Cell Structure and Function | 细胞结构的组成与功能

📚 Cell Structure and Function | 细胞结构的组成与功能

Cells are the fundamental units of life. Every organism, from a single-celled bacterium to a complex multicellular animal, is composed of cells that carry out essential processes. Understanding the structure and function of cellular components is a core requirement in biology, as it forms the foundation for topics such as metabolism, genetics, and physiology.

细胞是生命的基本单位。从单细胞细菌到复杂的多细胞动物，每一种生物体都由执行基本生命过程的细胞组成。理解细胞各组分的结构与功能是生物学的核心考点，也是学习新陈代谢、遗传学与生理学等主题的基础。

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1. The Cell Membrane | 细胞膜

The cell membrane, also known as the plasma membrane, surrounds the cell and acts as a selective barrier. According to the fluid mosaic model, the membrane is composed of a phospholipid bilayer with embedded proteins, cholesterol, and glycolipids. The hydrophobic tails face inward, while the hydrophilic heads face outward, creating a stable barrier in aqueous environments.

细胞膜也称质膜，包裹在细胞外，作为选择性屏障。根据“流体镶嵌模型”，细胞膜由磷脂双分子层构成，其中镶嵌有蛋白质、胆固醇和糖脂。疏水的脂肪酸尾部朝向内侧，亲水的头部朝向外侧，从而在水环境中形成稳定屏障。

• Phospholipids form the basic bilayer structure and provide fluidity.

• 磷脂构成基本的双分子层结构，并提供流动性。

• Proteins function as channels, carriers, receptors, and enzymes.

• 蛋白质担任通道、载体、受体和酶等功能。

• Cholesterol regulates fluidity and stability in animal cells.

• 胆固醇调节动物细胞膜的流动性与稳定性。

• Glycoproteins and glycolipids are involved in cell recognition and adhesion.

• 糖蛋白和糖脂参与细胞识别与黏附。

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2. The Nucleus | 细胞核

The nucleus is the control center of the cell. It is surrounded by a double membrane called the nuclear envelope, which contains nuclear pores that allow selective exchange of materials between the nucleus and the cytoplasm. Inside the nucleus, chromatin (DNA associated with histone proteins) carries genetic information, and the nucleolus is the site of ribosomal RNA synthesis and ribosome assembly.

细胞核是细胞的调控中心。它由双层核膜包裹，核膜上存在核孔，允许细胞核与细胞质之间进行选择性物质交换。细胞核内，染色质（DNA与组蛋白结合而成）携带遗传信息，而核仁是核糖体RNA合成与核糖体组装的场所。

• The nuclear envelope separates nuclear contents from the cytoplasm.

• 核膜将核内物质与细胞质分隔开来。

• Nuclear pores regulate the transport of mRNA and proteins.

• 核孔调控mRNA和蛋白质的转运。

• Chromatin condenses into chromosomes during cell division.

• 染色质在细胞分裂时凝缩为染色体。

• The nucleolus disappears during cell division and reforms after mitosis.

• 核仁在细胞分裂期间消失，有丝分裂后重新形成。

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3. Ribosomes | 核糖体

Ribosomes are the sites of protein synthesis. They are composed of ribosomal RNA and proteins, and may exist freely in the cytoplasm or be bound to the rough endoplasmic reticulum. Free ribosomes synthesise proteins that remain in the cytoplasm, while bound ribosomes produce proteins destined for secretion or for incorporation into membranes.

核糖体是蛋白质合成的场所。它们由核糖体RNA和蛋白质组成，既可以在细胞质中游离存在，也可以附着于粗面内质网上。游离核糖体合成保留在细胞质中的蛋白质，而附着核糖体合成用于分泌或嵌入细胞膜的蛋白质。

• Prokaryotic ribosomes are 70S, while eukaryotic ribosomes are 80S.

• 原核生物核糖体为70S，真核生物核糖体为80S。

• Each ribosome has a large and a small subunit.

• 每个核糖体由大亚基和小亚基组成。

• Ribosomes translate mRNA sequences into polypeptide chains.

• 核糖体将mRNA序列翻译为多肽链。

mRNA → Polypeptide (protein) | mRNA → 多肽（蛋白质）

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4. Endoplasmic Reticulum | 内质网

The endoplasmic reticulum (ER) is a network of membranous tubules and flattened sacs called cisternae. There are two types: rough ER, which has ribosomes attached and is involved in protein synthesis and modification, and smooth ER, which lacks ribosomes and is involved in lipid synthesis, carbohydrate metabolism, and detoxification.

内质网是由膜性管道和扁平囊状结构（称为潴泡）组成的网络系统。内质网分为两种：粗面内质网表面附着核糖体，参与蛋白质的合成与加工；滑面内质网表面无核糖体，参与脂质合成、糖类代谢和解毒过程。

• Rough ER synthesises and modifies proteins destined for secretion.

• 粗面内质网合成并加工面向分泌的蛋白质。

• Smooth ER synthesises phospholipids and steroid hormones.

• 滑面内质网合成磷脂和类固醇激素。

• Smooth ER in liver cells detoxifies drugs and metabolic waste.

• 肝细胞中的滑面内质网参与药物和代谢废物的解毒。

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5. Golgi Apparatus | 高尔基体

The Golgi apparatus consists of a stack of flattened membrane sacs known as cisternae. It functions as the cell’s processing, sorting, and packaging centre. Proteins and lipids received from the ER are chemically modified, then packaged into vesicles for transport to their final destinations, such as the plasma membrane, lysosomes, or extracellular space.

高尔基体由一叠扁平膜囊（潴泡）构成，是细胞的加工、分选和包装中心。从内质网接收的蛋白质和脂质在此进行化学修饰，随后被包装进囊泡，运往最终目的地，如质膜、溶酶体或细胞外空间。

• The Golgi apparatus modifies proteins through glycosylation and phosphorylation.

• 高尔基体通过糖基化和磷酸化对蛋白质进行修饰。

• It sorts proteins into different vesicles targeted to specific locations.

• 它将蛋白质分选入不同囊泡，定向运输到特定位置。

• It also synthesises some polysaccharides, such as pectin and hemicellulose in plant cells.

• 它还合成某些多糖，如植物细胞中的果胶和半纤维素。

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6. Mitochondria | 线粒体

Mitochondria are double-membraned organelles known as the powerhouse of the cell. The inner membrane is folded into cristae, which greatly increase its surface area for oxidative phosphorylation. The matrix contains enzymes for the Krebs cycle, as well as mitochondrial DNA and ribosomes. Mitochondria are the site of aerobic respiration, producing ATP from carbohydrates and fats.

线粒体是双层膜细胞器，被称为细胞的“动力工厂”。其内膜向内折叠形成嵴，大幅增加了氧化磷酸化的表面积。基质中含有三羧酸循环所需的酶，以及线粒体DNA和核糖体。线粒体是有氧呼吸的场所，利用糖类和脂肪合成ATP。

• The outer membrane is smooth and permeable to small molecules.

• 外膜平滑，对小分子具有通透性。

• The inner membrane contains electron transport chain complexes and ATP synthase.

• 内膜上含有电子传递链复合物和ATP合酶。

• The matrix is the site of the Krebs cycle and fatty acid oxidation.

• 基质是三羧酸循环和脂肪酸氧化的场所。

• Mitochondria are semi-autonomous and can divide by binary fission.

• 线粒体是半自主细胞器，可通过二分裂方式增殖。

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7. Chloroplasts | 叶绿体

Chloroplasts are organelles found in plant cells and some protists, and are the sites of photosynthesis. They are surrounded by a double membrane and contain an internal system of thylakoid membranes arranged in stacks called grana. Surrounding the grana is the stroma, which contains enzymes for the Calvin cycle. Chlorophyll pigments located in the thylakoid membranes capture light energy.

叶绿体存在于植物细胞和某些原生生物中，是光合作用的场所。它由双层膜包裹，内部具有由类囊体膜组成的系统，类囊体堆叠成基粒。基粒周围的基质中含有卡尔文循环所需的酶。类囊体膜上的叶绿素色素负责捕获光能。

• Thylakoid membranes are the site of the light-dependent reactions.

• 类囊体膜是光依赖反应的场所。

• The stroma is the site of the light-independent reactions (Calvin cycle).

• 基质是光非依赖反应（卡尔文循环）的场所。

• Chloroplasts contain their own DNA and ribosomes.

• 叶绿体含有自身的DNA和核糖体。

• The double membrane controls the movement of metabolites in and out.

• 双层膜控制代谢物进出叶绿体。

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

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8. Lysosomes and Peroxisomes | 溶酶体与过氧化物酶体

Lysosomes are membrane-bound organelles containing hydrolytic enzymes that digest macromolecules, worn-out organelles, and foreign materials. They maintain an acidic internal pH, optimal for enzyme activity. Peroxisomes are small organelles containing oxidative enzymes that break down fatty acids and amino acids, and catalyse the detoxification of hydrogen peroxide (H₂O₂) into water and oxygen.

溶酶体是含有水解酶的膜结合细胞器，可消化大分子、衰老细胞器和外来物质。其内部维持酸性pH，以提供酶的最适活性条件。过氧化物酶体是含有氧化酶的小型细胞器，可分解脂肪酸和氨基酸，并催化过氧化氢（H₂O₂）分解为水和氧气。

• Lysosomal enzymes are optimally active at acidic pH (around 4.5–5.0).

• 溶酶体酶在酸性pH（约4.5–5.0）条件下活性最佳。

• Peroxisomes protect the cell from oxidative damage.

• 过氧化物酶体保护细胞免受氧化损伤。

• Lysosomes are involved in apoptosis (programmed cell death).

• 溶酶体参与细胞凋亡（程序性细胞死亡）。

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9. Cytoskeleton | 细胞骨架

The cytoskeleton is a dynamic network of protein filaments that extends throughout the cytoplasm. It provides structural support, enables intracellular transport, and facilitates cell movement. The three main components are microfilaments (actin filaments), microtubules (tubulin polymers), and intermediate filaments.

细胞骨架是一个贯穿细胞质的动态蛋白质纤维网络。它提供结构支撑、促进细胞内物质运输，并参与细胞运动。三大组成部分是微丝（肌动蛋白纤维）、微管（微管蛋白聚合物）和中间纤维。

• Microfilaments are involved in muscle contraction, cell division, and cytoplasmic streaming.

• 微丝参与肌肉收缩、细胞分裂和细胞质流动。

• Microtubules form the mitotic spindle and provide tracks for vesicle transport.

• 微管构成有丝分裂纺锤体，并为囊泡运输提供轨道。

• Intermediate filaments provide mechanical strength and maintain cell shape.

• 中间纤维提供机械强度并维持细胞形态。

• Cilia and flagella are microtubule-based structures that enable cell motility.

• 纤毛和鞭毛是以微管为基础的结构，赋予细胞运动能力。

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10. Cell Wall and Extracellular Matrix | 细胞壁与胞外基质

Plant cells are surrounded by a rigid cell wall composed primarily of cellulose, hemicellulose, and pectin. The cell wall provides structural support, prevents excessive water uptake, and protects against pathogens. In animal cells, the extracellular matrix (ECM) is composed of proteins such as collagen and fibronectin, and polysaccharides like hyaluronic acid, providing support and mediating cell signalling.

植物细胞外有坚硬的细胞壁，主要由纤维素、半纤维素和果胶组成。细胞壁提供结构支撑，防止细胞过度吸水，并抵御病原体入侵。动物细胞外基质主要由胶原蛋白、纤连蛋白等蛋白质以及透明质酸等多糖构成，提供支持并介导细胞信号传递。

• Cellulose microfibrils provide tensile strength to the plant cell wall.

• 纤维素微纤丝赋予植物细胞壁抗张强度。

• Plasmodesmata are cytoplasmic channels connecting neighbouring plant cells.

• 胞间连丝是连接相邻植物细胞的细胞质通道。

• The ECM influences cell growth, migration, and differentiation.

• 胞外基质影响细胞生长、迁移和分化。

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11. Protein Secretion Pathway | 蛋白质分泌途径

Cells that secrete proteins, such as pancreatic acinar cells and plasma cells, follow a well-defined pathway. The process begins with the synthesis of a protein on ribosomes attached to the rough ER, followed by transport to the Golgi apparatus for modification and packaging, and finally exocytosis via secretory vesicles at the plasma membrane.

分泌蛋白质的细胞，如胰腺腺泡细胞和浆细胞，遵循一条明确的通路。过程始于粗面内质网附着核糖体上的蛋白质合成，随后转运至高尔基体进行加工和包装，最终通过分泌囊泡在质膜处进行胞吐作用。

Nucleus → Rough ER → Golgi Apparatus → Vesicle → Plasma Membrane → Exocytosis

细胞核 → 粗面内质网 → 高尔基体 → 囊泡 → 质膜 → 胞吐

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12. Comparative Summary Table | 比较总结表

The table below summarises the key structures, locations, and functions of the major cell components.

下表总结了主要细胞组件的结构、位置与功能。

Organelle | 细胞器	Structure | 结构	Key Function | 主要功能
Nucleus | 细胞核	Double membrane, pores | 双层膜、核孔	Stores genetic information | 储存遗传信息
Ribosome | 核糖体	RNA + protein | RNA和蛋白质	Protein synthesis | 蛋白质合成
Rough ER | 粗面内质网	Membrane sacs with ribosomes | 带核糖体的膜囊	Protein synthesis and processing | 蛋白质合成与加工
Smooth ER | 滑面内质网	Membrane sacs without ribosomes | 无核糖体的膜囊	Lipid synthesis and detoxification | 脂质合成与解毒
Golgi apparatus | 高尔基体	Stacked cisternae | 叠层潴泡	Modification and packaging | 加工与包装
Mitochondrion | 线粒体	Double membrane, cristae | 双层膜、嵴	Aerobic respiration / ATP synthesis | 有氧呼吸 / ATP合成
Chloroplast | 叶绿体	Double membrane, thylakoids | 双层膜、类囊体	Photosynthesis | 光合作用
Lysosome | 溶酶体	Membrane-bound, hydrolytic enzymes | 膜结合、水解酶	Intracellular digestion | 细胞内消化
Peroxisome | 过氧化物酶体	Single membrane, oxidase enzymes | 单层膜、氧化酶	Oxidation and detoxification | 氧化与解毒

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In conclusion, each organelle has a specialised structure that enables its specific function. The coordination between these organelles ensures the survival, growth, and reproduction of the cell. A clear understanding of the structure–function relationship of cellular components is essential for exam success.

总之，每种细胞器都有与其功能相适应的特殊结构。细胞器之间的协调配合保证了细胞的存活、生长与繁殖。清晰理解细胞组分的结构与功能关系是取得考试成功的关键。

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