📚 Core Concepts in CIE Pre-U Biology | CIE Pre-U 生物学核心知识点梳理
Cambridge Pre-U Biology is a rigorous course that bridges the gap between secondary education and university-level biology. It demands a deep understanding of fundamental principles, from the molecular machinery inside cells to the dynamics of entire ecosystems. This article distils the core concepts that every Pre-U candidate must master, presenting each idea in a clear, bilingual format to support learners aiming for high grades.
剑桥 Pre-U 生物是一门严谨的课程，它填补了中学教育与大学水平生物学之间的差距。该课程要求学生深刻理解从细胞内部分子机器到整个生态系统动态的基本原理。本文凝练了每位 Pre-U 考生必须掌握的核心概念，以清晰的双语形式呈现，助力学员冲击高分。
1. Cell Structure and Organelles | 细胞结构与细胞器
All living organisms are composed of cells, which are classified as prokaryotic or eukaryotic. Prokaryotic cells lack a membrane-bound nucleus and organelles, while eukaryotic cells possess a true nucleus and compartmentalised structures. Key organelles include the rough endoplasmic reticulum (rER) for protein synthesis, the smooth endoplasmic reticulum (sER) for lipid synthesis, the Golgi apparatus for protein modification and sorting, mitochondria for aerobic respiration, and chloroplasts for photosynthesis in plants.
所有生物体都由细胞组成，细胞可分为原核细胞和真核细胞。原核细胞缺乏有膜包被的细胞核和细胞器，而真核细胞具有真正的细胞核和区室化结构。关键细胞器包括负责蛋白质合成的粗面内质网 (rER)、负责脂质合成的滑面内质网 (sER)、负责蛋白质修饰和分选的高尔基体、进行有氧呼吸的线粒体，以及植物中进行光合作用的叶绿体。
The endosymbiotic theory states that mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotic cells. Evidence includes the double membrane of these organelles, their own circular DNA, and ribosomes similar to those of bacteria.
内共生学说认为线粒体和叶绿体起源于被原始真核细胞吞噬的自由生活的原核生物。证据包括这些细胞器的双层膜结构、自身环状 DNA 以及与细菌相似的核糖体。
2. Biological Macromolecules | 生物大分子
The four major groups of biological macromolecules are carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates are polymers of monosaccharides such as glucose; starch and glycogen serve as energy storage, while cellulose provides structural support in plant cell walls. Lipids, including triglycerides and phospholipids, are hydrophobic molecules that form membranes and store energy. The phospholipid bilayer is the fundamental structure of cell membranes, with hydrophilic phosphate heads facing outward and hydrophobic fatty acid tails facing inward.
四大类生物大分子是碳水化合物、脂质、蛋白质和核酸。碳水化合物是单糖（如葡萄糖）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。脂质包括甘油三酯和磷脂，是形成膜并储存能量的疏水分子。磷脂双分子层是细胞膜的基本结构，亲水磷酸头朝外，疏水脂肪酸尾朝内。
Proteins are polymers of amino acids linked by peptide bonds. The primary structure is the linear sequence of amino acids; the secondary structure includes α-helices and β-pleated sheets; the tertiary structure is the overall three-dimensional folding; and the quaternary structure involves the assembly of multiple polypeptide subunits. The sequence of amino acids determines a protein's final shape and function.
蛋白质是通过肽键连接的氨基酸聚合物。一级结构是氨基酸的线性序列；二级结构包括 α-螺旋和 β-折叠片；三级结构是整体的三维折叠；四级结构涉及多条多肽亚基的组装。氨基酸序列决定了蛋白质的最终形状和功能。
3. Enzyme Kinetics and Inhibition | 酶动力学与抑制
Enzymes are biological catalysts that lower the activation energy of biochemical reactions. The active site is a specific region where the substrate binds, forming an enzyme-substrate complex. The induced-fit model states that the active site undergoes conformational change upon substrate binding, which optimally positions catalytic residues. Enzyme activity is affected by temperature, pH, and substrate concentration.
酶是降低生化反应活化能的生物催化剂。活性位点是底物结合的特异区域，形成酶-底物复合物。诱导契合模型认为活性位点在底物结合时发生构象变化，从而最佳地定位催化残基。酶活性受温度、pH 和底物浓度的影响。
Competitive inhibitors bind to the active site, competing with the substrate. This inhibition can be overcome by increasing substrate concentration. Non-competitive inhibitors bind to an allosteric site, causing a change in the enzyme's shape that reduces catalytic activity regardless of substrate level. The Michaelis–Menten model describes enzyme kinetics, where V₀ represents initial velocity, Vmax the maximum velocity, and Km the substrate concentration at half Vmax.
竞争性抑制剂与活性位点结合，同底物竞争，可通过提高底物浓度来克服。非竞争性抑制剂与别构位点结合，引起酶形状改变，无论底物浓度如何都会降低催化活性。米氏动力学模型描述了酶促反应动力学，其中 V₀ 代表初速度，Vmax 为最大速度，Km 是达到一半 Vmax 时的底物浓度。
4. Membrane Transport and Signalling | 膜转运与信号传导
The fluid mosaic model describes cell membranes as a dynamic phospholipid bilayer with embedded proteins and cholesterol. Passive transport includes simple diffusion, facilitated diffusion through channel or carrier proteins, and osmosis — the net movement of water across a semipermeable membrane from a region of higher water potential to lower water potential. Active transport requires ATP to move substances against their concentration gradient. Bulk transport, such as endocytosis and exocytosis, moves large molecules via vesicles.
流动镶嵌模型将细胞膜描述为嵌入蛋白质和胆固醇的动态磷脂双分子层。被动运输包括简单扩散、通过通道蛋白或载体蛋白的协助扩散以及渗透——水通过半透膜从水势较高区域向水势较低区域的净移动。主动运输需要 ATP 来逆浓度梯度转运物质。胞吞和胞吐等大分子运输通过囊泡移动大分子。
Cell signalling involves receptor proteins that detect extracellular ligands. G-protein-coupled receptors (GPCRs) activate intracellular second messengers such as cAMP. Signal transduction pathways often involve phosphorylation cascades that amplify the signal and lead to cellular responses such as gene expression changes or metabolic adjustments.
细胞信号传导涉及检测胞外配体的受体蛋白。G 蛋白偶联受体 (GPCRs) 激活胞内第二信使如 cAMP。信号转导途径常涉及磷酸化级联反应，放大信号并导致基因表达变化或代谢调整等细胞应答。
5. The Cell Cycle and Mitosis | 细胞周期与有丝分裂
The cell cycle consists of interphase (G₁, S, G₂) and mitotic (M) phase. During G₁ the cell grows and synthesises proteins; in S phase DNA replicates; and in G₂ the cell prepares for division. Mitosis is divided into prophase, metaphase, anaphase, and telophase, followed by cytokinesis. Mitosis produces two genetically identical daughter nuclei, essential for growth, tissue repair, and asexual reproduction.
细胞周期包括间期（G₁ 期、S 期、G₂ 期）和分裂期（M 期）。G₁ 期细胞生长并合成蛋白质；S 期 DNA 复制；G₂ 期细胞为分裂做准备。有丝分裂分为前期、中期、后期和末期，随后进行胞质分裂。有丝分裂产生两个遗传上相同的子细胞核，对生长、组织修复和无性生殖至关重要。
Regulation of the cell cycle involves checkpoints controlled by cyclins and cyclin-dependent kinases (CDKs). The G₁/S checkpoint ensures DNA is undamaged before replication; the G₂/M checkpoint verifies completion of DNA replication and damage repair; the metaphase checkpoint confirms all chromosomes are correctly attached to the spindle. Dysregulation can lead to uncontrolled proliferation and cancer.
细胞周期的调控涉及由细胞周期蛋白和细胞周期蛋白依赖性激酶 (CDKs) 控制的检查点。G₁/S 检查点在复制前确保 DNA 无损伤；G₂/M 检查点核实 DNA 复制和损伤修复已完成；中期检查点确认所有染色体已正确附着在纺锤体上。调控失调可导致不受控的增殖和癌症。
6. 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 consists of two successive divisions: meiosis I separates homologous chromosomes, and meiosis II separates sister chromatids. Crossing over during prophase I and independent assortment of homologous pairs during metaphase I generate extensive genetic diversity in gametes.
减数分裂将染色体数目减半，产生单倍体配子。它包含两个连续的分裂：减数分裂 I 分离同源染色体，减数分裂 II 分离姐妹染色单体。前期 I 的交叉互换以及中期 I 同源染色体对的独立分配在配子中产生大量遗传多样性。
Fertilisation randomly combines gametes, further increasing variation. Errors in meiosis, such as non-disjunction, lead to aneuploidy, as seen in Down syndrome (trisomy 21). Linkage and recombination frequencies allow the mapping of gene loci on chromosomes.
受精作用随机组合配子，进一步增加了变异。减数分裂中的错误，如不分离，会导致非整倍体，例如唐氏综合征（21 三体）。连锁和重组频率可用于绘制染色体上基因位点的图谱。
7. DNA Replication and the Central Dogma | DNA 复制与中心法则
e unwinds the double helix; DNA polymerase adds complementary nucleotides in the 5′→3′ direction. The leading strand is synthesised continuously, while the lagging strand is formed in Okazaki fragments, later joined by DNA ligase.
DNA 复制是半保留的：每个新 DNA 分子包含一条原始链和一条新合成的链。解旋酶解开双螺旋；DNA 聚合酶沿 5′→3′方向添加互补核苷酸。前导链连续合成，而后随链以冈崎片段的形式合成，随后由 DNA 连接酶连接。
The central dogma of molecular biology states that DNA is transcribed into mRNA, which is then translated into protein. Transcription by RNA polymerase produces an mRNA strand complementary to the template DNA strand. In eukaryotes, pre-mRNA undergoes splicing to remove introns and join exons. Translation occurs on ribosomes: tRNA molecules bring amino acids corresponding to the mRNA codons, and peptide bonds form the polypeptide chain.
分子生物学的中心法则指出，DNA 被转录为 mRNA，然后 mRNA 被翻译成蛋白质。RNA 聚合酶进行的转录产生与模板 DNA 链互补的 mRNA 链。在真核生物中，前体 mRNA 经过剪接去除内含子并连接外显子。翻译在核糖体上进行：tRNA 分子携带与 mRNA 密码子对应的氨基酸，肽键形成多肽链。
8. Gene Regulation in Prokaryotes and Eukaryotes | 原核与真核生物的基因调控
In prokaryotes, gene expression is often controlled through operons. The lac operon in E. coli contains structural genes for lactose metabolism, regulated by a repressor and an activator (CAP). In the absence of lactose, the repressor binds to the operator, preventing transcription. When lactose is present, allolactose binds to the repressor, inactivating it and allowing transcription to proceed only when glucose is low and cAMP levels are high.
在原核生物中，基因表达通常通过操纵子来调控。大肠杆菌的乳糖操纵子包含乳糖代谢的结构基因，受阻遏物和激活物（CAP）调控。无乳糖时，阻遏物与操纵基因结合，阻止转录。当乳糖存在时，异乳糖与阻遏物结合使其失活，且仅在葡萄糖含量低、cAMP 水平高时才允许转录进行。
Eukaryotic gene regulation is more complex and occurs at multiple levels: chromatin remodelling (acetylation and methylation of histones), transcription factors that bind promoter or enhancer regions, alternative splicing, mRNA stability, and post-translational modifications. Epigenetic changes, such as DNA methylation and histone modification, can alter gene expression without changing the nucleotide sequence and may be inherited.
真核生物的基因调控更为复杂，发生在多个层面：染色质重塑（组蛋白的乙酰化和甲基化）、与启动子或增强子区域结合的转录因子、可变剪接、mRNA 稳定性以及翻译后修饰。表观遗传变化，如 DNA 甲基化和组蛋白修饰，可在不改变核苷酸序列的情况下改变基因表达，并可能遗传。
9. Population Genetics and Evolution | 群体遗传学与进化
The Hardy-Weinberg principle describes the conditions under which allele and genotype frequencies remain constant in a population: no mutation, random mating, no gene flow, infinite population size, and no natural selection. For a gene with two alleles, p and q represent the frequencies of the dominant and recessive alleles, respectively. The genotype frequencies are given by:
哈代–温伯格原理描述了等位基因和基因型频率在种群中保持恒定的条件：无突变、随机交配、无基因流动、无限大的种群规模和无自然选择。对于具有两个等位基因的基因，p 和 q 分别代表显性和隐性等位基因的频率。基因型频率由下式给出：
p² + 2pq + q² = 1
p + q = 1
Natural selection acts on phenotypic variation and can lead to directional, stabilising, or disruptive selection. Genetic drift — random changes in allele frequencies — has a stronger effect in small populations. Speciation occurs when populations become reproductively isolated, either allopatrically (geographical barrier) or sympatrically (ecological or behavioural isolation).
自然选择作用于表型变异，可导致定向选择、稳定化选择或分裂选择。遗传漂变——等位基因频率的随机变化——在小种群中效应更强。当种群因异域隔离（地理障碍）或同域隔离（生态或行为隔离）而生殖隔离时，就会发生物种形成。
10. Energy Transfer in Ecosystems | 生态系统的能量传递
In an ecosystem, energy enters as sunlight and is captured by autotrophs (producers) through photosynthesis. The gross primary production (GPP) is the total energy fixed; net primary production (NPP) is GPP minus respiratory losses. Energy transfers between trophic levels are inefficient: typically only about 10% of the energy is passed on, with the rest lost as heat through respiration, uneaten parts, or excretion.
在生态系统中，能量以阳光形式进入，并由自养生物（生产者）通过光合作用捕获。总初级生产量 (GPP) 是固定的总能量；净初级生产量 (NPP) 是 GPP 减去呼吸损耗。营养级之间的能量传递效率很低：通常仅有约 10% 的能量被传递，其余以呼吸热、未食用部分或排泄物等形式散失。
Food chains and food webs illustrate feeding relationships. Ecological pyramids of energy, biomass, and numbers can be used to represent trophic structure. Nutrient cycles, such as the carbon and nitrogen cycles, involve the movement of elements between biotic and abiotic components. Decomposers and nitrifying bacteria are essential for recycling nutrients and maintaining ecosystem productivity.
食物链和食物网表示了摄食关系。能量金字塔、生物量金字塔和数量金字塔可用于表示营养结构。碳循环和氮循环等养分循环涉及元素在生物和非生物组分之间的移动。分解者和硝化细菌对于养分再循环和维持生态系统生产力至关重要。
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