扩散。
Ventilation involves the diaphragm and intercostal muscles. During inspiration, the diaphragm contracts (flattens) and the external intercostal muscles raise the ribcage, increasing thoracic volume and decreasing pressure, so air flows in. During expiration, the muscles relax, lung elastic recoil reduces volume and forces air out. Lung volumes can be measured with a spirometer: tidal volume, vital capacity, and residual volume are key measurements.
肺通气由膈肌和肋间肌完成。吸气时，膈肌收缩（变平），外肋间肌上提肋骨，增加胸腔容积，压力下降，空气进入。呼气时，肌肉舒张，肺的弹性回缩使容积减少，迫使气体排出。肺容量可用肺活量计测量：潮气量、肺活量和残气量是重要指标。
Smoking damages the respiratory system. Tar contains carcinogens and paralyses cilia, leading to mucus accumulation and infection. Nicotine is addictive and increases heart rate and blood pressure. Carbon monoxide binds irreversibly to haemoglobin, reducing oxygen transport. Smoking is linked to chronic obstructive pulmonary disease (COPD), lung cancer, and emphysema.
吸烟损害呼吸系统。焦油含有致癌物并使纤毛麻痹，导致黏液堆积和感染。尼古丁具有成瘾性，会加快心跳和升高血压。一氧化碳与血红蛋白不可逆结合，降低氧气运输能力。吸烟与慢性阻塞性肺病 (COPD)、肺癌和肺气肿密切相关。
10. Immunity | 免疫
The immune system defends the body against pathogens. Non-specific defences include physical barriers (skin, mucous membranes), phagocytosis by neutrophils and macrophages, and inflammation. Specific immune responses involve lymphocytes and produce immunological memory.
免疫系统保护身体免受病原体侵害。非特异性防御包括物理屏障（皮肤、黏膜）、中性粒细胞和巨噬细胞的吞噬作用以及炎症反应。特异性免疫应答由淋巴细胞参与，并产生免疫记忆。
Phagocytosis: a phagocyte is attracted to the pathogen, engulfs it into a phagosome, and fuses with a lysosome where hydrolytic enzymes digest the pathogen. Antigens from the pathogen are presented on the phagocyte's surface (antigen presentation), triggering the specific response.
吞噬作用：吞噬细胞被吸引到病原体处，将其吞入形成吞噬体，随后与溶酶体融合，水解酶将病原体消化。病原体的抗原被呈递到吞噬细胞表面（抗原呈递），启动特异性免疫。
B lymphocytes (activated by helper T cells or directly by antigens) divide by clonal expansion into plasma cells that secrete antibodies, and memory cells that persist for long-term immunity. This is humoral immunity. Antibodies are specific proteins that bind to antigens, neutralising or marking pathogens for destruction. T lymphocytes: helper T cells release cytokines to stimulate other lymphocytes; cytotoxic T cells kill infected body cells (cell-mediated immunity).
B 淋巴细胞（由辅助 T 细胞激活或直接被抗原激活）通过克隆扩增分化为分泌抗体的浆细胞和长期存活的记忆细胞。这属于体液免疫。抗体是特异性蛋白质，能与抗原结合，中和或标记病原体以便销毁。T 淋巴细胞：辅助 T 细胞释放细胞因子刺激其他淋巴细胞；细胞毒性 T 细胞杀灭受感染的体细胞（细胞介导免疫）。
Immunity can be active (the body produces its own antibodies after exposure to pathogen or vaccination) or passive (antibodies are transferred from another source, e.g. from mother to baby via placenta or breast milk). It can be natural (through infection or maternal transfer) or artificial (through vaccination or injection of antibodies). Vaccines contain weakened or dead pathogens, or their antigens, stimulating an immune response without causing disease, leading to memory cell production and rapid secondary response upon real infection.
免疫可分为主动免疫（身体在接触病原体或接种后自行产生抗体）和被动免疫（抗体从其他来源转移而来，如通过胎盘或母乳从母体传给婴儿）。也可分为自然免疫（通过感染或母体转移）和人工免疫（接种疫苗或注射抗体）。疫苗含有灭活或减毒病原体或其抗原，能激发免疫应答而不致病，产生记忆细胞，从而实现真正的感染后的快速二次应答。
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