A-Level WJEC Biology: Cell Structure Key Exam Focus | A-Level WJEC 生物：细胞结构考点精讲

📚 A-Level WJEC Biology: Cell Structure Key Exam Focus | A-Level WJEC 生物：细胞结构考点精讲

This article covers the core knowledge required for the WJEC A-Level Biology topic of cell structure, including the cell theory, microscopy, prokaryotic and eukaryotic cells, organelles, membrane architecture, and specialised cells. It also integrates exam-focused tips to help you maximise marks.

本文涵盖 WJEC A-Level 生物“细胞结构”所需的核心知识，包括细胞学说、显微镜检术、原核与真核细胞、细胞器、膜结构以及特化细胞，并融入考试技巧，助你拿到高分。

1. Cell Theory and Microscopy | 细胞学说与显微镜检术

The unified cell theory states that all living organisms are composed of one or more cells, the cell is the basic unit of life, and all cells arise from pre-existing cells.

统一的细胞学说指出：所有生物由一个或多个细胞组成，细胞是生命的基本单位，且所有细胞均来自已有细胞。

In WJEC exams, you must be able to compare light microscopes and electron microscopes. Light microscopes use visible light and have a maximum resolution of about 200 nm, while transmission electron microscopes (TEM) use electron beams to achieve a resolution of around 0.5 nm, enabling ultrastructure visualisation. Scanning electron microscopes (SEM) provide 3D surface images.

在 WJEC 考试中，你必须比较光学显微镜与电子显微镜。光学显微镜使用可见光，最高分辨率约为 200 nm；透射电子显微镜使用电子束，分辨率可达 0.5 nm，能观察超微结构；扫描电子显微镜则提供三维表面图像。

Remember the unit conversions: 1 mm = 1000 µm, 1 µm = 1000 nm. Magnification is the number of times larger an image is than the actual object, while resolution is the ability to distinguish two separate points as distinct. Electron microscopes have far greater resolution but require dead, stained specimens.

牢记单位换算：1 mm = 1000 µm，1 µm = 1000 nm。放大倍数是图像比实物放大的倍数，而分辨率是区分两个点的能力。电子显微镜分辨率极高，但需要杀死并染色样本。

2. Prokaryotic vs Eukaryotic Cells | 原核细胞与真核细胞的比较

Prokaryotic cells, such as bacteria, lack a membrane-bound nucleus and membrane-bound organelles. Their DNA is found as a single, circular chromosome in a region called the nucleoid, and they often contain small loops of DNA called plasmids. Ribosomes are 70S, and the cell wall is made of peptidoglycan (murein). Some have a capsule, flagella, or pili.

原核细胞（如细菌）没有膜包被的细胞核及膜性细胞器。其 DNA 为单一条环状染色体，位于拟核区，常含有小型 DNA 环状质粒。核糖体为 70S，细胞壁由肽聚糖组成。有些细菌具有荚膜、鞭毛或菌毛。

Eukaryotic cells (animal, plant, fungal, protoctist) have a true nucleus enclosed by a nuclear envelope, linear DNA associated with histones, and numerous membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus. Ribosomes are 80S (except in mitochondria and chloroplasts, which contain 70S ribosomes, supporting endosymbiotic theory).

真核细胞（动物、植物、真菌、原生生物）具有由核膜包被的真正细胞核，线性 DNA 与组蛋白结合，并拥有大量膜性细胞器，如线粒体、内质网和高尔基体。核糖体为 80S（但线粒体与叶绿体含 70S 核糖体，支持内共生学说）。

Feature	Prokaryote	Eukaryote
Nucleus	Absent	Present
DNA shape	Circular, naked	Linear, histone-bound
Ribosomes	70S	80S
Organelles	None	Many
Cell wall	Peptidoglycan	Cellulose (plants), chitin (fungi)

WJEC often asks for a table comparing these features, so memorise the key differences and provide precise terminology in your answers.

WJEC 常要求以表格比较这些特征，因此要熟记关键差异，并在答案中使用精确术语。

3. The Nucleus and Genetic Material | 细胞核与遗传物质

The nucleus is the control centre of the eukaryotic cell. It is surrounded by a double membrane called the nuclear envelope, which contains nuclear pores that regulate the passage of molecules such as mRNA and ribosomal subunits. Inside, the nucleolus synthesises ribosomal RNA (rRNA) and assembles ribosomes. Chromatin consists of DNA wound around histone proteins; during cell division, chromatin condenses into visible chromosomes.

细胞核是真核细胞的控制中心。它由双层核膜包裹，核膜上的核孔调控 mRNA 及核糖体亚基等分子出入。核仁合成核糖体 RNA 并组装核糖体。染色质由 DNA 缠绕组蛋白构成，在细胞分裂时凝聚成可见的染色体。

Examiners expect you to link structure to function: the nuclear pores allow selective transport, the nucleolus produces ribosome components for protein synthesis, and the linear organisation of genes enables precise gene regulation.

考官希望你建立结构与功能的联系：核孔实现选择性运输，核仁为蛋白质合成提供核糖体组分，线性基因排列利于精准调控。

4. Mitochondria and Chloroplasts | 线粒体与叶绿体

Mitochondria are the sites of aerobic respiration. They have a smooth outer membrane and a highly folded inner membrane forming cristae, which increase surface area for oxidative phosphorylation. The fluid matrix contains enzymes for the Krebs cycle, as well as mitochondrial DNA and 70S ribosomes. ATP synthase is embedded in the inner membrane.

线粒体是有氧呼吸的场所。其外膜光滑，内膜高度折叠形成嵴，增大氧化磷酸化的表面积。基质含三羧酸循环所需酶、线粒体 DNA 和 70S 核糖体。ATP 合酶位于内膜上。

Chloroplasts are found in plant and algal cells. They contain thylakoids stacked into grana, where the light-dependent reactions of photosynthesis occur. The stroma contains enzymes for the Calvin cycle, as well as chloroplast DNA and 70S ribosomes. Chlorophyll pigments in the thylakoid membranes capture light energy.

叶绿体存在于植物和藻类细胞中，类囊体堆叠成基粒，发生光合作用光反应。基质含卡尔文循环所需酶、叶绿体 DNA 和 70S 核糖体。类囊体膜上的叶绿素捕获光能。

The endosymbiotic theory proposes that mitochondria and chloroplasts evolved from free-living prokaryotes that were engulfed by ancestral eukaryotic cells. Evidence includes the presence of 70S ribosomes, double membranes, their own circular DNA, and the ability to replicate independently.

内共生学说认为，线粒体与叶绿体由被原始真核细胞吞噬的自由生活原核生物演化而来。证据包括拥有 70S 核糖体、双层膜、自身的环状 DNA 以及独立复制能力。

5. Endoplasmic Reticulum and Ribosomes | 内质网与核糖体

The endoplasmic reticulum (ER) is a network of membrane-bound tubules and sacs. Rough ER is studded with ribosomes on its cytosolic surface, giving it a ‘rough’ appearance. It is involved in protein synthesis and transport; polypeptides are threaded into the lumen for folding and modification. Smooth ER lacks ribosomes and is responsible for lipid synthesis, detoxification, and carbohydrate metabolism.

内质网是由膜构成的管状和扁平囊状网络。粗面内质网胞质面附着核糖体，参与蛋白质合成与运输，多肽进入内腔进行折叠修饰。滑面内质网无核糖体，负责脂质合成、解毒及碳水化合物代谢。

Ribosomes are the protein factories of the cell. Eukaryotic ribosomes are 80S, composed of a 60S large subunit and a 40S small subunit, while prokaryotic and organelle ribosomes are 70S (50S + 30S). They translate mRNA into polypeptides with the help of tRNA. Free ribosomes in the cytoplasm produce proteins for intracellular use, whereas those bound to the rough ER synthesise proteins destined for secretion or membrane insertion.

核糖体是细胞的蛋白质工厂。真核核糖体为 80S（60S 大亚基 + 40S 小亚基），原核及细胞器核糖体为 70S（50S + 30S）。它们在 tRNA 协助下将 mRNA 翻译成多肽。胞质游离核糖体合成胞内蛋白，粗面内质网附着核糖体则合成分泌蛋白或膜蛋白。

6. Golgi Apparatus and Vesicles | 高尔基体与囊泡

The Golgi apparatus consists of a stack of flattened, membrane-bound cisternae. It receives proteins and lipids from the ER in transport vesicles at its cis face. Within the Golgi, these molecules are chemically modified, for example by glycosylation (addition of carbohydrate groups). The modified products are then packaged into secretory vesicles that bud off from the trans face and are directed to their targets, such as the plasma membrane or lysosomes.

高尔基体由一叠扁平膜性囊泡组成。它在顺面通过运输囊泡接收来自内质网的蛋白质和脂质。在高尔基体内这些分子被化学修饰，例如糖基化。修饰后产物装入分泌囊泡，从反面出芽，定向运输到质膜或溶酶体等目标。

In addition to secretion, the Golgi apparatus forms lysosomes and synthesises certain polysaccharides, such as pectin for plant cell walls. In exam answers, emphasise its role in processing, sorting, and packaging, rather than just ‘transport’.

除了分泌，高尔基体还形成溶酶体并合成某些多糖，如植物细胞壁中的果胶。答案中要强调其加工、分选和包装功能，而不只是“运输”。

7. Lysosomes and Peroxisomes | 溶酶体与过氧化物酶体

Lysosomes are membrane-bound organelles containing hydrolytic enzymes (e.g. proteases, lipases, nucleases) that are active at an acidic pH. They digest worn-out organelles (autophagy), engulfed pathogens, and cellular debris. They also play a role in programmed cell death (apoptosis). The lysosomal membrane protects the rest of the cell from these enzymes.

溶酶体是含有水解酶（如蛋白酶、脂肪酶、核酸酶）的膜性细胞器，其酶在酸性 pH 下具有活性。它们消化老废细胞器（自噬）、吞噬的病原体和细胞碎屑，并参与细胞程序性死亡（凋亡）。溶酶体膜保护细胞其余部分免受酶的攻击。

Peroxisomes contain oxidative enzymes, such as catalase, which break down fatty acids and detoxify hydrogen peroxide (H₂O₂) into water and oxygen. They are particularly abundant in liver cells. Do not confuse their functions with those of lysosomes; peroxisomes are not part of the endomembrane system and do not bud from the Golgi.

过氧化物酶体含氧化酶如过氧化氢酶，可分解脂肪酸并将过氧化氢解毒为水和氧气。它们在肝细胞中尤其丰富。不要将其功能与溶酶体混淆；过氧化物酶体不属于内膜系统，也不从高尔基体出芽。

8. Cell Membrane Structure | 细胞膜结构

The fluid mosaic model describes the cell membrane as a phospholipid bilayer with embedded proteins. Phospholipids have hydrophilic phosphate heads facing outward and hydrophobic fatty acid tails facing inward, creating a selectively permeable barrier. The membrane is ‘fluid’ because lipids and proteins can move laterally.

流动镶嵌模型将细胞膜描述为嵌有蛋白质的磷脂双分子层。磷脂的亲水磷酸头朝外，疏水脂肪酸尾朝内，形成选择透过性屏障。膜具有“流动性”，因为脂质和蛋白质可侧向移动。

Integral proteins, such as channel proteins and carrier proteins, span the membrane and mediate transport. Glycoproteins and glycolipids on the outer surface act as recognition sites. Cholesterol in animal membranes reduces fluidity and increases stability. WJEC requires you to relate these components to functions like cell signalling and transport.

通道蛋白和载体蛋白等整合蛋白横跨全膜，介导物质转运。外侧的糖蛋白和糖脂作为识别位点。动物细胞膜中的胆固醇降低流动性、增加稳定性。WJEC 要求将这些组分与细胞信号转导和运输等功能相关联。

The core components: hydrophobic core prevents passage of ions and polar molecules, while channel proteins provide hydrophilic pores. The model also explains membrane fluidity for endocytosis and exocytosis.

核心组分：疏水核心阻止离子和极性分子通过，通道蛋白提供亲水孔道。该模型也解释了膜流动性对内吞和外排的作用。

9. Cell Wall and Vacuoles | 细胞壁与液泡

Plant cell walls are composed mainly of cellulose microfibrils embedded in a matrix of hemicellulose and pectin. The middle lamella, rich in calcium pectate, glues adjacent plant cells together. The cell wall provides structural support, maintains cell shape, and resists turgor pressure. Fungal cell walls contain chitin.

植物细胞壁主要由嵌在半纤维素和果胶基质中的纤维素微纤维组成。胞间层富含果胶钙，粘合相邻细胞。细胞壁提供结构支撑，维持细胞形态并抵抗膨压。真菌细胞壁含几丁质。

Plant cells also contain a large, permanent vacuole surrounded by a membrane called the tonoplast. The vacuole stores water, ions, pigments, and waste products, and it helps maintain turgor pressure, which is essential for plant support and growth. In exam answers, be clear that animal cells do not have a large permanent vacuole; they may have small, temporary vesicles.

植物细胞还有一个由液泡膜包围的中央大液泡，储存水、离子、色素和废物，并维持膨压，这对植物支撑和生长至关重要。答题时需明确，动物细胞没有大的永久液泡，可存在临时小囊泡。

10. Specialised Cells and Tissues | 特化细胞与组织

WJEC expects you to explain how eukaryotic cell structure is adapted to function. Red blood cells (erythrocytes) lose their nucleus and organelles to maximise space for haemoglobin, and their biconcave shape increases surface area for oxygen diffusion. Sperm cells have a streamlined head with an acrosome containing digestive enzymes, numerous mitochondria for energy, and a flagellum for propulsion.

WJEC 期望你能解释真核细胞结构如何适应功能。红细胞去核并丢失细胞器以最大化容纳血红蛋白，双凹圆盘状增加氧气扩散表面积。精子细胞有流线型头部及含酶的顶体，众多线粒体供能，鞭毛推进。

Palisade mesophyll cells in leaves are packed with chloroplasts and arranged vertically to maximise light absorption. Root hair cells have elongated projections to increase surface area for water and mineral uptake. Xylem vessel elements are dead, lignified cells forming continuous hollow tubes for water transport. Phloem sieve tubes have reduced cytoplasm and companion cells to assist translocation.

叶片栅栏细胞富含叶绿体、竖直排列以最大化吸收光能。根毛细胞有细长突起增大吸收水及矿物质的表面积。木质部导管为死细胞，木质化形成连续空管输导水分。韧皮部筛管细胞质稀薄，伴胞协助运输。

Always relate the adaptation directly to the function, using precise terms. For example, for red blood cells, mention the lack of nucleus to pack more haemoglobin, not simply ‘small size’.

回答时务必用精确术语直接将适应性联系功能，例如红细胞缺少细胞核以装载更多血红蛋白，而非简单说“体积小”。

11. Viruses as Acellular Particles | 病毒作为非细胞颗粒

Viruses are acellular, non-living entities that are much smaller than bacteria. They consist of a nucleic acid core (either DNA or RNA) surrounded by a protective protein coat called a capsid. Some viruses have an additional phospholipid envelope derived from the host cell membrane, studded with viral glycoproteins. Viruses lack cytoplasm, ribosomes, and metabolic machinery, so they must invade a host cell to replicate.

病毒是非细胞、非生命实体，远小于细菌。它们由核酸核心（DNA 或 RNA）和蛋白质衣壳组成。部分病毒还存在源自宿主细胞膜的磷脂包膜，镶嵌病毒糖蛋白。病毒缺乏细胞质、核糖体及代谢系统，因此必须侵入宿主细胞才能复制。

In the context of cell structure, WJEC may ask you to explain why viruses are not considered cells: they do not have a cellular organisation, do not carry out metabolic reactions independently, and cannot reproduce without hijacking host machinery. Be able to contrast viral structure with that of bacteria.

在细胞结构背景下，WJEC 可能会要求说明病毒为何不算细胞：它们不具有细胞构造，不能独立进行代谢反应，若不劫持宿主机制则无法繁殖。要能将病毒结构与细菌结构进行对比。

12. Exam Tips and Common Mistakes | 考试技巧与常见错误

When labelling organelles on diagrams, use rulers and ensure label lines point precisely to the structure. For electron micrographs, distinguish between organelles by their characteristic appearance; for instance, mitochondria are round with folded cristae, rough ER shows parallel, ribosome-studded membranes.

在图上标注细胞器时使用直尺，确保标注线精确指向结构。在电子显微照片中，通过特征区分细胞器；例如线粒体呈圆形且有折叠嵴，粗面内质网为平行的、附着核糖体的膜。

Avoid vague language like ‘mitochondria provide energy’; instead write ‘mitochondria are the sites of aerobic respiration, producing ATP through oxidative phosphorylation’. Similarly, do not say the Golgi body ‘sends’ proteins; describe it as ‘chemically modifying proteins and packaging them into vesicles for secretion’. Using accurate scientific vocabulary gains marks.

避免使用模糊语言，如“线粒体提供能量”；应写明“线粒体是有氧呼吸的场所，通过氧化磷酸化产生 ATP”。同样，不要说高尔基体“送出”蛋白质，而应描述为“化学修饰蛋白质并将其包裹入囊泡以备分泌”。准确的科学术语能够得分。

Common pitfalls: confusing resolution with magnification, muddling 70S and 80S ribosomes, and forgetting that plant cells lack centrioles (except in some lower plants). Also, be precise about membrane structures: the nuclear envelope is a double membrane, whereas the mitochondrial inner membrane is highly folded but still a double membrane overall. Draw comparison tables to aid revision.

常见陷阱：混淆分辨率与放大倍数，分不清 70S 和 80S 核糖体，忘记植物细胞没有中心粒（某些低等植物除外）。同时要精确描述膜结构：核膜为双层膜，线粒体内膜高度折叠，但整体仍是双层膜。绘制比较表格辅助复习。

Practise explaining the relationships between organelles, such as how a protein is synthesised on the rough ER, processed in the Golgi, and secreted via exocytosis. These pathway questions are common in WJEC papers.

练习解释细胞器之间的联系，例如蛋白质如何在粗面内质网合成，经高尔基体加工，再通过胞吐分泌。这类通路问题在 WJEC 试卷中很常见。

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A-Level WJEC Biology: Cell Structure Key Exam Focus | A-Level WJEC 生物：细胞结构 考点精讲