Analysis of Cell Ultrastructure | 细胞超微结构解析

📚 Analysis of Cell Ultrastructure | 细胞超微结构解析

The term “ultrastructure” refers to the detailed structure of a cell as revealed by electron microscopy. Unlike light microscopy, which is limited by a resolution of approximately 200 nm, electron microscopes can achieve resolutions down to 0.2 nm, allowing us to visualise organelles such as mitochondria, ribosomes and the endoplasmic reticulum in extraordinary detail. Understanding ultrastructure is fundamental to linking cellular form with physiological function.

“超微结构”一词指的是通过电子显微镜所揭示的细胞精细结构。光学显微镜的分辨率极限约为 200 nm，而电子显微镜的分辨率可达 0.2 nm，使我们能够观察到线粒体、核糖体、内质网等细胞器的精细形态。理解超微结构是将细胞形态与生理功能联系起来的基础。

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1. Overview of Cellular Organisation | 细胞组织的总览

All living organisms are composed of cells, which fall into two broad categories: prokaryotic cells (bacteria and archaea) and eukaryotic cells (protists, fungi, plants and animals). Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles, whereas eukaryotic cells possess a highly compartmentalised internal structure. Compartmentalisation is a key evolutionary advantage: it allows incompatible biochemical reactions to occur simultaneously within the same cell.

所有生物体均由细胞构成，细胞可分为两大类：原核细胞（细菌和古菌）与真核细胞（原生生物、真菌、植物和动物）。原核细胞没有膜包裹的细胞核和膜性细胞器，而真核细胞具有高度区室化的内部结构。区室化是一个关键的进化优势：它允许互不相容的生化反应在同一细胞内同时进行。

Resolution of light microscope ≈ 200 nm; Resolution of electron microscope ≈ 0.2 nm

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2. The Nucleus | 细胞核

The nucleus is the largest organelle in most eukaryotic cells and serves as the control centre. It is surrounded by a double membrane called the nuclear envelope, which is perforated by nuclear pores approximately 9 nm in diameter. These pores allow the selective transport of mRNA, ribosomal subunits and proteins between the nucleus and the cytoplasm. The nucleolus, a dense region within the nucleus, is the site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

细胞核是大多数真核细胞中最大的细胞器，是细胞的调控中心。它由称为核被膜的双层膜包裹，核被膜上分布着直径约 9 nm 的核孔。核孔允许 mRNA、核糖体亚基和蛋白质在细胞核与细胞质之间选择性转运。核仁是细胞核内致密的区域，是核糖体 RNA（rRNA）合成和核糖体组装的场所。

Within the nucleus, DNA is associated with histone proteins to form chromatin. During interphase, chromatin exists in a partially condensed state; during cell division, it condenses further to form visible chromosomes. The nuclear lamina, a meshwork of intermediate filaments beneath the nuclear envelope, provides structural support and regulates chromatin organisation.

在细胞核内，DNA 与组蛋白结合形成染色质。在间期，染色质处于部分凝集状态；在细胞分裂时，染色质进一步凝集成可见的染色体。核纤层是核被膜下方由中间丝构成的网状结构，为细胞核提供结构支撑并调节染色质的组织。

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3. Mitochondria | 线粒体

Mitochondria are the sites of aerobic respiration and ATP synthesis. Each mitochondrion is bounded by two membranes: a smooth outer membrane and an inner membrane that is folded into cristae. The cristae greatly increase the surface area available for the electron transport chain and ATP synthase complexes. The fluid-filled space enclosed by the inner membrane is called the matrix, which contains enzymes for the Krebs cycle, mitochondrial DNA (mtDNA), and 70S ribosomes.

线粒体是有氧呼吸和 ATP 合成的场所。每个线粒体由两层膜包裹：光滑的外膜和折叠形成嵴的内膜。嵴大大增加了电子传递链和 ATP 合酶复合物可用的表面积。内膜所包围的液态空间称为基质，其中含有三羧酸循环所需的酶、线粒体 DNA（mtDNA）和 70S 核糖体。

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP (maximum)

The presence of mtDNA and 70S ribosomes supports the endosymbiotic theory, which proposes that mitochondria evolved from aerobic bacteria engulfed by ancestral eukaryotic cells. The inner membrane is impermeable to most small ions, so specific transport proteins are required to move pyruvate, ADP and phosphate into the matrix.

线粒体中 mtDNA 和 70S 核糖体的存在支持内共生学说，该学说认为线粒体由被原始真核细胞吞噬的好氧细菌演化而来。内膜对大多数小离子不通透，因此需要特定的转运蛋白将丙酮酸、ADP 和磷酸运入基质。

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4. Chloroplasts | 叶绿体

Chloroplasts are the organelles responsible for photosynthesis in plant cells and green algae. Like mitochondria, they are surrounded by a double membrane. Internally, they contain a system of membranous sacs called thylakoids, which are stacked into structures known as grana (singular: granum). The thylakoid membrane contains chlorophyll and the photosystems required for the light-dependent reactions of photosynthesis.

叶绿体是植物细胞和绿藻中进行光合作用的细胞器。与线粒体类似，叶绿体也由双层膜包裹。其内部包含称为类囊体的膜性囊泡系统，类囊体堆叠形成基粒。类囊体膜上含有叶绿素和光合作用光反应所需的光系统。

The stroma is the semi-fluid matrix surrounding the thylakoids; it contains enzymes for the Calvin cycle (light-independent reactions), as well as chloroplast DNA and ribosomes. The presence of starch grains and lipid droplets in the stroma indicates that chloroplasts are sites of carbohydrate synthesis and temporary storage. Chloroplasts are capable of limited movement within the cell, a phenomenon known as cyclosis or cytoplasmic streaming, which optimises light capture.

基质是围绕类囊体的半流体基质，含有卡尔文循环（光独立反应）所需的酶，以及叶绿体 DNA 和核糖体。基质中淀粉粒和脂滴的存在表明叶绿体是碳水化合物合成和临时储存的场所。叶绿体能够在细胞内进行有限的移动，这种现象称为胞质环流，有助于优化光能捕获。

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5. Endoplasmic Reticulum | 内质网

The endoplasmic reticulum (ER) is an extensive network of membranous tubules and flattened sacs called cisternae, continuous with the nuclear envelope. The rough endoplasmic reticulum (RER) is studded with ribosomes on its cytoplasmic surface, giving it a “rough” appearance. RER is specialised for the synthesis of proteins destined for secretion, lysosomes or the plasma membrane. These proteins are co-translationally inserted into the ER lumen, where they undergo folding and initial glycosylation.

内质网（ER）是由膜性小管和扁平囊状结构（称为潴泡）组成的广泛网络，与核被膜相连。粗面内质网（RER）的细胞质表面附着有核糖体，使其呈现”粗糙”外观。RER 专门合成分泌蛋白、溶酶体蛋白和质膜蛋白。这些蛋白质在翻译过程中被共翻译插入 ER 腔，在那里进行折叠和初始糖基化。

The smooth endoplasmic reticulum (SER) lacks ribosomes and is involved in lipid and steroid synthesis, calcium ion storage, and detoxification of drugs and poisons. In liver cells, SER is abundant to support detoxification; in muscle cells, a specialised form called the sarcoplasmic reticulum stores Ca²⁺ ions essential for muscle contraction.

滑面内质网（SER）没有核糖体，参与脂质和类固醇的合成、钙离子储存以及药物和毒物的解毒。肝细胞中 SER 丰富以支持解毒功能；肌细胞中特化的 SER 称为肌浆网，储存肌肉收缩所必需的 Ca²⁺ 离子。

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6. Golgi Apparatus | 高尔基体

The Golgi apparatus consists of a stack of flattened, curved cisternae, usually 4–8 in number, with distinct cis (receiving) and trans (sending) faces. Vesicles from the ER fuse with the cis face, delivering proteins for further processing. As proteins move through the Golgi stack, they undergo post-translational modifications including glycosylation, phosphorylation and proteolytic cleavage. Modified proteins are then packaged into vesicles at the trans face for transport to their final destinations.

高尔基体由一叠扁平弯曲的潴泡组成，通常有 4–8 层，具有明显的顺面（接收面）和反面（发送面）。来自 ER 的囊泡与顺面融合，将蛋白质送入进一步加工。蛋白质在通过高尔基体层叠的过程中经历翻译后修饰，包括糖基化、磷酸化和蛋白水解切割。修饰后的蛋白质在反面被包装成囊泡，运往最终目的地。

In plant cells, the Golgi apparatus also synthesises pectin and hemicelluloses for the cell wall. The number and size of Golgi stacks varies with cell type: cells that actively secrete proteins, such as pancreatic acinar cells and goblet cells, have particularly prominent Golgi apparatuses.

在植物细胞中，高尔基体还合成细胞壁所需的果胶和半纤维素。高尔基体的数量和大小因细胞类型而异：活跃分泌蛋白质的细胞，如胰腺腺泡细胞和杯状细胞，具有特别发达的高尔基体。

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7. Lysosomes | 溶酶体

Lysosomes are membrane-bound organelles containing over 50 types of hydrolytic enzymes, including proteases, nucleases, lipases and glycosidases. These enzymes are most active at acidic pH (around 4.8), which is maintained by a vacuolar H⁺-ATPase that pumps protons into the lumen. The lysosomal membrane protects the rest of the cell from these destructive enzymes; leakage of lysosomal enzymes can trigger apoptosis (programmed cell death).

溶酶体是含有 50 多种水解酶的膜性细胞器，包括蛋白酶、核酸酶、脂肪酶和糖苷酶。这些酶在酸性 pH（约 4.8）条件下活性最高，该酸性环境由液泡型 H⁺-ATP 酶将质子泵入腔内来维持。溶酶体膜保护细胞的其他部分免受这些破坏性酶的侵害；溶酶体酶泄漏可触发细胞凋亡（程序性细胞死亡）。

Lysosomes perform three major functions: intracellular digestion of macromolecules (autophagy of damaged organelles), phagocytosis of bacteria and cellular debris (in white blood cells), and exocytosis of waste materials. In sperm cells, the acrosome is a specialised lysosome that releases enzymes to penetrate the egg’s outer layer during fertilisation.

溶酶体执行三大功能：大分子的胞内消化（自噬受损细胞器）、吞噬细菌和细胞碎片（在白细胞中），以及通过胞吐排出废物。在精子细胞中，顶体是特化的溶酶体，在受精过程中释放酶以穿透卵子的外层。

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8. Ribosomes | 核糖体

Ribosomes are non-membranous particles composed of ribosomal RNA (rRNA) and proteins. Each ribosome has two subunits: a large subunit and a small subunit. Eukaryotic 80S ribosomes (60S + 40S subunits) are found in the cytoplasm and on the RER, whereas prokaryotic 70S ribosomes (50S + 30S subunits) are found in bacteria, mitochondria and chloroplasts. The difference in size and structure between 70S and 80S ribosomes is exploited by certain antibiotics, such as streptomycin and chloramphenicol, which selectively inhibit 70S ribosomes.

核糖体是由核糖体 RNA（rRNA）和蛋白质组成的非膜性颗粒。每个核糖体有两个亚基：大亚基和小亚基。真核生物的 80S 核糖体（60S + 40S 亚基）存在于细胞质和 RER 上，而原核生物的 70S 核糖体（50S + 30S 亚基）存在于细菌、线粒体和叶绿体中。70S 和 80S 核糖体在大小和结构上的差异被某些抗生素利用，如链霉素和氯霉素可选择性地抑制 70S 核糖体。

Ribosomes are the sites of protein synthesis, where mRNA codons are translated into amino acid sequences. Polyribosomes (polysomes) are clusters of ribosomes simultaneously translating a single mRNA molecule, greatly increasing the efficiency of protein production.

核糖体是蛋白质合成的场所，mRNA 密码子在此被翻译成氨基酸序列。多聚核糖体是同时翻译同一条 mRNA 分子的核糖体簇，大大提高了蛋白质合成的效率。

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9. Cytoskeleton | 细胞骨架

The cytoskeleton is a dynamic network of protein filaments that provides structural support, intracellular transport and cell motility. It is composed of three main types of filaments: microtubules, microfilaments and intermediate filaments.

细胞骨架是一个动态的蛋白丝网络，提供结构支撑、胞内运输和细胞运动功能。它由三种主要类型的丝组成：微管、微丝和中间丝。

• Microtubules (diameter ≈ 25 nm) are hollow tubes of α- and β-tubulin dimers. They form the mitotic spindle during cell division, the structural core of cilia and flagella, and the tracks along which motor proteins (kinesin and dynein) transport vesicles.

  微管（直径约 25 nm）是由 α- 和 β-微管蛋白二聚体组成的中空管。它们在细胞分裂时形成纺锤体，构成纤毛和鞭毛的结构核心，也是马达蛋白（驱动蛋白和动力蛋白）运输囊泡的轨道。

• Microfilaments (diameter ≈ 7 nm) are polymers of actin. They are involved in muscle contraction, cell division (cytokinesis via the contractile ring), and cell shape changes such as those during cell migration.

  微丝（直径约 7 nm）是肌动蛋白的聚合物。它们参与肌肉收缩、细胞分裂（通过收缩环进行胞质分裂）以及细胞迁移过程中的形态变化。

• Intermediate filaments (diameter ≈ 10 nm) are rope-like fibres made of various proteins (e.g., keratin, vimentin). They provide mechanical strength and maintain cell integrity; nuclear lamins are a type of intermediate filament.

  中间丝（直径约 10 nm）是由多种蛋白质（如角蛋白、波形蛋白）组成的绳索状纤维。它们提供机械强度并维持细胞完整性；核纤层蛋白是中间丝的一种。

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10. Cell Surface Membrane | 细胞表面膜

The plasma membrane is a phospholipid bilayer with embedded proteins, approximately 7.5 nm thick. According to the fluid mosaic model (Singer and Nicolson, 1972), phospholipids and proteins can move laterally within the bilayer. The membrane is selectively permeable: small non-polar molecules (O₂, CO₂) diffuse freely, whereas ions and large polar molecules require transport proteins (channels or carriers).

质膜是厚度约 7.5 nm 的磷脂双分子层，其中嵌有蛋白质。根据流动镶嵌模型（Singer 和 Nicolson，1972），磷脂和蛋白质可以在双分子层内进行侧向移动。膜具有选择透过性：小的非极性分子（O₂、CO₂）可自由扩散，而离子和大极性分子需要转运蛋白（通道蛋白或载体蛋白）的帮助。

Membrane proteins serve diverse functions: transport proteins move substances across the membrane; receptor proteins transmit signals from the extracellular environment; enzymes catalyse membrane-associated reactions; and glycoproteins/glycolipids function in cell recognition and adhesion. Cholesterol molecules in animal cell membranes modulate fluidity, preventing the bilayer from becoming too rigid at low temperatures or too fluid at high temperatures.

膜蛋白具有多种功能：转运蛋白跨膜运输物质；受体蛋白传递来自细胞外环境的信号；酶催化膜相关反应；糖蛋白和糖脂参与细胞识别和黏附。动物细胞膜中的胆固醇分子调节膜的流动性，防止双分子层在低温下过于僵硬或在高温下过于流动。

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11. Plant Cell Wall and Plasmodesmata | 植物细胞壁与胞间连丝

Plant cells are surrounded by a rigid cell wall composed primarily of cellulose microfibrils embedded in a matrix of hemicelluloses, pectins and glycoproteins. The primary cell wall is thin and extensible, permitting cell growth; the secondary cell wall, deposited in some mature cells, is thicker and may contain lignin for additional strength. Plasmodesmata are cytoplasmic channels that pass through the cell wall, connecting the cytoplasm of adjacent cells. They allow the transport of small molecules and signalling macromolecules, facilitating symplastic communication.

植物细胞被坚硬的细胞壁包围，细胞壁主要由嵌入半纤维素、果胶和糖蛋白基质中的纤维素微纤丝组成。初生细胞壁薄且可延展，允许细胞生长；次生细胞壁沉积在某些成熟细胞中，较厚且可能含有木质素以增加强度。胞间连丝是穿过细胞壁的细胞质通道，连接相邻细胞的细胞质，允许小分子和信号大分子的运输，促进共质体通讯。

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12. Comparative Table of Prokaryotic and Eukaryotic Cells | 原核与真核细胞比较表

Feature | 特征	Prokaryotic | 原核细胞	Eukaryotic | 真核细胞
Nucleus | 细胞核	Absent (nucleoid region) | 无（拟核区）	Present, membrane-bound | 有，膜包被
Membrane-bound organelles | 膜性细胞器	Absent | 无	Present (mitochondria, ER, Golgi, etc.) | 有（线粒体、内质网、高尔基体等）
Ribosome type | 核糖体类型	70S | 70S	80S (cytoplasmic); 70S (mitochondria/chloroplasts) | 80S（细胞质）；70S（线粒体/叶绿体）
Cell wall | 细胞壁	Peptidoglycan (bacteria) | 肽聚糖（细菌）	Cellulose (plants); chitin (fungi); none (animals) | 纤维素（植物）；几丁质（真菌）；无（动物）
DNA organisation | DNA 组织	Circular, naked | 环状，裸露	Linear, associated with histones | 线性，与组蛋白结合
Typical size | 典型大小	1–10 μm | 1–10 μm	10–100 μm | 10–100 μm

Recognising these structural differences is essential for exam questions involving cell identification, antibiotic selectivity and evolutionary relationships. For example, the presence of 70S ribosomes in mitochondria is explained by endosymbiosis, and the sensitivity of bacterial cells to antibiotics that target 70S ribosomes arises from their prokaryotic nature.

识别这些结构差异对于涉及细胞鉴定、抗生素选择性和进化关系的考试题目至关重要。例如，线粒体中存在 70S 核糖体可用内共生学说解释；细菌细胞对靶向 70S 核糖体的抗生素敏感，则源于其原核特性。

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