📚 Pre-U CAIE Biology: Core Concepts Review | Pre-U CAIE 生物：核心知识点梳理
This comprehensive review covers the essential topics required for the Cambridge Pre-U Biology syllabus. It is designed to consolidate your understanding of cell biology, biochemistry, genetics, physiology, and ecology. Each section presents key facts in English immediately followed by a paired Chinese translation to support bilingual learners and reinforce memory retention.
这份全面的复习资料涵盖了剑桥 Pre-U 生物课程必需的核心主题，旨在帮助你巩固细胞生物学、生物化学、遗传学、生理学和生态学知识。每个要点先以英文呈现，紧接中文双语对照，强化双语学习与记忆。
1. Cell Structure and Organelles | 细胞结构与细胞器
All living organisms are composed of cells. Eukaryotic cells possess a true nucleus enclosed by a nuclear envelope and contain membrane-bound organelles such as mitochondria, the endoplasmic reticulum, the Golgi apparatus, lysosomes, and, in plants, chloroplasts. Prokaryotic cells lack a nucleus and membrane-bound organelles; their DNA is free in the cytoplasm as a circular chromosome, and they often have a cell wall containing peptidoglycan.
所有生物都是由细胞构成的。真核细胞具有由核膜包裹的真正细胞核，并含有膜包被的细胞器，如线粒体、内质网、高尔基体、溶酶体以及植物细胞中的叶绿体。原核细胞没有细胞核和膜包被的细胞器；其DNA以环状染色体的形式游离在细胞质中，且通常具有含肽聚糖的细胞壁。
Key organelles and their functions include: the nucleus (stores genetic information and controls cell activities), mitochondria (site of aerobic respiration, producing ATP), ribosomes (protein synthesis), rough ER (protein transport and processing), smooth ER (lipid synthesis), Golgi apparatus (modifies, sorts, and packages proteins), lysosomes (digestion of macromolecules and worn-out organelles), and chloroplasts (site of photosynthesis in plant cells).
关键细胞器及其功能包括：细胞核（储存遗传信息，调控细胞活动）、线粒体（有氧呼吸场所，产生ATP）、核糖体（蛋白质合成）、粗面内质网（蛋白质运输与加工）、滑面内质网（脂质合成）、高尔基体（修饰、分拣和包装蛋白质）、溶酶体（消化大分子及衰老细胞器）以及叶绿体（植物细胞光合作用场所）。
| Organelle | Function |
|---|---|
| Mitochondrion | Aerobic respiration / ATP production |
| Ribosome | Translation / protein synthesis |
| Golgi apparatus | Modification and packaging of proteins into vesicles |
| Chloroplast | Photosynthesis / light-dependent and light-independent reactions |
2. Biological Molecules: Carbohydrates, Lipids, Proteins | 生物分子：碳水化合物、脂质、蛋白质
Carbohydrates are composed of saccharide monomers. Monosaccharides such as glucose (C₆H₁₂O₆) are the simplest units. Disaccharides form by a glycosidic bond (e.g., maltose from two glucose units, sucrose from glucose and fructose). Polysaccharides like starch, glycogen, and cellulose are polymers of glucose; starch and glycogen serve as energy storage, while cellulose provides structural support in plant cell walls.
碳水化合物由糖类单体组成。单糖如葡萄糖（C₆H₁₂O₆）是最简单的单元。二糖通过糖苷键形成（例如两个葡萄糖形成麦芽糖，葡萄糖与果糖形成蔗糖）。多糖如淀粉、糖原和纤维素都是葡萄糖的聚合物；淀粉和糖原用于储存能量，而纤维素为植物细胞壁提供结构支撑。
Lipids are non-polar macromolecules including triglycerides (fats) and phospholipids. Triglycerides consist of one glycerol molecule esterified to three fatty acids; they function as long-term energy stores and thermal insulators. Phospholipids have a hydrophilic phosphate head and two hydrophobic fatty acid tails, spontaneously forming bilayers in aqueous environments—the basis of cell membranes.
脂质是非极性大分子，包括甘油三酯（脂肪）和磷脂。甘油三酯由一分子甘油与三分子脂肪酸通过酯键结合而成；它们作为长期储能物质和隔热层。磷脂具有亲水的磷酸头部和两条疏水的脂肪酸尾部，在水环境中自发形成双分子层——这是细胞膜的基础。
Proteins are polymers of amino acids joined by peptide bonds. The sequence of amino acids determines the primary structure, which folds into secondary structures (α-helix and β-pleated sheet) stabilised by hydrogen bonds. The tertiary structure is the overall 3D shape held by hydrogen bonds, ionic bonds, disulfide bridges, and hydrophobic interactions. Quaternary structure arises when more than one polypeptide chain associates, as in haemoglobin.
蛋白质是由氨基酸通过肽键连接成的多聚体。氨基酸序列决定一级结构，进而折叠为二级结构（α-螺旋和β-折叠片层），由氢键稳定。三级结构是整体的三维形态，由氢键、离子键、二硫键以及疏水相互作用维持。四级结构出现于多条多肽链结合时，如血红蛋白。
3. Enzymes: Kinetics and Inhibition | 酶：动力学与抑制作用
Enzymes are biological catalysts that lower activation energy without being consumed. They possess a specific active site complementary to the substrate, forming an enzyme-substrate complex. The induced-fit model states that the active site changes shape slightly to accommodate the substrate, facilitating the reaction. Factors affecting enzyme activity include temperature, pH, enzyme concentration, and substrate concentration.
酶是生物催化剂，能降低活化能而自身不被消耗。酶具有与底物互补的特异性活性位点，形成酶‑底物复合物。诱导契合模型指出，活性位点会略微改变形状以容纳底物，从而促进反应。影响酶活性的因素包括温度、pH、酶浓度和底物浓度。
Inhibition can be competitive or non-competitive. Competitive inhibitors bind to the active site, blocking substrate binding; this effect can be overcome by increasing substrate concentration. Non-competitive inhibitors bind to an allosteric site, altering the enzyme's shape so the substrate can no longer bind effectively; increasing substrate concentration does not reverse this inhibition.
抑制作用分为竞争性抑制和非竞争性抑制。竞争性抑制剂与活性位点结合，阻碍底物结合；提高底物浓度可克服这种抑制。非竞争性抑制剂结合于酶的别构位点，改变酶的形状，使底物无法有效结合；增加底物浓度不能逆转这类抑制。
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), glycolipids, and glycoproteins. The bilayer is fluid, allowing lateral movement of lipids and proteins. Membrane proteins function as channels, carriers, receptors, and enzymes. Cholesterol modulates membrane fluidity and stability.
细胞膜可用流动镶嵌模型描述：由磷脂双分子层和镶嵌其中的蛋白质、胆固醇（动物细胞）、糖脂与糖蛋白构成。双分子层具有流动性，允许脂质和蛋白质侧向移动。膜蛋白的功能包括通道、载体、受体和酶。胆固醇调节膜的流动性和稳定性。
Transport across membranes may be passive or active. Passive transport includes diffusion (e.g., oxygen, carbon dioxide) and facilitated diffusion via channel or carrier proteins; no energy is required. Osmosis is the net movement of water across a selectively permeable membrane from higher to lower water potential. Active transport uses ATP to move substances against their concentration gradient, e.g., the Na⁺/K⁺ pump. Bulk transport occurs through endocytosis and exocytosis.
跨膜运输可分为被动运输和主动运输。被动运输包括扩散（如氧气、二氧化碳）和通过通道蛋白或载体蛋白的易化扩散，均不消耗能量。渗透是水通过选择透过性膜从高水势向低水势的净移动。主动运输利用ATP逆浓度梯度转运物质，例如钠‑钾泵。大分子运输通过胞吞作用与胞吐作用实现。
5. Cell Cycle and Mitosis | 细胞周期与有丝分裂
The cell cycle consists of interphase (G₁, S, G₂) and mitotic phase (mitosis and cytokinesis). During S phase, DNA replication occurs, so each chromosome consists of two sister chromatids. Mitosis is divided into prophase, metaphase, anaphase, and telophase. It produces two genetically identical daughter nuclei, essential for growth, repair, and asexual reproduction.
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期进行DNA复制，因此每条染色体由两条姐妹染色单体组成。有丝分裂分为前期、中期、后期和末期。它产生两个遗传上相同的子细胞核，对生长、修复和无性繁殖至关重要。
- Prophase: chromatin condenses into chromosomes, nuclear envelope breaks down, spindle fibres form.
- Metaphase: chromosomes align at the metaphase plate, attached to spindle fibres via kinetochores.
- Anaphase: sister chromatids separate and are pulled to opposite poles.
- Telophase: nuclear envelopes reform, chromosomes decondense.
- 前期：染色质凝缩为染色体，核膜解体，纺锤体形成。
- 中期：染色体排列在赤道板上，通过动粒与纺锤丝相连。
- 后期：姐妹染色单体分开，被拉向细胞两极。
- 末期：核膜重新形成，染色体解凝缩。
6. Meiosis and Genetic Variation | 减数分裂与遗传变异
Meiosis involves two successive divisions, producing four haploid gametes. Meiosis I separates homologous chromosomes, while Meiosis II separates sister chromatids. Key events include crossing over during prophase I and independent assortment of chromosomes in metaphase I. These processes generate genetic diversity among offspring.
减数分裂包含两次连续分裂，产生四个单倍体配子。减数第一次分裂分离同源染色体，减数第二次分裂分离姐妹染色单体。关键事件包括前期I中的交叉互换以及中期I中染色体的自由组合。这些过程在后代中产生遗传多样性。
Crossing over is the exchange of alleles between non-sister chromatids of homologous chromosomes, forming recombinant chromatids. Independent assortment describes the random orientation of homologous pairs at metaphase I, leading to different combinations of maternal and paternal chromosomes in gametes. Random fertilisation further amplifies variation.
交叉互换是同源染色体非姐妹染色单体之间等位基因的交换，形成重组染色单体。自由组合指的是中期I同源染色体对的随机取向，导致配子中母源与父源染色体的不同组合。随机受精进一步增加了变异。
7. DNA Replication and Protein Synthesis | DNA复制与蛋白质合成
DNA replication is semi-conservative: each new DNA molecule consists of one original strand and one newly synthesised strand. The enzyme helicase unwinds the double helix, and DNA polymerase adds complementary nucleotides in the 5' to 3' direction. The leading strand is synthesised continuously, while the lagging strand forms as Okazaki fragments joined by DNA ligase.
DNA复制是半保留的：每个新的DNA分子含有一条模板链和一条新合成的链。解旋酶解开双螺旋，DNA聚合酶沿5'到3'方向添加互补核苷酸。前导链连续合成，后随链则形成冈崎片段，由DNA连接酶连接。
Protein synthesis involves transcription and translation. During transcription, RNA polymerase synthesises a complementary mRNA strand from the DNA template. In eukaryotes, pre-mRNA undergoes splicing to remove introns. Translation occurs at ribosomes: tRNA molecules with anticodons complementary to mRNA codons deliver specific amino acids for polypeptide elongation.
蛋白质合成包括转录和翻译。转录过程中，RNA聚合酶以DNA为模板合成互补的mRNA链。在真核生物中，前体mRNA经过剪接去除内含子。翻译在核糖体上进行：携带反密码子的tRNA与mRNA密码子互补配对，运送特定氨基酸以延伸多肽链。
8. Genetics: Mendelian and Beyond | 遗传学：孟德尔定律及延伸
Mendel's law of segregation states that allele pairs separate during gamete formation, so each gamete carries only one allele for each gene. The law of independent assortment asserts that genes for different traits are distributed to gametes independently, provided they lie on different chromosomes. Monohybrid crosses yield a 3:1 phenotypic ratio in the F₂ generation when both parents are heterozygous.
孟德尔分离定律指出，等位基因在配子形成时分离，每个配子只携带每个基因的一个等位基因。自由组合定律表明，控制不同性状的基因独立地分配到配子中，前提是它们位于不同的染色体上。双亲均为杂合子时，单基因杂交在F₂代产生3:1的表现型比例。
Dihybrid crosses produce a 9:3:3:1 phenotypic ratio for unlinked genes. Linkage disrupts independent assortment; genes on the same chromosome tend to be inherited together. Recombination frequency can be used to map gene loci. Codominance and incomplete dominance deviate from complete dominance; multiple alleles and epistasis further complicate inheritance patterns.
双基因杂交的非连锁基因产生9:3:3:1的表现型比例。连锁干扰自由组合；位于同一染色体上的基因倾向于共同遗传。重组频率可用于绘制基因座图谱。共显性和不完全显性偏离完全显性；复等位基因和上位效应使遗传模式更加复杂。
9. Evolution and Speciation | 进化与物种形成
Natural selection acts on heritable variation within a population. Individuals with advantageous traits are more likely to survive and reproduce, passing those alleles to the next generation. Over time, allele frequencies change, leading to adaptation. The modern synthesis integrates Darwinian natural selection with Mendelian genetics and population genetics.
自然选择作用于种群内的可遗传变异。具有有利性状的个体更可能存活并繁殖，将这些等位基因传给下一代。随时间推移，等位基因频率改变，导致适应。现代综合进化论将达尔文的自然选择学说与孟德尔遗传学和群体遗传学结合起来。
Speciation occurs when populations become reproductively isolated and diverge genetically. Allopatric speciation results from a physical barrier preventing gene flow; sympatric speciation occurs within the same geographical area, often through polyploidy or behavioural isolation. Prezygotic and postzygotic isolating mechanisms prevent interbreeding.
当种群生殖隔离并发生遗传分化时，便会形成新物种。异域物种形成由物理屏障阻断基因流引起；同域物种形成在同一地理区域内发生，常通过多倍体化或行为隔离实现。合子前和合子后隔离机制阻止了种群间的杂交。
10. Plant Physiology: Photosynthesis and Transpiration | 植物生理学：光合作用与蒸腾作用
Photosynthesis converts light energy into chemical energy in the chloroplasts. The light-dependent reactions occur in the thylakoid membranes, where photolysis of water releases electrons, protons, and oxygen; ATP and NADPH are generated. The light-independent reactions (Calvin cycle) take place in the stroma, using ATP and NADPH to fix CO₂ and produce triose phosphate, which can be converted to glucose and other carbohydrates.
光合作用在叶绿体中将光能转化为化学能。光依赖反应发生在类囊体膜上，水的光解释放电子、质子和氧气，并生成ATP和NADPH。光不依赖反应（卡尔文循环）在基质中进行，利用ATP和NADPH固定CO₂，生成磷酸丙糖，后者可转化为葡萄糖及其他碳水化合物。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Transpiration is the loss of water vapour from leaves via stomata, driven by the water potential gradient. It creates a transpiration pull that moves water and dissolved minerals up the xylem in a continuous column (cohesion-tension theory). Factors affecting transpiration rate include light intensity, temperature, humidity, and wind speed. Stomatal closure is regulated by guard cells responding to light and CO₂ concentration.
蒸腾作用是水蒸气通过气孔从叶片散失的过程，由水势梯度驱动。蒸腾拉力使水分和溶解的矿物质在木质部中形成连续水柱向上运输（内聚力‑张力理论）。影响蒸腾速率的因素包括光强、温度、湿度和风速。气孔关闭由保卫细胞响应光照和CO₂浓度进行调节。
11. Mammalian Physiology: Circulation and Respiration | 哺乳动物生理学：循环与呼吸
The mammalian circulatory system is a closed, double circulation in which blood passes through the heart twice per complete circuit. The right side pumps deoxygenated blood to the lungs (pulmonary circulation); the left side pumps oxygenated blood to the rest of the body (systemic circulation). The heart comprises four chambers: right atrium, right ventricle, left atrium, and left ventricle. The cardiac cycle is coordinated by the sinoatrial node (pacemaker), which initiates electrical impulses leading to atrial and ventricular contraction.
哺乳动物循环系统为封闭式双循环，血液在一次完整的循环中两次经过心脏。右心将缺氧血泵至肺部（肺循环）；左心将富氧血泵送至全身（体循环）。心脏包括四个腔室：右心房、右心室、左心房和左心室。心动周期由窦房结（起搏器）协调，它发出电冲动引起心房和心室收缩。
Gas exchange occurs in the alveoli of the lungs. Oxygen diffuses from alveolar air into the blood, while carbon dioxide diffuses in the opposite direction, down concentration gradients. Haemoglobin in red blood cells binds oxygen for transport. Breathing is driven by the diaphragm and intercostal muscles, regulated by the respiratory centre in the medulla oblongata responding to blood CO₂ levels.
气体交换发生在肺部的肺泡中。氧气顺浓度梯度从肺泡气扩散入血液，二氧化碳则反向扩散。红细胞中的血红蛋白结合氧气进行运输。呼吸由膈肌和肋间肌驱动，受延髓呼吸中枢调控，该中枢对血液中CO₂浓度敏感。
12. Ecology and Energy Flow | 生态学与能量流动
Ecology studies the interactions between organisms and their environment. Populations are groups of the same species in a defined area; a community includes all populations in an ecosystem. Energy flows through ecosystems in food chains and food webs. Producers (autotrophs) convert light energy into biomass; primary consumers feed on producers, secondary consumers on primary consumers, and so on.
生态学研究生物与其环境之间的相互作用。种群是某一区域内同种个体的集合；群落包括生态系统中的所有种群。能量在生态系统中沿食物链和食物网流动。生产者（自养生物）将光能转化为生物量；初级消费者以生产者為食，次级消费者以初级消费者為食，以此类推。
Energy transfer between trophic levels is inefficient: typically only about 10% of energy is passed on; the rest is lost as heat through respiration, undigested material, and excretion. Energy is not recycled, but nutrients such as carbon and nitrogen are. The carbon cycle involves photosynthesis, respiration, decomposition, and combustion. The nitrogen cycle includes nitrogen fixation, nitrification, denitrification, and ammonification, all driven by microorganisms.
营养级间的能量传递效率很低，通常只有约10%的能量被传递；其余部分通过呼吸、未消化物质和排泄以热的形式散失。能量不可循环再利用，但碳、氮等物质可以。碳循环包括光合作用、呼吸作用、分解作用和燃烧。氮循环包括固氮作用、硝化作用、反硝化作用和氨化作用，均由微生物驱动。
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