📚 A-Level Biology: Cell Biology & Microscopy Techniques | A-Level 生物：细胞生物学与显微镜技术
The cell is the fundamental unit of all living organisms. Mastering cell structure and the techniques used to study it is essential for the CIE A-Level Biology examinations. This article provides a comprehensive revision guide, placing particular emphasis on microscopy techniques, magnification, and organelle function.
细胞是一切生物体的基本结构单位。掌握细胞结构以及研究细胞的技术手段，是 CIE A-Level 生物考试的关键。本文是一份全面的复习指南，重点介绍显微镜技术、放大倍数与细胞器功能。
1. The Cell Theory | 细胞学说
The cell theory was established in the 19th century by Schleiden, Schwann, and Virchow, and it remains the organising principle of biology. It rests on three fundamental claims, all of which you are expected to apply when comparing different cell types in the CIE examination.
细胞学说由施莱登、施万和菲尔肖于 19 世纪建立，至今仍是生物学的组织原则。该学说基于三项基本主张，在 CIE 考试中，你需要运用这些观点来比较不同类型的细胞。
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All living organisms are composed of one or more cells.
一切生物体均由一个或多个细胞构成。
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The cell is the basic structural and functional unit of life.
细胞是生命的基本结构与功能单位。
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All cells arise from pre-existing cells through cell division.
所有细胞都通过细胞分裂来源于先前已经存在的细胞。
Notable exceptions exist, such as viruses, which are acellular and considered non-living under most definitions. However, A-Level questions rarely ask you to debate this; instead, focus on how the structure of a cell supports its function.
例外情况确实存在，例如病毒无细胞结构，在大多数定义下被视为非生命体。但 A-Level 试题很少要求你争论这一点；相反，重点在于理解细胞结构如何支持其功能。
2. Prokaryotic vs Eukaryotic Cells | 原核细胞与真核细胞
One of the most frequently tested distinctions in CIE Biology is the difference between prokaryotic and eukaryotic cells. Prokaryotic cells, such as bacteria, lack a true nucleus and membrane-bound organelles. Eukaryotic cells, found in plants, animals and fungi, possess a membrane-bound nucleus and many specialised organelles.
CIE 生物考试中最常考的区别之一就是原核细胞与真核细胞的差异。原核细胞（如细菌）没有真正的细胞核和膜包围的细胞器；而真核细胞存在于植物、动物和真菌中，具有膜包围的细胞核和多种特化细胞器。
| Feature | Prokaryotic 原核细胞 | Eukaryotic 真核细胞 |
| Nucleus | Absent; DNA free in cytoplasm | Present; enclosed by nuclear envelope |
| Membrane-bound organelles | Absent | Present |
| Ribosomes | 70S (smaller) | 80S in cytoplasm; 70S in mitochondria & chloroplasts |
| Cell wall | Peptidoglycan (murein) | Cellulose in plants; chitin in fungi; absent in animals |
| Typical size | 0.5 – 5 μm | 10 – 100 μm |
| DNA arrangement | Circular DNA; no histones | Linear DNA associated with histone proteins |
In addition, prokaryotic cells divide by binary fission, while eukaryotic cells divide by mitosis or meiosis. Prokaryotes may also possess plasmids — small circles of DNA that carry additional genes, such as antibiotic resistance.
此外，原核细胞通过二分裂进行增殖，而真核细胞通过有丝分裂或减数分裂增殖。原核细胞还可能含有质粒——携带额外基因（如抗生素抗性基因）的小型环状 DNA。
3. Key Organelles and Their Functions | 关键细胞器及其功能
Each organelle in a eukaryotic cell has a specific role. The CIE syllabus tests both the structure and function of organelles, and you must be able to link the two. Below are the most important organelles for the examination.
真核细胞中的每种细胞器都有特定功能。CIE 考纲考查细胞器的结构与功能，并且要求你能够将二者联系起来。以下是考试中最重要的一些细胞器。
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Nucleus: contains chromosomes (DNA + histone proteins); the nucleolus synthesises ribosomal RNA and assembles ribosomes. It controls all cellular activities through gene expression.
细胞核：含有染色体（DNA 与组蛋白）；核仁负责合成核糖体 RNA 并组装核糖体。细胞核通过基因表达控制所有细胞活动。
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Mitochondrion: double-membrane organelle with folded cristae and a fluid matrix. It is the site of aerobic respiration — the Krebs cycle and oxidative phosphorylation occur here, generating ATP.
线粒体：具有双层膜，内膜折叠形成嵴，内部充满液态基质。线粒体是有氧呼吸的场所——三羧酸循环和氧化磷酸化在此进行，生成 ATP。
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Chloroplast: contains thylakoid membranes stacked into grana, surrounded by stroma. It is the site of photosynthesis; light-dependent reactions occur on thylakoid membranes, while the Calvin cycle occurs in the stroma.
叶绿体：由堆叠成基粒的类囊体膜和基质构成。叶绿体是光合作用的场所——光反应在类囊体膜上进行，而卡尔文循环在基质中完成。
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Rough Endoplasmic Reticulum (RER): studded with ribosomes; folds and transports newly synthesised proteins for secretion.
粗面内质网（RER）：表面附着核糖体；负责折叠和运输新合成的分泌蛋白。
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Smooth Endoplasmic Reticulum (SER): synthesises lipids and steroids; also involved in detoxification of drugs and toxins.
滑面内质网（SER）：负责合成脂质和类固醇；还参与药物和毒素的解毒过程。
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Golgi Apparatus: modifies, packages, and sorts proteins and lipids into vesicles; also forms lysosomes.
高尔基体：对蛋白质和脂质进行加工、包装和分选，将其装入囊泡；同时负责形成溶酶体。
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Lysosomes: contain hydrolytic enzymes (lysozyme, proteases) that digest waste materials, dead organelles, and invading pathogens.
溶酶体：含有水解酶（溶菌酶、蛋白酶），用于消化废物、衰老的细胞器以及入侵的病原体。
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Ribosomes: composed of rRNA and protein; the site of protein synthesis (translation). They are free in the cytoplasm or attached to the RER.
核糖体：由 rRNA 和蛋白质组成；是蛋白质合成（翻译）的场所。核糖体游离于细胞质中或附着于粗面内质网上。
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Centrioles: pair of cylindrical structures composed of microtubules; they organise the mitotic spindle during cell division in animal cells.
中心粒：由微管构成的一对圆柱形结构；在动物细胞分裂时负责组织纺锤体。
You should also remember the plant-specific structures: a cell wall made of cellulose for structural support, and a large central vacuole for turgor pressure and storage.
你还应记住植物特有的结构：由纤维素构成的细胞壁以提供结构支撑，以及用于维持膨压和储存物质的大型中央液泡。
4. Light Microscopy | 光学显微镜技术
solution limit of approximately 200 nm (0.2 μm). This means that two objects closer than 200 nm cannot be distinguished as separate structures under the LM.
光学显微镜利用可见光和玻璃透镜放大标本。其最高放大倍率约为 ×1500，分辨率极限约为 200 nm（0.2 μm）。这意味着在光学显微镜下，相距小于 200 nm 的两个物体无法被分辨为独立结构。
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Light microscopes allow observation of living cells and dynamic processes such as cytoplasmic streaming and cell division.
光学显微镜可以观察活细胞以及细胞质流动、细胞分裂等动态过程。
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Only larger organelles can be seen, such as the nucleus, chloroplasts, and stained mitochondria. Ribosomes, lysosomes, and internal membrane structures are too small to resolve.
只能看到较大的细胞器，如细胞核、叶绿体和经染色的线粒体；核糖体、溶酶体及内部膜结构因太小而无法分辨。
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Specimens require a thin section to allow light to pass through; staining increases contrast but may kill living cells.
标本需切成薄片以便光线穿透；染色可增强对比度，但可能杀死活细胞。
Common exam question: explain why organelles such as ribosomes cannot be seen with a light microscope. The answer must state that ribosomes are smaller than the resolution limit of 200 nm and therefore cannot be resolved as separate structures.
常见考题：解释为什么核糖体等细胞器无法用光学显微镜观察到。答案必须指出：核糖体小于光学显微镜 200 nm 的分辨率极限，因此无法被分辨为独立结构。
5. Electron Microscopy | 电子显微镜技术
Electron microscopes use a beam of electrons instead of visible light. Electrons have a much shorter wavelength, enabling far higher resolution. There are two main types you need to know for CIE A-Level Biology.
电子显微镜使用电子束代替可见光。电子的波长比可见光短得多，因此能够实现高得多的分辨率。CIE A-Level 生物考试要求你掌握两种主要类型。
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Transmission Electron Microscope (TEM) -and-white images. It can magnify up to ×500,000 and resolve details down to about 0.5 nm.
透射电子显微镜（TEM）：电子穿透超薄标本，揭示内部超微结构的二维图像（黑白）。最大放大倍数可达 ×500,000，可分辨小至约 0.5 nm 的结构细节。
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Scanning Electron Microscope (SEM) llected to form a three-dimensional image of the surface topography. Resolution is about 3–10 nm, with magnification up to ×100,000.
扫描电子显微镜（SEM）：电子束扫描标本表面，收集散射电子形成表面形貌的三维图像。分辨率约为 3–10 nm，最大放大倍数可达 ×100,000。
Both types of electron microscope operate in a vacuum, so specimens must be fixed, dehydrated and coated with heavy metal stains. Consequently
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