IB生物 细胞结构 细胞膜 呼吸 光合 分裂
细胞生物学是IB生物课程的核心基石，Paper 1和Paper 2中至少有30%的题目直接或间接考察细胞相关知识点。IB HL学生还需要额外掌握原核细胞基因结构、光合作用光系统的详细反应机制、以及有氧呼吸各阶段的酶和中间产物。
Cell biology forms the bedrock of the IB Biology syllabus ： at least 30% of questions across Paper 1 and Paper 2 directly or indirectly test cellular knowledge. HL students must additionally master prokaryotic gene structure, detailed photochemical reactions of both photosystems, and the enzymes and intermediates of every stage of aerobic respiration. This guide covers the full IB Core Topic 1: Cell Biology with a focus on the concepts that examiners consistently target.
1. 原核细胞 vs 真核细胞 Prokaryotic vs Eukaryotic Cells
IB考试中最常见的比较题就是原核和真核细胞的区别。关键得分点：原核细胞没有膜结合的细胞器（无核膜、无线粒体、无内质网），DNA是裸露的环状分子位于拟核区，核糖体为70S型。真核细胞具有区室化（compartmentalisation）的优势：不同代谢反应可以在特定细胞器内独立进行，互不干扰。IB HL还要求掌握原核细胞的细胞壁由肽聚糖（peptidoglycan）构成，与植物细胞壁的纤维素完全不同。
The most common IB comparison question contrasts prokaryotic and eukaryotic cells. Key marking points: prokaryotes lack membrane-bound organelles (no nuclear membrane, no mitochondria, no ER), DNA is a naked circular molecule in the nucleoid region, and ribosomes are 70S. Eukaryotes benefit from compartmentalisation ： different metabolic reactions can proceed independently in dedicated organelles without interference. IB HL requires knowing that prokaryotic cell walls are made of peptidoglycan, entirely distinct from the cellulose of plant cell walls. When drawing, label the cell wall, plasma membrane, cytoplasm, pili, flagella, ribosomes (70S), and nucleoid with naked DNA ： examiners deduct marks for missing any of these seven structures.
2. 真核细胞超微结构 Eukaryotic Ultrastructure
IB要求学生能够识别并标注电子显微镜下的细胞器。重点关注：粗面内质网（rER）上有核糖体附着，负责蛋白质合成和运输；滑面内质网（sER）无核糖体，合成脂质和类固醇；高尔基体修饰、分拣、包装蛋白质进入囊泡；线粒体是需氧呼吸的场所，内膜向内折叠形成嵴（cristae）以增加表面积；叶绿体是光合作用场所，内部有类囊体堆叠形成的基粒（grana）。
IB expects students to identify and label organelles from electron micrographs. Key organelles: rough endoplasmic reticulum (rER) has attached ribosomes and synthesises and transports proteins; smooth endoplasmic reticulum (sER) lacks ribosomes and produces lipids and steroids; the Golgi apparatus modifies, sorts, and packages proteins into vesicles; mitochondria are the site of aerobic respiration, with inner membrane folded into cristae to maximise surface area; chloroplasts carry out photosynthesis, containing thylakoids stacked into grana. A common exam trap: the nucleus has a double membrane with nuclear pores ： students frequently draw a single membrane and lose the mark. Remember that ribosomes (80S in eukaryotes) are NOT membrane-bound organelles; they are macromolecular complexes.
3. 细胞膜的流动镶嵌模型 Fluid Mosaic Model
细胞膜的基本结构是磷脂双分子层（phospholipid bilayer），其中亲水头部朝外、疏水尾部朝内。蛋白质镶嵌在其中，分为内在蛋白（integral proteins）和外周蛋白（peripheral proteins）。"流动"指的是磷脂和蛋白质可以在膜平面内横向移动；"镶嵌"指的是蛋白质不均匀地穿插在磷脂双分子层中。胆固醇分子在动物细胞膜中调节流动性：高温时限制磷脂运动、低温时防止磷脂过度紧密排列。
The membrane's basic structure is a phospholipid bilayer with hydrophilic heads facing outward and hydrophobic tails facing inward. Proteins are embedded throughout: integral proteins span the membrane, while peripheral proteins attach to the surface. "Fluid" refers to the lateral movement of phospholipids and proteins within the plane of the membrane; "mosaic" describes the uneven, patchwork distribution of proteins throughout the bilayer. Cholesterol molecules in animal cell membranes modulate fluidity: at high temperatures they restrict phospholipid movement, and at low temperatures they prevent phospholipids from packing too tightly. HL students should be able to explain how the hydrophobic core prevents ions and large polar molecules from crossing without transport proteins ： this selective permeability is the functional consequence of the phospholipid bilayer structure.
4. 细胞膜运输机制 Membrane Transport Mechanisms
IB重点考察四种跨膜运输方式。被动运输无需能量：（1）简单扩散（simple diffusion）：：小分子如O₂、CO₂直接穿过磷脂双分子层，顺着浓度梯度；（2）促进扩散（facilitated diffusion）：：通过通道蛋白（channel proteins）或载体蛋白（carrier proteins），同样顺浓度梯度。主动运输需要ATP能量：（3）主动运输（active transport）：：载体蛋白逆浓度梯度转运分子，如钠钾泵（Na⁺/K⁺ pump）每消耗1个ATP将3个Na⁺泵出、2个K⁺泵入；（4）胞吞胞吐（endocytosis/exocytosis）：：通过囊泡进行大分子或颗粒的跨膜运输。水分子通过水通道蛋白（aquaporins）的渗透（osmosis）是IB常考的特殊案例。
IB examines four membrane transport mechanisms. Passive transport requires no energy: (1) simple diffusion ： small molecules like O₂ and CO₂ cross the phospholipid bilayer directly, moving down their concentration gradient; (2) facilitated diffusion ： using channel proteins or carrier proteins, also down the concentration gradient. Active transport requires ATP: (3) active transport ： carrier proteins move molecules against the concentration gradient, for example the Na⁺/K⁺ pump uses 1 ATP to expel 3 Na⁺ and import 2 K⁺ per cycle, generating the resting membrane potential essential for nerve impulses; (4) endocytosis/exocytosis ： bulk transport of macromolecules or particles via vesicles. Osmosis of water through aquaporins is a frequently tested special case. A key distinction: facilitated diffusion and active transport both use carrier proteins, but only active transport requires energy and moves substances against the gradient ： confusing these two costs marks every exam session.
5. 细胞呼吸概述 Cellular Respiration Overview
细胞呼吸是将葡萄糖中的化学能转化为ATP的过程。IB Syllabus将呼吸分为四个阶段：糖酵解（glycolysis）在细胞质中进行，将1个葡萄糖（6C）分解为2个丙酮酸（3C），净产2个ATP和2个NADH；环节反应（link reaction）在线粒体基质中进行，丙酮酸被氧化为乙酰辅酶A（acetyl-CoA），释放CO₂并产生NADH；克雷布斯循环（Krebs cycle）在线粒体基质中完成乙酰辅酶A的彻底氧化，每圈产生2个CO₂、3个NADH、1个FADH₂和1个ATP；氧化磷酸化（oxidative phosphorylation）在线粒体内膜上进行，NADH和FADH₂的电子通过电子传递链传递，最终将O₂还原为H₂O，同时驱动化学渗透（chemiosmosis）产生大量ATP。
Cellular respiration converts the chemical energy in glucose into ATP. The IB syllabus divides respiration into four stages: glycolysis occurs in the cytoplasm, splitting 1 glucose (6C) into 2 pyruvate (3C) with a net yield of 2 ATP and 2 NADH; the link reaction takes place in the mitochondrial matrix, oxidising pyruvate to acetyl-CoA and releasing CO₂ while generating NADH; the Krebs cycle completes the oxidation of acetyl-CoA in the mitochondrial matrix, yielding 2 CO₂, 3 NADH, 1 FADH₂, and 1 ATP per turn; oxidative phosphorylation on the inner mitochondrial membrane passes electrons from NADH and FADH₂ through the electron transport chain to reduce O₂ to H₂O, while the energy released drives chemiosmosis ： protons pumped into the intermembrane space flow back through ATP synthase, producing approximately 34 ATP molecules per glucose. HL students must also know that NADH and FADH₂ carry hydrogen atoms to the electron transport chain: think of them as "hydrogen taxis" shuttling high-energy electrons to the cristae.
6. 光合作用 Photosynthesis
光合作用是IB生物最复杂的主题之一，分为光反应（light-dependent reactions）和暗反应（light-independent reactions / Calvin cycle）。光反应在类囊体膜上进行：光系统II（PSII）吸收光能分解水分子（photolysis），释放O₂、H⁺和电子；电子通过电子传递链传递到光系统I（PSI），同时H⁺被泵入类囊体腔，通过ATP合酶回流产生ATP（化学渗透）；PSI吸收光能后产生还原型NADP（NADPH）。暗反应在叶绿体基质中进行：CO₂通过Rubisco酶固定到RuBP（5C）上生成不稳定的6C中间体，立即分解为2个3-磷酸甘油酸（PGA），随后在ATP和NADPH驱动下还原为磷酸丙糖（G3P），部分G3P再生RuBP以维持循环。
Photosynthesis is one of the most complex IB Biology topics, divided into light-dependent reactions and light-independent reactions (Calvin cycle). Light-dependent reactions occur on the thylakoid membrane: Photosystem II (PSII) absorbs light energy to split water molecules (photolysis), releasing O₂, H⁺, and electrons; electrons travel through the electron transport chain to Photosystem I (PSI), while H⁺ is pumped into the thylakoid lumen, flowing back through ATP synthase to produce ATP (chemiosmosis); PSI absorbs light energy and produces reduced NADP (NADPH). The Calvin cycle takes place in the stroma: CO₂ is fixed by the enzyme Rubisco onto RuBP (5C) forming an unstable 6C intermediate that immediately splits into 2 molecules of phosphoglycerate (PGA), subsequently reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH; some G3P regenerates RuBP to sustain the cycle while the rest is used to synthesise glucose and other organic molecules. HL students must be able to draw and label both photosystems, trace the path of electrons, and explain how cyclic photophosphorylation (PSI only) produces additional ATP without NADPH.
7. 细胞周期与有丝分裂 Cell Cycle and Mitosis
细胞周期包括间期（interphase）和分裂期（M phase）。间期分为G₁期（细胞生长和蛋白质合成）、S期（DNA复制，每条染色体变为两条姐妹染色单体）和G₂期（准备分裂，合成微管蛋白）。有丝分裂分为四个阶段：前期（prophase）：：染色体凝聚可见，核膜解体，纺锤体形成；中期（metaphase）：：染色体排列在赤道板（metaphase plate）上，纺锤丝附着在着丝粒上；后期（anaphase）：：姐妹染色单体被纺锤丝拉开移向两极；末期（telophase）：：核膜重新形成，染色体去凝聚。胞质分裂（cytokinesis）在动物细胞中通过收缩环完成，在植物细胞中通过细胞板形成完成。IB特别强调细胞分裂对生长、组织修复和无性繁殖的重要性。
The cell cycle consists of interphase and M phase. Interphase is divided into G₁ (cell growth and protein synthesis), S (DNA replication ： each chromosome becomes two sister chromatids held at the centromere), and G₂ (preparation for division, tubulin synthesis for spindle fibres). Mitosis proceeds through four stages: prophase ： chromosomes condense and become visible, nuclear membrane breaks down, spindle fibres form; metaphase ： chromosomes align at the metaphase plate with spindle fibres attached to centromeres; anaphase ： sister chromatids are pulled apart to opposite poles by spindle fibres; telophase ： nuclear membranes reform, chromosomes decondense. Cytokinesis follows: in animal cells via a cleavage furrow (contractile ring of actin and myosin), in plant cells via a cell plate formed from Golgi-derived vesicles. IB emphasises the importance of mitosis for growth, tissue repair, and asexual reproduction. A frequently tested concept: uncontrolled mitosis leads to tumour formation ： tumour suppressor genes and oncogenes regulate the cell cycle checkpoints at G₁/S and G₂/M transitions.
8. IB Paper 1 & Paper 2 答题策略 Exam Strategy
Paper 1由30道多项选择题组成（SL是30题，HL是40题），备考策略：练习排除明显错误选项，注意单元转化和数据解读题：：IB常常把容易混淆的概念放在两个选项中进行对比。Paper 2的Section A包括数据分析题（data-based question），要求根据图表计算百分比变化、描述趋势并给出生物学解释：：务必引用图表中的具体数据。Section B的论述题（extended response）按得分点评分：8分题目通常需要写8个独立的关键点，不要在一个点上啰嗦。对于"draw and label"题目，用铅笔绘制并清晰标注：：未标注的结构不得分。
Paper 1 has 30 multiple-choice questions (SL; HL has 40). Strategy: practise eliminating clearly wrong options ： IB often places two closely related concepts as contrasting choices. Watch for unit conversions and data interpretation questions. Paper 2 Section A includes data-based questions: calculate percentage change, describe trends, and give biological explanations ： always quote specific numerical data from the graph or table provided. Section B extended responses are marked point by point: an 8-mark question typically expects 8 distinct key points; do not elaborate excessively on one point. For "draw and label" questions, use a sharp pencil and annotate clearly ： unlabelled structures earn zero marks. Strong responses use command terms precisely: "outline" means give a brief account or summary (2-3 marks), "explain" means give a detailed account including reasons or causes (4-5 marks), and "compare" requires similarities AND differences with explicit reference to both items compared.
9. Key Bilingual Terms 关键双语术语
Prokaryotic cell 原核细胞 | Eukaryotic cell 真核细胞 | Nucleus 细胞核 | Mitochondrion 线粒体 | Chloroplast 叶绿体 | Ribosome 核糖体 | Rough endoplasmic reticulum 粗面内质网 | Golgi apparatus 高尔基体 | Vesicle 囊泡 | Phospholipid bilayer 磷脂双分子层 | Fluid mosaic model 流动镶嵌模型 | Integral protein 内在蛋白 | Cholesterol 胆固醇 | Simple diffusion 简单扩散 | Facilitated diffusion 促进扩散 | Osmosis 渗透 | Active transport 主动运输 | Endocytosis 胞吞 | Glycolysis 糖酵解 | Krebs cycle 克雷布斯循环 | Oxidative phosphorylation 氧化磷酸化 | Electron transport chain 电子传递链 | Chemiosmosis 化学渗透 | ATP synthase ATP合酶 | Photosystem 光系统 | Photolysis 光解 | Calvin cycle 卡尔文循环 | Rubisco 核酮糖二磷酸羧化酶 | Interphase 间期 | Sister chromatids 姐妹染色单体 | Centromere 着丝粒 | Spindle fibre 纺锤丝 | Cell cycle 细胞周期 | Mitosis 有丝分裂 | Cytokinesis 胞质分裂
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