📚 Pre-U CCEA Biology: Core Concepts Review | Pre-U CCEA 生物：核心知识点梳理
This revision guide distils the essential topics tested in the Pre-U CCEA Biology specification, from molecular building blocks to whole ecosystems. Each section pairs a concise English explanation with its Chinese counterpart, ensuring bilingual clarity for independent learners.
这份复习指南浓缩了 Pre-U CCEA 生物考试的核心内容，从分子构件到完整生态系统。每个部分都采用英文解释与中文对照的形式，为自学者提供清晰的双语学习支持。
1. Cell Structure and Function | 细胞结构与功能
All organisms are composed of cells, which fall into two fundamental types: prokaryotic and eukaryotic. Prokaryotes (e.g. bacteria) lack membrane-bound organelles and a nucleus, whereas eukaryotes (e.g. animal and plant cells) possess a distinct nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and in plants, chloroplasts and a large central vacuole. Ribosomes are present in both, but they differ in size (70S in prokaryotes, 80S in eukaryotes).
所有生物都由细胞构成，细胞分为两种基本类型：原核细胞和真核细胞。原核生物（如细菌）没有膜包裹的细胞器和细胞核，而真核生物（如动物和植物细胞）拥有明显的细胞核、线粒体、内质网、高尔基体，植物细胞还有叶绿体和大液泡。核糖体两者都有，但大小不同（原核为 70S，真核为 80S）。
The endosymbiotic theory explains the origin of mitochondria and chloroplasts: they were once free-living prokaryotes engulfed by an ancestral eukaryotic cell. Evidence includes their double membrane, own circular DNA, and ribosomes similar to bacterial ones.
内共生学说解释了线粒体和叶绿体的起源：它们曾经是自由生活的原核生物，被原始真核细胞吞噬。证据包括双层膜结构、自己的环状 DNA 以及与细菌相似的核糖体。
The table below summarises key differences between prokaryotic and eukaryotic cells.
下表总结了原核细胞与真核细胞的主要差异。
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | Absent; DNA in nucleoid region | Present; DNA enclosed by nuclear envelope |
| Membrane-bound organelles | None | Multiple (mitochondria, ER, etc.) |
| Ribosomes | 70S | 80S (plus 70S in organelles) |
| Cell wall | Peptidoglycan (bacteria) | Cellulose (plants), chitin (fungi), absent in animals |
| DNA shape | Circular | Linear chromosomes |
2. Biological Molecules | 生物分子
Carbohydrates, lipids, proteins, and nucleic acids are the four major classes of biological macromolecules. Carbohydrates are made from monosaccharide monomers (e.g. glucose) and serve as energy sources (starch, glycogen) or structural components (cellulose). Lipids, including triglycerides and phospholipids, are hydrophobic molecules vital for membrane structure and long-term energy storage. Proteins are polymers of amino acids and perform a vast range of functions including catalysis, transport, and signalling. Nucleic acids (DNA and RNA) store and transmit hereditary information.
碳水化合物、脂质、蛋白质和核酸是四类主要的生物大分子。碳水化合物由单糖单体（如葡萄糖）构成，可作为能量来源（淀粉、糖原）或结构成分（纤维素）。脂质包括甘油三酯和磷脂，是疏水性分子，对膜结构和长期能量储存至关重要。蛋白质是氨基酸的聚合物，功能多样，包括催化、运输和信号传导。核酸（DNA 和 RNA）储存并传递遗传信息。
Polymers are synthesised through condensation reactions, which release a water molecule each time a new bond forms. Breakdown occurs via hydrolysis, where water is added to split bonds. For example, a dipeptide forms when two amino acids condense, releasing H₂O; during digestion, peptide bonds are hydrolysed.
聚合物通过缩合反应合成，每形成一个新的化学键就释放一个水分子。分解则通过水解进行，即加入水分子来断裂化学键。例如，两个氨基酸缩合形成二肽，同时放出一分子 H₂O；消化过程中，肽键被水解。
A quick reference for monomer-polymer relationships is shown below.
以下是单体与聚合物关系的快速参考。
| Macromolecule | Monomer | Bond type | Example polymer |
|---|---|---|---|
| Carbohydrate | Monosaccharide (glucose) | Glycosidic bond | Starch, cellulose |
| Protein | Amino acid | Peptide bond | Enzyme, haemoglobin |
| Nucleic acid | Nucleotide | Phosphodiester bond | DNA, mRNA |
| Lipid (triglyceride) | Glycerol + 3 fatty acids | Ester bond | Fats and oils |
3. Enzymes | 酶
Enzymes are globular proteins that act as biological catalysts, lowering activation energy without being consumed. The active site of an enzyme is complementary to its specific substrate. The lock-and-key model describes a rigid active site, while the induced-fit model proposes a conformational change upon substrate binding, which strains bonds and facilitates the reaction.
酶是球状蛋白质，作为生物催化剂降低活化能而自身不被消耗。酶的活性部位与其特定的底物互补。锁钥模型描述了刚性的活性部位，而诱导契合模型则认为底物结合引起酶构象改变，使化学键受到应力从而促进反应。
The general reaction can be written as:
E + S ⇌ ES → E + P
where E = enzyme, S = substrate, ES = enzyme-substrate complex, P = product. Note the formation of the ES complex is reversible, but product formation is essentially irreversible.
其中 E = 酶，S = 底物，ES = 酶-底物复合物，P = 产物。注意 ES 复合物的形成是可逆的，但产物形成基本上是不可逆的。
Factors affecting enzyme activity include temperature, pH, substrate concentration, and inhibitor presence. Each enzyme has an optimal temperature and pH. Temperatures above the optimum cause denaturation (permanent loss of tertiary structure). Competitive inhibitors resemble the substrate and occupy the active site, whereas non-competitive inhibitors bind elsewhere, altering the active site’s shape.
影响酶活性的因素包括温度、pH、底物浓度和抑制剂的存在。每种酶都有最适温度和 pH。超过最适温度会导致变性（三级结构的永久丧失）。竞争性抑制剂与底物相似，占据活性部位；非竞争性抑制剂结合在别处，改变活性部位的形状。
4. Cell Membranes and Transport | 细胞膜与运输
The fluid mosaic model describes the plasma membrane as a phospholipid bilayer with embedded proteins, cholesterol (in animal cells), and carbohydrate chains. Phospholipids are amphipathic, with hydrophilic phosphate heads facing outward and hydrophobic fatty acid tails inward, forming a selective barrier. Membrane proteins include channel proteins, carrier proteins, and pumps, which facilitate transport, recognition, and cell signalling.
液态镶嵌模型将质膜描述为磷脂双分子层，其中镶嵌着蛋白质、胆固醇（动物细胞）和糖链。磷脂是两亲性的，亲水的磷酸头部朝外，疏水的脂肪酸尾部朝内，形成选择通透性屏障。膜蛋白包括通道蛋白、载体蛋白和泵，它们介导物质运输、细胞识别和信号传导。
Small, non-polar molecules (e.g. O₂, CO₂) diffuse directly through the bilayer. Water moves via osmosis through aquaporins or the bilayer. Facilitated diffusion uses channel or carrier proteins to move substances down the concentration gradient without energy. Active transport requires ATP to pump solutes against their gradient, e.g. the Na⁺/K⁺ pump. Endocytosis and exocytosis handle large particles or bulk fluid transport.
小的非极性分子（如 O₂、CO₂）可直接穿过双分子层扩散。水通过水通道蛋白或双分子层进行渗透。协助扩散利用通道蛋白或载体蛋白顺浓度梯度运输物质，不消耗能量。主动运输需要 ATP 将溶质逆浓度梯度泵出，例如 Na⁺/K⁺ 泵。胞吞和胞吐用于大颗粒或批量液体运输。
The comparison below clarifies the four main transport mechanisms.
下面对比了四种主要运输机制。
| Process | Protein needed? | Energy (ATP)? | Direction |
|---|---|---|---|
| Simple diffusion | No | No | Down concentration gradient |
| Facilitated diffusion | Yes (channel or carrier) | No | Down concentration gradient |
| Osmosis (water) | Aquaporins (sometimes) | No | Down water potential gradient |
| Active transport | Yes (pump) | Yes | Against concentration gradient |
5. Cell Division and Genetics | 细胞分裂与遗传
Mitosis produces two genetically identical diploid daughter cells and is responsible for growth, repair, and asexual reproduction. The stages are prophase, metaphase, anaphase, and telophase. Meiosis, by contrast, yields four genetically distinct haploid gametes through two consecutive divisions, introducing variation through crossing over and independent assortment.
有丝分裂产生两个遗传上相同的二倍体子细胞，负责生长、修复和无性生殖。其阶段为前期、中期、后期和末期。减数分裂则通过两次连续分裂产生四个遗传上不同的单倍体配子，通过交叉和独立分配引入变异。
Monohybrid inheritance follows Mendel’s law of segregation. If a gene has two alleles, a Punnett square can predict offspring ratios. For a heterozygous cross (Aa × Aa), the expected genotypic ratio is 1 AA : 2 Aa : 1 aa, and the phenotypic ratio is 3:1 when the dominant allele is fully expressed. Codominance and multiple alleles (e.g. ABO blood groups) extend this pattern.
单基因杂交遵循孟德尔分离定律。如果一个基因有两个等位基因，庞纳特方格可以预测后代比例。对于杂合子自交（Aa × Aa），预期基因型比例为 1 AA : 2 Aa : 1 aa，当显性等位基因完全表达时，表型比例为 3:1。共显性和复等位基因（如 ABO 血型）扩展了这种模式。
Sex linkage is illustrated by the inheritance of colour blindness or haemophilia, where the recessive allele is carried on the X chromosome. A carrier female (XᴺXⁿ) and unaffected male (XᴺY) produce sons with a 50% chance of being affected.
性连锁可通过红绿色盲或血友病的遗传说明，隐性等位基因位于 X 染色体上。携带者女性（XᴺXⁿ）与正常男性（XᴺY）婚配，儿子有 50% 的概率患病。
6. DNA Replication and Gene Expression | DNA 复制与基因表达
DNA replication is semi-conservative: each new double helix consists of one original strand and one newly synthesised strand. DNA helicase unwinds the double helix, and DNA polymerase adds complementary nucleotides to the 3′ end of the growing strand. The leading strand is synthesised continuously, while the lagging strand forms in short Okazaki fragments, later joined by DNA ligase.
DNA 复制是半保留的：每个新的双螺旋由一条原始链和一条新合成的链组成。DNA 解旋酶解开双螺旋，DNA 聚合酶将互补核苷酸添加到生长链的 3′ 端。前导链连续合成，而后随链形成短的冈崎片段，随后由 DNA 连接酶连接。
Gene expression proceeds via transcription and translation. During transcription, RNA polymerase synthesises a complementary mRNA strand from a DNA template. In eukaryotes, the primary transcript is modified (5′ cap, poly-A tail, splicing). Translation occurs on ribosomes: tRNA molecules carry specific amino acids, and their anticodons pair with mRNA codons, allowing the ribosome to polymerise the polypeptide chain.
基因表达通过转录和翻译进行。转录时，RNA 聚合酶以 DNA 为模板合成互补的 mRNA 链。在真核生物中，初级转录物经过修饰（5′ 帽、poly-A 尾、剪接）。翻译在核糖体上进行：tRNA 分子携带特定的氨基酸，其反密码子与 mRNA 密码子配对，使核糖体聚合生成多肽链。
The genetic code is degenerate (most amino acids are encoded by more than one codon), universal, and non-overlapping. Key codons include AUG (start) and UAA, UAG, UGA (stop).
遗传密码具有简并性（大多数氨基酸由多个密码子编码）、通用性和不重叠性。关键密码子包括 AUG（起始）和 UAA、UAG、UGA（终止）。
7. Respiration and Energy | 呼吸与能量
Aerobic respiration consists of four stages: glycolysis (cytoplasm), link reaction, Krebs cycle, and oxidative phosphorylation (mitochondrial matrix and inner membrane). Glycolysis splits glucose (6C) into two molecules of pyruvate (3C), yielding a net gain of 2 ATP and 2 reduced NAD. In the link reaction, pyruvate is decarboxylated and oxidised to acetyl CoA, releasing CO₂ and more reduced NAD.
有氧呼吸包括四个阶段：糖酵解（细胞质）、链接反应、克里布斯循环和氧化磷酸化（线粒体基质和内膜）。糖酵解将葡萄糖（6C）分解为两个丙酮酸分子（3C），净生成 2 个 ATP 和 2 个还原型 NAD。在链接反应中，丙酮酸脱羧并氧化为乙酰辅酶 A，释放 CO₂ 和更多的还原型 NAD。
The Krebs cycle completes the oxidation of acetyl CoA to CO₂, generating reduced NAD, reduced FAD, and a small amount of ATP (GTP). In oxidative phosphorylation, the electron transport chain transfers electrons from reduced coenzymes to oxygen, with the energy released used to pump protons across the inner mitochondrial membrane. The resulting proton gradient drives ATP synthase to produce up to ~28-30 ATP molecules per glucose.
克里布斯循环将乙酰辅酶 A 完全氧化为 CO₂，生成还原型 NAD、还原型 FAD 和少量 ATP（GTP）。在氧化磷酸化中，电子传递链将电子从还原型辅酶传递到氧，释放的能量用于将质子泵过线粒体内膜。形成的质子梯度驱动 ATP 合酶，每分子葡萄糖最多可产生约 28-30 个 ATP。
Overall equation for aerobic respiration:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP)
In anaerobic respiration, only glycolysis continues, and pyruvate is reduced to lactate (animals) or ethanol and CO₂ (plants/yeast), regenerating NAD⁺ for glycolysis. The ATP yield is only 2 per glucose.
8. Photosynthesis | 光合作用
Photosynthesis occurs in the chloroplasts of plant cells. The light-dependent reactions take place on the thylakoid membranes, where chlorophyll absorbs light energy to split water (photolysis), releasing O₂, and to generate ATP and reduced NADP. Electrons move through photosystems II and I and the electron transport chain, driving chemiosmotic ATP synthesis.
光合作用发生在植物细胞的叶绿体中。光依赖反应在类囊体膜上进行，叶绿素吸收光能分解水（光解），释放 O₂，并生成 ATP 和还原型 NADP。电子通过光系统 II 和 I 及电子传递链传递，驱动化学渗透合成 ATP。
The Calvin cycle (light-independent reactions) uses ATP and reduced NADP to fix CO₂ into carbohydrate. The enzyme RuBisCO catalyses the carboxylation of ribulose bisphosphate (RuBP) to form two molecules of glycerate-3-phosphate (GP), which are then reduced to triose phosphate (TP). Some TP regenerates RuBP, while the rest is used to synthesise glucose, starch, and other organic compounds.
卡尔文循环（光不依赖反应）利用 ATP 和还原型 NADP 将 CO₂ 固定为碳水化合物。RuBisCO 酶催化核酮糖二磷酸（RuBP）的羧化反应，生成两分子甘油酸-3-磷酸（GP），随后被还原为磷酸三碳糖（TP）。部分 TP 再生 RuBP，其余用于合成葡萄糖、淀粉和其他有机物。
The simplified summary equation for photosynthesis is:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Chlorophyll a and b primarily absorb red and blue-violet light, reflecting green light, which explains why leaves appear green. Accessory pigments such as carotenoids extend the absorption spectrum and protect against photo-oxidation.
9. Homeostasis and Hormones | 稳态与激素
Homeostasis is the maintenance of a stable internal environment. Negative feedback mechanisms detect deviations from a set point and initiate corrective responses. In blood glucose regulation, when glucose rises, β-cells in the pancreatic islets secrete insulin, promoting glucose uptake and glycogen synthesis. When glucose falls, α-cells secrete glucagon, stimulating glycogenolysis and gluconeogenesis. Diabetes mellitus results from impaired insulin production (Type 1) or insulin resistance (Type 2).
稳态是维持稳定的内部环境。负反馈机制检测偏离设定点的变化并启动纠正反应。在血糖调节中，血糖升高时，胰岛 β 细胞分泌胰岛素，促进葡萄糖摄取和糖原合成；血糖降低时，α 细胞分泌胰高血糖素，刺激糖原分解和糖异生。糖尿病源于胰岛素分泌受损（1 型）或胰岛素抵抗（2 型）。
Thermoregulation in mammals involves both physiological and behavioural responses. When core temperature rises, vasodilation, sweating, and reduced metabolic rate promote heat loss. When it drops, vasoconstriction, shivering, and increased metabolism generate and conserve heat. The hypothalamus acts as the thermoregulatory centre.
哺乳动物的体温调节涉及生理和行为反应。核心温度升高时，血管舒张、出汗和代谢率降低促进散热；温度下降时，血管收缩、战栗产热和代谢增强则产生并保存热量。下丘脑是体温调节中枢。
Plant hormones (e.g. auxins, gibberellins, ethylene) coordinate growth and responses. Auxin (IAA) promotes cell elongation in shoots and is redistributed in response to unilateral light, causing phototropism. A high concentration of auxin inhibits growth in roots, causing geotropism.
植物激素（如生长素、赤霉素、乙烯）协调生长和响应。生长素（IAA）促进茎中细胞伸长，并在单侧光照下重新分布，导致向光性。高浓度生长素抑制根的生长，导致向地性。
10. Ecology and Populations | 生态学与种群
An ecosystem comprises a community of organisms interacting with their abiotic environment. Energy flows from producers through consumers in food chains, with only around 10% of energy transferred from one trophic level to the next. The rest is lost as heat through respiration, movement, and undigested material. This pyramid of energy explains why food chains rarely exceed four or five trophic levels.
生态系统由生物群落与其非生物环境相互作用构成。能量从生产者通过消费者沿食物链流动，每个营养级仅约 10% 的能量传递到下一级，其余以热、运动和未消化物质等形式散失。能量金字塔解释了食物链很少超过四到五个营养级的原因。
Population size can be estimated using the mark-release-recapture method (Lincoln index):
N = (M × C) ÷ R
where N = estimated population size, M = number initially marked, C = total caught in second sample, R = number of marked individuals recaptured. Assumptions include no migration, equal mixing, and no effect of marking on survival.
种群大小可用标记重捕法（林肯指数）估算：N = (M × C) ÷ R，其中 N 为估算的种群大小，M 为最初标记数，C 为第二次捕获总数，R 为重捕到的已标记个体数。假设包括无迁入迁出、充分混合以及标记不影响存活。
The carbon cycle illustrates nutrient recycling: CO₂ is fixed by photosynthesis, passed through food webs, and returned to the atmosphere via respiration, decomposition, and combustion. Decomposers (bacteria, fungi) play a vital role in mineralisation, releasing inorganic nutrients for autotrophs.
碳循环展示了营养物质的再循环：CO₂ 通过光合作用固定，经食物网传递，再通过呼吸、分解和燃烧返回大气。分解者（细菌、真菌）在矿化过程中发挥关键作用，为自养生物释放无机营养。
Population growth can be exponential (ideal conditions) or logistic (limited resources), where carrying capacity is the maximum stable population size the environment can sustain. Factors limiting growth include density-dependent (competition, predation, disease) and density-independent (climate, natural disasters) factors.
种群增长可以是指数型（理想条件）或逻辑斯谛型（资源有限），环境容纳量是环境能维持的最大稳定种群大小。限制增长的因素包括密度制约因素（竞争、捕食、疾病）和非密度制约因素（气候、自然灾害）。
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