📚 AS CAIE Biology: Core Knowledge Summary | AS CAIE 生物：核心知识点梳理
For students taking the Cambridge International AS Level Biology (9700), building a solid foundation of core concepts is essential. This article summarises the key topics you will encounter, from cell structure to immunity, highlighting essential principles and terminology. Each section is presented in both English and Chinese to support bilingual learners, ensuring you can grasp the material confidently and perform well in your examinations.
对于修读剑桥国际 AS Level 生物 (9700) 的学生来说，打好核心概念的基础至关重要。本文梳理了你会遇到的关键知识点，从细胞结构到免疫学，突出基本原理和术语。每个小节都以英文和中文配对呈现，帮助双语学习者牢固掌握内容，在考试中取得理想成绩。
1. Cell Structure | 细胞结构
All living organisms are composed of cells, which are the basic structural and functional units of life. Eukaryotic cells, such as those of animals, plants, and fungi, contain a true nucleus enclosed by a nuclear envelope, as well as membrane-bound organelles like mitochondria, the endoplasmic reticulum (ER), and the Golgi apparatus. The mitochondrion is the site of aerobic respiration, producing ATP. The rough ER is studded with ribosomes and synthesises proteins, while the smooth ER is involved in lipid synthesis. The Golgi apparatus modifies, sorts, and packages proteins for secretion. Plant cells also possess a cell wall made of cellulose, a large permanent vacuole for storage and support, and chloroplasts for photosynthesis. Prokaryotic cells, such as bacteria, lack a nucleus and membrane-bound organelles; their DNA lies free in the cytoplasm as a circular chromosome, and they may have plasmids. Ribosomes are smaller (70S) in prokaryotes compared to those in eukaryotes (80S). Understanding the ultrastructure and function of these organelles is fundamental, as it links closely to processes like respiration, photosynthesis, and protein synthesis.
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。真核细胞，如动物细胞、植物细胞和真菌细胞，含有由核膜包裹的真正细胞核，以及膜包裹的细胞器，例如线粒体、内质网和高尔基体。线粒体是有氧呼吸的场所，产生 ATP。粗面内质网上附有核糖体，负责合成蛋白质；滑面内质网参与脂质合成。高尔基体对蛋白质进行修饰、分选和包装以便分泌。植物细胞还具有由纤维素构成的细胞壁，一个起储存和支持作用的大液泡，以及进行光合作用的叶绿体。原核细胞（如细菌）没有细胞核和膜包裹的细胞器；其 DNA 作为环状染色体游离在细胞质中，并可能含有质粒。原核生物的核糖体（70S）比真核生物的（80S）小。理解这些细胞器的超微结构和功能是基础，因为它与呼吸作用、光合作用和蛋白质合成等过程密切相关。
2. Biological Molecules | 生物分子
Biological molecules are the building blocks of life and include carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates consist of carbon, hydrogen, and oxygen, with a general formula Cₓ(H₂O)ₓ. Monosaccharides such as glucose (C₆H₁₂O₆) are the simplest sugars. Two monosaccharides join by a glycosidic bond to form disaccharides like sucrose (glucose + fructose) and maltose (glucose + glucose). Polysaccharides, such as starch, glycogen, and cellulose, are polymers of glucose. Starch, a storage molecule in plants, is composed of amylose (unbranched, α-1,4 glycosidic bonds) and amylopectin (branched, α-1,4 and α-1,6 bonds). Glycogen, a storage molecule in animals, is highly branched. Cellulose, a structural polysaccharide in plant cell walls, has β-1,4 glycosidic bonds, making it straight and rigid; many hydrogen bonds form between parallel cellulose molecules, giving strength.
生物分子是生命的基本组成部分，包括碳水化合物、脂类、蛋白质和核酸。碳水化合物由碳、氢和氧组成，通式为 Cₓ(H₂O)ₓ。单糖如葡萄糖（C₆H₁₂O₆）是最简单的糖。两个单糖通过糖苷键连接形成双糖，如蔗糖（葡萄糖+果糖）和麦芽糖（葡萄糖+葡萄糖）。多糖，如淀粉、糖原和纤维素，是葡萄糖的聚合物。淀粉是植物体内储藏的分子，由直链淀粉（无分支，α-1,4 糖苷键）和支链淀粉（分支，α-1,4 和 α-1,6 糖苷键）组成。糖原是动物体内储藏的分子，高度分支。纤维素是植物细胞壁的结构性多糖，具有 β-1,4 糖苷键，使得分子链呈直线且刚硬；平行的纤维素分子之间形成大量氢键，增加强度。
Lipids are diverse hydrophobic molecules. Triglycerides consist of one glycerol molecule ester-linked to three fatty acid chains; they are used for energy storage and insulation. Phospholipids are similar but have one fatty acid replaced by a phosphate group, giving them a hydrophilic head and hydrophobic tails, making them ideal for forming cell membranes. Proteins are polymers of amino acids joined by peptide bonds. There are four levels of protein structure: primary (sequence of amino acids), secondary (α-helices and β-pleated sheets held by hydrogen bonds), tertiary (3D folding due to hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges), and quaternary (assembly of multiple polypeptide chains, e.g., haemoglobin). Water is a key molecule in biology due to its polarity, high specific heat capacity, high latent heat of vaporisation, cohesive and adhesive properties, and ability to act as a solvent. These properties are crucial for temperature regulation, transport, and metabolic reactions.
脂类是一类多样的疏水分子。甘油三酯由一个甘油分子与三条脂肪酸链通过酯键连接而成，用于能量储存和隔热。磷脂与之类似，但有一条脂肪酸被磷酸基团取代，使其具有亲水头部和疏水尾巴，非常适合形成细胞膜。蛋白质是由氨基酸通过肽键连接而成的聚合物。蛋白质结构分为四个层次：一级结构（氨基酸序列），二级结构（由氢键维持的 α-螺旋和 β-折叠），三级结构（因氢键、离子键、疏水作用和二硫键形成的三维折叠），以及四级结构（多条多肽链的组合，如血红蛋白）。水因其极性、高比热容、高蒸发潜热、内聚力和附着力以及作为溶剂的能力，成为生物学中的关键分子。这些特性对于温度调节、运输和代谢反应至关重要。
3. Enzymes | 酶
Enzymes are globular proteins that act as biological catalysts, speeding up metabolic reactions by lowering the activation energy (Eₐ). They possess an active site with a specific shape complementary to their substrate, explained by the lock-and-key hypothesis. The induced-fit model refines this by suggesting that the active site molds itself around the substrate upon binding, placing strain on bonds and lowering activation energy more effectively. Enzyme activity is affected by temperature, pH, enzyme concentration, and substrate concentration. As temperature increases, kinetic energy rises and collisions occur more frequently, increasing the rate of reaction up to an optimum temperature (often around 37–40 °C in humans); beyond this, hydrogen bonds in the enzyme break, causing denaturation and loss of active site shape. Similarly, each enzyme has an optimum pH; deviations alter ionic charges and disrupt the tertiary structure, leading to denaturation. Increasing substrate concentration raises the rate until all active sites are occupied, at which point Vₘₐₓ is reached. Competitive inhibitors resemble the substrate and bind to the active site, blocking it; their effect can be overcome by increasing substrate concentration. Non-competitive inhibitors bind to an allosteric site, altering the active site’s shape, and cannot be overcome by adding more substrate. Understanding enzyme kinetics is vital for grasping metabolic pathways and regulation.
酶是球状蛋白质，作为生物催化剂，通过降低活化能 (Eₐ) 来加速代谢反应。它们具有一个活性部位，其特定形状与底物互补，可用锁钥假说解释。诱导契合模型对此进行了改进，认为活性部位在底物结合时会围绕底物发生构象变化，对化学键施加应力，从而更有效地降低活化能。酶活性受温度、pH、酶浓度和底物浓度的影响。随着温度升高，动能增加，碰撞更频繁，反应速率上升，直至达到最适温度（人体中通常在 37–40 °C 左右）；超过此温度，酶内部的氢键断裂，导致变性，活性部位形状丧失。同样地，每种酶有最适 pH；偏离最适值会改变离子电荷，破坏三级结构，导致变性。增加底物浓度会提高反应速率，直到所有活性部位被占据，此时达到最大反应速率 Vₘₐₓ。竞争性抑制剂与底物结构相似，结合到活性部位，将其阻塞；其效果可通过增加底物浓度来克服。非竞争性抑制剂结合到别构部位，改变活性部位的形状，且无法通过添加更多底物来克服。理解酶动力学对于掌握代谢途径和调控至关重要。
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 embedded proteins, cholesterol (in animal cells), and glycoproteins. Phospholipids arrange themselves so that hydrophilic heads face outward and hydrophobic tails face inward, forming a selectively permeable barrier. Cholesterol provides stability and regulates fluidity. Membrane proteins include channel proteins and carrier proteins for facilitated diffusion and active transport. Transport across membranes occurs via several mechanisms. Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, down a concentration gradient, and is a passive process. Oxygen and carbon dioxide cross membranes by simple diffusion. Facilitated diffusion uses channel proteins (forming pores) or carrier proteins (changing shape) to move larger or charged molecules like glucose and ions down their gradient, without energy input. Osmosis is the diffusion of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Water potential is measured in kilopascals (kPa); pure water has a water potential of 0 kPa, and adding solutes lowers it to negative values. In plant cells, turgor pressure builds up when water enters by osmosis, pushing the cell membrane against the cell wall; plasmolysis occurs when water leaves the cell and the membrane pulls away from the wall. Active transport requires energy in the form of ATP to move substances against their concentration gradient, using specific carrier proteins; e.g., the sodium-potassium pump. Bulk transport involves endocytosis (taking in materials by membrane invagination to form vesicles) and exocytosis (secreting substances via vesicles fusing with the membrane).
细胞膜可用流动镶嵌模型描述：由磷脂双分子层及其嵌入的蛋白质、胆固醇（动物细胞）和糖蛋白组成。磷脂排列成亲水头部朝外、疏水尾部朝内的形式，形成选择性通透屏障。胆固醇提供稳定性并调节膜的流动性。膜蛋白包括用于易化扩散和主动转运的通道蛋白和载体蛋白。物质跨膜运输有多种机制。扩散是粒子从高浓度区域向低浓度区域净移动，顺着浓度梯度进行，是被动过程。氧气和二氧化碳通过简单扩散穿过膜。易化扩散则利用通道蛋白（形成孔道）或载体蛋白（改变形状）来运输较大或带电的分子，如葡萄糖和离子，顺浓度梯度且不消耗能量。渗透是水分子通过部分透性膜从较高水势区域向较低水势区域的扩散。水势以千帕 (kPa) 为单位；纯水的水势为 0 kPa，添加溶质会使其降至负值。在植物细胞中，水通过渗透进入时产生膨压，将细胞膜推向细胞壁；当水离开细胞、细胞膜脱离细胞壁时，发生质壁分离。主动转运需要 ATP 形式的能量，利用特定的载体蛋白逆浓度梯度运输物质，例如钠钾泵。大块运输包括内吞（通过膜内陷形成囊泡摄取物质）和外排（通过囊泡与膜融合分泌物质）。
5. Cell Division (Mitosis) | 细胞分裂（有丝分裂）
The cell cycle consists of interphase and mitotic phase. Interphase includes G₁ (cell growth and protein synthesis), S (DNA replication), and G₂ (final preparation for division). During S phase, each chromosome is duplicated to form two identical sister chromatids joined at a centromere. Mitosis is nuclear division producing two genetically identical daughter nuclei, essential for growth, repair, and asexual reproduction. It is divided into prophase, metaphase, anaphase, and telophase. In prophase, chromatin condenses into visible chromosomes, the nuclear envelope breaks down, and spindle fibres begin to form from the centrosomes. In metaphase, chromosomes line up along the cell’s equator (metaphase plate), attached to spindle fibres via their centromeres. In anaphase, centromeres split and sister chromatids are pulled to opposite poles by shortening spindle fibres, becoming individual chromosomes. In telophase, chromosomes decondense, nuclear envelopes re-form around each set, and spindle fibres disassemble. Cytokinesis (division of cytoplasm) follows, in animal cells by a cleavage furrow and in plant cells by a cell plate forming a new cell wall. Uncontrolled cell division due to mutations in genes regulating the cell cycle can lead to cancer. Understanding mitosis is crucial for explaining growth, tissue regeneration, and reproduction in organisms.
细胞周期包含间期和分裂期。间期包括 G₁ 期（细胞生长和蛋白质合成）、S 期（DNA 复制）和 G₂ 期（分裂的最后准备）。在 S 期，每条染色体复制成两条相同的姐妹染色单体，通过着丝粒相连。有丝分裂是细胞核分裂，产生两个遗传上完全相同的子核，对生长、修复和无性繁殖至关重要。它分为前期、中期、后期和末期。前期，染色质凝缩成可见的染色体，核膜解体，纺锤体纤维开始从中心体发出。中期，染色体排列在细胞赤道板（中期板）上，通过着丝粒与纺锤丝相连。后期，着丝粒分裂，姐妹染色单体被缩短的纺锤丝拉向两极，成为独立的染色体。末期，染色体去凝缩，每组染色体周围重新形成核膜，纺锤丝解体。随后是胞质分裂（细胞质分裂），动物细胞通过分裂沟进行，植物细胞则通过形成细胞板产生新的细胞壁。由于调控细胞周期的基因突变而导致的不受控制的细胞分裂可引发癌症。理解有丝分裂对于解释生物体的生长、组织再生和繁殖至关重要。
6. Nucleic Acids and Protein Synthesis | 核酸与蛋白质合成
Nucleic acids, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are polymers of nucleotides. A nucleotide consists of a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base. In DNA, the bases are adenine (A), thymine (T), cytosine (C), and guanine (G). DNA is a double helix: two antiparallel strands held together by hydrogen bonds between complementary base pairs—A with T (two hydrogen bonds), C with G (three hydrogen bonds). This structure ensures stability and accurate replication. DNA replication is semiconservative: each original strand serves as a template for a new complementary strand. The enzyme DNA helicase unwinds and unzips the double helix; DNA polymerase then adds free nucleotides in a 5′ to 3′ direction, forming phosphodiester bonds. The leading strand is synthesised continuously, while the lagging strand is formed in Okazaki fragments that are later joined by DNA ligase.
核酸，DNA（脱氧核糖核酸）和 RNA（核糖核酸），是核苷酸的聚合物。一个核苷酸由一个磷酸基团、一个戊糖（DNA 中为脱氧核糖，RNA 中为核糖）和一个含氮碱基组成。在 DNA 中，碱基为腺嘌呤 (A)、胸腺嘧啶 (T)、胞嘧啶 (C) 和鸟嘌呤 (G)。DNA 是双螺旋结构：两条反平行的链通过互补碱基对之间的氢键连接在一起——A 与 T 配对（两个氢键），C 与 G 配对（三个氢键）。这种结构保证了稳定性和精确复制。DNA 复制是半保留的：每条原始链作为合成新互补链的模板。DNA 解旋酶解旋并解开双螺旋；随后 DNA 聚合酶沿 5′ 到 3′ 方向添加游离核苷酸，形成磷酸二酯键。前导链是连续合成的，而滞后链则以冈崎片段形式合成，随后由 DNA 连接酶连接。
Protein synthesis involves transcription and translation. In transcription, an enzyme RNA polymerase binds to a region of DNA, unwinds it, and synthesises a complementary mRNA strand using one of the DNA strands as a template (A–U, C–G). The mRNA then carries the genetic code out of the nucleus to a ribosome in the cytoplasm. Translation occurs when the ribosome reads the mRNA in sets of three bases called codons. Transfer RNA (tRNA) molecules carry specific amino acids; each tRNA has an anticodon that base-pairs with the complementary codon on the mRNA. The ribosome facilitates the formation of peptide bonds between adjacent amino acids, building a polypeptide chain. The genetic code is degenerate (more than one codon can code for the same amino acid) and universal. This process underlies how genes determine the structure and function of proteins, including enzymes and structural components.
蛋白质合成涉及转录和翻译。在转录中，RNA 聚合酶与 DNA 的一段区域结合，解开双链，并以一条 DNA 链为模板合成互补的 mRNA 链（A–U，C–G）。随后 mRNA 将遗传密码携带出细胞核，进入细胞质中的核糖体。翻译发生时，核糖体以三个碱基为一组（密码子）读取 mRNA。转运 RNA (tRNA) 分子携带特定氨基酸；每个 tRNA 有一个反密码子，与 mRNA 上的互补密码子进行碱基配对。核糖体促进相邻氨基酸之间形成肽键，构建多肽链。遗传密码具有简并性（不止一个密码子可编码同一种氨基酸）和通用性。这个过程说明了基因如何决定蛋白质（包括酶和结构组分）的结构和功能。
7. Transport in Plants | 植物运输
Vascular plants possess two transport systems: xylem and phloem. Xylem tissue transports water and dissolved mineral ions from the roots upwards to the leaves. It is composed of dead cells with no end walls or cytoplasm, forming hollow tubes strengthened by lignin. The cohesion-tension theory explains water movement in the xylem. Transpiration (evaporation of water from the stomata of leaves) creates a negative pressure or tension at the top of the plant. This tension pulls water up through the xylem because water molecules exhibit strong cohesion (hydrogen bonding between water molecules) and adhesion (water molecules sticking to the xylem walls). The continuous column of water is maintained from roots to leaves. Transpiration rate is affected by light intensity, temperature, humidity, and wind speed; it can be measured using a potometer. Mineral ions are actively taken up by root hair cells, lowering water potential in the root and promoting water uptake by osmosis via the symplast and apoplast pathways.
维管植物拥有两种运输系统：木质部和韧皮部。木质部组织将水和溶解的矿物质离子从根部向上运输到叶片。它由死细胞组成，没有端壁和细胞质，形成由木质素加固的中空管道。内聚力-张力假说解释了木质部中水的移动。蒸腾作用（水分从叶片气孔蒸发）在植物顶部产生负压或张力。这种张力将水沿着木质部向上拉动，因为水分子表现出强大的内聚力（水分子之间的氢键）和附着力（水分子粘附在木质部壁上）。从根部到叶片维持着连续的水柱。蒸腾速率受光照强度、温度、湿度和风速的影响；可用蒸腾计测量。矿质离子由根毛细胞主动吸收，降低根部水势，并通过共质体和质外体途径促进水分通过渗透进入。
Phloem tissue transports assimilates, mainly sucrose and amino acids, from sources (where they are produced, e.g., leaves) to sinks (where they are used or stored, e.g., roots, fruits). This process is called translocation and is explained by the mass flow hypothesis. At the source, sucrose is actively loaded into sieve tubes, reducing water potential and causing water to enter from the xylem by osmosis. The increased hydrostatic pressure pushes the phloem sap toward the sink, where sucrose is unloaded, water follows out, and pressure drops. The continuous pressure gradient drives bulk flow. Sieve tube elements are living cells but lack nuclei and many organelles; they are associated with companion cells that provide metabolic support. Understanding plant transport systems explains how plants obtain water and nutrients and distribute the products of photosynthesis.
韧皮部组织运输同化物，主要是蔗糖和氨基酸，从源（产生部位，如叶片）到库（利用或储存部位，如根、果实）。这一过程称为转运，可用集流假说解释。在源端，蔗糖被主动装载到筛管中，降低水势，导致水分通过渗透从木质部进入。增加的静水压将韧皮部汁液推向库端，在那里蔗糖被卸出，水分随之流出，压力下降。持续的静水压梯度驱动集流。筛管分子是活细胞，但缺乏细胞核和许多细胞器；它们与伴胞相连，由伴胞提供代谢支持。理解植物运输系统可以解释植物如何获取水分和养分，以及如何分配光合作用产物。
8. Transport in Mammals | 哺乳动物运输
The mammalian circulatory system is a closed, double circulation consisting of the pulmonary circulation (to and from the lungs) and the systemic circulation (to and from the rest of the body). The heart is a muscular organ with four chambers: right atrium, right ventricle, left atrium, and left ventricle. Deoxygenated blood returns from the body into the right atrium via the vena cavae, passes to the right ventricle, and is pumped to the lungs through the pulmonary artery. Oxygenated blood returns from the lungs via the pulmonary veins to the left atrium, enters the left ventricle, and is pumped through the aorta to the body. Valves (atrioventricular and semilunar) prevent backflow. The cardiac cycle includes systole (contraction) and diastole (relaxation). The sinoatrial node (SAN) acts as the pacemaker, initiating a wave of electrical activity that causes atrial contraction; the atrioventricular node (AVN) delays the impulse before it spreads through the bundle of His and Purkinje fibres to the ventricles, causing ventricular contraction. The sequence coordinates efficient pumping.
哺乳动物的循环系统是一个封闭的双循环系统，包括肺循环（往返肺部）和体循环（往返身体其他部分）。心脏是一个肌肉器官，有四个腔室：右心房、右心室、左心房和左心室。脱氧血液从身体经上下腔静脉返回右心房，进入右心室，通过肺动脉泵入肺部。含氧血液从肺部经肺静脉返回左心房，进入左心室，通过主动脉泵送到全身。瓣膜（房室瓣和半月瓣）防止血液倒流。心动周期包括收缩期和舒张期。窦房结 (SAN) 充当起搏器，启动电活动波，导致心房收缩；房室结 (AVN) 将冲动延迟，随后经房室束和浦肯野纤维传至心室，引起心室收缩。这一序列协调了高效的泵血。
Blood is composed of plasma, red blood cells (erythrocytes), white blood cells (leucocytes), and platelets. Red blood cells contain haemoglobin, a quaternary protein with four polypeptide chains and iron-containing haem groups that bind oxygen in the lungs and release it in respiring tissues. The oxygen dissociation curve shows how haemoglobin’s affinity for oxygen changes with partial pressure of oxygen; it is sigmoidal due to cooperative binding. Carbon dioxide is transported in the blood as hydrogen carbonate ions (HCO₃⁻), dissolved in plasma, and bound to haemoglobin as carbaminohaemoglobin. The Bohr effect describes how increased carbon dioxide concentration lowers pH, reducing haemoglobin’s affinity for oxygen and promoting oxygen release to active tissues. Tissue fluid forms at the arterial end of capillaries by filtration under high hydrostatic pressure, and returns at the venous end due to osmotic pull of plasma proteins; the lymphatic system drains excess fluid back to the blood. Transport in mammals ensures efficient delivery of oxygen and nutrients and removal of wastes.
血液由血浆、红细胞、白细胞和血小板组成。红细胞含有血红蛋白，这是一种具有四条多肽链的四级结构蛋白质，每条链含有含铁的血红素基团，在肺部结合氧并在呼吸组织释放氧。氧解离曲线显示血红蛋白对氧的亲和力如何随氧分压变化；它呈 S 形是由于协同结合。二氧化碳在血液中以碳酸氢根离子 (HCO₃⁻) 形式运输、溶解在血浆中，以及与血红蛋白结合形成氨基甲酸血红蛋白。玻尔效应描述了二氧化碳浓度升高会降低 pH，降低血红蛋白对氧的亲和力，促进氧向活跃组织释放。组织液在毛细血管动脉端因高静水压通过滤过形成，在静脉端因血浆蛋白的渗透吸引而返回；淋巴系统将多余的液体送回血液。哺乳动物的运输系统确保了氧气和营养物质的有效输送以及废物的清除。
9. Gas Exchange | 气体交换
Gas exchange is the process by which organisms take in oxygen from the environment and release carbon dioxide. In humans, this occurs in the alveoli of the lungs. The mammalian gas exchange system includes the nasal cavity, trachea, bronchi, bronchioles, and alveolar ducts ending in clusters of alveoli. The trachea and bronchi are supported by rings of cartilage to prevent collapse; ciliated epithelial cells and goblet cells (which secrete mucus) line the airways to trap and move pathogens and particles toward the throat. Alveoli are well adapted for gas exchange: they have a large surface area, very thin walls (one cell thick squamous epithelium), are surrounded by an extensive capillary network, and are moist. Oxygen diffuses from the alveolar air into the blood down a concentration gradient; carbon dioxide diffuses in the opposite direction. Ventilation is the movement of air in and out of the lungs. During inspiration, the external intercostal muscles contract, and the diaphragm contracts and flattens, increasing thoracic volume and decreasing pressure, drawing air in. During expiration, these muscles relax (and during forced expiration, internal intercostal muscles contract and abdominal muscles contract), decreasing volume and pushing air out.
气体交换是生物体从环境中获取氧气并释放二氧化碳的过程。在人体中，这一过程发生在肺的肺泡中。哺乳动物的气体交换系统包括鼻腔、气管、支气管、细支气管以及末端成簇的肺泡管。气管和支气管由软骨环支撑以防止塌陷；纤毛上皮细胞和杯状细胞（分泌黏液）排列在气道内，用以捕获病原体和颗粒并将其推向咽喉。肺泡非常适合气体交换：它们具有大面积表面积、极薄的壁（单层鳞状上皮）、被丰富的毛细血管网包围，并且保持湿润。氧气顺浓度梯度从肺泡空气中的扩散到血液中；二氧化碳以相反方向扩散。通气是空气进出肺的过程。吸气时，外肋间肌收缩，膈肌收缩并变平，胸腔体积增大，压力降低，空气被吸入。呼气时，这些肌肉松弛（而在用力呼气时，内肋间肌收缩，腹肌收缩），体积减小，将空气推出。
In other organisms, gas exchange surfaces vary. Fish use gills with a countercurrent flow system, where water flows over the gill lamellae in the opposite direction to blood flow, maintaining a concentration gradient along the entire lamella and maximising oxygen uptake. Insects have a tracheal system: spiracles lead to tracheae reinforced by chitin, which branch into finer tracheoles that deliver oxygen directly to tissues and remove carbon dioxide; ventilation may be assisted by body movements. Plants take in carbon dioxide and release oxygen through stomata in leaves; these pores are controlled by guard cells. Gas exchange in all organisms relies on diffusion and therefore requires a large surface area, thin exchange surface, concentration gradient maintenance, and moisture.
在其他生物中，气体交换表面各有不同。鱼类使用带有逆流交换系统的鳃，水流经鳃薄片的方向与血流方向相反，从而沿整个薄片维持浓度梯度，最大限度地摄取氧气。昆虫拥有气管系统：气门通向由几丁质加固的气管，气管分支为更细的微气管，将氧气直接输送到组织并排出二氧化碳；通风可通过身体运动辅助。植物通过叶片上的气孔吸收二氧化碳并释放氧气；这些孔隙由保卫细胞控制。所有生物的气体交换都依靠扩散，因此需要大面积表面积、薄交换面、浓度梯度的维持和湿润环境。
10. Infectious Diseases and Immunity | 传染病与免疫
Infectious diseases are caused by pathogens: bacteria, viruses, fungi, and protoctists. Bacteria are prokaryotic cells that can reproduce rapidly by binary fission and may produce toxins. Viruses are acellular and consist of genetic material (DNA or RNA) inside a protein coat (capsid); they invade host cells and use the host’s machinery to replicate. Examples of diseases: cholera (bacterium), tuberculosis (bacterium), HIV/AIDS (virus), and malaria (protoctist). Transmission can be direct (contact, droplet inhalation) or indirect (contaminated food, vectors). The body has several defence mechanisms. Non-specific defences include physical barriers like skin and mucous membranes, phagocytosis by macrophages and neutrophils, and the inflammatory response. Specific immune response involves lymphocytes: B cells mature in bone marrow and produce antibodies that are specific to antigens. Each B cell is programmed to make one type of antibody. Upon binding to its specific antigen, a B cell is activated and clones into plasma cells (secreting large quantities of antibodies) and memory cells (providing long-term immunity).
传染病由病原体引起：细菌、病毒、真菌和原生生物。细菌是原核细胞，可通过二分裂快速繁殖，并可能产生毒素。病毒是非细胞结构，由蛋白质外壳（衣壳）内的遗传物质（DNA 或 RNA）组成；它们侵入宿主细胞并利用宿主的机制进行复制。疾病举例：霍乱（细菌）、结核病（细菌）、HIV/艾滋病（病毒）和疟疾（原生生物）。传播途径可以是直接的（接触、飞沫吸入）或间接的（污染食物、媒介）。人体有多种防御机制。非特异性防御包括物理屏障，如皮肤和黏膜，巨噬细胞和中性粒细胞的吞噬作用，以及炎症反应。特异性免疫应答涉及淋巴细胞：B 细胞在骨髓中成熟，并产生针对抗原的特异性抗体。每个 B 细胞只产生一种抗体。当与特异性抗原结合后，B 细胞被激活并克隆成浆细胞（分泌大量抗体）和记忆细胞（提供长期免疫）。
T cells mature in the thymus. Helper T cells release cytokines that activate B cells and cytotoxic T cells. Cytotoxic T cells kill infected cells. Antibodies defend the body by neutralising toxins, agglutinating pathogens, and marking them for phagocytosis. The primary immune response is slower and produces fewer antibodies; the secondary response is faster and stronger due to memory cells. Natural active immunity results from infection; artificial active immunity from vaccination. Vaccines contain antigens or weakened pathogens that stimulate an immune response without causing disease, creating memory cells. Passive immunity involves receiving antibodies from another source (e.g., across the placenta or in breast milk, or by injection of antiserum), providing immediate but short-lived protection. Understanding immunity is critical for designing strategies to prevent and control infectious diseases. Antibiotics are used to treat bacterial infections by targeting bacterial cell structures (e.g., cell wall synthesis) without harming host cells, but they are ineffective against viruses.
T 细胞在胸腺中成熟。辅助性 T 细胞释放细胞因子，激活 B 细胞和细胞毒性 T 细胞。细胞毒性 T 细胞杀死受感染的细胞。抗体通过中和毒素、凝集病原体以及标记病原体以供吞噬等方式防御身体。初次免疫应答较慢，产生抗体较少；二次应答因记忆细胞的存在而更快更强。自然主动免疫源于感染；人工主动免疫来自疫苗接种。疫苗含有抗原或减毒病原体，能刺激免疫应答而不引起疾病，从而产生记忆细胞。被动免疫涉及从其他来源获取抗体（如通过胎盘或母乳传递，或注射抗血清），提供即时但短暂的保护。理解免疫对于设计预防和控制传染病的策略至关重要。抗生素用于治疗细菌感染，通过靶向细菌细胞结构（如细胞壁合成）而不损害宿主细胞，但对病毒无效。
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