Cell Structure: Key Points for IB & CIE Biology | 细胞结构：IB与CIE生物考点精讲

📚 Cell Structure: Key Points for IB & CIE Biology | 细胞结构：IB与CIE生物考点精讲

Understanding cell structure is fundamental to all topics in IB and CIE Biology. This revision guide breaks down every essential concept – from the differences between prokaryotic and eukaryotic cells to the function of individual organelles – with paired English and Chinese explanations to strengthen your bilingual understanding. We focus on exam-style precision, common misconceptions, and how to apply your knowledge to data-based questions.

理解细胞结构是所有 IB 和 CIE 生物学课题的基础。本篇复习指南将逐一讲解每个核心概念——从原核细胞与真核细胞的区别，到各个细胞器的功能——并配有中英文对照说明，帮助你加强双语理解。我们聚焦于考试要求的精确度、常见的误解，以及如何将知识应用到数据分析题中。

1. The Cell Theory | 细胞学说

The cell theory states that all living organisms are composed of cells, the cell is the basic unit of life, and all cells arise from pre‑existing cells. Exceptions exist that you must know for the exam: striated muscle fibres are multinucleated, fungal hyphae may be aseptate (multinucleate and continuous), and giant algae like Acetabularia can be unicellular yet macroscopic. Viruses are non‑cellular and therefore do not conform to the cell theory.

细胞学说指出：所有生物体都由细胞构成，细胞是生命的基本单位，所有细胞都来源于已有的细胞。考试中必须知道的例外包括：横纹肌纤维是多核的，真菌菌丝可能是无隔膜的（多核且连续），而像伞藻这样的巨型藻类可以是单细胞且宏观可见。病毒是非细胞结构，因此不符合细胞学说。

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2. Prokaryotic vs Eukaryotic Cells | 原核细胞与真核细胞的比较

Prokaryotic cells (bacteria and archaea) lack a true nucleus and membrane‑bound organelles. Their DNA is a naked, circular molecule located in a nucleoid region. They possess 70S ribosomes (smaller than the 80S ribosomes of eukaryotes), a cell wall containing peptidoglycan (in most bacteria), and may have flagella, pili, and a capsule. The plasma membrane often contains infoldings called mesosomes (in some prokaryotes) but the existence of mesosomes as distinct organelles is debated.

原核细胞（细菌和古菌）没有真正的细胞核和膜包被的细胞器。它们的 DNA 是裸露的环状分子，位于拟核区。它们具有 70S 核糖体（比真核细胞的 80S 核糖体小），细胞壁含有肽聚糖（大多数细菌），并可能拥有鞭毛、菌毛和荚膜。质膜常含有内折结构（在某些原核生物中称为间体），但间体作为独立细胞器的存在尚有争议。

Eukaryotic cells are characterised by a nucleus enclosed by a double membrane with pores, linear DNA associated with histone proteins, and many membrane‑bound organelles such as endoplasmic reticulum, Golgi apparatus, mitochondria, and (in photosynthetic organisms) chloroplasts. They are typically larger (10–100 µm) compared to prokaryotes (0.1–5 µm). Compartmentalisation allows for the separation of incompatible biochemical reactions and the formation of distinct microenvironments.

真核细胞的特点是具有由双层膜包围并带有核孔的细胞核，线性 DNA 与组蛋白结合，以及许多膜包被的细胞器，如内质网、高尔基体、线粒体和（在光合生物中的）叶绿体。与原核细胞（0.1–5 µm）相比，它们通常更大（10–100 µm）。细胞内的区室化使得不相容的生化反应能够分隔开来，并形成独特的微环境。

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3. The Nucleus and Ribosomes | 细胞核与核糖体

The nucleus is the largest organelle in most eukaryotic cells. It is surrounded by a nuclear envelope (two phospholipid bilayers) perforated by nuclear pores that regulate the passage of mRNA, ribosomal subunits, and proteins. The nucleolus is a dense region where rRNA is synthesised and ribosomal subunits are assembled. Euchromatin (loosely packed DNA) is the site of active transcription, while heterochromatin is condensed and transcriptionally inactive.

细胞核是大多数真核细胞中最大的细胞器。它由核膜（两层磷脂双分子层）包围，核膜上有核孔，调控 mRNA、核糖体亚基和蛋白质的进出。核仁是一个致密区域，在此合成 rRNA 并组装核糖体亚基。常染色质（松散包装的 DNA）是活跃转录的部位，而异染色质则高度凝集、转录不活跃。

Ribosomes are the sites of protein synthesis (translation). They can be free in the cytoplasm (producing proteins for intracellular use) or bound to the rough endoplasmic reticulum (synthesising proteins for secretion or membrane insertion). Ribosomes are composed of two subunits: large (50S and 60S in prokaryotes and eukaryotes respectively) and small (30S and 40S). Remember that in prokaryotes the subunits combine to form a 70S ribosome, while in eukaryotes they form an 80S ribosome. The Svedberg unit (S) is not additive because it depends on sedimentation rate.

核糖体是蛋白质合成（翻译）的场所。它们可以游离在细胞质中（合成供细胞内使用的蛋白质），或附着在粗面内质网上（合成用于分泌或嵌入膜的蛋白质）。核糖体由两个亚基组成：大亚基（原核生物为 50S，真核生物为 60S）和小亚基（30S 和 40S）。请记住，在原核生物中，亚基组合形成 70S 核糖体，而在真核生物中则形成 80S 核糖体。Svedberg 单位（S）不具有加和性，因为它取决于沉降速率。

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4. Endomembrane System – ER, Golgi, Vesicles | 内膜系统 – 内质网、高尔基体、囊泡

The endomembrane system is a network of membranes responsible for protein modification, packaging, and lipid transport. It includes the nuclear envelope, rough and smooth endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and the plasma membrane. The rough ER (with ribosomes) synthesises proteins that enter its lumen for folding and modification (e.g., glycosylation). The smooth ER lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium ion storage.

内膜系统是一个负责蛋白质修饰、包装和脂质运输的膜网络。它包括核膜、粗面内质网和滑面内质网、高尔基体、溶酶体、囊泡以及质膜。粗面内质网（附有核糖体）合成蛋白质，这些蛋白质进入其内腔进行折叠和修饰（如糖基化）。滑面内质网没有核糖体，参与脂质合成、解毒和钙离子储存。

Proteins are transported from the ER to the cis face of the Golgi apparatus via vesicles. The Golgi modifies, sorts, and packages proteins into secretory vesicles that bud from its trans face. The Golgi also synthesises lysosomes and is involved in the transport of lipids and the formation of glycolipids and glycoproteins. Vesicles move between these compartments directed by motor proteins along the cytoskeleton.

蛋白质通过囊泡从内质网运输到高尔基体的顺面。高尔基体对蛋白质进行修饰、分选和包装，形成分泌囊泡从它的反面出芽。高尔基体也合成溶酶体，并参与脂质的运输以及糖脂和糖蛋白的形成。囊泡在这些区室之间移动，由马达蛋白沿细胞骨架引导。

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5. Mitochondria – Structure and Function | 线粒体 – 结构与功能

Mitochondria are the sites of aerobic respiration and ATP synthesis. They have a double membrane: the outer membrane is smooth, while the inner membrane is highly folded into cristae, increasing the surface area for oxidative phosphorylation. The intermembrane space has a low pH due to the proton gradient. The matrix contains enzymes for the Krebs cycle, mitochondrial DNA (circular and naked), and 70S ribosomes – evidence for the endosymbiotic origin of mitochondria.

线粒体是有氧呼吸和 ATP 合成的场所。它们具有双层膜：外膜光滑，而内膜高度折叠形成嵴，增加了氧化磷酸化的表面积。由于质子梯度的存在，膜间隙的 pH 值较低。基质中含有三羧酸循环所需的酶、线粒体 DNA（环状、裸露）和 70S 核糖体——这是线粒体内共生起源的证据。

In the exam you may be asked to interpret electron micrographs: recognize the double membrane and cristae. Also, be prepared to explain how the structure of mitochondria supports their function – for example, the inner membrane folded into cristae accommodates many electron transport chains and ATP synthase complexes.

在考试中，你可能需要解读电镜照片：识别出双层膜和嵴。此外，要准备好解释线粒体的结构如何支持其功能——例如，内折成嵴的内膜能容纳大量的电子传递链和 ATP 合酶复合物。

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6. Chloroplasts and Photosynthesis | 叶绿体与光合作用

Chloroplasts are found in plants and algae and carry out photosynthesis. Like mitochondria, they have a double membrane, their own circular DNA, and 70S ribosomes. Internal to the inner membrane are thylakoids – flattened membrane sacs stacked into grana. Thylakoid membranes contain chlorophyll and other pigments for light‑dependent reactions. The stroma is the fluid‑filled space where the Calvin cycle takes place. Starch grains may be visible as stored carbohydrate.

叶绿体存在于植物和藻类中，进行光合作用。与线粒体一样，它们具有双层膜、自己的环状 DNA 和 70S 核糖体。在内膜内部是类囊体——扁平的膜囊堆叠形成基粒。类囊体膜上含有叶绿素和其他光合色素，以进行光依赖反应。基质是充满液体的空间，其中发生卡尔文循环。可以观察到淀粉粒作为储存的碳水化合物。

Under the electron microscope, a chloroplast shows stacks of grana connected by stromal lamellae. You should be able to relate the large thylakoid surface area to the efficiency of light capture, and the stroma volume to the enzymatic reactions of the Calvin cycle.

在电子显微镜下，叶绿体显示出被基质片层连接的基粒堆。你应当能够将巨大的类囊体表面积与高效捕光联系起来，并将基质体积与卡尔文循环的酶促反应联系起来。

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7. Cytoskeleton, Centrioles, and Cell Movement | 细胞骨架、中心粒与细胞运动

The cytoskeleton is a dynamic network of protein fibres that maintains cell shape, anchors organelles, and enables movement. It consists of three types of filaments: microtubules (made of tubulin), microfilaments (actin filaments), and intermediate filaments (e.g., keratin). Microtubules form the spindle during mitosis and are the main structural components of cilia and flagella (9+2 arrangement).

细胞骨架是一个由蛋白质纤维构成的动态网络，负责维持细胞形状、锚定细胞器并实现运动。它由三种纤丝组成：微管（由微管蛋白构成）、微丝（肌动蛋白丝）和中间丝（如角蛋白）。微管在有丝分裂时形成纺锤体，并且是纤毛和鞭毛的主要结构成分（9+2 排列）。

Centrioles are cylindrical structures composed of nine triplets of microtubules (9+0 arrangement). They are found in the centrosome region of animal cells and help organise the mitotic spindle. Plant cells lack centrioles but still form spindles. Cilia and flagella are motile extensions of the cell surface; cilia are usually many and short, while flagella are longer and fewer. The sliding microtubule model explains their beating motion using dynein motor proteins.

中心粒是由九组三联体微管（9+0 排列）组成的圆柱形结构。它们位于动物细胞的中心体区域，帮助组织有丝分裂纺锤体。植物细胞没有中心粒，但仍能形成纺锤体。纤毛和鞭毛是细胞表面的运动延伸物；纤毛通常数量多且短，而鞭毛较长且数量少。微管滑动模型利用动力蛋白马达蛋白来解释它们的摆动运动。

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8. Cell Walls and Extracellular Matrix | 细胞壁与细胞外基质

In plants, a primary cell wall is made of cellulose microfibrils embedded in a matrix of hemicellulose and pectin. It provides structural support while allowing expansion. Some cells later secrete a secondary cell wall containing lignin for added strength. Middle lamella, rich in pectin, glues adjacent cells together. Plasmodesmata are channels through the cell walls that allow direct cytoplasmic communication and transport of molecules between plant cells.

在植物中，初生细胞壁由嵌入半纤维素和果胶基质中的纤维素微纤丝构成。它提供结构支持，同时允许细胞扩张。一些细胞随后分泌含有木质素的次生细胞壁以增加强度。胞间层富含果胶，将相邻细胞粘合在一起。胞间连丝是穿过细胞壁的通道，允许植物细胞之间直接的细胞质通信和分子运输。

Animal cells do not have a cell wall but are surrounded by an extracellular matrix (ECM). The ECM is a network of glycoproteins (e.g., collagen, fibronectin) and proteoglycans, attached to the cell membrane via integrins. The ECM provides structural support, regulates cell behaviour (signalling, adhesion, migration), and plays a key role in tissue differentiation.

动物细胞没有细胞壁，但被细胞外基质（ECM）包围。ECM 是由糖蛋白（如胶原蛋白、纤连蛋白）和蛋白聚糖构成的网络，通过整合素附着到细胞膜上。ECM 提供结构支持，调节细胞行为（信号传导、粘附、迁移），并在组织分化中起关键作用。

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9. Microscopy – Light vs Electron | 显微镜技术 – 光学与电子显微镜

You must be able to compare light microscopes (LM) and electron microscopes (EM). Light microscopes use visible light and glass lenses; maximum resolution is about 200 nm, maximum magnification about 1500×. They can observe living specimens and colour can be seen using dyes. Electron microscopes use beams of electrons and electromagnetic lenses; resolution can reach 0.1 nm (TEM), magnification up to 2 million×. Samples must be dead due to the vacuum and staining with heavy metals. TEM gives internal details (cross‑sections), while SEM gives 3D surface views.

你必须能够比较光学显微镜（LM）和电子显微镜（EM）。光学显微镜使用可见光和玻璃透镜；最高分辨率约为 200 nm，最高放大倍数约 1500×。它可以观察活体标本，并且可以通过染料看到颜色。电子显微镜使用电子束和电磁透镜；分辨率可达 0.1 nm（TEM），放大倍数可达 200 万倍。由于真空和重金属染色，样品必须是死的。TEM 可显示内部细节（横截面），而 SEM 可提供三维表面视图。

Be comfortable converting units: 1 mm = 1000 µm, 1 µm = 1000 nm. Magnification = image size ÷ actual size. Scale bar questions are common. Resolution, not magnification, is the limiting factor in distinguishing adjacent structures.

要熟练掌握单位换算：1 mm = 1000 µm，1 µm = 1000 nm。放大倍数 = 图像尺寸 ÷ 实际尺寸。比例尺问题是常见的。限制相邻结构区分的是分辨率，而不是放大倍数。

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10. Specialised Cells and Structure–Function Relationship | 特化细胞与结构-功能关系

Cells differentiate to perform specific functions. IB/CIE exams frequently ask you to relate the structure of a specialised cell to its role. For example: red blood cells are biconcave and lack a nucleus and mitochondria – maximising space for haemoglobin and allowing flexibility (but they rely on anaerobic respiration). Palisade mesophyll cells contain many chloroplasts arranged vertically to absorb sunlight efficiently. Neurons have a long axon with myelin sheath for rapid saltatory conduction. Sperm cells have a streamlined head with an acrosome (containing enzymes) and a flagellum for motility.

细胞分化以执行特定功能。IB/CIE 考试经常要求你将特化细胞的结构与其作用联系起来。例如：红细胞呈双凹形，没有细胞核和线粒体——最大化血红蛋白的空间并具有柔韧性（但它们依赖无氧呼吸）。栅栏组织细胞含有许多叶绿体，垂直排列以有效吸收阳光。神经元有长轴突和髓鞘，用于快速跳跃传导。精子细胞有流线型的头部和顶体（含酶），以及用于运动的鞭毛。

Understanding structure–function relationships is key for data‑based questions where you might be given an unfamiliar cell type. Always look at the density of mitochondria (energy need), the presence of extensive ER (protein/lipid synthesis), lots of Golgi (secretion), or highly folded membranes (transport or enzyme attachment).

理解结构-功能关系是应对数据分析题的关键，你可能会被给到一个不熟悉的细胞类型。始终注意线粒体的密度（能量需求）、大量内质网的存在（蛋白质/脂质合成）、丰富的高尔基体（分泌）或高度折叠的膜（运输或酶附着）。

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11. Exam Tips and Common Misconceptions | 考试技巧与常见误区

Many students lose marks by confusing ‘cell wall’ and ‘cell membrane’ in terms of composition and function. Plant cell walls are primarily cellulose; fungal cell walls are chitin; bacterial cell walls are peptidoglycan. Animal cells never have a cell wall.

许多学生在组成和功能方面混淆“细胞壁”和“细胞膜”而失分。植物细胞壁主要是纤维素；真菌细胞壁是几丁质；细菌细胞壁是肽聚糖。动物细胞永远没有细胞壁。

Avoid writing simply “the cell gets bigger” when explaining why surface area‑to‑volume ratio limits cell size. Instead, state: as a cell increases in size, its volume increases more rapidly than its surface area (volume ∝ r³, surface area ∝ r²). This limits the rate of exchange (nutrients, wastes, heat) across the membrane, so cells remain small or adopt flattened/elongated shapes to compensate.

在解释为什么表面积与体积之比限制了细胞大小时，不要简单地写“细胞变大了”。而应说明：随着细胞体积增大，其体积的增长速率快于表面积（体积 ∝ r³，表面积 ∝ r²）。这限制了跨膜的物质交换速率（养分、废物、热量），因此细胞保持较小，或采取扁平/拉长的形状来补偿。

Another common error is misidentifying organelles in electron micrographs. Practise distinguishing smooth ER (tubular, no ribosomes) from rough ER (flattened sacs with dark dots), and free ribosomes (dark small dots) from glycogen granules. In chloroplasts, do not confuse starch grains with grana stacks.

另一个常见错误是在电子显微镜照片中错误识别细胞器。练习区分滑面内质网（管状，无核糖体）和粗面内质网（带有黑点的扁平囊），以及游离核糖体（暗色小点）与糖原颗粒。在叶绿体中，不要将淀粉粒与基粒堆混淆。

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12. Comparing Cell Types – Quick Reference Table | 细胞类型对比速查表

Use this table to review the key features tested in multiple‑choice and structured questions. Be aware that some structures (like vacuoles) vary between plant and animal cells; large central vacuoles are typical of mature plant cells but small, temporary vacuoles may exist in animal cells.

利用这张表格复习选择题和结构题中常考的关键特征。请注意，某些结构（如液泡）在植物和动物细胞之间存在差异；大型中央液泡是成熟植物细胞的典型特征，而动物细胞中可能存在小型、暂时性的液泡。

Feature	Prokaryote	Eukaryote – Animal	Eukaryote – Plant
Nucleus	Absent (nucleoid)	Present	Present
DNA form	Circular, naked	Linear, histone‑bound	Linear, histone‑bound
Ribosomes	70S	80S	80S (70S in chloroplast/mitochondria)
Cell wall	Peptidoglycan (most)	Absent	Cellulose
Chloroplasts	Absent	Absent	Present
Membrane‑bound organelles	None	Many	Many
Typical size	0.1‑5 µm	10‑100 µm	10‑100 µm
Mode of division	Binary fission	Mitosis	Mitosis

Careful comparison of these features will ensure you answer diagram‑based questions accurately and quickly. When in doubt, refer back to the principle of compartmentalisation and size measurement techniques.

仔细比较这些特征将确保你准确、快速地回答示意图题目。如有疑问，请回顾区室化原理和尺寸测量技术。

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