📚 Organelles in A-Level CIE Biology: Key Points | A-Level CIE 生物：细胞器 考点精讲
Mastering the structure and function of eukaryotic organelles is a cornerstone of the CIE A-Level Biology syllabus. This detailed revision guide breaks down every key organelle, compares eukaryotic and prokaryotic cells, and explains how viruses challenge the conventional idea of a cell. Carefully matched English–Chinese paragraphs will help you grasp the precise terminology and build exam-ready understanding.
掌握真核细胞器的结构和功能是 CIE A-Level 生物大纲的核心。这篇详细复习指南逐一剖析每个关键细胞器，比较真核与原核细胞，并解释病毒如何挑战传统的细胞概念。精心配对的英中段落将帮助你掌握精确术语，建立应试所需的全面理解。
1. Overview of Cell Organelles | 细胞器总览
In eukaryotic cells, membrane-bound compartments called organelles perform specialised tasks, allowing division of labour. The nucleus stores genetic information, mitochondria generate ATP, ribosomes synthesise proteins, and the endomembrane system (endoplasmic reticulum, Golgi body, vesicles) modifies and transports molecules. CIE exam questions often ask you to relate the structure of an organelle to its function.
在真核细胞中，称为细胞器的膜包被隔室执行特定任务，实现了分工合作。细胞核储存遗传信息，线粒体产生 ATP，核糖体合成蛋白质，内膜系统（内质网、高尔基体、囊泡）修饰和运输分子。CIE 考题常要求你将细胞器的结构与其功能联系起来。
Remember that not all organelles are bound by a single membrane: ribosomes are non-membranous, mitochondria and chloroplasts have double membranes, and the nucleus has a nuclear envelope with two membranes. The concept of compartmentalisation is crucial for regulating metabolic pathways, maintaining different internal environments, and increasing efficiency.
请记住，并非所有细胞器都由单层膜包裹：核糖体无膜，线粒体和叶绿体具有双层膜，细胞核则有双层核膜。区室化的概念对于调节代谢途径、维持不同的内部环境和提高效率至关重要。
2. The Nucleus | 细胞核
The nucleus is the largest organelle and contains nearly all the cell’s genetic material in the form of chromatin (DNA wrapped around histone proteins). It is surrounded by a nuclear envelope consisting of two phospholipid bilayers perforated by nuclear pores. These pores allow mRNA and ribosome subunits to exit and certain proteins to enter. The nucleolus, a dense region within the nucleus, is the site of ribosomal RNA (rRNA) synthesis and ribosome assembly.
细胞核是最大的细胞器，几乎包含细胞所有的遗传物质，以染色质（DNA 缠绕组蛋白）的形式存在。它由双层磷脂双层构成的核膜包围，核膜上分布着核孔。核孔允许 mRNA 和核糖体亚基输出，以及特定蛋白质进入。核仁是细胞核内一个致密区域，是核糖体 RNA (rRNA) 合成和核糖体组装的场所。
In an exam answer, you must connect these features to the central role of the nucleus: controlling gene expression and mediating the transmission of genetic information during cell division. The separation of transcription (in the nucleus) from translation (on ribosomes in the cytoplasm) is a key distinction in eukaryotes.
在考试作答时，你必须将这些结构特征与细胞核的核心作用联系起来：调控基因表达，并在细胞分裂时介导遗传信息的传递。转录（在细胞核内）与翻译（在细胞质核糖体上）的分隔是真核生物的一个关键特征。
3. Mitochondria | 线粒体
Mitochondria are often described as the powerhouses of the cell because they produce most of the cell’s ATP via aerobic respiration. They are oval-shaped organelles with two membranes: a smooth outer membrane and a highly folded inner membrane. The folds, called cristae, greatly increase the surface area for the electron transport chain and ATP synthase. The matrix enclosed by the inner membrane contains enzymes for the Krebs cycle, as well as mitochondrial DNA and ribosomes.
线粒体常被称为细胞的能量工厂，因为它们通过有氧呼吸产生细胞的大部分 ATP。它们是卵形细胞器，具有两层膜：光滑的外膜和高度折叠的内膜。内膜的折叠称为嵴，极大地增加了电子传递链和 ATP 合酶所需的表面积。内膜围成的基质含有克雷布斯循环的酶，以及线粒体 DNA 和核糖体。
The presence of their own circular DNA and 70S ribosomes supports the endosymbiotic theory, which states that mitochondria evolved from free-living prokaryotes that were engulfed by ancestral eukaryotic cells. In CIE papers, you may be asked to explain how the inner membrane is adapted for oxidative phosphorylation.
它含有自己的环状 DNA 和 70S 核糖体，这支持了内共生学说，即线粒体源于被原始真核细胞吞噬的独立生活的原核生物。在 CIE 考卷中，你可能会被要求解释内膜如何适应于氧化磷酸化过程。
4. Chloroplasts | 叶绿体
Chloroplasts are the sites of photosynthesis in plant cells and some algae. Like mitochondria, they have a double membrane and their own DNA and 70S ribosomes, also consistent with endosymbiosis. Inside the chloroplast, a system of flattened, membrane-bound sacs called thylakoids is arranged in stacks known as grana. The thylakoid membranes contain chlorophyll and carry out the light-dependent reactions. The stroma, the fluid-filled space surrounding the thylakoids, contains enzymes for the Calvin cycle and other synthetic reactions.
叶绿体是植物细胞和某些藻类进行光合作用的场所。与线粒体相似，它们也具有双层膜、自己的 DNA 和 70S 核糖体，同样符合内共生学说。在叶绿体内部，由扁平的膜包囊状结构——类囊体——堆叠成基粒。类囊体膜含有叶绿素，进行光依赖反应。基质是围绕类囊体的充液空间，含有卡尔文循环和其他合成反应所需的酶。
Under a light microscope, grana are visible as green dots; electron micrographs reveal the detailed organisation. When describing the structure–function relationship, always link the large surface area of thylakoids to light absorption, and the stroma to the water-soluble enzymes of the Calvin cycle.
在光学显微镜下，基粒显示为绿色的点；电子显微照片可揭示其细微结构。在描述结构–功能关系时，始终将类囊体的大表面积与光吸收联系起来，将基质与卡尔文循环的水溶性酶联系起来。
5. Endoplasmic Reticulum (ER) | 内质网
The endoplasmic reticulum is a network of interconnected membrane sheets and tubules that extends from the nuclear envelope throughout the cytoplasm. Rough ER (RER) is studded with ribosomes on its cytosolic surface; these ribosomes synthesise proteins destined for secretion, for incorporation into membranes, or for use in lysosomes. The lumen of the RER folds and modifies these proteins. Smooth ER (SER) lacks ribosomes and is involved in lipid synthesis, detoxification of drugs and poisons, and calcium ion storage.
内质网是一个由相互连接的膜片和管道组成的网络，从核膜延伸到整个细胞质。粗糙内质网 (RER) 在其胞质面上附着有核糖体；这些核糖体合成用于分泌、插入膜中或进入溶酶体的蛋白质。RER 腔对这些蛋白质进行折叠和修饰。滑面内质网 (SER) 无核糖体，参与脂质合成、药物和毒物的解毒以及钙离子储存。
In exam answers, be precise: the RER provides a pathway for transporting proteins via vesicles to the Golgi body. The extensive membrane surface of the SER in liver cells, for instance, is an adaptation for efficient detoxification.
考试作答时需精确：RER 提供了一条通过囊泡将蛋白质运输到高尔基体的通路。例如，肝细胞中 SER 膜面积的增大是为了高效解毒的一种适应。
6. The Golgi Apparatus | 高尔基体
The Golgi apparatus (or Golgi body) is a stack of flattened, membrane-bound sacs called cisternae. It receives proteins and lipids from the ER in transport vesicles at its cis face, modifies them (e.g., by adding carbohydrate groups to form glycoproteins), packages them, and dispatches them in vesicles that bud off from its trans face. These vesicles may deliver their contents to the plasma membrane for secretion, to lysosomes, or to other organelles.
高尔基体（或称高尔基器）是一叠扁平的膜包囊状结构，称为扁平囊。它在其顺面接收来自内质网的运输囊泡中的蛋白质和脂质，对其进行修饰（例如添加糖基以形成糖蛋白），包装它们，并从其反面以出芽形成的囊泡将它们派遣出去。这些囊泡可能将其内容物运送到细胞膜以供分泌，或送至溶酶体及其他细胞器。
CIE questions might ask you to describe the sequence of events from protein synthesis on the RER to secretion via the Golgi. Emphasise the role of the Golgi as a processing, sorting, and dispatching centre.
CIE 考题可能会让你描述从 RER 上蛋白质合成到经由高尔基体分泌的全过程。要强调高尔基体作为加工、分拣和转运中心的作用。
7. Lysosomes | 溶酶体
Lysosomes are spherical, single-membrane organelles containing hydrolytic enzymes that break down proteins, nucleic acids, lipids, and carbohydrates. These enzymes function optimally at an acidic pH of about 5.0 maintained inside the lysosome by proton pumps. Lysosomes digest material taken into the cell by endocytosis, recycle worn-out organelles through autophagy, and can trigger programmed cell death (apoptosis).
溶酶体是球形的单层膜细胞器，含有水解酶，可分解蛋白质、核酸、脂质和碳水化合物。这些酶在溶酶体内由质子泵维持的约 pH 5.0 的酸性环境中发挥最佳活性。溶酶体消化通过内吞作用进入细胞的物质，通过自噬回收破损的细胞器，并可引发程序性细胞死亡（凋亡）。
In phagocytic white blood cells, lysosomes fuse with phagosomes to destroy ingested pathogens. The containment of powerful digestive enzymes within a membrane protects the rest of the cell from damage.
在吞噬性白细胞中，溶酶体与吞噬体融合，以消灭摄入的病原体。强大的消化酶被包裹在膜内，保护细胞其余部分免受损伤。
8. Ribosomes | 核糖体
Ribosomes are not membrane-bound; they consist of two subunits made of rRNA and proteins. Eukaryotic ribosomes are 80S (composed of 40S and 60S subunits), while prokaryotic ribosomes, mitochondrial ribosomes, and chloroplast ribosomes are 70S. Ribosomes translate mRNA into polypeptide chains. Free ribosomes in the cytosol produce proteins that function within the cytoplasm, while ribosomes bound to the RER manufacture secretory and membrane proteins.
核糖体无膜包裹，由 rRNA 和蛋白质组成的两个亚基构成。真核生物的核糖体是 80S（由 40S 和 60S 亚基组成），而原核生物、线粒体和叶绿体的核糖体是 70S。核糖体将 mRNA 翻译为多肽链。细胞质中的游离核糖体生产在胞质内发挥作用的蛋白质，而附着于 RER 上的核糖体则制造分泌蛋白和膜蛋白。
This difference in sedimentation coefficient (S) is a classic exam point. Antibiotics such as tetracycline can selectively target 70S ribosomes, disrupting bacterial protein synthesis without harming the host's 80S ribosomes.
沉降系数 (S) 的差异是经典的考点。四环素等抗生素能选择性地作用于 70S 核糖体，破坏细菌蛋白质合成而不伤害宿主的 80S 核糖体。
9. Vacuoles | 液泡
Plant cells typically possess a large, permanent central vacuole surrounded by a single membrane called the tonoplast. This vacuole stores water, ions, sugars, pigments, and waste products. Its most important role is maintaining turgor pressure against the cell wall, which supports the plant and keeps cells firm. Animal cells may have small, transient vacuoles or vesicles for transport and storage, but never a single large central vacuole.
植物细胞通常含有一个大型的永久性中央液泡，由单层膜——液泡膜——包裹。该液泡储存水、离子、糖、色素和废物。它最重要的作用是维持对细胞壁的膨压，从而支撑植物并使细胞保持坚挺。动物细胞可能有小型的临时液泡或囊泡用于运输和储存，但绝不会有一个单独的大中央液泡。
In protists, contractile vacuoles collect and expel excess water to osmoregulate, a feature often highlighted in CIE practical-related questions on unicellular organisms.
在原生生物中，收缩泡收集并排出多余水分以进行渗透调节，这一特征常常在 CIE 涉及单细胞生物的实践相关题目中出现。
10. The Cytoskeleton | 细胞骨架
The cytoskeleton is a dynamic network of protein filaments that provides mechanical support, maintains cell shape, and enables movement. It comprises three main types: microtubules (made of tubulin), microfilaments (actin filaments), and intermediate filaments. Microtubules form the spindle apparatus during mitosis, serve as tracks for motor proteins (kinesin, dynein) to transport vesicles, and are the core of cilia and flagella. Microfilaments drive cell crawling and cytokinesis.
细胞骨架是由蛋白质纤维构成的动态网络，提供机械支撑、维持细胞形状并实现运动。它包含三种主要类型：微管（由微管蛋白构成）、微丝（肌动蛋白丝）和中间丝。微管在有丝分裂时形成纺锤体，充当马达蛋白（驱动蛋白、动力蛋白）运输囊泡的轨道，也是纤毛和鞭毛的核心。微丝驱动细胞爬行和胞质分裂。
CIE expects you to link the 9+2 arrangement of microtubules in cilia and flagella to their bending mechanism, and to appreciate that the cytoskeleton is not a permanent, rigid structure but constantly remodels.
CIE 期望你能将纤毛和鞭毛中微管的“9+2”排列与其弯曲机制联系起来，并理解细胞骨架并非永久性刚性结构，而是不断重塑的。
11. Prokaryotic vs Eukaryotic Cells | 原核细胞与真核细胞的比较
Prokaryotic cells (bacteria and archaea) are much smaller than typical eukaryotic cells and lack a true nucleus. Their DNA is a single circular chromosome lying free in a region called the nucleoid, and they may contain small loops of DNA called plasmids. Prokaryotes have 70S ribosomes, no membrane-bound organelles, and a cell wall made of peptidoglycan (in bacteria). Some possess a capsule, flagella with a rotary structure, and pili.
原核细胞（细菌和古菌）比典型的真核细胞小得多，且无真正的细胞核。它们的 DNA 是单个环状染色体，游离在称为拟核的区域，并可能含有称为质粒的小环状 DNA。原核生物具有 70S 核糖体，没有膜包裹的细胞器，细胞壁由肽聚糖构成（细菌）。有些具有荚膜、旋转结构的鞭毛和菌毛。
In contrast, eukaryotic cells have a nucleus enclosed by an envelope, 80S ribosomes, membrane-bound organelles, and, in plants and fungi, cell walls of cellulose and chitin respectively. A common exam question asks you to list several differences or to interpret electron micrographs to identify cell type.
相比之下，真核细胞具有核膜包围的细胞核、80S 核糖体、膜包裹的细胞器，并且在植物和真菌中，细胞壁分别由纤维素和几丁质构成。常见的考题要求你列出若干区别，或通过电子显微照片判断细胞类型。
| Feature | Eukaryote | Prokaryote |
|---|---|---|
| Nucleus | Present, with double membrane | Absent; nucleoid region |
| DNA | Linear, associated with histones | Circular, naked; plasmids common |
| Ribosomes | 80S (except organelles) | 70S |
| Membrane-bound organelles | Present (mitochondria, ER, etc.) | Absent |
| Cell wall (if present) | Cellulose (plants), chitin (fungi) | Peptidoglycan (bacteria) |
| Size | Typically 10–100 μm | Typically 0.5–5 μm |
12. Viruses: Acellular Entities | 病毒：非细胞实体
Viruses are not cells and are not made of organelles, yet CIE expects you to know their basic structure and to compare them with cellular life. A virus consists of genetic material (either DNA or RNA, never both) enclosed in a protein coat called a capsid. Some viruses have an additional lipid envelope derived from the host cell membrane. Viruses lack ribosomes, cytoplasm, and a metabolic system; they cannot carry out protein synthesis or generate ATP independently and must infect a host cell to replicate.
病毒不是细胞，也不由细胞器构成，但 CIE 期望你了解它们的基本结构，并与细胞生物进行比较。病毒由遗传物质（DNA 或 RNA，不同时具备两者）和包裹它的蛋白质外壳——衣壳——组成。部分病毒还有一个来自宿主细胞膜的额外脂质包膜。病毒缺乏核糖体、细胞质和代谢系统；它们不能独立进行蛋白质合成或产生 ATP，必须感染宿主细胞才能复制。
Understanding why viruses are considered non-living helps reinforce the characteristics of life that cells exhibit. In essays, you can contrast the simplicity of a virus with the complexity of even the smallest prokaryote.
理解为什么病毒被视为非生命体，有助于强化细胞所表现出的生命特征。在长篇问答中，你可以将病毒的简单性与即使是最小的原核生物的复杂性进行对比。
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