📚 IB WJEC Biology: Cell Structure – Key Concepts Explained | IB WJEC 生物：细胞结构 考点精讲
Cell structure is a fundamental topic in both IB and WJEC biology specifications. Understanding the ultrastructure of prokaryotic and eukaryotic cells, the functions of organelles, and the principles of techniques like microscopy not only builds a solid foundation for molecular biology but also sharpens your skills in data analysis and comparison questions. This article breaks down all the essential concepts, clears up common misconceptions, and provides targeted exam advice to help you aim for the top grades.
细胞结构是 IB 和 WJEC 生物课程的核心基础。掌握原核与真核细胞的超微结构、各细胞器的功能以及显微镜等研究技术的原理，不仅能为你后续的分子生物学学习打下坚实基础，还能提升你在数据分析与比较型题目中的解题能力。本文系统梳理所有核心考点，澄清常见误区，并提供针对性考试建议，助你冲刺高分。
1. Cell Theory | 细胞学说
Cell theory states three core principles: all living organisms are composed of one or more cells, the cell is the basic unit of structure and organisation in organisms, and all cells arise from pre‑existing cells. This theory was developed through the work of scientists such as Robert Hooke, who first observed cells in cork, and Louis Pasteur, whose swan‑neck flask experiments disproved spontaneous generation. In IB and WJEC exams, you may be asked to explain how exceptions—like striated muscle fibres, giant algae, and aseptate fungal hyphae—appear to challenge the theory, but in fact these are considered ‘atypical examples’ that do not contradict the principles, because they are still made of cells or derived from cells.
细胞学说包含三条核心原则：所有生物体都由一个或多个细胞构成，细胞是生物体结构和组织的基本单位，所有细胞都来自已存在的细胞。这一理论经由多位科学家的工作而建立，例如罗伯特·胡克首次在软木中观察到细胞，路易·巴斯德的鹅颈瓶实验否定了自然发生说。在 IB 和 WJEC 考试中，你可能会被要求解释一些特殊例子——如横纹肌纤维、巨型藻类和缺隔菌丝——如何看似挑战细胞学说，但实际上它们被视为“非典型例子”，并不违背原则，因为它们仍由细胞构成或源自细胞。
2. Prokaryotic vs Eukaryotic Cells | 原核细胞与真核细胞的比较
Prokaryotic cells are smaller (typically 1–5 µm) and lack a membrane‑bound nucleus; their DNA is a single circular chromosome found in a nucleoid region. Eukaryotic cells (10–100 µm) have a true nucleus enclosed by a nuclear envelope and numerous membrane‑bound organelles. Both cell types share ribosomes (70S in prokaryotes, 80S in eukaryotes), a cell membrane, cytoplasm, and genetic material. A common exam question asks you to compare and contrast prokaryotic and eukaryotic cells using a table. Remember that prokaryotes may also have plasmids, a capsule, flagella, and pili. Note: members of the kingdom Archaea, though prokaryotic, have some biochemical features closer to eukaryotes, but for this specification you only need to distinguish Bacteria and Eukaryota clearly.
原核细胞较小（通常 1–5 µm），缺乏膜包被的细胞核；其 DNA 为一条环形染色体，位于拟核区域。真核细胞（10–100 µm）具有由核膜包裹的真正细胞核和众多膜包被的细胞器。两类细胞共有的结构包括核糖体（原核为 70S，真核为 80S）、细胞膜、细胞质和遗传物质。考试中常见的问题之一是要求你用表格比较和对比原核与真核细胞。请记住，原核细胞还可能具有质粒、荚膜、鞭毛和菌毛。注意：古细菌虽然属于原核生物，但其某些生化特征更接近真核生物，不过在课程范围内你只要清晰区分细菌与真核生物即可。
| Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
| Nucleus | Absent (nucleoid region) | Present (nuclear envelope) |
| DNA | Single circular chromosome | Multiple linear chromosomes |
| Membrane‑bound organelles | None | Many (e.g., mitochondria, ER, Golgi) |
| Ribosomes | 70S (50S + 30S) | 80S (60S + 40S) |
| Cell wall | Peptidoglycan (most bacteria) | Cellulose (plants), chitin (fungi), none in animals |
3. The Cell Membrane | 细胞膜
The cell membrane is described by the fluid mosaic model. It consists of a phospholipid bilayer with hydrophobic fatty acid tails facing inward and hydrophilic phosphate heads facing the aqueous environments. Embedded proteins include integral proteins (spanning the membrane) and peripheral proteins (on the surface). Cholesterol molecules are intercalated between phospholipids in animal cells, regulating membrane fluidity. Membrane carbohydrates form glycoproteins and glycolipids that function in cell recognition. The membrane is selectively permeable, allowing small, non‑polar molecules to diffuse freely, while polar molecules and ions require transport proteins. You must be able to explain how the structure of membrane components relates to their function—for example, the hydrophobic core acting as a barrier to ions and large polar solutes.
细胞膜可以用流动镶嵌模型来描述。它由磷脂双分子层构成，疏水性脂肪酸尾部朝向内侧，亲水性磷酸头部面向两侧的水相环境。嵌入其中的蛋白质包括整合蛋白（跨膜）和外周蛋白（位于表面）。动物细胞膜中，胆固醇分子插在磷脂之间，调节膜的流动性。膜上的碳水化合物形成糖蛋白和糖脂，承担细胞识别的功能。细胞膜具有选择透过性，允许小的非极性分子自由扩散，而极性分子和离子则需要转运蛋白的帮助。你必须能够解释膜组分结构与其功能的关系——例如，疏水核心如何成为离子和大分子极性溶质的屏障。
4. Nucleus and Ribosomes | 细胞核与核糖体
The nucleus is surrounded by a double membrane called the nuclear envelope, which possesses nuclear pores that regulate the passage of molecules such as mRNA and ribosomal subunits. Inside, chromatin (DNA wrapped around histone proteins) is present; during cell division, chromatin condenses into chromosomes. The nucleolus is a dense region where ribosomal RNA (rRNA) is synthesised and combined with proteins to form ribosomal subunits. Ribosomes are the sites of protein synthesis. Eukaryotic ribosomes (80S) are either free in the cytoplasm, producing proteins for intracellular use, or bound to the rough endoplasmic reticulum, synthesising proteins destined for secretion or membrane insertion. Prokaryotic 70S ribosomes are slightly smaller and are targeted by certain antibiotics.
细胞核由称为核膜的双层膜包裹，核膜上分布有核孔，调控着 mRNA 和核糖体亚基等分子的出入。在细胞核内部，染色质（DNA 缠绕在组蛋白上）存在；细胞分裂时，染色质凝缩成染色体。核仁是一个致密区域，进行 rRNA 的合成，并与蛋白质组装成核糖体亚基。核糖体是蛋白质合成的场所。真核生物的 80S 核糖体或游离于细胞质中，合成供细胞内使用的蛋白质，或附着在粗面内质网上，合成分泌蛋白或膜蛋白。原核生物的 70S 核糖体略小，是某些抗生素的作用靶点。
5. Endomembrane System (ER & Golgi) | 内膜系统（内质网与高尔基体）
The endomembrane system includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and the cell membrane. Rough ER is studded with ribosomes and is involved in protein folding and modification; it also produces transport vesicles. Smooth ER lacks ribosomes and functions in lipid synthesis, carbohydrate metabolism, and detoxification. The Golgi apparatus receives vesicles from the ER, modifies proteins (e.g., glycosylation), sorts them, and packages them into vesicles for transport to the cell membrane or other organelles. IB exams often ask you to trace the pathway of a protein, such as a digestive enzyme, from synthesis to secretion: ribosome → rough ER → transport vesicle → Golgi → secretory vesicle → cell membrane (exocytosis). WJEC may also focus on the role of vesicles in intracellular transport.
内膜系统包括核膜、内质网、高尔基体、溶酶体、囊泡和细胞膜。粗面内质网上附着核糖体，参与蛋白质的折叠与修饰，并产生运输囊泡。滑面内质网无核糖体，功能涉及脂质合成、碳水化合物代谢和解毒作用。高尔基体接收来自内质网的囊泡，对蛋白质进行修饰（如糖基化），将其分选并包装到囊泡中，再运输至细胞膜或其他细胞器。IB 考试常要求你追溯一种蛋白质（如消化酶）从合成到分泌的路径：核糖体 → 粗面内质网 → 运输囊泡 → 高尔基体 → 分泌囊泡 → 细胞膜（胞吐）。WJEC 考试同样常关注囊泡在胞内运输中的作用。
6. Mitochondria | 线粒体
Mitochondria are the sites of aerobic respiration, producing ATP through the Krebs cycle and oxidative phosphorylation. They have a double membrane: the outer membrane is smooth, while the inner membrane is highly folded into cristae to increase surface area for ATP synthase and the electron transport chain. The matrix contains enzymes for the Krebs cycle, mitochondrial DNA (circular and without histones), and 70S ribosomes—evidence supporting the endosymbiotic theory. Be ready to equate structural adaptations with function: the intermembrane space creates a proton gradient for chemiosmosis, and the folded cristae maximise ATP production. Diagrams should label outer membrane, inner membrane, crista, matrix, and intermembrane space.
线粒体是有氧呼吸的场所，通过克雷布斯循环和氧化磷酸化生成 ATP。它们具有双层膜：外膜平滑，内膜高度折叠成嵴，以增加表面积供 ATP 合酶和电子传递链使用。基质中含有克雷布斯循环的酶、线粒体 DNA（环状、无组蛋白）和 70S 核糖体——这些证据支持内共生学说。你要准备好将结构适应性与功能联系起来：膜间隙为化学渗透建立质子梯度，折叠的嵴最大化了 ATP 的产量。画图时需标注外膜、内膜、嵴、基质和膜间隙。
7. Chloroplasts (for plant cells) | 叶绿体（植物细胞）
Chloroplasts are the organelles responsible for photosynthesis in plant cells and some protists. They are surrounded by a double membrane and contain an internal system of thylakoids stacked into grana, where the light‑dependent reactions occur. The stroma is the fluid‑filled space containing enzymes for the Calvin cycle, as well as circular DNA and 70S ribosomes. Chlorophyll and other photosynthetic pigments are embedded in the thylakoid membrane. Like mitochondria, chloroplasts are believed to have arisen by endosymbiosis. In exam questions, you could be asked to compare chloroplasts and mitochondria—both have double membranes, their own DNA, and ribosomes, but chloroplasts have a third membrane system (thylakoids) and are larger.
叶绿体是植物细胞和一些原生生物进行光合作用的细胞器。它们被双层膜包裹，内部具有由类囊体堆叠成的基粒，光反应在此进行。基质是充满液体的空间，含有卡尔文循环的酶以及环状 DNA 和 70S 核糖体。叶绿素和其他光合色素镶嵌在类囊体膜上。与线粒体一样，叶绿体被认为起源于内共生。考试中，可能要求你比较叶绿体和线粒体——二者都具有双层膜、自身 DNA 和核糖体，但叶绿体多了一层类囊体膜系统，且体积更大。
8. Cytoskeleton | 细胞骨架
The cytoskeleton is a network of protein fibres that provides mechanical support, maintains cell shape, and enables movement. It comprises three main types: microfilaments (actin filaments) involved in muscle contraction, cell division (cleavage furrow), and amoeboid movement; microtubules (tubulin polymers) that form the spindle fibres during mitosis, act as tracks for organelle movement, and make up cilia and flagella (9+2 arrangement); and intermediate filaments that provide tensile strength and anchor organelles. IB students should understand the role of microtubules in vesicle transport and mitosis; WJEC may also stress the link between cytoskeleton components and human diseases (e.g., certain muscular dystrophies are linked to defects in cytoskeletal proteins).
细胞骨架是由蛋白质纤维构成的网络，提供机械支持，维持细胞形态并实现运动。它包含三种主要类型：微丝（肌动蛋白丝）参与肌肉收缩、细胞分裂（分裂沟）和变形运动；微管（微管蛋白聚合体）在有丝分裂中形成纺锤丝，充当细胞器运动的轨道，并构成纤毛和鞭毛（9+2 排列）；中间丝提供抗拉强度并锚定细胞器。IB 学生应理解微管在囊泡运输和有丝分裂中的作用；WJEC 也可能强调细胞骨架成分与人类疾病的关联（例如，某些肌营养不良与细胞骨架蛋白的缺陷有关）。
9. Cell Wall and Extracellular Matrix | 细胞壁与细胞外基质
Plant cells possess a rigid cell wall outside the cell membrane, primarily composed of cellulose fibres embedded in a matrix of hemicellulose and pectin. The wall provides structural support, prevents excessive water uptake, and maintains turgor pressure. Fungi have cell walls made of chitin, and bacterial cell walls contain peptidoglycan. In animals, there is no cell wall, but cells secrete an extracellular matrix (ECM) composed mainly of collagen and glycoproteins; the ECM is involved in tissue support, cell adhesion, and cell‑to‑cell communication. IB and WJEC may ask about the role of the ECM in epithelial or connective tissues.
植物细胞在细胞膜外具有坚硬的细胞壁，其主要成分是纤维素纤维嵌入半纤维素和果胶的基质中。细胞壁提供结构支持，防止过度吸水，并维持膨压。真菌的细胞壁由几丁质构成，细菌细胞壁含有肽聚糖。动物没有细胞壁，但细胞分泌主要由胶原蛋白和糖蛋白组成的细胞外基质（ECM）；ECM 参与组织支持、细胞粘附和细胞间通讯。IB 和 WJEC 可能考到 ECM 在上皮组织或结缔组织中的作用。
10. Techniques for Studying Cells | 研究细胞的技术
Light microscopy uses visible light and glass lenses to magnify images up to about 1000×. It can show living cells and tissue organisation but has limited resolution (approx. 200 nm). Electron microscopy (EM) uses a beam of electrons; transmission EM (TEM) provides high‑resolution images of internal ultrastructure, while scanning EM (SEM) gives 3D surface views, both achieving resolutions down to 0.5 nm. A key skill is calculating magnification and actual size using the formula: Magnification = Image size ÷ Actual size. You must be able to convert units (mm, µm, nm). Freeze‑fracture and cryo‑electron microscopy are advanced techniques mentioned in IB HL that help visualise membrane proteins.
光学显微镜使用可见光和玻璃透镜将图像放大最多约 1000 倍。它可以观察活细胞和组织结构，但分辨率有限（约 200 nm）。电子显微镜使用电子束；透射电镜提供内部超微结构的高分辨率图像，扫描电镜给出三维表面视图，二者分辨率均可达到 0.5 nm。关键技能是使用公式（放大率 = 图像尺寸 ÷ 实际尺寸）计算放大倍数和实际大小。你必须能够进行单位换算（mm，µm，nm）。冰冻断裂和冷冻电镜是 IB HL 中提及的先进技术，有助于观察膜蛋白。
Magnification = Image size ÷ Actual size
11. Exam Tips and Common Misconceptions | 考试技巧与常见误区
Misconception 1: ‘All cells have a nucleus.’ Prokaryotic cells do not. Instead, refer to the nucleoid region. Misconception 2: ‘Mitochondria and chloroplasts are the same size.’ Chloroplasts are generally larger and contain thylakoids, which mitochondria lack. Misconception 3: ‘Cell wall and cell membrane are the same.’ The cell wall is an external, non‑living structure in plants, fungi, and bacteria; the cell membrane is a living, selectively permeable bilayer present in all cells. In drawing and labelling questions, always use a sharp pencil, clear lines, and accurate proportions; don’t forget to include a title and scale bar where required. For ‘compare’ questions, use a table or structured paragraphs highlighting similarities and differences. When explaining a process, use sequential linking words (first, then, next, finally) and refer to specific structures by name.
误区一：“所有细胞都有细胞核。”原核细胞没有，应使用拟核区域描述。误区二：“线粒体和叶绿体大小相同。”叶绿体通常更大，含有类囊体，而线粒体没有。误区三：“细胞壁和细胞膜是一样的。”细胞壁是植物、真菌和细菌的外层非生命结构；细胞膜是所有细胞都具有的、有生命的、具选择透过性的双分子层。在绘图与标注题中，始终使用削尖的铅笔、清晰的线条和准确的比例；不要忘记按要求添加标题和比例尺。遇到“比较”类题目，可使用表格或结构分段突出相同点与不同点。在解释过程时，运用表示顺序的连接词（首先、然后、接着、最后），并明确称谓具体结构名称。
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