📚 AS CCEA Biology: Core Concepts Summary | AS CCEA 生物：核心知识点梳理
This article provides a structured summary of the essential topics covered in the AS CCEA Biology specification. It highlights key concepts from both Unit AS 1: Molecules and Cells and Unit AS 2: Organisms and Biodiversity. Each section pairs fundamental explanations with clear bilingual notes, helping students consolidate knowledge and prepare effectively for examinations.
本文系统地梳理了 AS CCEA 生物课程的核心知识点，涵盖单元 AS 1（分子与细胞）和单元 AS 2（生物体与生物多样性）。每个部分均以中英双语对照的形式解析关键概念，帮助学生巩固基础，高效备考。
1. Biological Molecules and Water | 生物分子与水
Living organisms are built from a few key biological molecules: carbohydrates, lipids, proteins and nucleic acids. Carbohydrates exist as monosaccharides (e.g. glucose, C₆H₁₂O₆), disaccharides (e.g. sucrose) and polysaccharides (starch, glycogen, cellulose). Lipids, such as triglycerides, are composed of glycerol and three fatty acids; phospholipids are amphipathic and form the basis of cell membranes. Proteins are polymers of amino acids linked by peptide bonds, folding into primary, secondary, tertiary and quaternary structures. Water is a polar molecule that acts as a universal solvent, has a high specific heat capacity and strong cohesion due to hydrogen bonding, which makes it vital for transport, temperature regulation and metabolic reactions.
生物体由几类重要的生物分子构成：碳水化合物、脂质、蛋白质和核酸。碳水化合物分为单糖（如葡萄糖 C₆H₁₂O₆）、二糖（如蔗糖）和多糖（淀粉、糖原、纤维素）。脂质如甘油三酯由甘油和三个脂肪酸组成；磷脂具有双亲性，是细胞膜的基础。蛋白质是由氨基酸通过肽键连接而成的多聚体，折叠形成一级至四级结构。水是极性分子，作为通用溶剂，因氢键而具有高比热容和强内聚力，对运输、体温调节和代谢反应至关重要。
2. Enzymes and Factors Affecting Activity | 酶及其活性影响因素
Enzymes are globular proteins that act as biological catalysts, lowering activation energy without being consumed. Their activity depends on the specific three-dimensional shape of the active site. The induced-fit model describes how the active site molds around the substrate. Factors affecting enzyme action include temperature, pH, substrate concentration and inhibitor presence. Competitive inhibitors resemble the substrate and bind to the active site; non-competitive inhibitors bind elsewhere and alter the enzyme’s shape. Denaturation occurs at extreme pH or high temperature, causing permanent loss of function.
酶是球状蛋白，作为生物催化剂降低活化能而不被消耗。其活性依赖于活性位点特定的三维形状。诱导契合模型描述了活性位点如何围绕底物形变。影响酶活性的因素包括温度、pH、底物浓度以及抑制剂的存在。竞争性抑制剂结构与底物相似，占据活性位点；非竞争性抑制剂结合在别处，改变酶的形状。极端 pH 或高温会导致变性，使酶永久失活。
3. Cell Structure: Prokaryotes and Eukaryotes | 细胞结构：原核与真核细胞
Eukaryotic cells possess a true nucleus enclosing linear DNA, and membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus and (in plants) chloroplasts and a large permanent vacuole. Prokaryotic cells, like bacteria, lack a membrane-bound nucleus; their circular DNA lies free in the cytoplasm, and they contain 70S ribosomes. Organelles perform specialised functions: mitochondria are the sites of aerobic respiration; chloroplasts carry out photosynthesis; ribosomes synthesise proteins. Recognising these structures under light and electron microscopes and linking structure to function is a core skill.
真核细胞具有真正的细胞核，内含线性 DNA，以及膜包被的细胞器，如线粒体、内质网、高尔基体和（植物中的）叶绿体及大液泡。原核细胞（如细菌）没有膜包被的细胞核，环状 DNA 游离在细胞质中，核糖体为 70S 型。各细胞器分工明确：线粒体是有氧呼吸的场所；叶绿体进行光合作用；核糖体合成蛋白质。识别光镜和电镜下这些结构并将结构关联功能，是一项核心技能。
4. Cell Membranes and Transport | 细胞膜与物质运输
The fluid-mosaic model describes the cell membrane as a phospholipid bilayer with embedded proteins, cholesterol (in animals) and glycoproteins. It is selectively permeable. Transport across membranes can be passive – diffusion (including facilitated diffusion via channel and carrier proteins) and osmosis – or active, requiring energy in the form of ATP. Active transport moves substances against their concentration gradient using carrier proteins. Bulk transport, such as endocytosis and exocytosis, involves vesicle formation. Osmosis is the net movement of water from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
流动镶嵌模型将细胞膜描述为磷脂双分子层，其中嵌有蛋白质、胆固醇（动物细胞）和糖蛋白。膜具有选择透过性。跨膜运输可以是被动过程——扩散（包括经通道蛋白和载体蛋白的协助扩散）和渗透——也可以是主动运输，需要 ATP 供能。主动运输利用载体蛋白逆浓度梯度转运物质。胞吞和胞吐等批量运输涉及囊泡的形成。渗透是水分子通过半透膜从较高水势区域向较低水势区域的净移动。
5. The Cell Cycle and Mitosis | 细胞周期与有丝分裂
The cell cycle comprises interphase (G₁, S, G₂) and mitotic phase. During S phase, DNA replicates, so each chromosome consists of two sister chromatids held at the centromere. Mitosis produces two genetically identical diploid daughter cells for growth and repair. The stages are prophase (chromosomes condense, nuclear envelope breaks down), metaphase (chromosomes align at the equator, spindle fibres attach), anaphase (sister chromatids separate to opposite poles) and telophase (nuclei reform, chromosomes decondense). Cytokinesis follows, dividing the cytoplasm. Cancer can result from uncontrolled cell division due to mutations in genes regulating the cycle.
细胞周期包括分裂间期（G₁、S、G₂）和分裂期。S 期 DNA 复制，每条染色体包含两条由着丝粒连接的姐妹染色单体。有丝分裂产生两个遗传相同的二倍体子细胞，用于生长和修复。各阶段为：前期（染色体凝集，核膜解体）、中期（染色体排列在赤道板，纺锤丝附着）、后期（姐妹染色单体分离移向两极）和末期（核膜重建，染色体解旋）。随后胞质分裂将细胞质分开。癌症可能源于调控周期的基因突变导致细胞分裂失控。
6. DNA Replication and Protein Synthesis | DNA复制与蛋白质合成
DNA replication is semi-conservative: each new molecule contains one original strand and one newly synthesised strand. The enzyme DNA helicase unwinds the double helix; DNA polymerase adds complementary nucleotides following base-pairing rules (A–T, C–G). The leading strand is synthesised continuously, the lagging strand in Okazaki fragments. In protein synthesis, transcription produces mRNA complementary to the template strand of DNA. In eukaryotes, pre-mRNA is spliced to remove introns. Translation occurs on ribosomes: tRNA molecules carrying specific amino acids bind to mRNA codons via anticodons, forming a polypeptide chain. The genetic code is degenerate and universal.
DNA 复制是半保留的：每个新分子包含一条原始链和一条新合成链。DNA 解旋酶解开双螺旋，DNA 聚合酶按照碱基配对规则（A–T，C–G）添加互补核苷酸。前导链连续合成，后随链以冈崎片段方式合成。在蛋白质合成中，转录产生与 DNA 模板链互补的 mRNA。真核生物中，前体 mRNA 需剪接除去内含子。翻译在核糖体上进行：携带特定氨基酸的 tRNA 分子通过反密码子与 mRNA 密码子结合，形成多肽链。遗传密码具有简并性和通用性。
7. Meiosis and Genetic Variation | 减数分裂与遗传变异
Meiosis reduces the chromosome number by half to produce haploid gametes. It involves two divisions: meiosis I separates homologous chromosomes, while meiosis II separates sister chromatids. Crossing over in prophase I and independent assortment of homologous pairs in metaphase I generate genetic variation. The random fertilisation of gametes further increases variation. Errors in meiosis can lead to chromosome mutations such as non-disjunction, causing conditions like Down syndrome (trisomy 21). Meiosis is essential for sexual reproduction and contributes to the diversity needed for natural selection.
减数分裂将染色体数目减半，产生单倍体配子。它包含两次分裂：第一次分裂分离同源染色体，第二次分裂分离姐妹染色单体。前期 I 的交叉互换和中期 I 同源染色体的独立分配产生遗传变异。配子的随机受精进一步增加变异。减数分裂中的错误可导致染色体突变，如不分离，引起唐氏综合征（21 三体）等。减数分裂对有性生殖至关重要，并为自然选择提供所需的多样性。
8. Monohybrid Inheritance and Sex Linkage | 单基因遗传与伴性遗传
Monohybrid inheritance involves a single gene with alleles. Using Punnett squares, we can predict phenotypic ratios from crosses. Dominant alleles mask recessive ones in heterozygotes. The expected 3:1 F₂ ratio in a monohybrid cross assumes random fertilisation and independent assortment. Sex linkage refers to genes located on sex chromosomes, most often the X chromosome. Because human males have only one X chromosome, recessive X-linked alleles are always expressed in males, leading to conditions like haemophilia and red-green colour blindness. Pedigree analysis helps deduce inheritance patterns.
单基因遗传涉及具有等位基因的单个基因。利用庞纳特方格可以预测杂交的表型比例。在杂合子中，显性等位基因掩盖隐性等位基因。单基因杂交预期的 3∶1 F₂ 表型比建立在随机受精和独立分配的基础之上。伴性遗传指位于性染色体（常为 X 染色体）上的基因。由于人类男性只有一条 X 染色体，隐性 X 连锁等位基因在男性中总是表达，导致血友病和红绿色盲等。系谱分析有助于推断遗传模式。
9. Biodiversity and Classification | 生物多样性与分类
Biodiversity encompasses species diversity, genetic diversity within species, and habitat diversity. Species are defined as groups of organisms that can interbreed to produce fertile offspring. Classification organises organisms into a hierarchy: domain, kingdom, phylum, class, order, family, genus and species. The binomial naming system uses genus and species. The three domains are Bacteria, Archaea and Eukarya. Techniques such as DNA barcoding and phylogenetic trees help reveal evolutionary relationships. High biodiversity contributes to ecosystem stability, but human activities such as deforestation and pollution reduce it.
生物多样性包括物种多样性、物种内的遗传多样性以及栖息地多样性。物种被定义为能相互交配并产生可育后代的生物群体。分类将生物体组织为域、界、门、纲、目、科、属和种的等级体系。双名法使用属名和种名。三个域为细菌、古菌和真核生物域。DNA 条形码和系统发育树等技术有助于揭示进化关系。高生物多样性有助于生态系统稳定，但森林砍伐和污染等人类活动使其降低。
10. Transport Systems in Plants | 植物的运输系统
Xylem vessels transport water and dissolved mineral ions from roots to leaves. Water movement is driven by transpiration pull, cohesion between water molecules and adhesion to xylem walls. This cohesion-tension theory explains how water rises against gravity. Phloem translocates sucrose and amino acids from sources (e.g. leaves) to sinks (e.g. roots, fruits) in a process called translocation. The mass flow hypothesis suggests that active loading of sucrose into phloem at the source lowers water potential, drawing water in by osmosis and creating a pressure gradient that moves sap. Sieve tube elements and companion cells are key components.
木质部导管将水和溶解的无机离子从根运输到叶。水分的移动由蒸腾拉力、水分子间的内聚力以及对导管壁的附着力驱动。内聚力-张力理论解释了水逆重力上升的机制。韧皮部将蔗糖和氨基酸从源（如叶片）转运到库（如根、果实），这一过程称为运输作用。压力流动假说认为，在源处活跃地装载蔗糖进入韧皮部会降低水势，通过渗透吸水产生压力梯度，推动汁液流动。筛管分子和伴胞是其中的关键结构。
11. The Mammalian Circulatory System | 哺乳动物的循环系统
Mammals have a closed, double circulatory system. The heart comprises four chambers: right and left atria and ventricles. Deoxygenated blood flows from the right ventricle to the lungs via the pulmonary artery; oxygenated blood returns via the pulmonary vein to the left atrium, then is pumped by the left ventricle through the aorta to the body. Cardiac muscle is myogenic; the sinoatrial node initiates the heartbeat. Arteries carry blood away from the heart at high pressure, veins return blood at low pressure with valves to prevent backflow, and capillaries allow exchange of substances. Blood transports oxygen (on haemoglobin), carbon dioxide, nutrients, hormones and waste.
哺乳动物具有封闭的双循环系统。心脏由四个腔室组成：左、右心房和心室。缺氧血从右心室经肺动脉流向肺部；含氧血经肺静脉返回左心房，然后由左心室泵入主动脉输送到全身。心肌是肌源性的；窦房结发起心跳。动脉将血液在高压下运离心脏，静脉在低压下将血液送回，内有瓣膜防止倒流，毛细血管则进行物质交换。血液运输氧气（结合在血红蛋白上）、二氧化碳、营养物质、激素和代谢废物。
12. Gas Exchange Adaptations | 气体交换的适应
Efficient gas exchange surfaces have a large surface area, are thin, moist and well supplied with blood. In humans, alveoli in the lungs provide a huge surface area. Breathing movements (ventilation) maintain concentration gradients. Fish gills use a countercurrent flow system where blood and water flow in opposite directions, maximising oxygen uptake. Insects have a tracheal system delivering oxygen directly to tissues; spiracles can open and close to reduce water loss. Plants exchange gases through stomata, which are controlled by guard cells. The opening and closing mechanism is influenced by light, CO₂ concentration and water availability.
高效的气体交换表面具有较大的表面积，薄且湿润，并有良好的血液供应。人类肺中的肺泡提供了巨大的表面积。呼吸运动（通气）维持浓度梯度。鱼鳃利用逆流交换系统，血液与水流方向相反，从而最大化氧气的吸收。昆虫拥有气管系统将氧气直接输送到组织；气门可开闭以减少水分流失。植物通过气孔进行气体交换，气孔由保卫细胞控制开闭。气孔的开启与关闭受光照、CO₂ 浓度和水分供应的影响。
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