📚 2.1 Cell Structure Exam Practice | 细胞结构真题精练
Mastering cell structure is a cornerstone of A-Level Biology. This revision article provides a targeted selection of real exam-style questions and model answers to help you understand the concepts and boost your confidence. We will cover microscopy, organelles, prokaryotic vs eukaryotic cells, cell fractionation, and essential exam techniques.
掌握细胞结构是 A-Level 生物学的基础。本文精选典型真题和参考答案，帮助你理解概念、树立信心。内容包括显微镜技术、细胞器、原核与真核细胞对比、细胞分馏，以及关键的答题技巧。
1. Overview of Question Types & Marking Points | 考题类型与得分点概述
In A-Level Biology, questions on cell structure can range from simple recall of organelle functions to data analysis using micrographs and calibration curves. The command words ‘describe’, ‘explain’, ‘compare’, and ‘evaluate’ require different levels of detail. Always note the mark allocation to guide your answer length.
在 A-Level 生物中，细胞结构题目形式多样，从简单的细胞器功能回忆到运用显微照片和校准曲线进行数据分析。指令词如“描述”、“解释”、“比较”、“评估”对答案的详略要求不同。务必留意题目分值，以控制答案长度。
For example, a 3-mark ‘describe’ question expects three clear points; a 4-mark ‘explain’ question requires linking structure to function. Use scientific terminology (e.g., cristae, cisternae, nucleoid) precisely to gain credit. Diagrams must be fully labelled with straight lines, no arrowheads.
例如，3 分的“描述”题要求写出三个明确的要点；4 分的“解释”题则需要将结构与功能联系起来。准确使用科学术语（如嵴、扁平膜囊、拟核）才能得分。画图时必须用直线标注，不得使用箭头。
2. Magnification & Resolution Exam Practice | 放大倍率与分辨率真题练习
Magnification refers to how much larger an object appears compared to its actual size, while resolution is the ability to distinguish two separate points. The formula for magnification is:
放大倍率指物体比实际尺寸放大的倍数，分辨率则是区分两个点的能力。放大倍率的计算公式为：
magnification = image size ÷ actual size
Note: image size and actual size must be in the same units (e.g., mm). When working with electron micrographs, you are often given a scale bar.
注意：图像尺寸与实际尺寸单位必须一致（如 mm）。处理电子显微照片时，通常会提供比例尺。
Example: An electron micrograph shows a chloroplast with a scale bar marked 1 µm. The scale bar measures 20 mm on the image. Calculate the magnification. Give your answer to the appropriate number of significant figures.
例题： 一张叶绿体的电子显微照片上标有 1 µm 比例尺。比例尺在照片上的长度为 20 mm。计算放大倍率，答案需给出适当有效数字。
Solution: Convert 20 mm to µm: 20 mm = 20 000 µm. Magnification = image size / actual size = 20 000 µm / 1 µm = ×20 000. Answer: ×20 000 (or 20 000x).
解答： 将 20 mm 换算为 µm：20 mm = 20 000 µm。放大倍率 = 图像尺寸 / 实际尺寸 = 20 000 µm / 1 µm = ×20 000。答案：×20 000（或 20 000×）。
Common mistakes: forgetting to convert units, or using the calibration formula incorrectly. Always check that your answer is sensible — a 4 cm long micrograph of a 2 µm mitochondrion should yield a magnification around ×20 000.
常见错误：忘记换算单位或误用校准公式。务必检查答案是否合理——例如一张 4 cm 长的线粒体显微照片，实际大小为 2 µm，放大倍率约 ×20 000。
3. Eukaryotic Organelles: Structure & Function | 真核细胞器：结构与功能
Eukaryotic cells contain membrane-bound organelles such as nucleus, mitochondria, rough and smooth endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and ribosomes (though ribosomes are not membrane-bound). The table below summarises their key features:
真核细胞含有具膜细胞器，如细胞核、线粒体、粗面和滑面内质网、高尔基体、溶酶体，以及核糖体（核糖体无膜）。下表总结其关键特征：
| Organelle | Structure | Function |
|---|---|---|
| Nucleus | Double membrane with nuclear pores; contains chromatin | Controls cell activities; stores genetic information |
| Mitochondrion | Double membrane; inner membrane folded into cristae; matrix | Aerobic respiration; ATP synthesis |
| Rough ER | Network of flattened sacs studded with ribosomes | Protein synthesis and transport |
| Smooth ER | Network of tubules lacking ribosomes | Lipid synthesis; detoxification |
| Golgi apparatus | Stack of flattened cisternae | Modifies, sorts and packages proteins for secretion |
| Lysosome | Membrane-bound vesicle containing hydrolytic enzymes | Digestion of worn-out organelles and engulfed pathogens |
| Ribosome | Two subunits made of rRNA and protein; 80S in eukaryotes | Protein synthesis |
| Chloroplast (plant) | Double membrane; thylakoids stacked into grana; stroma | Photosynthesis |
| Large central vacuole (plant) | Membrane-bound sac containing cell sap | Storage; maintains turgor pressure |
The above table lists the main eukaryotic organelles and their roles. In an exam, you may be asked to identify these organelles from micrographs or diagrams and state one function.
上表列出了主要真核细胞器及其功能。考试中可能会要求你从显微照片或图示中识别这些细胞器，并说明一项功能。
Quick-fire question (2 marks): State one structural feature of a mitochondrion and explain how it is adapted for ATP production.
速答题 (2分)： 说出线粒体的一个结构特征，并解释其如何适应 ATP 的生成。
Answer: Cristae: the folded inner membrane provides a large surface area for the electron transport chain and ATP synthase. Alternatively, the matrix contains enzymes of the Krebs cycle.
答案： 嵴：折叠的内膜为电子传递链和 ATP 合酶提供了极大的表面积。或其基质含有克雷布斯循环的酶。
4. Prokaryotic vs Eukaryotic Cells | 原核细胞与真核细胞对比
Prokaryotic cells (bacteria and archaeans) are smaller and simpler than eukaryotic cells. They lack a nucleus and membrane-bound organelles. Instead, they have a single circular DNA molecule free in the cytoplasm, a region called the nucleoid. They may also contain plasmids, 70S ribosomes, a cell wall made of peptidoglycan (in bacteria), and sometimes a capsule or flagella.
原核细胞（细菌和古菌）比真核细胞更小、更简单。它们没有细胞核和膜包被的细胞器。相反，有一条环状DNA分子游离在细胞质中，形成拟核。它们还可能含有质粒、70S 核糖体、由肽聚糖构成的细胞壁（细菌），有时还有荚膜或鞭毛。
A typical six-mark exam question: ‘Compare and contrast the structure of a prokaryotic cell and a eukaryotic cell.’ To score full marks, you must mention both similarities and differences using clear comparative language. Tabulate your answer if the question permits.
一道典型的 6 分考题：“比较和对比原核细胞和真核细胞的结构。” 要获得满分，必须使用明确的比较语言，同时提及相同点和不同点。如题目允许，可用表格形式作答。
Similarities: both have cytoplasm, cell membrane, ribosomes, and DNA as genetic material. Differences: prokaryotes have no nucleus, no membrane-bound organelles, circular DNA, 70S ribosomes, cell wall of peptidoglycan; eukaryotes have a nucleus, membrane-bound organelles, linear DNA associated with histones, 80S ribosomes, cell wall of cellulose (plants) or no cell wall (animals).
相同点：都有细胞质、细胞膜、核糖体和遗传物质DNA。不同点：原核细胞无细胞核、无膜包细胞器、环状DNA、70S 核糖体、肽聚糖细胞壁；真核细胞有细胞核、具膜细胞器、与组蛋白结合的线性DNA、80S 核糖体、植物细胞壁由纤维素构成（动物无细胞壁）。
Exam tip: Do not say prokaryotes ‘have no mitochondria’ without stating what they use instead — they carry out respiration on the cell membrane. Mention that some prokaryotes have mesosomes (infoldings) increasing surface area.
答题技巧： 不要说原核细胞“没有线粒体”而不说明其替代结构——它们在细胞膜上进行呼吸作用。可提及一些原核生物存在中膜体（内褶）以增加表面积。
5. Endosymbiotic Theory: Evidence from Mitochondria & Chloroplasts | 内共生理论：来自线粒体和叶绿体的证据
The endosymbiotic theory states that mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by an ancestral eukaryotic cell. Evidence includes: double membranes, own circular DNA, 70S ribosomes, and they divide by binary fission. The inner membrane of mitochondria and chloroplasts shows similarities to bacterial membranes.
内共生理论认为，线粒体和叶绿体曾经是自由生活的原核生物，被原始真核细胞吞入。证据包括：双层膜、自身环状DNA、70S 核糖体，且通过二分裂方式繁殖。线粒体和叶绿体的内膜与细菌膜相似。
Common exam question (2 marks): ‘Give two pieces of evidence supporting the endosymbiotic origin of mitochondria.’ Answer: Mitochondria contain 70S ribosomes similar to those in bacteria; mitochondrial DNA is circular and not associated with histones. Do not just say ‘own DNA’—specify the features.
常考题 (2分)： “给出两个支持线粒体内共生起源的证据。” 答案：线粒体含有与细菌相似的 70S 核糖体；线粒体DNA 是环状且不与组蛋白结合。不要仅仅回答“有自身的DNA”——必须说明特征。
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