AS Cambridge Biology: Core Knowledge Review — AS剑桥生物：核心知识点梳理

📚 AS Cambridge Biology: Core Knowledge Review | AS剑桥生物：核心知识点梳理

The AS Level Cambridge Biology syllabus (9700) lays the foundation for understanding life at the molecular, cellular, and systems level. This review covers the 11 core topics that every AS candidate must master, from the intricate structure of cells to the coordinated defence mechanisms of the immune system. Whether you are preparing for Paper 1 (multiple choice), Paper 2 (structured questions), or Paper 3 (practical skills), a solid grasp of these fundamentals is essential for success.

AS阶段剑桥生物课程（9700）为学生理解生命的分子、细胞和系统层面奠定了基础。本文梳理了AS考生必须掌握的11个核心主题，从精密的细胞结构到免疫系统的协同防御机制。无论你是在准备Paper 1（选择题）、Paper 2（结构化问答题）还是Paper 3（实验技能），扎实掌握这些基础知识都是取得好成绩的关键。

1. Cell Structure | 细胞结构

All living organisms are composed of cells, the basic structural and functional units of life. Cambridge AS Biology requires students to compare prokaryotic and eukaryotic cells in detail. Eukaryotic cells, found in animals, plants, fungi, and protoctists, possess a true nucleus enclosed by a nuclear envelope, along with membrane-bound organelles such as mitochondria, the endoplasmic reticulum, the Golgi apparatus, and lysosomes. Prokaryotic cells, such as bacteria, lack a nucleus and membrane-bound organelles; their genetic material exists as a circular DNA molecule in the nucleoid region, and they possess 70S ribosomes rather than the 80S ribosomes found in eukaryotes.

所有生物体都由细胞构成，细胞是生命的基本结构和功能单位。剑桥AS生物课程要求学生详细比较原核细胞和真核细胞。真核细胞存在于动物、植物、真菌和原生生物中，拥有被核膜包裹的真正细胞核，以及线粒体、内质网、高尔基体和溶酶体等膜结合细胞器。原核细胞（如细菌）没有细胞核和膜结合细胞器，其遗传物质以环状DNA分子形式存在于拟核区域，并且拥有70S核糖体而非真核生物中的80S核糖体。

A key skill is the ability to identify organelles from electron micrographs and to relate structure to function. For example, the extensive cristae of mitochondria increase the surface area for oxidative phosphorylation, while the stacked thylakoids of chloroplasts maximise light absorption for photosynthesis. The relative sizes of organelles and cells should also be understood — students must be comfortable converting between millimetres, micrometres, and nanometres using the eyepiece graticule and stage micrometer calibration method.

一项关键技能是能够从电子显微照片中识别细胞器，并将结构与功能联系起来。例如，线粒体广泛的嵴增加了氧化磷酸化的表面积，而叶绿体堆叠的类囊体最大限度地提高了光合作用的光吸收效率。细胞器与细胞的相对大小也是需要掌握的内容——学生必须能够熟练使用目镜测微尺和载物台测微尺校准法在毫米、微米和纳米之间进行换算。

Viruses occupy a unique position — they are non-cellular infectious particles consisting of nucleic acid (DNA or RNA) surrounded by a protein coat called a capsid. Since they lack cellular structure and cannot carry out metabolic processes independently, they are not classified as living organisms. Understanding viral structure is important for topics in infectious disease and immunity.

病毒占据着独特的位置——它们是非细胞结构的感染性颗粒，由核酸（DNA或RNA）和称为衣壳的蛋白质外壳组成。由于缺乏细胞结构且无法独立进行代谢活动，病毒不被归类为生物体。理解病毒结构对于感染性疾病和免疫学主题非常重要。

2. Biological Molecules | 生物分子

Life is built from a relatively small number of chemical elements — primarily carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. These combine to form the four major groups of biological macromolecules: carbohydrates, lipids, proteins, and nucleic acids. Each group is composed of smaller monomeric units: monosaccharides for carbohydrates, fatty acids and glycerol for lipids, amino acids for proteins, and nucleotides for nucleic acids.

生命由相对少量的化学元素构成——主要是碳、氢、氧、氮、磷和硫。这些元素结合形成四大类生物大分子：碳水化合物、脂质、蛋白质和核酸。每类大分子由较小的单体单元组成：碳水化合物的单糖，脂质的脂肪酸和甘油，蛋白质的氨基酸，以及核酸的核苷酸。

Carbohydrates serve as both energy sources and structural components. Glucose, a hexose monosaccharide with the formula C6H12O6, exists in two isomeric forms: alpha-glucose and beta-glucose. The orientation of the hydroxyl group on carbon-1 determines whether the polymer formed will be starch (from alpha-glucose), which is compact and ideal for energy storage in plants, or cellulose (from beta-glucose), which forms strong, straight chains suitable for cell wall construction. The glycosidic bonds linking monosaccharides — formed through condensation reactions and broken by hydrolysis — are a recurring theme throughout biochemistry.

碳水化合物既作为能量来源也作为结构组分。葡萄糖（分子式C6H12O6）是一种己糖单糖，以两种异构形式存在：α-葡萄糖和β-葡萄糖。碳-1上羟基的朝向决定了所形成的聚合物是淀粉（由α-葡萄糖构成，结构紧凑，适合植物储能）还是纤维素（由β-葡萄糖构成，形成坚固的直链，适合构建细胞壁）。通过缩合反应形成并通过水解反应断裂的糖苷键，是整个生物化学中反复出现的主题。

Lipids are diverse hydrophobic molecules including triglycerides (fats and oils), phospholipids, and steroids. Triglycerides are formed by ester bonds between glycerol and three fatty acid chains. Their high ratio of carbon-hydrogen bonds to carbon-oxygen bonds makes them excellent long-term energy stores — gram for gram, lipids release approximately twice the energy of carbohydrates. The distinction between saturated and unsaturated fatty acids (presence or absence of carbon-carbon double bonds) determines the physical properties of fats and their health implications.

脂质是多样的疏水分子，包括甘油三酯（脂肪和油）、磷脂和类固醇。甘油三酯由甘油与三条脂肪酸链之间形成的酯键构成。其高碳氢键与碳氧键比例使之成为出色的长期能量储存物质——以克为单位，脂质释放的能量约为碳水化合物的两倍。饱和与不饱和脂肪酸的区别（碳碳双键的有无）决定了脂肪的物理性质及其对健康的影响。

Proteins are the most structurally and functionally diverse macromolecules. They are polymers of amino acids joined by peptide bonds. AS students must know the general structure of an amino acid — an amino group (NH2), a carboxyl group (COOH), and a variable R group all attached to a central alpha carbon. The four levels of protein structure — primary (amino acid sequence), secondary (alpha-helices and beta-pleated sheets stabilised by hydrogen bonds), tertiary (3D folding maintained by hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges), and quaternary (multiple polypeptide subunits) — determine a protein’s specific function. Haemoglobin and collagen are classic AS examples of quaternary structure.

蛋白质是结构和功能最为多样的大分子。它们是由肽键连接的氨基酸聚合物。AS学生必须知道氨基酸的基本结构——氨基（NH2）、羧基（COOH）和可变的R基团都连接在中心α碳原子上。蛋白质结构的四个层次——一级结构（氨基酸序列）、二级结构（由氢键稳定的α-螺旋和β-折叠片）、三级结构（由氢键、离子键、疏水相互作用和二硫键维持的三维折叠）以及四级结构（多个多肽亚基）——决定了蛋白质的特定功能。血红蛋白和胶原蛋白是AS阶段关于四级结构的经典例子。

3. Enzymes | 酶

Enzymes are biological catalysts — globular proteins that dramatically accelerate the rate of metabolic reactions without being consumed in the process. AS candidates must thoroughly understand the induced-fit model, where the binding of a substrate induces a conformational change in the enzyme’s active site, moulding it into a precise complementary shape. This contrasts with the older lock-and-key model, which assumes a rigid, pre-formed active site.

酶是生物催化剂——能够在反应过程中不被消耗而大幅加速代谢反应速率的球状蛋白质。AS考生必须深入理解诱导契合模型，即底物的结合诱导酶活性位点发生构象变化，使其塑造成精确互补的形状。这与较旧的锁钥模型形成对比，后者假设活性位点是预先形成的刚性结构。

The factors affecting enzyme activity — temperature, pH, enzyme concentration, and substrate concentration — are central to the AS syllabus. Students must be able to interpret the characteristic bell-shaped curves of temperature and pH against reaction rate: low temperatures reduce kinetic energy and collision frequency; temperatures above the optimum cause denaturation as hydrogen bonds in the tertiary structure break; extreme pH values alter the ionisation of R groups in the active site, disrupting substrate binding. The concept of Vmax (maximum rate when all active sites are occupied) and the Michaelis-Menten constant (Km) is introduced at AS level as a way to quantify enzyme affinity for its substrate.

影响酶活性的因素——温度、pH、酶浓度和底物浓度——是AS课程的核心内容。学生必须能够解读温度/pH与反应速率之间典型的钟形曲线：低温降低动能和碰撞频率；超过最适温度会导致氢键断裂、三级结构破坏而发生变性；极端pH值会改变活性位点R基团的离子化状态，干扰底物结合。Vmax（所有活性位点被占据时的最大速率）和米氏常数（Km）的概念在AS阶段被引入，用于量化酶对底物的亲和力。

Enzyme inhibition is classified as competitive or non-competitive. Competitive inhibitors resemble the substrate and bind reversibly to the active site, an effect that can be overcome by increasing substrate concentration. Non-competitive inhibitors bind to an allosteric site, altering the shape of the active site without directly blocking it; increasing substrate concentration cannot reverse this inhibition. This distinction is frequently tested in AS exam questions, often with graphs showing Vmax and Km changes.

酶抑制作用分为竞争性抑制和非竞争性抑制。竞争性抑制剂与底物结构相似，可逆地结合于活性位点，这种效应可以通过增加底物浓度来克服。非竞争性抑制剂结合于别构位点，改变活性位点的形状而不直接阻断它；增加底物浓度无法逆转这种抑制。这一区别在AS考试题目中经常出现，通常以显示Vmax和Km变化的图表形式考查。

4. Cell Membranes and Transport | 细胞膜与物质运输

The fluid mosaic model describes the structure of cell membranes: a phospholipid bilayer with hydrophobic fatty acid tails facing inward and hydrophilic phosphate heads facing outward. Embedded within this bilayer are proteins — integral proteins spanning the membrane and peripheral proteins attached to the surface — along with cholesterol molecules (in animal cells) that regulate membrane fluidity, and glycoproteins and glycolipids involved in cell recognition and signalling.

流动镶嵌模型描述了细胞膜的结构：磷脂双分子层，疏水的脂肪酸尾部朝向内部，亲水的磷酸头部朝向外部。镶嵌在双分子层中的是蛋白质——贯穿膜层的整合蛋白和附着在表面的外周蛋白——以及调节膜流动性的胆固醇分子（动物细胞中）和参与细胞识别与信号传递的糖蛋白和糖脂。

Transport across membranes occurs through several mechanisms. Simple diffusion is the passive net movement of small, non-polar molecules (such as oxygen and carbon dioxide) down their concentration gradient. Facilitated diffusion uses channel proteins (for ions and water) or carrier proteins (for larger polar molecules like glucose) — still passive but protein-mediated. Osmosis is the specific case of water diffusing through a partially permeable membrane from a region of higher water potential (less negative) to a region of lower water potential (more negative). Active transport requires metabolic energy (ATP) to move substances against their concentration gradient via specific carrier proteins, such as the sodium-potassium pump. Endocytosis and exocytosis are bulk transport mechanisms involving vesicle formation and fusion.

跨膜运输通过多种机制发生。简单扩散是指小的非极性分子（如氧气和二氧化碳）沿浓度梯度的被动净移动。协助扩散使用通道蛋白（用于离子和水）或载体蛋白（用于较大的极性分子如葡萄糖）——仍然是被动运输，但由蛋白质介导。渗透作用是水分子通过部分渗透膜从较高水势区域（较不负）向较低水势区域（较负）扩散的特殊情况。主动运输需要代谢能量（ATP）通过特定的载体蛋白（如钠钾泵）逆浓度梯度移动物质。胞吞和胞吐是涉及囊泡形成和融合的批量运输机制。

5. The Mitotic Cell Cycle | 有丝分裂细胞周期

The cell cycle is a precisely regulated sequence of events by which a cell duplicates its contents and divides into two daughter cells. It consists of interphase (G1, S, and G2 phases) followed by mitosis and cytokinesis. During interphase, the cell grows, carries out normal metabolic activities, and replicates its DNA. The S (synthesis) phase is critical — each chromosome is duplicated to form two identical sister chromatids held together at the centromere.

细胞周期是一系列精确调控的事件序列，细胞通过这些事件复制其内容物并分裂为两个子细胞。它包括间期（G1、S和G2期），随后是有丝分裂和胞质分裂。在间期，细胞生长，进行正常代谢活动，并复制其DNA。S（合成）期至关重要——每条染色体被复制形成两条相同的姐妹染色单体，由着丝粒连接在一起。

Mitosis is divided into four stages for examination purposes: prophase (chromosomes condense and become visible, nuclear envelope breaks down, spindle fibres form), metaphase (chromosomes align at the metaphase plate equator, spindle fibres attach to centromeres), anaphase (centromeres split, sister chromatids are pulled to opposite poles by shortening spindle fibres), and telophase (chromosomes decondense, nuclear envelopes reform around each set). Cytokinesis follows, dividing the cytoplasm; in animal cells this occurs through a cleavage furrow, while in plant cells a cell plate forms.

有丝分裂为考试目的分为四个阶段：前期（染色体凝聚并变得可见，核膜解体，纺锤丝形成）、中期（染色体排列在赤道板中纬线上，纺锤丝附着于着丝粒）、后期（着丝粒分裂，姐妹染色单体被缩短的纺锤丝拉向两极）和末期（染色体解凝聚，每个染色体组周围重新形成核膜）。随后是胞质分裂，将细胞质分开；动物细胞中通过分裂沟完成，而植物细胞中形成细胞板。

Understanding the role of mitosis in growth, repair of tissues, and asexual reproduction is essential. AS students should also be introduced to the concept that cancer results from uncontrolled cell division due to mutations in genes that regulate the cell cycle, such as proto-oncogenes and tumour suppressor genes.

理解有丝分裂在生长、组织修复和无性繁殖中的作用至关重要。AS学生还应了解癌症是由于调节细胞周期的基因（如原癌基因和肿瘤抑制基因）发生突变导致细胞分裂失控而产生的概念。

6. Nucleic Acids and Protein Synthesis | 核酸与蛋白质合成

Nucleic acids — DNA and RNA — are polymers of nucleotides, each consisting of a pentose sugar, a phosphate group, and a nitrogenous base. DNA is a double helix of two antiparallel polynucleotide strands held together by hydrogen bonds between complementary base pairs: adenine (A) pairs with thymine (T) via two hydrogen bonds, and cytosine (C) pairs with guanine (G) via three hydrogen bonds. RNA is typically single-stranded and contains ribose instead of deoxyribose, with uracil (U) replacing thymine.

核酸——DNA和RNA——是核苷酸的聚合物，每个核苷酸由戊糖、磷酸基团和含氮碱基组成。DNA是由两条反平行的多核苷酸链构成的双螺旋结构，通过互补碱基对之间的氢键维系：腺嘌呤（A）与胸腺嘧啶（T）通过两个氢键配对，胞嘧啶（C）与鸟嘌呤（G）通过三个氢键配对。RNA通常是单链结构，含有核糖而非脱氧核糖，并且尿嘧啶（U）替代了胸腺嘧啶。

DNA replication is semi-conservative — each new DNA molecule consists of one original (parental) strand and one newly synthesised strand. The Meselson and Stahl experiment, which used heavy nitrogen (15N) isotopes and density-gradient centrifugation, provided elegant experimental evidence for this model. The enzyme DNA helicase unwinds the double helix, while DNA polymerase adds complementary nucleotides to the template strand in the 5′ to 3′ direction, forming a continuous leading strand and a discontinuous lagging strand composed of Okazaki fragments joined by DNA ligase.

DNA复制是半保留的——每个新DNA分子由一条原始（亲代）链和一条新合成的链组成。Meselson和Stahl实验使用了重氮（15N）同位素和密度梯度离心技术，为这一模型提供了优雅的实验证据。DNA解旋酶解开双螺旋，而DNA聚合酶沿5’到3’方向向模板链添加互补核苷酸，形成连续的前导链和由冈崎片段组成、由DNA连接酶连接的不连续后随链。

Protein synthesis involves two major stages: transcription and translation. During transcription, the enzyme RNA polymerase binds to a promoter region, unwinds a section of DNA, and synthesises a complementary messenger RNA (mRNA) strand. In eukaryotes, the pre-mRNA undergoes splicing — introns are removed and exons are joined — before the mature mRNA exits the nucleus. Translation occurs on ribosomes in the cytoplasm: the mRNA sequence is read in triplet codons, each specifying a particular amino acid. Transfer RNA (tRNA) molecules with complementary anticodons deliver the appropriate amino acids, and peptide bonds form between adjacent amino acids to build the polypeptide chain. The genetic code is universal, degenerate, and non-overlapping — key features that AS students must be prepared to explain.

蛋白质合成包括两个主要阶段：转录和翻译。转录过程中，RNA聚合酶结合于启动子区域，解开一段DNA，并合成一条互补的信使RNA（mRNA）链。在真核生物中，前体mRNA经历剪接——内含子被去除，外显子被连接——然后成熟的mRNA离开细胞核。翻译在细胞质的核糖体上进行：mRNA序列以三联密码子的方式被读取，每个密码子指定一个特定的氨基酸。具有互补反密码子的转运RNA（tRNA）分子递送相应的氨基酸，相邻氨基酸之间形成肽键以构建多肽链。遗传密码是通用的、简并的且不重叠的——这些关键特征是AS学生必须准备解释的内容。

7. Transport in Plants | 植物运输

Plants require efficient transport systems to move water, minerals, and the products of photosynthesis throughout the organism. The xylem and phloem are the two vascular tissues that form the plant transport system. Xylem vessels transport water and dissolved mineral ions from the roots to the leaves. Xylem vessel elements are dead at maturity, with their end walls broken down to form continuous hollow tubes. Their walls are strengthened with lignin, which provides structural support and prevents collapse under the tension created by transpiration.

植物需要高效的运输系统将水分、矿物质和光合作用产物运送到整个有机体。木质部和韧皮部是构成植物运输系统的两种维管组织。木质部导管将水分和溶解的矿物质离子从根部运输到叶片。木质部导管分子在成熟时是死细胞，其端壁被分解形成连续的中空管道。其细胞壁由木质素加固，提供结构支撑并防止在蒸腾作用产生的张力下塌陷。

The cohesion-tension theory explains water movement in the xylem. Transpiration from leaf mesophyll cells lowers water potential, creating a tension that pulls water up the xylem. Water molecules cohere to each other through hydrogen bonding, maintaining an unbroken column of water from roots to leaves. The rate of transpiration is affected by temperature, humidity, wind speed, and light intensity — students should be able to design and interpret experiments using a potometer to measure transpiration rates.

内聚力-张力理论解释了木质部中的水分运动。叶肉细胞的蒸腾作用降低了水势，产生将水分拉上木质部的张力。水分子通过氢键相互内聚，维持从根部到叶片不间断的水柱。蒸腾速率受温度、湿度、风速和光照强度的影响——学生应能够设计并解读使用蒸腾计测量蒸腾速率的实验。

Phloem tissue transports assimilates, principally sucrose and amino acids, from sources (where they are produced or stored, such as mature leaves) to sinks (where they are used or stored, such as roots, developing fruits, and meristems). The mass flow hypothesis, also called the pressure-flow model, proposes that active loading of sucrose into the sieve tube elements at the source lowers water potential, causing water to enter by osmosis and creating high hydrostatic pressure. At the sink, sucrose is unloaded, water leaves, and the pressure drops. The resulting pressure gradient drives the bulk flow of phloem sap from source to sink. Companion cells provide metabolic support for the enucleate sieve tube elements, including the ATP required for active loading.

韧皮部组织运输同化物，主要是蔗糖和氨基酸，从源（它们被生产或储存的地方，如成熟叶片）到库（它们被使用或储存的地方，如根部、发育中的果实和分生组织）。质量流假说（也称压力流模型）提出，在源端主动向筛管分子加载蔗糖会降低水势，使水分通过渗透进入并产生高静水压。在库端，蔗糖被卸载，水分离开，压力下降。由此产生的压力梯度驱动韧皮部汁液从源到库的集流运动。伴细胞为无核的筛管分子提供代谢支持，包括主动加载所需的ATP。

8. Transport in Mammals: The Circulatory System | 哺乳动物运输：循环系统

The mammalian circulatory system is a closed, double circulatory system. The heart is a muscular organ with four chambers: two thin-walled atria that receive blood and two thick-walled ventricles that pump blood out. The right side of the heart pumps deoxygenated blood to the lungs via the pulmonary artery (pulmonary circulation), while the left side pumps oxygenated blood to the rest of the body via the aorta (systemic circulation). The left ventricle has a thicker muscular wall than the right because it must generate sufficient pressure to propel blood throughout the entire body.

哺乳动物循环系统是一个封闭的双循环系统。心脏是一个具有四个腔室的肌肉器官：两个接受血液的薄壁心房和两个泵出血液的厚壁心室。心脏右侧通过肺动脉将缺氧血液泵送至肺部（肺循环），而左侧通过主动脉将富氧血液泵送至身体其他部位（体循环）。左心室拥有比右心室更厚的肌肉壁，因为它必须产生足够的压力推动血液流经全身。

The cardiac cycle consists of systole (contraction) and diastole (relaxation). The sinoatrial node (SAN) in the right atrium acts as the heart’s natural pacemaker, generating electrical impulses that spread across the atria, causing atrial systole. The impulse is then delayed at the atrioventricular node (AVN) before travelling down the bundle of His and Purkinje fibres to trigger ventricular systole from the apex upwards. Understanding this sequence and the associated pressure changes in the atria, ventricles, and major arteries is essential for interpreting electrocardiogram (ECG) traces.

心动周期由收缩期（收缩）和舒张期（舒张）组成。右心房中的窦房结（SAN）作为心脏的天然起搏器，产生电冲动并在心房中传播，引起心房收缩。随后冲动在房室结（AVN）处被延迟，然后沿希氏束和浦肯野纤维向下传导，从心尖向上触发心室收缩。理解这一序列以及心房、心室和主要动脉中相关的压力变化对于解读心电图（ECG）波形至关重要。

Blood vessels are specialised for their roles in circulation. Arteries carry blood away from the heart under high pressure; their walls contain thick layers of smooth muscle and elastic tissue that allow them to stretch and recoil, maintaining pressure during diastole. Veins return blood to the heart under low pressure; they have thinner walls, wider lumens, and valves to prevent backflow. Capillaries are the site of material exchange between blood and tissues; their walls consist of a single layer of endothelial cells, providing a short diffusion distance. AS students should relate the structure of each vessel type to its function and be able to recognise them from photomicrographs.

血管因其在循环中的作用而特化。动脉在高压下将血液从心脏运出；其壁含有厚的平滑肌和弹性组织层，使其能够伸展和回弹，在舒张期维持压力。静脉在低压下将血液送回心脏；其壁较薄，管腔较宽，并具有防止回流的瓣膜。毛细血管是血液与组织之间进行物质交换的场所；其壁由单层内皮细胞构成，提供了较短的扩散距离。AS学生应将每种血管的结构与其功能联系起来，并能从显微照片中识别它们。

9. Gas Exchange and Smoking | 气体交换与吸烟

The human gas exchange system is adapted for the efficient uptake of oxygen and removal of carbon dioxide. Air enters through the nasal passages and trachea, passes through the bronchi and bronchioles, and reaches the alveoli — millions of tiny air sacs where gas exchange occurs. The alveoli are ideally suited for their function: they provide a large total surface area (approximately 70 m2 in an adult), a thin diffusion barrier (the alveolar and capillary walls are each just one cell thick), a steep concentration gradient maintained by continuous blood flow and ventilation, and a moist surface for the dissolution of gases.

人体气体交换系统适应于高效摄取氧气和排出二氧化碳。空气通过鼻腔和气管进入，经过支气管和细支气管，到达肺泡——数百万个进行气体交换的微小气囊。肺泡非常适合其功能：它们提供巨大的总表面积（成人约70平方米），薄的扩散屏障（肺泡壁和毛细血管壁各只有一层细胞厚），由持续血流和通气维持的浓度梯度，以及供气体溶解的湿润表面。

AS students should understand the mechanism of ventilation. During inspiration, the external intercostal muscles contract (raising the rib cage) and the diaphragm contracts (flattening), increasing the volume of the thoracic cavity and decreasing its pressure below atmospheric pressure, so air flows in. During expiration at rest, these muscles relax passively; the elastic recoil of the lungs and chest wall reduces thoracic volume, increasing pressure and forcing air out. Forced expiration involves contraction of the internal intercostal muscles and abdominal muscles.

AS学生应理解通气的机制。吸气时，外肋间肌收缩（提起胸廓），膈肌收缩（变平），增加胸腔容积并将其压力降低至大气压以下，使空气流入。静息呼气时，这些肌肉被动松弛；肺和胸壁的弹性回缩减少胸腔容积，增加压力并排出空气。用力呼气涉及内肋间肌和腹肌的收缩。

The link between smoking and disease is a significant component of the syllabus. Tar, a component of cigarette smoke, paralyses and destroys cilia in the airways, impairing the mucociliary escalator and leading to chronic bronchitis. It is also the primary carcinogen responsible for lung cancer. Carbon monoxide binds irreversibly to haemoglobin with an affinity approximately 250 times greater than oxygen, reducing the oxygen-carrying capacity of blood. Nicotine is a vasoconstrictor that increases heart rate and blood pressure, contributing to cardiovascular disease and increasing the risk of thrombosis.

吸烟与疾病之间的联系是课程大纲的重要组成部分。焦油（香烟烟雾的成分之一）麻痹并破坏气道中的纤毛，损害黏膜纤毛清除系统，导致慢性支气管炎。它也是导致肺癌的主要致癌物。一氧化碳以约250倍于氧气的亲和力与血红蛋白不可逆结合，降低血液的携氧能力。尼古丁是一种血管收缩剂，会增加心率和血压，导致心血管疾病并增加血栓形成的风险。

10. Infectious Diseases | 感染性疾病

Infectious diseases are caused by pathogens — organisms that live in or on a host organism, causing harm. The four main types of pathogens covered at AS level are bacteria, viruses, fungi, and protoctists (protists). AS students are expected to study specific examples: cholera (caused by the bacterium Vibrio cholerae), tuberculosis (caused by Mycobacterium tuberculosis), malaria (caused by protoctists of the genus Plasmodium), and HIV/AIDS (caused by the human immunodeficiency virus).

感染性疾病由病原体引起——即寄生在宿主体内或体表并造成伤害的生物。AS阶段涵盖的四种主要病原体类型是细菌、病毒、真菌和原生生物（原虫）。AS学生需要学习具体案例：霍乱（由霍乱弧菌引起）、结核病（由结核分枝杆菌引起）、疟疾（由疟原虫属的原虫引起）以及HIV/AIDS（由人类免疫缺陷病毒引起）。

For each disease, students must know the causative organism, its mode of transmission, the symptoms and effects on the body, and methods of prevention and control. Cholera is transmitted via faecally contaminated water and food; the cholera toxin causes chloride ion channels in intestinal epithelial cells to open, leading to massive water loss through osmosis and potentially fatal dehydration. Tuberculosis is an airborne disease spread by droplet infection. Malaria is vector-borne, transmitted by female Anopheles mosquitoes. HIV is transmitted through the exchange of bodily fluids and attacks helper T cells, progressively destroying the immune system.

对于每种疾病，学生必须了解致病微生物、传播方式、对身体的症状和影响以及预防和控制方法。霍乱通过粪便污染的水和食物传播；霍乱毒素使肠道上皮细胞中的氯离子通道打开，渗透作用导致大量水分流失和可能致命的脱水。结核病是通过飞沫传播的呼吸道疾病。疟疾是媒介传播疾病，由雌性按蚊传播。HIV通过体液交换传播，攻击辅助T细胞，逐步摧毁免疫系统。

Antibiotics are a major weapon against bacterial infections. They work by targeting features specific to prokaryotic cells — such as bacterial cell wall synthesis (penicillin), protein synthesis (tetracycline), or nucleic acid synthesis — without harming eukaryotic host cells. The emergence of antibiotic resistance, driven by natural selection when antibiotics are overused or misused, is a growing global health concern that AS students should be prepared to discuss, including strategies such as completing prescribed courses and reducing unnecessary prescriptions.

抗生素是对抗细菌感染的主要武器。它们通过靶向原核细胞特有的特征发挥作用——如细菌细胞壁合成（青霉素）、蛋白质合成（四环素）或核酸合成——而不损害真核宿主细胞。抗生素耐药性的出现（由抗生素过度使用或滥用的自然选择驱动）是日益严重的全球健康问题，AS学生应准备讨论这一问题，包括完成处方疗程和减少不必要处方等策略。

11. Immunity | 免疫

The immune system provides defence against pathogens through a coordinated network of cells, tissues, and molecules. AS students must distinguish between non-specific (innate) and specific (adaptive) defences. Innate defences are present from birth and respond rapidly: physical barriers such as skin and mucous membranes, phagocytosis by neutrophils and macrophages, inflammation, and the action of natural killer cells. These responses do not differentiate between different pathogens.

免疫系统通过细胞、组织和分子的协同网络提供针对病原体的防御。AS学生必须区分非特异性（先天）和特异性（适应性）防御。先天防御从出生时就存在并快速响应：物理屏障如皮肤和黏膜、中性粒细胞和巨噬细胞的吞噬作用、炎症以及自然杀伤细胞的作用。这些反应不区分不同的病原体。

Specific immune responses involve lymphocytes (B cells and T cells) and are characterised by specificity and immunological memory. B lymphocytes produce antibodies — Y-shaped proteins (immunoglobulins) with variable regions that bind specifically to antigens on the pathogen surface. This is the humoral response. T lymphocytes are responsible for cell-mediated immunity: helper T cells release cytokines that activate B cells and cytotoxic T cells, while cytotoxic T cells directly destroy infected body cells displaying foreign antigens on their MHC class I molecules.

特异性免疫反应涉及淋巴细胞（B细胞和T细胞），具有特异性和免疫记忆的特点。B淋巴细胞产生抗体——具有可变区的Y形蛋白质（免疫球蛋白），可变区能够特异性结合病原体表面的抗原。这就是体液免疫反应。T淋巴细胞负责细胞介导免疫：辅助T细胞释放细胞因子激活B细胞和细胞毒性T细胞，而细胞毒性T细胞直接摧毁在其MHC I类分子上展示外来抗原的受感染体细胞。

Vaccination exploits the principle of immunological memory. By introducing a harmless form of an antigen (attenuated pathogen, killed pathogen, or antigenic fragment), the primary immune response is triggered, generating memory B and T cells. Upon subsequent exposure to the actual pathogen, the secondary response is faster, stronger, and more prolonged, often preventing disease entirely. The concept of herd immunity — where a sufficiently high proportion of a population is immunised to protect vulnerable individuals — is a key application of this principle in public health policy.

疫苗接种利用了免疫记忆的原理。通过引入无害形式的抗原（减毒病原体、灭活病原体或抗原片段），触发初次免疫反应，生成记忆B细胞和T细胞。当后续接触到真正的病原体时，二次反应更快、更强且更持久，通常能够完全预防疾病。群体免疫的概念——即人群中有足够高比例的人获得免疫以保护易感个体——是这一原理在公共卫生政策中的关键应用。

12. Exam Tips for AS Cambridge Biology | AS剑桥生物考试技巧

Success in AS Cambridge Biology requires more than factual recall — it demands the ability to apply knowledge to unfamiliar contexts, analyse data, and communicate scientific ideas precisely. Here are some key strategies for each paper type.

AS剑桥生物考试的成功不仅需要记忆事实——还需要将知识应用于陌生情境、分析数据并精确传达科学思想的能力。以下是针对每种试卷类型的关键策略。

Paper 1 (Multiple Choice): Work systematically through the 40 questions in 60 minutes. Read every option carefully — Cambridge examiners often include distractors that are partially correct but not the best answer. Pay special attention to negatively phrased questions (“Which of the following is NOT…”). If unsure, eliminate obviously wrong options first, then make an educated guess from the remainder. There is no penalty for incorrect answers.

Paper 1（选择题）：在60分钟内系统性地完成40道题目。仔细阅读每个选项——剑桥出题人经常会设置部分正确但并非最佳答案的干扰项。特别留意否定措辞的问题（”以下哪个不是……”）。如果不确定，首先排除明显错误的选项，然后从剩余的选项中做出有根据的猜测。错误答案没有惩罚。

Paper 2 (Structured Questions): Command words are critical. “Describe” requires stating what is observed without explanation; “Explain” demands reasons and mechanisms; “Suggest” invites you to apply knowledge to a new scenario. Note the mark allocation — a 3-mark question typically expects three distinct points. Use correct scientific terminology at all times; marks are regularly lost for vague or colloquial language. For graph questions, always read axis labels and units carefully before answering.

Paper 2（结构化问答题）：指令词至关重要。”Describe（描述）”要求陈述所观察到的情况而不加解释；”Explain（解释）”要求给出原因和机制；”Suggest（建议）”邀请你将知识应用于新情境。注意分值分配——一个3分的题目通常期望三个独立的要点。始终使用正确的科学术语；模糊或口语化的表达常常导致扣分。对于图表题，在作答前务必仔细阅读坐标轴标签和单位。

Paper 3 (Practical Skills): This paper tests experimental design, data handling, and evaluative skills. When asked to plan an investigation, clearly state the independent, dependent, and control variables. Describe how you would standardise control variables and ensure reliable results (e.g., replicates, large sample sizes). When evaluating data or experimental procedures, identify sources of error, suggest realistic improvements, and comment on the reliability and validity of conclusions. Practise drawing clear, well-labelled graphs with appropriate scales.

Paper 3（实验技能）：该试卷考查实验设计、数据处理和评估能力。当被要求设计调查方案时，明确陈述自变量、因变量和控制变量。描述如何标准化控制变量并确保结果可靠（如重复实验、大样本量）。在评估数据或实验程序时，识别误差来源，提出切实可行的改进意见，并评价结论的可靠性和有效性。练习绘制清晰、标注得当且比例合适的图表。

Mastering the AS Cambridge Biology syllabus is an ambitious undertaking, but a systematic approach to revision — actively testing yourself, drawing annotated diagrams, and practising past paper questions under timed conditions — will build both confidence and competence. Each of the eleven topics connects to the others in meaningful ways; recognising these interconnections is the hallmark of a high-achieving biology student.

掌握AS剑桥生物课程大纲是一项艰巨的任务，但系统的复习方法——主动自我测试、绘制带注释的图表以及在计时条件下练习历年试卷题目——将同时建立信心和能力。这十一个主题中的每一个都以有意义的方式相互关联；认识到这些内在联系是优秀生物学生的标志。