📚 Cell Structure and Function | 细胞结构与功能
Cells are the fundamental units of life. Understanding their structure and function is essential for mastering biology, as every physiological process ultimately depends on cellular organisation. This revision guide covers the key organelles, their roles, and the distinctions between cell types.
细胞是生命的基本单位。理解细胞的结构与功能是掌握生物学的关键，因为一切生理过程最终都依赖于细胞层面的组织。本篇复习指南涵盖了主要细胞器、它们的作用以及不同细胞类型之间的区别。
1. Prokaryotic vs Eukaryotic Cells | 原核细胞与真核细胞
All living organisms are made of either prokaryotic or eukaryotic cells. Prokaryotes (bacteria and archaea) lack a membrane-bound nucleus and organelles, while eukaryotes (plants, animals, fungi, protists) possess a true nucleus and compartmentalised organelles.
所有生物体由原核细胞或真核细胞构成。原核生物（细菌和古菌）没有膜包被的细胞核和细胞器，而真核生物（植物、动物、真菌、原生生物）具有真正的细胞核和分隔化的细胞器。
Key differences are summarised below:
关键区别如下表所示：
| Feature | 特征 | Prokaryotic | 原核 | Eukaryotic | 真核 |
|---|---|---|
| Nucleus | 细胞核 | Absent | 无 | Present | 有 |
| Size | 大小 | 0.5 – 5 μm | 0.5–5 微米 | 10 – 100 μm | 10–100 微米 |
| Membrane-bound organelles | 膜包被细胞器 | None | 无 | Many | 多种 |
| Ribosomes | 核糖体 | 70S | 70S | 80S (cytoplasm) | 80S（细胞质） |
| DNA | DNA | Circular, naked | 环状、裸露 | Linear, associated with histones | 线性、与组蛋白结合 |
Example exam question: State two features of prokaryotic cells that are absent in eukaryotic cells.
典型考题：说出原核细胞具有而真核细胞没有的两个特征。
2. Plasma Membrane Structure | 细胞膜的结构
The plasma membrane surrounds all cells, controlling the movement of substances in and out. According to the fluid mosaic model, it consists of a phospholipid bilayer with embedded proteins, cholesterol (in animal cells), and glycoproteins/glycolipids.
细胞膜包围所有细胞，控制物质进出。根据流动镶嵌模型，细胞膜由磷脂双分子层以及嵌入的蛋白质、胆固醇（动物细胞中）和糖蛋白/糖脂组成。
Phospholipids have hydrophilic heads and hydrophobic tails, forming a barrier to water-soluble molecules.
磷脂具有亲水头部和疏水尾部，形成阻止水溶性分子通过的屏障。
Proteins perform transport, enzymatic, signalling, and structural roles.
蛋白质执行运输、酶促、信号传递和结构功能。
Cholesterol regulates fluidity and stability in animal membranes.
胆固醇调节动物细胞膜的流动性和稳定性。
Fluid mosaic model: phospholipid bilayer + proteins + cholesterol | 流动镶嵌模型：磷脂双分子层 + 蛋白质 + 胆固醇
Membrane fluidity allows lateral movement of lipids and proteins, essential for cell growth, division, and vesicle formation.
膜的流动性允许脂质和蛋白质侧向移动，这对细胞生长、分裂和囊泡形成至关重要。
3. Nucleus and Genetic Control | 细胞核与遗传控制
The nucleus is the control centre of the eukaryotic cell, containing most of the genetic material in the form of chromatin. It is surrounded by a double membrane called the nuclear envelope, perforated by nuclear pores.
细胞核是真核细胞的控制中心，以染色质形式含有大部分遗传物质。它由称为核膜的双层膜包围，核膜上有核孔。
Nuclear envelope: double membrane continuous with the rough ER; controls exchange of materials.
核膜：与粗面内质网相连的双层膜；控制物质交换。
Nuclear pores: allow mRNA and ribosomal subunits to exit, and proteins to enter.
核孔：允许 mRNA 和核糖体亚基出核，蛋白质入核。
Nucleolus: site of rRNA synthesis and ribosome assembly.
核仁：rRNA 合成和核糖体组装的场所。
Chromatin is a complex of DNA and histone proteins. During cell division, chromatin condenses into visible chromosomes.
染色质是 DNA 和组蛋白的复合体。细胞分裂时，染色质凝缩为可见的染色体。
4. Ribosomes and Protein Synthesis | 核糖体与蛋白质合成
Ribosomes are non-membrane-bound organelles composed of rRNA and protein. They are the sites of translation, where mRNA is decoded to produce polypeptide chains.
核糖体是由 rRNA 和蛋白质构成的无膜细胞器。它们是翻译的场所，mRNA 在此被解码以产生多肽链。
Free ribosomes in the cytoplasm synthesise proteins used inside the cell.
细胞质中的游离核糖体合成用于细胞内部的蛋白质。
Bound ribosomes on the rough ER synthesise proteins destined for secretion or lysosomes.
附着在粗面内质网上的核糖体合成用于分泌或进入溶酶体的蛋白质。
70S ribosomes are found in prokaryotes, mitochondria and chloroplasts; 80S ribosomes in eukaryotic cytoplasm.
70S 核糖体存在于原核生物、线粒体和叶绿体中；80S 核糖体存在于真核细胞质中。
Translation: mRNA → polypeptide | 翻译：mRNA → 多肽
5. Endoplasmic Reticulum and Golgi Apparatus | 内质网与高尔基体
The endomembrane system includes the rough ER, smooth ER, Golgi apparatus, and vesicles. These organelles work together to synthesise, modify, and transport macromolecules.
内膜系统包括粗面内质网、滑面内质网、高尔基体和囊泡。这些细胞器协同合成、修饰和运输大分子。
Rough ER | 粗面内质网:
Studded with ribosomes; involved in protein folding and transport.
表面附着核糖体；参与蛋白质折叠和运输。
Proteins enter the ER lumen and may be modified (e.g., glycosylation).
蛋白质进入内质网腔，并可能被修饰（如糖基化）。
Smooth ER | 滑面内质网:
Lacks ribosomes; synthesises lipids and steroids.
无核糖体；合成脂质和类固醇。
In liver cells, it detoxifies drugs and toxins.
在肝细胞中，它代谢药物和毒素。
Golgi apparatus | 高尔基体:
Modifies proteins and lipids, and packages them into vesicles.
修饰蛋白质和脂质，并将它们包装成囊泡。
Produces lysosomes and secretory vesicles.
产生溶酶体和分泌囊泡。
Vesicles bud from the ER, fuse with the Golgi, and then travel to the plasma membrane for exocytosis.
囊泡从内质网出芽，与高尔基体融合，然后运往细胞膜进行胞吐。
6. Mitochondria and Energy Production | 线粒体与能量产生
Mitochondria are the powerhouses of aerobic eukaryotic cells. They carry out aerobic respiration, producing ATP from glucose and other substrates.
线粒体是需氧真核细胞的动力工厂。它们进行有氧呼吸，从葡萄糖和其他底物产生 ATP。
Double membrane: outer smooth, inner folded into cristae.
双层膜：外膜光滑，内膜折叠形成嵴。
Matrix: contains enzymes for Krebs cycle, mitochondrial DNA, and ribosomes.
基质：含有三羧酸循环所需的酶、线粒体 DNA 和核糖体。
Cristae increase surface area for oxidative phosphorylation (ATP synthase).
嵴增加氧化磷酸化（ATP 合酶）的表面积。
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP | 葡萄糖 + 氧气 → 二氧化碳 + 水 + ATP
Cells with high energy demand (e.g., muscle cells, neurons) contain many mitochondria.
能量需求高的细胞（如肌细胞、神经元）含有大量线粒体。
7. Chloroplasts and Photosynthesis | 叶绿体与光合作用
Chloroplasts are organelles found in plant cells and algae. They absorb light energy and convert it into chemical energy through photosynthesis.
叶绿体存在于植物细胞和藻类中。它们吸收光能，并通过光合作用将其转化为化学能。
Thylakoid membranes: contain chlorophyll; site of light-dependent reactions.
类囊体膜：含有叶绿素；光反应阶段发生在膜上。
Grana: stacks of thylakoids; increase surface area for light absorption.
基粒：类囊体的堆叠；增大光吸收的表面积。
Stroma: fluid around thylakoids; site of the Calvin cycle (light-independent reactions).
基质：类囊体周围的液体；卡尔文循环（暗反应）发生的场所。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ | 二氧化碳 + 水 → 葡萄糖 + 氧气
8. Lysosomes and Peroxisomes | 溶酶体与过氧化物酶体
Lysosomes are membrane-bound vesicles containing hydrolytic enzymes (lysozymes). They digest cellular waste, damaged organelles (autophagy), and engulfed pathogens.
溶酶体是含有水解酶（溶菌酶）的膜包囊泡。它们消化细胞废物、受损细胞器（自噬）和吞噬的病原体。
Enzymes work best at acidic pH (about 5), maintained by proton pumps.
酶在酸性 pH（约 5）下活性最高，该环境由质子泵维持。
If lysosomes burst, their enzymes can cause apoptosis (programmed cell death).
如果溶酶体破裂，其酶可能导致细胞凋亡（程序性细胞死亡）。
Peroxisomes contain enzymes such as catalase, which breaks down hydrogen peroxide (H₂O₂) into water and oxygen, protecting the cell from oxidative damage.
过氧化物酶体含有过氧化氢酶等酶，可将过氧化氢（H₂O₂）分解为水和氧气，保护细胞免受氧化损伤。
9. Cytoskeleton | 细胞骨架
The cytoskeleton is a network of protein fibres that maintains cell shape, enables movement, and organises organelles.
细胞骨架是蛋白质纤维网络，维持细胞形态、实现运动并组织细胞器。
Microtubules: hollow tubes of tubulin; provide tracks for vesicle transport and form spindle fibres during mitosis.
微管：由微管蛋白组成的中空管；为囊泡运输提供轨道，并在有丝分裂中形成纺锤丝。
Microfilaments: actin filaments; involved in muscle contraction, cell division, and cytoplasmic streaming.
微丝：肌动蛋白丝；参与肌肉收缩、细胞分裂和胞质环流。
Intermediate filaments: provide mechanical strength.
中间纤维：提供机械强度。
Cilia and flagella are microtubule-based structures that move fluids or propel cells.
纤毛和鞭毛是基于微管的结构，用于移动液体或推动细胞前进。
10. Cell Walls and Cell Junctions | 细胞壁与细胞连接
Plant cells, bacteria, and fungi have cell walls outside their plasma membrane, providing support and protection. Plant cell walls are made of cellulose; bacterial walls contain peptidoglycan; fungal walls contain chitin.
植物细胞、细菌和真菌在细胞膜外具有细胞壁，提供支撑和保护。植物细胞壁由纤维素构成；细菌细胞壁含肽聚糖；真菌细胞壁含几丁质。
Animal cells do not have cell walls, but they form specialised junctions:
动物细胞没有细胞壁，但形成特化的连接结构：
Tight junctions: seal adjacent cells to prevent leakage.
紧密连接：封闭相邻细胞以防止渗漏。
Desmosomes: anchor cells together, providing mechanical strength.
桥粒：将细胞锚定在一起，提供机械强度。
Gap junctions: allow small molecules and ions to pass directly between cells.
间隙连接：允许小分子和离子直接在细胞间通过。
In plant cells, plasmodesmata are cytoplasmic channels that connect neighbouring cells for communication and transport.
在植物细胞中，胞间连丝是连接相邻细胞的细胞质通道，用于通讯和运输。
11. Microscopy and Cell Studies | 显微镜与细胞研究
Observing cell structure requires different types of microscopes. The limits of resolution determine what structures can be seen.
观察细胞结构需要使用不同类型的显微镜。分辨率的极限决定了能看到哪些结构。
Light microscope: maximum resolution ~200 nm; can see nucleus, chloroplasts, mitochondria (as granules).
光学显微镜：最大分辨率约 200 纳米；可见细胞核、叶绿体、线粒体（呈颗粒状）。
Transmission electron microscope (TEM): resolution ~1 nm; reveals internal organelle structure.
透射电子显微镜：分辨率约 1 纳米；揭示细胞器内部结构。
Scanning electron microscope (SEM): produces 3D surface images.
扫描电子显微镜：产生三维表面图像。
Cell fractionation separates organelles by differential centrifugation, allowing biochemical analysis of each fraction.
细胞分级分离通过差速离心分离细胞器，从而对各组分进行生化分析。
12. Common Exam Pitfalls | 常见易错点
Many students confuse the functions of organelles. Here are the most frequent errors:
许多学生容易混淆细胞器的功能。以下是最常见的错误：
Mistaking smooth ER for protein synthesis – smooth ER makes lipids, while rough ER makes proteins.
误以为滑面内质网合成蛋白质——滑面内质网制造脂质，而粗面内质网制造蛋白质。
Stating that lysosomes "kill" bacteria – they contain enzymes that break them down after phagocytosis.
说溶酶体“杀死”细菌——实际是吞噬后其中的酶将其分解。
Forgetting that mitochondria have their own DNA and 70S ribosomes.
忘记线粒体拥有自己的 DNA 和 70S 核糖体。
Writing that chloroplasts contain DNA – yes, but also starch granules and lipid droplets.
写叶绿体含 DNA——对的，但还含有淀粉粒和脂滴。
Confusing resolution with magnification – resolution is the ability to distinguish two close points.
混淆分辨率与放大倍数——分辨率是区分两个邻近点的能力。
Tip: Always link structure to function. For example, the cristae of mitochondria increase surface area for ATP production.
提示：始终将结构与功能联系起来。例如，线粒体嵴增加表面积以利于 ATP 产生。
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