📚 Animal Cell Ultrastructure | 动物细胞的超微结构
The term “ultrastructure” refers to the detailed structure of a cell as revealed by electron microscopy. Unlike light microscopy, which can only resolve structures down to about 200 nm, electron microscopy reveals organelles and subcellular structures at the nanometre scale, providing A-Level biologists with a far more accurate picture of the animal cell’s internal organisation.
“超微结构”一词指的是通过电子显微镜揭示的细胞精细结构。与只能分辨约 200 nm 结构的光学显微镜不同，电子显微镜可以在纳米尺度上揭示细胞器及亚细胞结构，使 A-Level 生物学家能够更准确地认识动物细胞内部的精细组织。
1. Electron Microscopy: TEM vs SEM | 电子显微镜：透射电镜与扫描电镜
Transmission electron microscopy (TEM) directs a beam of electrons through an ultrathin specimen, producing a two-dimensional, high-resolution image of internal structures. Scanning electron microscopy (SEM) scans the surface of a specimen with electrons, generating a three-dimensional image of the cell’s exterior.
透射电子显微镜（TEM）让一束电子穿过超薄切片，生成内部结构的二维高分辨率图像。扫描电子显微镜（SEM）用电子束扫描标本表面，生成细胞外表面的三维图像。
For animal cell ultrastructure, TEM is the primary tool used in CIE A-Level Biology, as it visualises organelles such as mitochondria, ribosomes, and the endoplasmic reticulum in remarkable detail.
对于动物细胞超微结构研究，TEM 是 CIE A-Level 生物学中主要使用的工具，因为它能够以非凡的细节显示线粒体、核糖体和内质网等细胞器。
- TEM resolution: approximately 0.5 nm | TEM 分辨率：约 0.5 nm
- SEM resolution: approximately 10 nm | SEM 分辨率：约 10 nm
2. Plasma Membrane | 细胞膜
The plasma membrane is a thin, flexible boundary surrounding the animal cell, approximately 7–10 nm thick. Under TEM it appears as a trilaminar (“railway track”) structure: two dark lines sandwiching a light central region, corresponding to the phospholipid bilayer with associated proteins.
细胞膜是包裹动物细胞的薄而柔韧的边界，厚度约为 7–10 nm。在 TEM 下，它呈现三层（“铁轨”）结构：两条深色线夹着一条浅色中央区域，对应磷脂双分子层及其相关蛋白质。
Its functions include selective transport of substances, cell signalling via receptors, and cell–cell recognition through glycoproteins. In CIE exams, you should be able to identify the membrane in electron micrographs and explain how its structure relates to its functions.
其功能包括物质的选性转运、通过受体进行的细胞信号传导，以及通过糖蛋白完成的细胞间识别。在 CIE 考试中，你需要能够在电镜照片中识别细胞膜，并解释其结构如何与其功能相关联。
Phospholipid bilayer → Fluid mosaic model | 磷脂双分子层 → 流动镶嵌模型
3. Nucleus | 细胞核
The nucleus is the largest organelle in most animal cells, typically 5–10 µm in diameter. TEM reveals a double membrane (nuclear envelope) with pores, dense chromatin regions, and one or more nucleoli.
细胞核是大多数动物细胞中最大的细胞器，直径通常为 5–10 µm。TEM 显示其具有双层膜（核膜）、核孔、致密的染色质区域以及一个或多个核仁。
The nuclear envelope consists of two membranes separated by a perinuclear space. Nuclear pores (~40–100 nm in diameter) allow the selective exchange of mRNA, ribosomal subunits, and proteins between the nucleus and cytoplasm. Chromatin contains DNA complexed with histone proteins; during cell division it condenses into visible chromosomes.
核膜由两层膜组成，中间为核周间隙。核孔（直径约 40–100 nm）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交换。染色质由 DNA 与组蛋白结合而成；在细胞分裂期间，它凝缩为可见的染色体。
4. Mitochondria | 线粒体
Mitochondria are rod-shaped organelles, typically 1–10 µm in length. Each mitochondrion has an outer smooth membrane and an inner membrane folded into cristae, which greatly increase the surface area for oxidative phosphorylation.
线粒体是杆状细胞器，长度通常为 1–10 µm。每个线粒体具有光滑的外膜和折叠成嵴的内膜，嵴大大增大了氧化磷酸化的表面积。
The matrix, enclosed by the inner membrane, contains enzymes of the Krebs cycle, mitochondrial DNA (mtDNA), and ribosomes (70S). Mitochondria are the site of aerobic respiration, specifically the link reaction, the Krebs cycle, and oxidative phosphorylation. In electron micrographs, mitochondria can be identified by their double membrane and cristae.
内膜包绕的基质中含有三羧酸循环的酶、线粒体 DNA（mtDNA）和核糖体（70S）。线粒体是有氧呼吸的场所，具体包括连接反应、三羧酸循环和氧化磷酸化。在电镜照片中，线粒体可通过其双层膜和嵴来识别。
5. Ribosomes | 核糖体
Ribosomes are small, dense particles with a diameter of about 20–30 nm. Each ribosome consists of a large and a small subunit, both composed of ribosomal RNA (rRNA) and proteins. Animal cells contain 80S ribosomes (60S + 40S subunits), whereas mitochondria contain 70S ribosomes.
核糖体是直径约 20–30 nm 的小型致密颗粒。每个核糖体由大亚基和小亚基组成，两者均由核糖体 RNA（rRNA）和蛋白质构成。动物细胞含有 80S 核糖体（60S + 40S 亚基），而线粒体含有 70S 核糖体。
Ribosomes may be free in the cytoplasm (synthesising proteins for intracellular use) or bound to the rough endoplasmic reticulum (RER), where they synthesise proteins destined for secretion or incorporation into membranes. In ultrastructure diagrams, ribosomes appear as dark dots lining the RER or scattered in the cytosol.
核糖体可以游离于细胞质中（合成供细胞内使用的蛋白质），也可以附着于粗面内质网（RER）上，在那里合成用于分泌或嵌入膜的蛋白质。在超微结构图中，核糖体表现为排列在 RER 上或散布在胞质溶胶中的深色小点。
6. Endoplasmic Reticulum | 内质网
The endoplasmic reticulum (ER) is an extensive network of flattened membrane sacs (cisternae) extending from the nuclear envelope throughout the cytoplasm. Two types exist: rough ER (RER) and smooth ER (SER).
内质网（ER）是一个由扁平膜囊（潴泡）组成的广泛网络，从核膜延伸至整个细胞质。存在两种类型：粗面内质网（RER）和滑面内质网（SER）。
RER is studded with ribosomes on its cytoplasmic surface and is involved in protein synthesis, folding, and transport. Proteins synthesised here are packaged into vesicles and delivered to the Golgi apparatus. SER lacks ribosomes; it synthesises lipids, including phospholipids and steroids, and is involved in carbohydrate metabolism and detoxification of drugs and toxins.
RER 的细胞质表面附有核糖体，参与蛋白质的合成、折叠和转运。在此合成的蛋白质被包装成囊泡并运送至高尔基体。SER 没有核糖体；它合成脂质（包括磷脂和类固醇），并参与碳水化合物代谢以及药物和毒素的解毒。
| Feature | 特征 | Rough ER | 粗面内质网 | Smooth ER | 滑面内质网 |
| Ribosomes | 核糖体 | Present | 有 | Absent | 无 |
| Main function | 主要功能 | Protein synthesis | 蛋白质合成 | Lipid synthesis & detoxification | 脂质合成与解毒 |
7. Golgi Apparatus | 高尔基体
The Golgi apparatus consists of a stack of 4–8 flattened, curved cisternae, typically located near the nucleus. It has a cis face (receiving side) and a trans face (shipping side).
高尔基体由 4–8 层扁平、弯曲的潴泡堆叠而成，通常位于细胞核附近。它具有顺面（接收面）和反面（输出面）。
Vesicles from the ER fuse with the cis face; the Golgi then modifies proteins by glycosylation (adding carbohydrate groups), packages them into secretory vesicles, and sorts them for delivery to their final destinations — either within the cell or for secretion. In animal cells, the Golgi also produces lysosomes.
来自 ER 的囊泡与顺面融合；高尔基体随后通过糖基化（添加糖基）修饰蛋白质，将其包装成分泌囊泡，并分类运送至最终目的地——细胞内或分泌到细胞外。在动物细胞中，高尔基体还产生溶酶体。
8. Lysosomes | 溶酶体
Lysosomes are membrane-bound vesicles, approximately 0.1–1.0 µm in diameter, containing hydrolytic enzymes (lysozymes and digestive enzymes) that function optimally at acidic pH. They are formed by the Golgi apparatus and are abundant in animal cells, but rare or absent in plant cells.
溶酶体是膜结合囊泡，直径约 0.1–1.0 µm，含有在酸性 pH 下发挥最佳功能的水解酶（溶菌酶和消化酶）。它们由高尔基体形成，在动物细胞中丰富，而在植物细胞中罕见或不存在。
Lysosomes function in intracellular digestion (phagocytosis), autophagy (recycling of worn-out organelles), and apoptosis (programmed cell death). Their membrane prevents harmful enzymes from leaking into the cytoplasm; rupture of lysosomes leads to cellular self-digestion.
溶酶体的功能包括细胞内消化（吞噬作用）、自噬（回收衰老细胞器）和细胞凋亡（程序性细胞死亡）。其膜防止有害酶泄漏到细胞质中；溶酶体破裂会导致细胞自我消化。
9. Centrioles and Centrosome | 中心粒与中心体
Centrioles are cylindrical organelles, about 0.2 µm in diameter and 0.4 µm in length, composed of nine triplets of microtubules arranged in a ring. A pair of centrioles oriented at right angles forms the centrosome, located near the nucleus in animal cells.
中心粒是圆柱形细胞器，直径约 0.2 µm，长度约 0.4 µm，由九组微管三联体排列成环状。一对互成直角排列的中心粒构成中心体，位于动物细胞的细胞核附近。
During cell division, centrioles migrate to opposite poles of the cell and organise the spindle fibres that separate chromosomes. Centrioles are not present in most plant cells, which use alternative mechanisms to form the spindle.
在细胞分裂期间，中心粒迁移到细胞两极，组织纺锤体纤维以分离染色体。大多数植物细胞没有中心粒，而是利用其他机制形成纺锤体。
10. Cytoskeleton | 细胞骨架
The cytoskeleton is a dynamic network of protein filaments extending throughout the cytoplasm. It comprises three main components: microfilaments (actin, ~7 nm), intermediate filaments (~10 nm), and microtubules (~25 nm).
细胞骨架是贯穿细胞质的动态蛋白质丝网络。它包含三种主要成分：微丝（肌动蛋白，约 7 nm）、中间丝（约 10 nm）和微管（约 25 nm）。
The cytoskeleton maintains cell shape, anchors organelles, enables intracellular transport (via motor proteins such as kinesin and dynein moving along microtubules), and facilitates cell movement (e.g., cilia and flagella). It is also essential for chromosome movement during mitosis.
细胞骨架维持细胞形态、锚定细胞器、实现细胞内运输（通过动力蛋白如驱动蛋白和动力蛋白沿微管运动），并促进细胞运动（例如纤毛和鞭毛）。它也是有丝分裂期间染色体运动所必需的。
11. Animal vs Plant Cell Ultrastructure | 动物细胞与植物细胞超微结构比较
Although both animal and plant cells are eukaryotic, several key ultrastructural differences exist. Recognising these differences is essential for interpreting electron micrographs in CIE A-Level exams.
尽管动物细胞和植物细胞都是真核细胞，但存在若干关键的超微结构差异。识别这些差异对于解读 CIE A-Level 考试中的电镜照片至关重要。
| Feature | 特征 | Animal Cell | 动物细胞 | Plant Cell | 植物细胞 |
| Cell wall | 细胞壁 | Absent | 无 | Present (cellulose) | 有（纤维素） |
| Chloroplasts | 叶绿体 | Absent | 无 | Present | 有 |
| Large central vacuole | 中央大液泡 | Absent or small | 无或小 | Present | 有 |
| Centrioles | 中心粒 | Present | 有 | Absent in most | 大多数无 |
| Lysosomes | 溶酶体 | Common | 常见 | Rare | 罕见 |
| Storage of carbohydrates | 碳水化合物储存 | Glycogen | 糖原 | Starch | 淀粉 |
12. Exam Key Points and Common Errors | 考点总结与常见错误
In CIE A-Level Biology, examiners frequently test ultrastructure through labelled electron micrographs and short-answer questions. Students should be able to identify each organelle by its morphology and state its function precisely.
在 CIE A-Level 生物学中，考官经常通过标注电镜照片和简答题来考查超微结构。学生应当能够根据形态识别每种细胞器并准确说明其功能。
- Do not confuse rough ER with smooth ER — check for ribosomes on the surface | 不要混淆粗面内质网与滑面内质网——检查表面是否有核糖体
- Mitochondria have double membranes and cristae; chloroplasts have thylakoids — these are not the same | 线粒体有双层膜和嵴；叶绿体有类囊体——二者不同
- 70S ribosomes are found in mitochondria (and chloroplasts); 80S ribosomes are found in the cytoplasm of eukaryotic cells | 70S 核糖体存在于线粒体（和叶绿体）；80S 核糖体存在于真核细胞的细胞质中
- The Golgi apparatus modifies proteins — it does not synthesise them | 高尔基体修饰蛋白质——它不合成蛋白质
- Always use the term “ultrastructure” when referring to electron microscope images | 提到电子显微镜图像时务必使用“超微结构”一词
When drawing labelled diagrams, pay attention to membrane thickness and organelle proportions. Practice identifying organelles from electron micrographs, as this is a consistently assessed skill across CIE papers.
在绘制标注图时，注意膜厚度和细胞器比例。练习从电镜照片中识别细胞器，这是 CIE 试卷中持续考查的技能。
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