📚 Ultrastructure of an animal cell | 动物细胞的超微结构
Ultrastructure refers to the detailed organisation of a cell that is visible only with an electron microscope. For Cambridge A-Level Biology, you must be able to identify the main organelles of an animal cell and explain how their structures relate to their functions.
超微结构是指只有在电子显微镜下才能看到的细胞内部精细结构。在剑桥 A-Level 生物课程中，你需要能够识别动物细胞的主要细胞器，并解释它们的结构如何与功能相适应。
1. The meaning of ultrastructure | 超微结构的含义
Light microscopes have a limited resolving power of about 200 nm, so they cannot reveal membranes, ribosomes or the internal structure of organelles. The electron microscope uses a beam of electrons with a much shorter wavelength, giving a resolving power of about 0.1 nm and allowing biologists to see the ultrastructure of animal cells.
光学显微镜的分辨率极限约为 200 nm，因此无法显示膜结构、核糖体或细胞器的内部构造。电子显微镜使用波长极短的电子束，分辨率可达约 0.1 nm，使生物学家能够观察动物细胞的超微结构。
The magnification of an electron micrograph can be calculated from the equation below.
电子显微照片的放大倍数可以用下面的公式计算。
Magnification = Image size ÷ Actual size
For example, if a mitochondrion measures 20 mm in an image and its actual length is 2 μm, the magnification is 20 000 μm ÷ 2 μm = 10 000 ×.
例如，如果图像中的线粒体长度为 20 mm，实际长度为 2 μm，则放大倍数为 20 000 μm ÷ 2 μm = 10 000 ×。
2. Plasma membrane and cell surface | 质膜与细胞表面
The plasma membrane is a partially permeable barrier formed by a phospholipid bilayer. Embedded proteins act as channels, carriers, receptors and enzymes, while glycoproteins and glycolipids are involved in cell recognition and adhesion.
质膜是由磷脂双分子层形成的选择透过性屏障。嵌入其中的蛋白质可作为通道、载体、受体和酶，而糖蛋白和糖脂参与细胞识别与黏附。
Cholesterol molecules fit between the phospholipids, reducing fluidity at high temperatures and preventing the membrane from becoming too rigid at low temperatures. Some animal cells, such as those lining the small intestine, have microvilli to increase surface area for absorption.
胆固醇分子分布在磷脂之间，在高温下降低膜的流动性，在低温下防止膜变得过于僵硬。一些动物细胞，如小肠上皮细胞，具有微绒毛以增加吸收表面积。
3. Nucleus and nucleolus | 细胞核与核仁
The nucleus is bounded by a double membrane called the nuclear envelope, which is perforated by nuclear pores. These pores allow mRNA and ribosomal subunits to leave the nucleus and permit proteins such as histones to enter.
细胞核由双层膜——核膜包裹，核膜上分布着核孔。核孔允许 mRNA 和核糖体亚基离开细胞核，并允许组蛋白等蛋白质进入。
Inside the nucleus, chromatin consists of DNA wrapped around histone proteins. During cell division, chromatin condenses into visible chromosomes. The nucleolus is a dense region where ribosomal RNA is synthesised and ribosomal subunits are assembled.
细胞核内，染色质由 DNA 缠绕组蛋白构成。在细胞分裂过程中，染色质凝聚成可见的染色体。核仁是细胞核内的致密区域，负责合成核糖体 RNA 并组装核糖体亚基。
The nucleus stores genetic information and controls protein synthesis by directing which genes are transcribed.
细胞核储存遗传信息，并通过决定哪些基因被转录来控制蛋白质合成。
4. Mitochondria | 线粒体
Mitochondria are rod-shaped organelles surrounded by two membranes. The inner membrane is folded into cristae, which greatly increase the surface area for oxidative phosphorylation and ATP production.
线粒体是棒状细胞器，由两层膜包裹。内膜向内折叠形成嵴，大大增加了氧化磷酸化和 ATP 生成的表面积。
The matrix contains enzymes for the link reaction and the Krebs cycle, as well as mitochondrial DNA and 70S ribosomes. These features support the endosymbiotic theory and allow mitochondria to synthesise some of their own proteins.
线粒体基质中含有进行连接反应和克雷布斯循环的酶，还含有线粒体 DNA 和 70S 核糖体。这些特征支持内共生学说，并使线粒体能够合成自身的一部分蛋白质。
ATP produced by mitochondria is the main energy currency for active transport, muscle contraction, anabolic reactions and cell division.
线粒体产生的 ATP 是主动运输、肌肉收缩、合成代谢反应和细胞分裂的主要能量货币。
5. Ribosomes | 核糖体
Ribosomes are small non-membrane-bound organelles made of ribosomal RNA and protein. Each ribosome consists of a large subunit and a small subunit, which assemble around an mRNA molecule during translation.
核糖体是无膜包裹的小型细胞器，由核糖体 RNA 和蛋白质组成。每个核糖体由大亚基和小亚基组成，在翻译过程中围绕 mRNA 分子组装。
Eukaryotic animal cells contain 80S ribosomes free in the cytoplasm or attached to the rough endoplasmic reticulum. Free ribosomes synthesise proteins used inside the cell, while bound ribosomes produce proteins for secretion or for incorporation into membranes.
真核动物细胞含有 80S 核糖体，游离在细胞质中或附着在粗面内质网上。游离核糖体合成细胞内部使用的蛋白质，而附着核糖体则产生用于分泌或嵌入膜中的蛋白质。
6. Endoplasmic reticulum | 内质网
The rough endoplasmic reticulum (RER) is a network of flattened membrane sacs studded with ribosomes. It folds and transports newly synthesised proteins and is especially abundant in cells that secrete large amounts of protein, such as pancreatic cells.
粗面内质网是由扁平膜囊组成的网络，表面附着核糖体。它对新生蛋白质进行折叠和运输，在胰腺细胞等分泌大量蛋白质的细胞中尤为丰富。
The smooth endoplasmic reticulum (SER) lacks ribosomes and is involved in lipid and steroid synthesis, carbohydrate metabolism and detoxification of drugs. Liver cells contain extensive SER because they process many toxins.
光面内质网没有核糖体，参与脂质和类固醇的合成、碳水化合物代谢以及药物解毒。肝细胞含有大量光面内质网，因为它们需要处理许多有毒物质。
7. Golgi apparatus | 高尔基体
The Golgi apparatus consists of a stack of flattened membrane sacs called cisternae. It receives proteins from the RER in vesicles, modifies them by adding carbohydrate or phosphate groups, and packages them into secretory vesicles.
高尔基体由一叠扁平的膜囊（潴泡）组成。它通过囊泡接收来自粗面内质网的蛋白质，通过添加糖基或磷酸基团对其进行修饰，并将其包装成分泌囊泡。
The cis face receives vesicles from the RER, while the trans face releases them towards the plasma membrane for exocytosis or to other destinations such as lysosomes.
高尔基体的顺面接收来自粗面内质网的囊泡，反面则将其释放到质膜进行胞吐作用，或运往溶酶体等其他目的地。
8. Lysosomes | 溶酶体
Lysosomes are membrane-bound sacs containing hydrolytic enzymes such as proteases, lipases and nucleases. These enzymes work best at an acidic pH of about 5, which is maintained by proton pumps in the lysosomal membrane.
溶酶体是含有水解酶（如蛋白酶、脂肪酶和核酸酶）的膜包囊泡。这些酶在约 pH 5 的酸性环境中活性最高，溶酶体膜上的质子泵维持了这一酸性条件。
Lysosomes digest worn-out organelles in autophagy and break down material taken in by phagocytosis, as in white blood cells engulfing bacteria. Their membrane prevents the enzymes from destroying the rest of the cell.
溶酶体在自噬过程中消化衰老的细胞器，并分解通过吞噬作用摄入的物质，例如白细胞吞噬细菌。溶酶体膜可防止酶破坏细胞的其他部分。
9. Centrioles and centrosome | 中心粒与中心体
Animal cells contain a pair of centrioles located in the centrosome near the nucleus. Each centriole is a hollow cylinder made of nine triplets of microtubules arranged in a ring.
动物细胞在靠近细胞核的中心体内含有一对中心粒。每个中心粒是由九组三联体微管围成环状的空心圆柱体。
Before cell division, the centrioles replicate and move to opposite poles of the cell. They organise the spindle fibres that separate chromosomes during mitosis and meiosis.
在细胞分裂前，中心粒复制并移向细胞两极。它们组织纺锤丝，在有丝分裂和减数分裂过程中分离染色体。
Centrioles also form basal bodies that give rise to cilia and flagella in some animal cells, such as ciliated epithelial cells and sperm cells.
在某些动物细胞中，中心粒还能形成基体，产生纤毛和鞭毛，例如纤毛上皮细胞和精子细胞。
10. Cytoskeleton and cell inclusions | 细胞骨架与细胞内含物
The cytoskeleton is a network of protein fibres throughout the cytoplasm. Microfilaments made of actin are involved in cell movement and cytokinesis; intermediate filaments provide mechanical strength; microtubules made of tubulin guide vesicle transport and form the spindle.
细胞骨架是贯穿细胞质中的蛋白质纤维网络。由肌动蛋白组成的微丝参与细胞运动和胞质分裂；中间纤维提供机械强度；由微管蛋白组成的微管引导囊泡运输并形成纺锤体。
Animal cells may also contain non-living inclusions such as glycogen granules and lipid droplets, which store energy. These are not organelles because they are not surrounded by a membrane and have no metabolic activity of their own.
动物细胞还可能含有非生命的内含物，如糖原颗粒和脂滴，用于储存能量。它们不属于细胞器，因为没有膜包裹，也不具有自身的代谢活动。
11. Vesicles and vacuoles | 囊泡与液泡
Vesicles are small membrane-bound sacs that transport materials within the cell. They move proteins from the ER to the Golgi apparatus and from the Golgi to the plasma membrane, often along microtubule tracks.
囊泡是细胞内运输物质的小型膜包囊泡。它们将蛋白质从内质网运送到高尔基体，再从高尔基体运送到质膜，通常沿着微管轨道移动。
Animal cells may form food vacuoles by endocytosis and contain small temporary vacuoles for storage or osmotic regulation. Unlike plant cells, animal cells do not have a large permanent sap vacuole.
动物细胞可以通过胞吞作用形成食物泡，并含有用于储存或渗透调节的小型临时液泡。与植物细胞不同，动物细胞没有大型的永久性液泡。
12. Summary table of organelles | 细胞器功能汇总表
| Organelle | Main function | 主要功能 |
|---|---|
| Plasma membrane | Controls entry and exit of substances 控制物质进出 |
| Nucleus | Stores genetic information and controls protein synthesis 储存遗传信息并控制蛋白质合成 |
| Mitochondrion | Produces ATP by aerobic respiration 通过有氧呼吸产生 ATP |
| Ribosome | Site of protein synthesis 蛋白质合成的场所 |
| Rough endoplasmic reticulum | Folds and transports proteins made by bound ribosomes 折叠并运输附着核糖体合成的蛋白质 |
| Smooth endoplasmic reticulum | Synthesises lipids and detoxifies drugs 合成脂质并解毒药物 |
| Golgi apparatus | Modifies, sorts and packages proteins 修饰、分选和包装蛋白质 |
| Lysosome | Digests worn-out organelles and engulfed material 消化衰老细胞器和被吞噬的物质 |
| Centriole | Organises spindle fibres during cell division 在细胞分裂时组织纺锤丝 |
| Cytoskeleton | Provides support, movement and transport tracks 提供支持、运动和 Published by TutorHao | A-Level Biology Revision Series | aleveler.com 更多咨询请联系16621398022（同微信） CommentsMore posts |
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