📚 Cell Structure: Key Exam Points for IB and AQA Biology | 细胞结构：IB与AQA生物考点精讲
The cell is the fundamental unit of all living organisms, and a thorough knowledge of its structure is essential for success in both IB and AQA biology. This article distils the most examined concepts, draws comparisons, and highlights the practical skills and drawing requirements demanded by each specification.
细胞是所有生物体的基本单位，透彻理解其结构对于在IB和AQA生物考试中取得成功至关重要。本文提炼了最常考的概念，进行了对比，并突出了各课程大纲对实验技能和绘图的要求。
1. Comparing Prokaryotic and Eukaryotic Cells | 对比原核与真核细胞
Prokaryotic cells, exemplified by bacteria, are characterised by the absence of a membrane-bound nucleus. Their DNA is a single circular chromosome that lies free in the cytoplasm alongside smaller rings of DNA called plasmids. The cytoplasm also contains 70S ribosomes, which are smaller than those found in eukaryotes. The cell wall is composed of peptidoglycan, and some bacteria possess a protective capsule, flagella for locomotion, or pili for attachment.
以细菌为代表的原核细胞，其特征是没有膜结合的细胞核。它们的DNA是一个环状染色体，与称为质粒的小环状DNA一起游离在细胞质中。细胞质中还含有70S核糖体，这些核糖体比真核生物中的小。细胞壁由肽聚糖组成，有些细菌还具有保护性荚膜、用于运动的鞭毛或用于附着的菌毛。
Eukaryotic cells, in contrast, house their linear DNA associated with histone proteins within a true nucleus enclosed by a double membrane. These cells are larger and compartmentalised, containing membrane-bound organelles such as mitochondria, the endoplasmic reticulum, the Golgi apparatus, and often chloroplasts. Eukaryotic ribosomes are 80S, and cells may have a cellulose cell wall (plants), a chitin cell wall (fungi), or no wall (animals).
相反，真核细胞将与其组蛋白结合的线性DNA存储在由双层膜包裹的细胞核内。这些细胞更大且区域化，含有膜结合的细胞器，如线粒体、内质网、高尔基体，并且常有叶绿体。真核核糖体为80S，细胞可以具有纤维素细胞壁（植物）、几丁质细胞壁（真菌）或无壁（动物）。
Both IB and AQA require you to state that bacteria are prokaryotes lacking a nucleus and mitochondria; IB often presents a drawing of a Salmonella cell and asks for labels, while AQA expects you to recall that bacterial DNA is not enclosed within a nuclear membrane.
IB和AQA都要求说明细菌是没有细胞核和线粒体的原核生物；IB常给出一个沙门氏菌细胞的图并要求标注，而AQA期望你回忆起细菌DNA并不包被在核膜内。
2. Eukaryotic Organelles: Structure and Function | 真核细胞器：结构与功能
Nucleus: Surrounded by a nuclear envelope with pores, it contains chromatin and the nucleolus. It controls gene expression and mediates the passage of mRNA and ribosomes to the cytoplasm.
细胞核：由带有核孔的核膜包围，内含染色质和核仁。它控制基因表达，并介导mRNA和核糖体进入细胞质。
Rough Endoplasmic Reticulum (RER): A network of flattened sacs studded with 80S ribosomes. The RER processes and folds proteins destined for secretion or membrane insertion.
粗糙内质网（RER）：扁平的囊状网络，表面附着80S核糖体。RER加工并折叠将要分泌或嵌入膜的蛋白质。
Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; synthesises lipids, phospholipids, and steroids, and detoxifies certain chemicals.
光滑内质网（SER）：无核糖体；合成脂类、磷脂和类固醇，并解毒某些化学物质。
Golgi Apparatus: Stacks of cisternae that modify proteins received from the RER, package them into vesicles for transport to the plasma membrane or for lysosomal delivery.
高尔基体：扁平的潴泡堆叠，对来自RER的蛋白质进行修饰，并将其包装成囊泡以运往质膜或送入溶酶体。
Mitochondrion: Double-membrane organelle with inner folds called cristae; site of aerobic respiration and ATP synthesis. It contains 70S ribosomes and its own circular DNA.
线粒体：具有叫作嵴的内膜折叠的双膜细胞器；是有氧呼吸和ATP合成的场所。它含有70S核糖体和自己的环状DNA。
Chloroplast (plant and algal cells): Double membrane, internal thylakoid membranes stacked into grana, and stroma fluid. It performs photosynthesis and also contains 70S ribosomes and DNA.
叶绿体（植物和藻类细胞）：双层膜，内部类囊体膜堆叠成基粒，及基质液体。它进行光合作用，同样含有70S核糖体和DNA。
Lysosomes: Membrane-bound vesicles containing hydrolytic enzymes for digestion of macromolecules and worn-out organelles.
溶酶体：含有水解酶的膜结合囊泡，用于消化大分子和衰老的细胞器。
Ribosomes: Sites of protein synthesis; 80S in the cytoplasm and RER, 70S in mitochondria and chloroplasts.
核糖体：蛋白质合成的场所；细胞质和RER中为80S，线粒体和叶绿体中为70S。
When drawing organelles for IB, maintain correct relative sizes and always use a ruler and sharp pencil. AQA practicals often ask you to identify organelles from electron micrographs.
在IB中绘制细胞器时，要保持正确的相对大小，并使用直尺和尖铅笔。AQA实验常要求根据电子显微照片识别细胞器。
3. Cell Membrane Structure and Transport | 细胞膜结构与运输
The fluid mosaic model describes the plasma membrane as a phospholipid bilayer in which proteins are embedded. Phospholipid heads are hydrophilic and face the aqueous environments, while hydrophobic tails face inward. Cholesterol stabilises membrane fluidity, and glycoproteins act as recognition sites.
流动镶嵌模型将质膜描述为磷脂双分子层，其中嵌有蛋白质。磷脂头部亲水，朝向水环境，而疏水尾部朝内。胆固醇稳定膜流动性，糖蛋白则作为识别位点。
Passive transport includes simple diffusion, facilitated diffusion (via channel or carrier proteins), and osmosis. Active transport requires ATP to move substances against the concentration gradient, using pump proteins such as the Na⁺/K⁺ pump. Bulk transport, namely endocytosis and exocytosis, moves large particles across the membrane via vesicle formation.
被动运输包括简单扩散、协助扩散（通过通道或载体蛋白）和渗透。主动运输需要ATP逆浓度梯度移动物质，利用如Na⁺/K⁺泵这样的泵蛋白。批量运输，即胞吞和胞吐，通过形成囊泡来移动大颗粒物质。
In osmosis questions, be precise: water moves from a region of higher water potential (less negative) to one of lower water potential (more negative) through a partially permeable membrane. IB often asks students to draw a labelled plasma membrane, while AQA may combine permeability with beetroot practical tasks.
在渗透问题中要精确：水通过部分通透膜从水势较高（负值较小）的区域向水势较低（负值较大）的区域移动。IB常要求学生绘制标记的质膜图，而AQA可能将通透性与甜菜根实验任务结合。
4. Microscopy and Magnification Calculations | 显微镜与放大倍数计算
Optical (light) microscopes use visible light and can resolve down to about 200 nm; they are used for staining and live specimen observation. Electron microscopes (TEM and SEM) use electron beams and achieve resolution down to 0.5 nm or better, revealing ultrastructure.
光学显微镜使用可见光，分辨率低至约200 nm；用于染色和活体标本观察。电子显微镜（TEM和SEM）使用电子束，分辨率可达0.5 nm或更佳，能揭示超微结构。
Magnification = size of image ÷ actual size of specimen. You must be able to convert units: 1 cm = 10 mm = 10,000 µm = 10,000,000 nm. IB commonly gives a scale bar and requires an actual size calculation, while AQA frequently embeds these calculations into practical write‑ups.
放大倍数 = 图像大小 ÷ 标本实际大小。你必须能够进行单位换算：1 cm = 10 mm = 10,000 µm = 10,000,000 nm。IB常给出比例尺并要求计算实际大小，而AQA经常把这些计算嵌入实验报告中。
An eyepiece graticule calibrated with a stage micrometer allows precise measurement. Remember: magnification alone does not guarantee a clear image; resolution is equally important.
用台微尺校准目镜测微尺可以进行精确测量。记住：仅有放大倍数并不能保证图像清晰；分辨率同样重要。
5. Endosymbiotic Theory | 内共生学说
The endosymbiotic theory proposes that mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by ancestral eukaryotic cells and formed a symbiotic relationship. The evidence includes: both organelles have double membranes; they contain their own circular DNA, which is not wrapped around histones; they possess 70S ribosomes; they divide by binary fission independently of the host cell’s mitosis; and they are roughly the size of prokaryotes.
内共生学说提出，线粒体和叶绿体曾是被祖先真核细胞吞噬的自由生活的原核生物，并形成了共生关系。证据包括：两种细胞器都有双层膜；它们含有自身的环状DNA，没有组蛋白包裹；它们拥有70S核糖体；它们以二分裂方式分裂，与宿主细胞的有丝分裂独立；并且它们大小大致与原核生物相当。
IB explicitly expects you to discuss these pieces of evidence, whereas AQA at A‑level may require you to outline the theory and its supporting observations.
IB明确要求讨论这些证据，而AQA的A‑level可能要求概述该理论及其支持的观察结果。
6. Viruses as Non-Living Structures | 病毒作为非生命结构
Viruses are acellular and non‑living obligate parasites. They consist of genetic material (DNA or RNA) enclosed in a protein coat called a capsid. Some have a lipid envelope derived from the host cell membrane, with embedded glycoprotein spikes. They do not carry out metabolism, respond to stimuli, or reproduce independently.
病毒是无细胞结构的非生命专性寄生物。它们由包被在称为衣壳的蛋白质外壳内的遗传物质（DNA或RNA）组成。一些具有源自宿主细胞膜的脂质包膜，并嵌有糖蛋白刺突。它们不进行代谢、不对外界刺激作出反应，也不能独立繁殖。
IB often contrasts viruses with prokaryotic cells, highlighting the absence of cytoplasm, ribosomes, and enzyme activity. AQA gives prominence to specific viruses such as HIV and tobacco mosaic virus, structuring questions around their structures and life cycles.
IB常将病毒与原核细胞进行对比，强调病毒没有细胞质、核糖体和酶活性。AQA突显特定病毒，如HIV和烟草花叶病毒，并围绕它们的结构和生活周期设计问题。
7. Comparison of Plant and Animal Cells | 植物细胞与动物细胞的比较
Both plant and animal eukaryotic cells share a nucleus, mitochondria, ER, Golgi, cytoplasm, and 80S ribosomes. The key differences are that plant cells possess a rigid cellulose cell wall outside the membrane, a large permanent vacuole filled with cell sap, and chloroplasts in photosynthetic tissues. Animal cells lack these structures but contain centrioles and, typically, more lysosomes.
植物和动物真核细胞都有细胞核、线粒体、内质网、高尔基体、细胞质和80S核糖体。关键区别在于，植物细胞在细胞膜外有刚性的纤维素细胞壁、一个充满细胞液的大的永久液泡，以及光合组织中的叶绿体。动物细胞缺少这些结构，但含有中心粒，并且通常有更多的溶酶体。
When preparing microscope slides, AQA students look for these differences in onion epidermis and cheek cell specimens. IB requires accurate, scaled drawings from light microscope observations.
在准备显微镜玻片时，AQA学生会从洋葱表皮和口腔粘膜细胞标本中观察这些差异。IB要求根据光学显微镜观察绘制精确、按比例的图。
8. Specialised Cells and Adaptations | 特化细胞与适应
Red blood cells: biconcave disc shape increases surface area for oxygen uptake; no nucleus or mitochondria in mammals, maximising haemoglobin space.
红细胞：双凹盘状增加了氧气摄取的表面积；哺乳动物的红细胞无核无线粒体，使血红蛋白空间最大化。
Sperm cells: streamlined head with an acrosome containing enzymes to penetrate the egg, numerous mitochondria in the mid‑piece providing ATP for flagellum movement.
精子细胞：流线型的头部和含有穿透卵子的酶的顶体，中段大量线粒体为鞭毛运动提供ATP。
Root hair cells: long, thin protrusion vastly increasing surface area for water and mineral absorption; large vacuole and abundant mitochondria.
根毛细胞：细长的突起大大增加了吸收水和矿物质的表面积；有大的液泡和丰富的线粒体。
Neurones: long axon insulated by myelin sheath, dendrites for receiving signals, and terminal buttons for transmitting neurotransmitters.
神经元：长的轴突由髓鞘绝缘，树突接收信号，终末扣释放神经递质。
Both specifications link structure to function; IB may ask for sketches of specialised cells alongside an explanation of adaptations, while AQA commonly embeds such questions in exchange and transport topics.
两个大纲都将结构与功能联系起来；IB可能要求绘制特化细胞草图并解释适应特征，而AQA通常将此类问题嵌入交换和运输主题中。
9. Experimental Techniques: Cell Fractionation | 实验技术：细胞分级分离
Cell fractionation allows the separation of organelles by density. First, tissue is homogenised in a cold, isotonic buffer to break cell membranes while preserving organelles. The homogenate is filtered and then subjected to differential centrifugation. At low speeds, nuclei pellet out; increasing speeds successively isolate mitochondria, chloroplasts, and microsomal fractions.
细胞分级分离可根据密度分离细胞器。首先，在冷的等渗缓冲液中匀浆组织，以打破细胞膜并保存细胞器。匀浆液过滤后，进行差速离心。在低速下，细胞核沉淀；逐渐提高速度可依次分离线粒体、叶绿体和微粒体组分。
This technique is emphasised more in A‑level programmes, but IB may refer to it when discussing organelle function and research methods. Always recall the importance of temperature and tonicity to prevent organelle damage.
这项技术在A‑level课程中更受重视，但IB在讨论细胞器功能和研究方法时可能提及。时刻记住温度和渗透压的重要性，以防止细胞器受损。
10. IB Specific Requirements: Drawing Eukaryotic Cells | IB 特定要求：绘制真核细胞
IB Biology places strong emphasis on producing clear, labelled diagrams from observations. The generalised drawing of a liver cell must show nucleus, mitochondria, RER, ribosomes, lysosomes, and Golgi in correct relative proportions. The chloroplast drawing requires grana and thylakoid membranes; the mitochondrion should clearly show cristae and a smooth outer membrane. Annotations, not just labels, are rewarded, for example noting that cristae increase surface area for ATP synthase.
IB生物非常强调根据观察绘制清晰、标注齐全的图。肝细胞的通用图必须按正确的相对比例显示细胞核、线粒体、RER、核糖体、溶酶体和高尔基体。叶绿体图需画出基粒和类囊体膜；线粒体应清晰显示嵴和光滑的外膜。注解而不只是标签会得到加分，例如注明嵴增大了ATP合酶的表面积。
Always avoid shading and sketchy lines; use a pencil sharply. Magnification scale is critical—never draw a mitochondrion larger than the nucleus unless specifically magnified for a detail insert.
始终避免阴影和潦草线条；使用削尖的铅笔。放大比例至关重要——切勿将线粒体画得比细胞核还大，除非是专门放大的细节插图。
11. AQA Required Practicals | AQA 规定实验
AQA’s specification demands several microscopy‑based practicals. For ‘Using a light microscope to observe and record animal and plant cells’, students prepare temporary mounts of onion epidermis (stained with iodine in potassium iodide) and human cheek cells (stained with methylene blue). They must identify cytoplasm, nuclei, cell walls, and vacuoles, and calculate cell size using a calibrated eyepiece graticule.
AQA大纲要求多项基于显微镜的实践操作。在“使用光学显微镜观察和记录动植物细胞”实验中，学生制作洋葱表皮临时装片（用碘化钾碘液染色）和人口腔上皮细胞装片（用亚甲基蓝染色）。他们必须识别细胞质、细胞核、细胞壁和液泡，并用已校准的目镜测微尺计算细胞大小。
Additional practicals may involve investigating the effect of salt concentration on plant tissue to demonstrate osmosis, linking observations to cell membrane permeability.
额外的实验可能涉及探究盐浓度对植物组织的影响以证明渗透作用，将观察结果与细胞膜通透性关联起来。
12. Common Misconceptions and Exam Tips | 常见误解与考试技巧
One frequent error is stating that ‘all bacteria have a capsule’ – many do, but not all. Similarly, students confuse the cell wall with the cell membrane; the wall is a rigid external structure, not a selectively permeable barrier. Mixing up ribosome sedimentation coefficients is another pitfall: 70S in prokaryotes and organelles, 80S in eukaryotic cytoplasm. Also, magnification is not the same as resolution, and a larger image does not necessarily mean more detail.
一个常见的错误是说“所有细菌都有荚膜”——许多有，但并非所有都有。类似地，学生将细胞壁与细胞膜混淆；细胞壁是刚性的外部结构，而非选择通透性屏障。混淆核糖体沉降系数是另一个陷阱：原核生物和细胞器中是70S，真核细胞质中是80S。此外，放大倍数不等于分辨率，更大的图像并不一定意味着更多细节。
For success in IB, practise timed drawing and annotation, linking structure to function explicitly. For AQA, focus on key definitions, the fluid mosaic model, and the quantitative skills of magnification and unit conversion. Understand the limits of light microscopes and justify why electron microscopes are needed to see ribosomes or thylakoid membranes.
为了在IB中取得成功，要练习限时绘图和注释，并明确地将结构与功能联系起来。对于AQA，要专注于关键定义、流动镶嵌模型，以及放大倍数和单位换算的定量技能。理解光学显微镜的局限性，并证明为什么需要电子显微镜才能看到核糖体或类囊体膜。
Under exam pressure, carefully read the command terms: ‘draw’ means a pencil diagram with clear lines, ‘label’ requires straight indicator lines touching the structure, and ‘annotate’ asks for a brief note explaining function or relevance.
在考试压力下，仔细阅读指令词：“绘制”意味着用铅笔画出清晰的线条图，“标注”需要用直的指示线触及结构，而“注解”要求附上简短注释解释功能或相关性。
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