📚 A-Level Biology: Organelles Key Points Review | 细胞器考点精讲
Cell organelles are the specialised subunits within a cell that perform specific functions essential for life. In A-Level Biology, understanding the structure and function of these organelles is fundamental to grasping how cells maintain homeostasis, produce energy, synthesise proteins, and replicate. This article provides a comprehensive yet concise review of the key organelles examined at A-Level, with a focus on eukaryotic cells while also clarifying the differences with prokaryotes. Mastering these concepts will not only help in exams but also deepen your appreciation of cellular organisation.
细胞器是细胞内具有特定功能的专门亚单位，对维持生命至关重要。在A-Level生物学中，理解这些细胞器的结构和功能是掌握细胞如何维持稳态、产生能量、合成蛋白质以及复制的基础。本文全面而简明地回顾了A-Level考试中重点考查的细胞器，重点介绍真核细胞，并明确了与原核细胞的区别。掌握这些概念不仅有助于考试，还会加深你对细胞组织结构的理解。
1. Introduction to Cell Organelles | 细胞器概述
Eukaryotic cells contain numerous membrane-bound organelles, each compartmentalising specific biochemical processes. This compartmentalisation increases efficiency by preventing interference between incompatible reactions, and allows optimal conditions to be maintained in each organelle. The organelles studied at A-Level include the nucleus, mitochondria, ribosomes, rough and smooth endoplasmic reticulum, Golgi apparatus, lysosomes, chloroplasts, permanent vacuole, and centrioles. Prokaryotic cells, by contrast, lack membrane-bound organelles and have a much simpler structure.
真核细胞含有许多由膜包被的细胞器，每个细胞器分隔特定的生化过程。这种分隔通过防止不相容反应之间的干扰而提高了效率，并使每种细胞器内能维持最佳条件。A-Level课程中学习的细胞器包括细胞核、线粒体、核糖体、粗面和滑面内质网、高尔基体、溶酶体、叶绿体、中央大液泡和中心粒。相反，原核细胞缺乏膜结合的细胞器，结构要简单得多。
The presence or absence of organelles is a key distinction between eukaryotic and prokaryotic organisms. For example, bacteria (prokaryotes) have no nucleus, mitochondria, or chloroplasts; their DNA is free in the cytoplasm, and energy production occurs across the cell membrane. In contrast, eukaryotes, including animal and plant cells, fungi, and protists, possess these complex structures. Understanding this difference is a common exam question, often requiring students to draw and label electron micrographs or compare the two cell types in a table.
细胞器的有无是区分真核生物和原核生物的关键。例如，细菌（原核生物）没有细胞核、线粒体或叶绿体；它们的DNA游离在细胞质中，能量产生发生在细胞膜上。相反，真核生物——包括动植物细胞、真菌和原生生物——拥有这些复杂结构。理解这一区别是常见的考试题目，通常要求学生绘制并标注电子显微镜照片，或用表格比较两种细胞类型。
2. Nucleus: Structure and Function | 细胞核的结构与功能
The nucleus is the largest organelle in most eukaryotic cells and serves as the control centre. It is surrounded by a double membrane called the nuclear envelope, which contains numerous nuclear pores. These pores regulate the passage of molecules such as mRNA and ribosomal subunits between the nucleus and the cytoplasm. Inside the nucleus, chromatin (DNA associated with histone proteins) stores the genetic information, and a dense region known as the nucleolus is the site of ribosomal RNA (rRNA) synthesis and ribosome assembly.
细胞核是大多数真核细胞中最大的细胞器，起控制中心的作用。它被称作核被膜的双层膜包围，核被膜上含有许多核孔。这些核孔调控分子（如mRNA和核糖体亚基）在细胞核与细胞质之间的进出。在细胞核内部，染色质（与组蛋白结合的DNA）储存遗传信息，一个致密区域即核仁是核糖体RNA（rRNA）合成和核糖体组装的场所。
Exam questions often ask for the function of the nucleus: to contain the genetic material, to replicate DNA prior to cell division, and to transcribe DNA into mRNA for protein synthesis. The double membrane of the nuclear envelope is continuous with the rough endoplasmic reticulum, and the pores are essential for the transport of large molecules. A common mistake students make is confusing the nucleolus with the nucleus; remember that the nucleolus is a sub-region without a membrane, and its role is exclusively linked to ribosome biogenesis.
考试题目常问细胞核的功能：容纳遗传物质，在细胞分裂前复制DNA，以及将DNA转录为mRNA用于蛋白质合成。核被膜的双层膜与粗面内质网相连，核孔对大分子运输必不可少。学生常犯的错误是将核仁与整个细胞核混淆；记住核仁是一个无膜的亚区域，其作用仅与核糖体的生成相关。
3. Mitochondria: The Powerhouse | 线粒体：细胞的能量工厂
Mitochondria are double-membrane organelles responsible for aerobic respiration, producing adenosine triphosphate (ATP). The inner membrane is highly folded into cristae, which dramatically increase the surface area available for oxidative phosphorylation. The matrix enclosed by the inner membrane contains enzymes for the Krebs cycle, mitochondrial DNA (mtDNA), and ribosomes. The evidence for the endosymbiotic origin of mitochondria includes their double membrane, own circular DNA, and prokaryotic-like ribosomes.
线粒体是双层膜结构的细胞器，负责有氧呼吸并产生三磷酸腺苷（ATP）。其内膜高度折叠形成嵴，极大地增加了用于氧化磷酸化的表面积。内膜包围的基质含有克雷布斯循环所需的酶、线粒体DNA（mtDNA）和核糖体。线粒体内共生起源的证据包括其双层膜、自身的环状DNA和类似原核生物的核糖体。
In A-Level exams, you need to relate the structure of mitochondria to their function. For instance, the intermembrane space allows the build-up of a proton gradient, and ATP synthase enzymes are embedded in the inner membrane. Cells with high energy demands, such as muscle fibres and sperm cells, contain large numbers of mitochondria. When describing the role of mitochondria, always specify that they produce ATP through aerobic respiration, not that they ‘produce energy’ generically.
在A-Level考试中，你需要将线粒体的结构与其功能联系起来。例如，膜间隙允许质子梯度的建立，ATP合酶嵌在内膜上。能量需求高的细胞，如肌纤维和精子细胞，含有大量线粒体。在描述线粒体的作用时，一定要具体说明它们通过有氧呼吸产生ATP，而不是笼统地说“产生能量”。
4. Ribosomes: Protein Synthesis Factories | 核糖体：蛋白质合成工厂
Ribosomes are non-membrane-bound organelles composed of ribosomal RNA and proteins. They consist of a large and a small subunit and are the sites of translation, where the genetic code from mRNA is used to assemble amino acids into polypeptide chains. Ribosomes are found either free in the cytoplasm or attached to the rough endoplasmic reticulum. Free ribosomes typically synthesise proteins for intracellular use, while bound ribosomes produce proteins for secretion or insertion into membranes.
核糖体是无膜包被的细胞器，由核糖体RNA和蛋白质组成。它们由大亚基和小亚基构成，是翻译发生的场所——在这里，mRNA上的遗传密码被用来将氨基酸组装成多肽链。核糖体或游离在细胞质中，或附着在粗面内质网上。游离核糖体通常合成供细胞内使用的蛋白质，而附着核糖体则产生用于分泌或嵌入膜中的蛋白质。
Prokaryotic and eukaryotic ribosomes differ in size: eukaryotes have 80S ribosomes, while prokaryotes have 70S ribosomes. This difference is exploited by antibiotics such as tetracycline, which selectively inhibit bacterial protein synthesis. In exam answers, always clarify that ribosomes themselves do not produce proteins; they provide the platform on which translation occurs, involving mRNA, tRNA, and various translation factors. Emphasise that they are not membrane-bound, so they are present in all living cells.
原核生物与真核生物的核糖体大小不同：真核生物为80S核糖体，原核生物为70S核糖体。这一差异被四环素等抗生素所利用，可选择性地抑制细菌蛋白质合成。在答题时，务必阐明核糖体本身并不“制造”蛋白质；它们提供了翻译发生的平台，涉及mRNA、tRNA和多种翻译因子。要强调它们没有膜包被，因此存在于所有活细胞中。
5. Endoplasmic Reticulum: Rough and Smooth | 内质网：粗面与滑面
The endoplasmic reticulum (ER) is a network of membrane-bound tubules and flattened sacs called cisternae, continuous with the nuclear envelope. Rough ER (RER) is studded with ribosomes on its cytoplasmic surface, giving it a ‘rough’ appearance under an electron microscope. Its primary function is the folding and modification of newly synthesised proteins, as well as their transport in vesicles to the Golgi apparatus. Smooth ER (SER) lacks ribosomes and is involved in lipid synthesis, detoxification of drugs and poisons, and carbohydrate metabolism.
内质网是由膜包被的管状结构和称为潴泡的扁平囊构成的网络，与核被膜相连。粗面内质网在其胞质面布满核糖体，在电子显微镜下呈现“粗糙”外观。其主要功能是对新合成的蛋白质进行折叠和修饰，以及通过囊泡将这些蛋白质运输到高尔基体。滑面内质网没有核糖体，参与脂质合成、药物和毒物的解毒以及碳水化合物代谢。
An exam favourite is the role of the ER in the synthesis of secretory proteins. The polypeptide chain is synthesised by ribosomes on the RER and enters the ER lumen, where chaperone proteins assist folding, and enzymes may add carbohydrate groups (glycosylation). Smooth ER is especially abundant in liver cells (for detoxification) and in cells that secrete steroid hormones (e.g., in the testes and ovaries). When comparing the two, focus on the presence or absence of ribosomes and the distinct functions.
考试常考内质网在分泌蛋白合成中的作用。多肽链由RER上的核糖体合成，进入内质网腔后，伴侣蛋白辅助折叠，酶可能添加碳水化合物基团（糖基化）。滑面内质网在肝细胞（负责解毒）和分泌类固醇激素的细胞（如睾丸和卵巢）中特别丰富。比较两者时，应重点关注有无核糖体以及功能的差异。
6. Golgi Apparatus: Processing and Transport | 高尔基体：加工与转运
The Golgi apparatus consists of a stack of flattened membrane-bound cisternae that are not physically connected. It receives proteins from the rough ER at its cis face and modifies them as they travel through the stack. Modifications include further glycosylation, phosphorylation, and the sorting of proteins into different vesicles at the trans face. These vesicles then transport proteins to their destinations, such as lysosomes, the plasma membrane, or secretory vesicles for exocytosis.
高尔基体由一堆扁平的膜包被潴泡组成，这些潴泡在物理上并不相连。它在顺面接收来自粗面内质网的蛋白质，并在蛋白质穿过潴泡堆的过程中对其进行修饰。修饰过程包括进一步的糖基化、磷酸化，以及在反面将蛋白质分拣到不同的囊泡中。这些囊泡随后将蛋白质运送到目的地，如溶酶体、细胞质膜或用于胞吐的分泌囊泡。
The structure of the Golgi is perfectly adapted for its role: the cis face receives and the trans face dispatches. Enzymes within each cisterna perform stepwise modifications, much like an assembly line. You may be asked to describe the production of extracellular enzymes or glycoproteins, which involves the RER → Golgi → vesicle pathway. Remember that the Golgi also synthesises lysosomes by packaging hydrolytic enzymes into membrane-bound vesicles. It is important to note that the Golgi is not just a transport hub but also a centre for chemical modification of macromolecules.
高尔基体的结构与其功能完美适应：顺面接收蛋白质，反面分派。每个潴泡内的酶进行分步修饰，很像一条装配线。你可能会被要求描述胞外酶或糖蛋白的生成过程，这涉及RER→高尔基体→囊泡的途径。记住，高尔基体还通过将水解酶包装到膜结合的囊泡中来合成溶酶体。重要的是要认识到，高尔基体不仅是运输枢纽，还是大分子化学修饰的中心。
7. Lysosomes: Digestive Compartments | 溶酶体：消化间隔
Lysosomes are spherical membrane-bound organelles containing a variety of hydrolytic enzymes (such as proteases, lipases, and nucleases) that function at an acidic pH optimal for these enzymes. They break down worn-out organelles, engulfed pathogens, and cellular debris. The membrane of the lysosome protects the rest of the cell from these digestive enzymes. Lysosomes are formed by the Golgi apparatus and are particularly abundant in phagocytic cells like macrophages and neutrophils.
溶酶体是球形的膜包被细胞器，含有多种水解酶（如蛋白酶、脂肪酶和核酸酶），这些酶在适合它们的酸性pH环境下发挥功能。它们分解衰老的细胞器、被吞噬的病原体和细胞碎片。溶酶体的膜保护细胞其余部分不受这些消化酶的损害。溶酶体由高尔基体形成，在巨噬细胞和中性粒细胞等吞噬细胞中含量尤其丰富。
A-level questions often link lysosomes to phagocytosis and autophagy. During phagocytosis, a phagosome fuses with a lysosome to form a phagolysosome, where the ingested material is digested. In autophagy, damaged organelles are enclosed in a double membrane, which then fuses with a lysosome. Additionally, lysosomal enzymes are critical in apoptosis (programmed cell death). A common misconception is that lysosomes are present only in animal cells; while rare in plants, plant cells do have lytic vacuoles that perform similar functions.
A-Level题目常将溶酶体与吞噬作用和自噬联系起来。在吞噬过程中，吞噬体与溶酶体融合形成吞噬溶酶体，被摄入的物质在其中消化。在自噬中，受损细胞器被双层膜包裹，随后与溶酶体融合。此外，溶酶体酶在细胞凋亡（程序性细胞死亡）中也至关重要。一个常见的误解是溶酶体只存在于动物细胞中；虽然植物中较为少见，但植物细胞确实具有执行类似功能的溶酶体样液泡。
8. Chloroplasts and Photosynthesis | 叶绿体与光合作用
Chloroplasts are large, double-membrane organelles found in plant cells and algae. They are the site of photosynthesis, where light energy is converted into chemical energy in the form of glucose. Inside the chloroplast, a third membrane system forms disc-shaped structures called thylakoids, which are stacked into grana. The stroma, the fluid surrounding the thylakoids, contains enzymes for the Calvin cycle, as well as chloroplast DNA and ribosomes. The thylakoid membranes house chlorophyll and the electron transport chains required for the light-dependent reactions.
叶绿体是存在于植物细胞和藻类中的大型双层膜细胞器。它们是光合作用的场所，将光能转化为化学能并储存在葡萄糖中。在叶绿体内部，第三套膜系统形成称为类囊体的圆盘状结构，堆叠成基粒。包围类囊体的基质中含有卡尔文循环所需的酶、叶绿体DNA和核糖体。类囊体膜上含有叶绿素以及光依赖性反应所需的电子传递链。
Like mitochondria, chloroplasts contain their own circular DNA and 70S ribosomes, supporting the endosymbiotic theory. In exams, you must link the structure of chloroplasts to their function: the large surface area of thylakoid membranes for light absorption, the proton gradient generated across the thylakoid membrane for ATP synthesis, and the stroma for carbon fixation. Also, note that chloroplasts are a type of plastid; others include amyloplasts for starch storage and chromoplasts for pigment storage.
与线粒体相似，叶绿体含有自身的环状DNA和70S核糖体，支持内共生学说。在考试中，你必须将叶绿体的结构与其功能联系起来：类囊体膜的大面积有利于光吸收，跨类囊体膜形成的质子梯度用于ATP合成，基质则用于碳固定。还需注意，叶绿体是质体的一种；其他还有储存淀粉的造粉体和储存色素的色质体。
9. Vacuoles and Cell Wall in Plant Cells | 植物细胞的液泡与细胞壁
Plant cells are distinguished by the presence of a large central vacuole and a rigid cell wall. The permanent vacuole is a single, fluid-filled sac bound by a membrane called the tonoplast. It stores water, ions, sugars, pigments, and waste products. The vacuole also maintains turgor pressure against the cell wall, which is essential for structural support in non-woody plants. By controlling the osmotic concentration of the vacuole, plant cells can regulate their water potential.
植物细胞的特征是存在一个大型中央液泡和坚硬的细胞壁。中央液泡是一个由称为液泡膜的膜所包围的、充满液体的囊状结构。它储存水分、离子、糖类、色素和废物。液泡还通过向细胞壁施加膨压来维持非木质化植物的结构支撑。通过控制液泡的渗透浓度，植物细胞可以调节其水势。
The cell wall surrounds the plasma membrane and is primarily composed of cellulose. It provides mechanical strength, prevents osmotic lysis, and allows the development of high turgor pressure. The cell wall is fully permeable to water and solutes, unlike the selectively permeable plasma membrane. A-level questions often ask about the role of the cell wall in pathogen defence and as a structural component. Plasmodesmata, channels through the cell wall, allow communication and transport between adjacent plant cells.
细胞壁包围在细胞质膜之外，主要由纤维素构成。它提供机械强度，防止渗透裂解，并允许形成高膨压。细胞壁对水和溶质是完全通透的，这与具有选择透过性的细胞质膜不同。A-Level题目常问细胞壁在病原防御和作为结构组分方面的作用。胞间连丝是穿过细胞壁的通道，允许相邻植物细胞间进行通讯和运输。
10. Centrioles and the Cytoskeleton | 中心粒与细胞骨架
Centrioles are cylindrical organelles found in most animal cells, typically in pairs near the nucleus. Each centriole is composed of nine triplets of microtubules arranged in a ring. They play a crucial role in organising the spindle fibres during cell division (mitosis and meiosis) and are part of the centrosome, the main microtubule-organising centre of animal cells. Centrioles are also involved in the formation of cilia and flagella, where they act as basal bodies.
中心粒是大多数动物细胞中发现的圆柱形细胞器，通常成对存在于细胞核附近。每个中心粒由九组三连微管排列成环状。它们在细胞分裂（有丝分裂和减数分裂）中组织纺锤体纤维，起关键作用，也是动物细胞主要微管组织中心——中心体的一部分。中心粒还参与纤毛和鞭毛的形成，作为基体发挥作用。
Note that plant cells generally lack centrioles, yet still form spindle fibres during division. This is a common comparison point: animal cells have centrioles at the poles of the spindle, while plant cells do not. In exam answers, avoid suggesting that centrioles synthesise the spindle fibres; they merely organise the microtubules. The cytoskeleton as a whole includes microfilaments, intermediate filaments, and microtubules, providing cell shape, mechanical support, and intracellular transport pathways.
注意，植物细胞通常缺乏中心粒，但在分裂时仍能形成纺锤体纤维。这是一个常见的比较点：动物细胞在纺锤体两极有中心粒，而植物细胞没有。在答题时，要避免说中心粒合成纺锤体纤维；它们只是组织微管。整个细胞骨架包括微丝、中间丝和微管，提供细胞形状、机械支撑和胞内运输路径。
11. Prokaryotic vs Eukaryotic Cells | 原核与真核细胞比较
Prokaryotic cells (bacteria and archaea) are much smaller than eukaryotic cells and lack membrane-bound organelles. Their genetic material is a single, circular DNA molecule located in a region called the nucleoid. Plasmids—small circles of DNA—are also often present. Prokaryotes have 70S ribosomes, a cell wall containing peptidoglycan (in bacteria), and some possess a flagellum for movement. They may have infoldings of the plasma membrane called mesosomes, though their function is debated. Respiration and photosynthesis (if present) occur on the cell membrane or its infoldings.
原核细胞（细菌和古菌）比真核细胞小得多，且缺乏膜结合的细胞器。它们的遗传物质是位于称为拟核区域的单个环状DNA分子。通常还存在质粒——小环状DNA。原核生物具有70S核糖体、含有肽聚糖的细胞壁（细菌），有些还拥有用于运动的鞭毛。它们可能有细胞质膜的内褶，称为间体，但其功能存在争议。呼吸作用和光合作用（如果有的话）发生在细胞膜或其内褶上。
Exam questions frequently ask for a comparison of prokaryotic and eukaryotic cells in a table. Include features such as size, DNA form, membrane-bound organelles, ribosome size, cell wall composition, and method of cell division (binary fission vs mitosis). Also be prepared to interpret electron micrographs and identify whether a cell is prokaryotic or eukaryotic based on the presence or absence of a nucleus and other organelles. A key exam tip: do not confuse plasmids with mitochondrial DNA; both are small circles, but their location and function differ.
考试题常要求以表格形式比较原核细胞和真核细胞。列出的特征包括大小、DNA形态、膜结合细胞器、核糖体大小、细胞壁组成以及细胞分裂方式（二分裂与有丝分裂）。还要准备好解读电子显微照片，根据有无细胞核和其他细胞器来识别该细胞是原核还是真核细胞。一个关键的考试技巧：不要把质粒与线粒体DNA混淆；两者都是小环状，但定位和功能不同。
12. Protein Secretion Pathway | 蛋白质分泌途径
The synthesis and secretion of proteins in eukaryotic cells provide an excellent example of how organelles cooperate. The process begins with gene transcription in the nucleus; the mRNA exits via nuclear pores and binds to ribosomes on the rough ER. The polypeptide chain enters the RER lumen, where it folds and undergoes glycosylation. Transport vesicles then carry the protein to the Golgi apparatus for further modification and sorting. Finally, secretory vesicles fuse with the plasma membrane, releasing the protein by exocytosis.
真核细胞中蛋白质的合成与分泌是细胞器如何协同工作的绝佳范例。这一过程开始于细胞核内的基因转录；mRNA通过核孔离开，与粗面内质网上的核糖体结合。多肽链进入RER腔后进行折叠和糖基化。随后，运输囊泡将蛋白质运送到高尔基体进行进一步修饰和分拣。最后，分泌囊泡与细胞质膜融合，通过胞吐作用将蛋白质释放出去。
This pathway is a classic A-Level topic. Make sure you can name the organelles in the correct order: nucleus → RER → Golgi → vesicles → plasma membrane. Energy in the form of ATP from mitochondria is required for the formation and movement of vesicles along the cytoskeleton. Also, be able to explain how the structure of each organelle facilitates these steps: the nuclear pores allow mRNA export, the RER provides a lumen for modification, the Golgi has polarity and packaging ability, and vesicles have specific membrane proteins for targeting and fusion.
这一途径是A-Level的经典主题。务必能按正确顺序说出这些细胞器：细胞核→RER→高尔基体→囊泡→细胞质膜。囊泡沿细胞骨架的形成和移动需要线粒体提供的ATP形式的能量。同时，要能解释每个细胞器的结构如何促进这些步骤：核孔允许mRNA输出，RER提供修饰所需的腔，高尔基体具有极性和包装能力，囊泡具有特定的膜蛋白用于靶向和融合。
13. Endosymbiotic Theory | 内共生学说
The endosymbiotic theory explains the origin of mitochondria and chloroplasts. It proposes that these organelles evolved from free-living prokaryotes that were engulfed by an ancestral eukaryotic cell and established a symbiotic relationship. Evidence supporting this theory includes: both organelles have a double membrane; they contain their own circular DNA, which is not associated with histones; they possess 70S ribosomes, similar to prokaryotes; they replicate by binary fission independently of the host cell cycle; and antibiotics that inhibit bacterial protein synthesis also affect these organelles.
内共生学说解释了线粒体和叶绿体的起源。该学说认为，这些细胞器是由自由生活的原核生物被原始真核细胞吞入，并形成共生关系进化而来。支持这一理论的证据包括：这两种细胞器都具有双层膜；它们含有自身的环状DNA，不与组蛋白结合；它们拥有与原核生物相似的70S核糖体；它们通过二分裂方式独立于宿主细胞周期进行复制；而且抑制细菌蛋白质合成的抗生素同样会作用于这些细胞器。
A-level exam questions often ask for at least three pieces of evidence for the endosymbiotic theory. You must connect each piece of evidence to the conclusion that mitochondria and chloroplasts were once independent organisms. Be precise: saying ‘they have their own DNA’ is insufficient; specify that the DNA is circular and lacks histones, resembling a bacterial chromosome. Remember that the theory does not apply to other organelles like the ER or Golgi, which likely arose from infoldings of the plasma membrane.
A-Level考试常要求列出至少三条支持内共生学说的证据。你必须将每条证据与线粒体和叶绿体曾经是独立生物这一结论联系起来。表达要准确：只说“它们有自己的DNA”是不够的；要具体说明其DNA是环状的，缺乏组蛋白，类似于细菌染色体。记住，该学说不适用于内质网或高尔基体等其他细胞器，它们可能起源于细胞质膜的内褶。
14. Microscopy and Identifying Organelles | 显微技术与细胞器识别
Correctly identifying organelles from electron micrographs is a vital skill for A-Level Biology. In transmission electron microscopy (TEM) images, membrane-bound organelles appear with distinct profiles: the nucleus is large with a double membrane and nuclear pores; mitochondria have a double membrane with cristae; RER appears as flattened sacs with dots (ribosomes); the Golgi shows curved stacks of cisternae; chloroplasts contain grana stacks; and lysosomes appear as dense, spherical vesicles. Knowing these visual cues can save valuable time in the exam.
从电子显微照片中正确识别细胞器是A-Level生物学的一项重要技能。在透射电子显微镜图像中，膜结合细胞器具有独特的轮廓：细胞核大，带有双层膜和核孔；线粒体有双层膜和嵴；粗面内质网呈现为带有小点（核糖体）的扁平囊；高尔基体显示出弯曲的潴泡堆；叶绿体包含基粒堆；溶酶体呈现为致密的球形囊泡。掌握这些视觉标志可在考试中节省宝贵时间。
You may be asked to calculate the actual size of an organelle using a scale bar or magnification formula: Actual size = Image size ÷ Magnification. Always express your answer in appropriate units (µm or nm) and convert carefully. Also, be prepared to describe how artefacts can arise during sample preparation for electron microscopy and how this might influence the interpretation of images. Confusing RER with SER can be avoided by checking for ribosomes; remember, smooth ER has a more tubular, network-like appearance without attached particles.
你可能需要利用比例尺或放大倍数公式计算细胞器的实际大小：实际大小 = 图像大小 ÷ 放大倍数。答案单位务必恰当（µm或nm）并仔细换算。还要准备好描述在制备电镜样品时人工假象如何产生，以及这可能如何影响图像的解读。通过检查核糖体可以避免将RER与SER混淆；记住，滑面内质网外观更加管状、网状，没有附着的颗粒。
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