📚 Ultrastructure of an animal cell | 动物细胞超微结构
Understanding the ultrastructure of an animal cell means looking at the fine details of its components as revealed by the electron microscope. This goes beyond what can be seen with a light microscope and reveals membrane-bound organelles, non-membrane-bound structures, and the complex cytoskeleton that together carry out all the activities needed for life. Mastering these details is essential for A-Level Biology because it underpins topics such as protein synthesis, respiration, cell division, and transport across membranes.
了解动物细胞的超微结构，意味着要借助电子显微镜观察其内部组分的精细细节。这超出了光学显微镜的分辨能力，揭示了有膜细胞器、无膜结构以及复杂的细胞骨架，它们共同完成生命所需的一切活动。掌握这些细节对A-Level生物学至关重要，因为它是蛋白质合成、呼吸作用、细胞分裂和跨膜运输等主题的基础。
1. Introduction to Ultrastructure | 超微结构导论
The term ‘ultrastructure’ refers to the detailed architecture of a cell that can only be visualised using an electron microscope. In an animal cell, this includes organelles such as the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus, as well as structures like ribosomes and centrioles that do not have membranes.
“超微结构”一词指只能用电子显微镜观察到的细胞精细结构。在动物细胞中，这包括细胞核、线粒体、内质网和高尔基体等细胞器，以及无膜的核糖体和中心粒等结构。
Electron microscopy allows magnification up to ×500 000 or more, resolving objects as small as 0.5 nm. This reveals membrane bilayers, internal cristae of mitochondria, and the tiny subunits of ribosomes, which are invisible with a light microscope.
电子显微镜可将物体放大至 500 000 倍以上，分辨率可达 0.5 nm。它能够显示膜的磷脂双分子层、线粒体内部嵴以及核糖体的微小亚基，这些在光学显微镜下均无法看见。
2. Cell Surface Membrane | 细胞表面膜
The cell surface membrane, or plasma membrane, forms the boundary of the cell. It is composed mainly of phospholipids arranged in a bilayer, with proteins and cholesterol molecules inserted within it. This structure is described by the fluid mosaic model.
细胞表面膜，即质膜，构成细胞的边界。它主要由排列成双层的磷脂组成，其中嵌入了蛋白质和胆固醇分子。这一结构可用流动镶嵌模型描述。
Under the electron microscope, the membrane appears as two dark lines separated by a lighter region – a ‘trilaminar’ appearance – representing the hydrophilic phosphate heads and the hydrophobic lipid tails. The membrane is about 7–8 nm thick.
在电子显微镜下，膜呈现为两条暗线夹一条亮区的“三层”结构，分别代表亲水的磷酸头部和疏水的脂质尾部。膜的厚度约为 7–8 nm。
Membrane proteins include channel proteins, carrier proteins, receptors, and enzymes. Cholesterol maintains membrane fluidity and stability. Glycoproteins and glycolipids on the outer surface form the glycocalyx, involved in cell recognition and adhesion.
膜蛋白包括通道蛋白、载体蛋白、受体和酶。胆固醇维持膜的流动性和稳定性。外表面的糖蛋白和糖脂构成糖萼，参与细胞识别和黏附。
3. Cytoplasm and Cytosol | 细胞质与胞质溶胶
The cytoplasm consists of the cytosol – a watery solution containing ions, metabolites, and macromolecules – and the organelles suspended within it. The cytosol is not simply a passive fluid; it is the site of many metabolic pathways, such as glycolysis, and part of protein synthesis.
细胞质由胞质溶胶（一种含有离子、代谢物和大分子的水样溶液）以及悬浮其中的细胞器组成。胞质溶胶并非简单的被动液体，它是糖酵解等许多代谢途径以及蛋白质合成部分过程的发生场所。
Electron micrographs show the cytosol as a granular background due to the presence of ribosomes, enzyme complexes, and cytoskeletal filaments. Approximately 70–80% of the cytosol is water, and its consistency can vary from a sol to a gel.
电子显微镜图像中，由于存在核糖体、酶复合体和细胞骨架纤维，胞质溶胶呈现颗粒状背景。其含水量约 70–80%，稠度可在溶胶和凝胶间变化。
4. Nucleus | 细胞核
The nucleus is the largest organelle in most animal cells, typically 10–20 µm in diameter. It is surrounded by a nuclear envelope, a double membrane perforated by nuclear pores that regulate the passage of molecules such as mRNA and ribosome subunits.
细胞核是多数动物细胞中最大的细胞器，直径通常为 10–20 µm。它由核膜包围，核膜为双层膜，其上布满核孔，可调控 mRNA 和核糖体亚基等分子的进出。
Inside the nucleus, chromatin – DNA associated with histone proteins – is visible as electron-dense patches. During cell division, chromatin condenses into distinct chromosomes. The nucleolus, a non-membrane-bound region inside the nucleus, is the site of ribosomal RNA synthesis and ribosome assembly.
在细胞核内部，染色质（与组蛋白结合的 DNA）呈现为电子致密斑块。细胞分裂时，染色质凝聚形成清晰的染色体。核仁是核内一个无膜区域，负责核糖体 RNA 的合成与核糖体组装。
5. Ribosomes | 核糖体
Ribosomes are tiny organelles, about 20–25 nm in diameter, composed of two subunits made of ribosomal RNA and protein. They are sites of protein synthesis, translating the genetic code carried by mRNA into polypeptide chains.
核糖体是直径约 20–25 nm 的微小细胞器，由两个亚基组成，亚基由核糖体 RNA 和蛋白质构成。它们是蛋白质合成场所，将 mRNA 携带的遗传密码翻译成多肽链。
In animal cells, ribosomes are found free in the cytosol, where they synthesise proteins for use within the cell, or bound to the rough endoplasmic reticulum, where they manufacture proteins for secretion or for incorporation into membranes.
在动物细胞中，核糖体可游离存在于胞质溶胶中，合成细胞内使用的蛋白质，也可附着在粗面内质网上，制造用于分泌或嵌入膜的蛋白质。
Both types of ribosomes are structurally identical and are assembled in the nucleolus. Their small size means they appear as tiny dots in electron micrographs, often clustered as polyribosomes on a single mRNA strand.
两种核糖体结构相同，均在核仁中组装。由于其体积小，在电镜图像中呈现为微小点状，常以多聚核糖体形式聚集在一条 mRNA 链上。
6. Rough Endoplasmic Reticulum | 粗面内质网
The rough endoplasmic reticulum (RER) is a network of membrane-bound flattened sacs, called cisternae, studded with ribosomes on their cytoplasmic surface. It is continuous with the outer membrane of the nuclear envelope.
粗面内质网是由膜包围的扁平囊泡（称为潴泡）构成的网络结构，其胞质面附着大量核糖体。它与核膜外层相连。
The main function of the RER is to synthesise, fold, and modify proteins destined for secretion, for the lysosomes, or for insertion into the cell membrane. As the polypeptide enters the RER lumen, chaperone proteins assist folding, and enzymes may add carbohydrate chains for glycoprotein formation.
粗面内质网的主要功能是合成、折叠和修饰那些将被分泌、进入溶酶体或嵌入细胞膜的蛋白质。当多肽进入 RER 腔后，分子伴侣协助折叠，酶可能添加糖链形成糖蛋白。
The RER is abundant in cells that actively produce export proteins, such as plasma cells that secrete antibodies, or acinar cells of the pancreas that secrete digestive enzymes.
活跃合成输出蛋白的细胞富含粗面内质网，例如分泌抗体的浆细胞，或分泌消化酶的胰腺腺泡细胞。
7. Smooth Endoplasmic Reticulum | 滑面内质网
The smooth endoplasmic reticulum (SER) consists of a more tubular, branched network of membranes without ribosomes. It is involved in lipid and steroid synthesis, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions.
滑面内质网由更管状的、分支的膜网络组成，无核糖体附着。它参与脂质与类固醇合成、碳水化合物代谢、药物和毒物的解毒以及钙离子储存。
Cells that synthesise steroid hormones, such as those in the adrenal cortex and gonads, possess extensive smooth ER. In liver cells, the smooth ER contains enzymes that detoxify harmful compounds and break down glycogen.
合成类固醇激素的细胞，如肾上腺皮质和性腺细胞，拥有大量滑面内质网。在肝细胞中，滑面内质网含有解毒酶和糖原分解酶。
In muscle cells, a specialised form of smooth ER called the sarcoplasmic reticulum regulates calcium ion concentration during muscle contraction and relaxation.
在肌细胞中，一种特化的滑面内质网——肌质网，在肌肉收缩与舒张过程中调节钙离子浓度。
8. Golgi Apparatus | 高尔基体
The Golgi apparatus is a stack of curved, membrane-bound flattened sacs, each called a cisterna, along with associated vesicles. It has a cis face, receiving vesicles from the ER, and a trans face, from which modified and sorted products exit.
高尔基体是一叠弯曲的、由膜包围的扁平囊泡（每个称潴泡）及相关小泡。它有一面为顺面，接收来自内质网的囊泡；另一面为反面，经过修饰分选后的产物从此处离开。
Proteins and lipids arriving from the ER are chemically modified – for example, by addition of carbohydrate groups or by proteolytic cleavage – before being packaged into vesicles and dispatched to their destinations, including the cell surface, lysosomes, or secretory vesicles.
由内质网运来的蛋白质和脂质在此经过化学修饰（如添加糖基或蛋白水解切割），然后被包装进小泡，分送至目的地，包括细胞表面、溶酶体或分泌小泡。
The Golgi also synthesises some polysaccharides and plays a key role in forming lysosomes. Its precise structure is seen only with the electron microscope, revealing the polarized stack and budding vesicles.
高尔基体还能合成某些多糖，并在溶酶体形成中起关键作用。只有在电子显微镜下才能清晰看到其极性堆叠结构和出芽小泡。
9. Mitochondria | 线粒体
Mitochondria are rod-shaped or oval organelles, typically 1–2 µm long, bounded by two membranes. The outer membrane is smooth, while the inner membrane is highly folded into cristae that project into the matrix. These cristae greatly increase the surface area for the reactions of aerobic respiration.
线粒体呈杆状或椭圆形，长约 1–2 µm，由两层膜包围。外膜光滑，内膜高度折叠形成嵴，伸入基质中。这些嵴极大地增加了有氧呼吸反应的表面积。
The matrix contains enzymes of the Krebs cycle, mitochondrial DNA, ribosomes (70S), and granules. The inner membrane houses the electron transport chain and ATP synthase, which are responsible for oxidative phosphorylation and ATP production.
基质中含有三羧酸循环的酶、线粒体 DNA、核糖体（70S）和颗粒。内膜上分布着电子传递链和 ATP 合酶，负责氧化磷酸化和 ATP 的生成。
Mitochondria are often described as the ‘powerhouses’ of the cell because they produce most of the ATP needed for cellular activities. Cells with high energy demands, such as muscle cells and sperm cells, contain large numbers of mitochondria.
线粒体常被称为细胞的“动力工厂”，因为它们产生细胞活动所需的大部分 ATP。能量需求高的细胞，如肌细胞和精子，含有大量线粒体。
10. Lysosomes | 溶酶体
Lysosomes are spherical, membrane-bound organelles, mostly 0.1–0.5 µm in diameter, containing a variety of hydrolytic enzymes that operate at an acidic pH of about 5. These enzymes include proteases, lipases, nucleases, and glycosidases.
溶酶体是球形的有膜细胞器，直径多在 0.1–0.5 µm，内含多种水解酶，在约 pH 5 的酸性环境中工作。这些酶包括蛋白酶、脂肪酶、核酸酶和糖苷酶。
They function in intracellular digestion, breaking down worn-out organelles (autophagy), digesting material taken in by endocytosis, and in programmed cell death (apoptosis). The membrane protects the rest of the cell from destructive enzymes.
它们在胞内消化中起作用，分解衰老细胞器（自噬作用）、消化胞吞摄入的物质，以及参与程序性细胞死亡（凋亡）。溶酶体膜保护细胞其余部分免受酶的破坏。
Lysosomes are formed by budding from the Golgi apparatus. In electron micrographs they appear as dense bodies, often with heterogeneous contents, reflecting partially digested material.
溶酶体由高尔基体出芽形成。在电镜图像中，它们呈致密小体，内容物常不均匀，反映其正在消化的物质。
11. Centrioles and Cytoskeleton | 中心粒与细胞骨架
Centrioles are small, cylindrical structures found in the centrosome near the nucleus of most animal cells. Each centriole consists of nine triplets of microtubules arranged in a ring. A pair of centrioles, oriented at right angles, is involved in organising the spindle during cell division.
中心粒是位于大多数动物细胞核附近中心体内的微小的圆筒状结构。每个中心粒由九组三联微管环状排列而成。一对相互垂直的中心粒参与细胞分裂时纺锤体的组织。
The cytoskeleton is a dynamic network of protein filaments extending throughout the cytoplasm. It is composed of three main types: microfilaments (actin), intermediate filaments, and microtubules (tubulin).
细胞骨架是遍布细胞质的蛋白质纤维动态网络。它由三种主要类型构成：微丝（肌动蛋白）、中间丝和微管（微管蛋白）。
Microfilaments serve in cell movement, muscle contraction, and cytokinesis. Intermediate filaments provide mechanical strength. Microtubules maintain cell shape, form tracks for organelle movement by motor proteins, and assemble into centrioles, cilia, and flagella.
微丝参与细胞运动、肌肉收缩和胞质分裂。中间丝提供机械强度。微管维持细胞形态，为马达蛋白驱动细胞器运动提供轨道，并组装成中心粒、纤毛和鞭毛。
12. Summary of Functions | 功能总览
An integrated view of all these ultrastructures is essential for understanding an animal cell’s life. The nucleus stores genetic information and controls protein synthesis. Ribosomes translate mRNA. The RER and Golgi constitute the endomembrane system for protein secretion. Mitochondria supply ATP. Lysosomes recycle and digest. The cytoskeleton maintains architecture and enables movement.
将所有超微结构整合在一起看待，对于理解动物细胞的生命活动至关重要。细胞核储存遗传信息并控制蛋白质合成。核糖体翻译 mRNA。粗面内质网和高尔基体构成蛋白质分泌的内膜系统。线粒体提供 ATP。溶酶体负责循环和消化。细胞骨架维持结构并实现运动。
Electron microscopy reveals that no organelle works in isolation. For example, a protein destined for secretion is synthesised on RER, transported through the Golgi, and released from the cell by exocytosis – an interplay that depends on the cytoskeleton and energy from mitochondria.
电子显微镜揭示，没有哪个细胞器是孤立工作的。例如，一种要分泌的蛋白质在粗面内质网上合成，经高尔基体运输，再通过胞吐作用释放到细胞外——这一协同过程依赖细胞骨架和线粒体提供的能量。
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