📚 A-Level CCEA Biology: Organelles – Key Points | 细胞器考点精讲
Organelles are the specialised, membrane-bound or non-membrane-bound compartments inside a cell that each perform distinct biochemical roles. For the CCEA A-Level Biology course, you must know the ultrastructure of eukaryotic organelles, link their structures to functions, and compare them with prokaryotic organisation. This revision guide walks through every important organelle, highlighting examiner favourite details.
细胞器是细胞内部特化的、有膜或无膜包裹的区室，各自执行不同的生化功能。在 CCEA A-Level 生物学课程中，你需要掌握真核细胞器的超微结构，理解结构如何决定功能，并能与原核细胞进行比较。本复习指南逐一梳理重要细胞器，点出考官偏爱的细节。
1. Overview and Classification of Organelles | 细胞器概述与分类
Eukaryotic cells possess both membrane-bound organelles (nucleus, mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, lysosomes) and non-membrane-bound organelles (ribosomes, centrioles, cytoskeletal fibres). These compartments allow segregation of incompatible metabolic reactions and create optimised microenvironments.
真核细胞既有膜结合的细胞器（细胞核、线粒体、叶绿体、内质网、高尔基体、溶酶体），也有无膜细胞器（核糖体、中心粒、细胞骨架纤维）。这种区室化使不相容的代谢反应得以分隔，并创造出优化的微环境。
Membrane-bound organelles are formed from phospholipid bilayers with embedded proteins, enabling controlled transport via vesicles and carrier proteins. Non-membrane organelles rely on protein-protein or protein-RNA interactions for their assembly.
膜结合的细胞器由磷脂双分子层和嵌入蛋白质构成，可通过囊泡和载体蛋白实现受控运输。无膜细胞器则依赖蛋白质-蛋白质或蛋白质-RNA相互作用完成组装。
| Organelle Type | Examples | Key Feature |
|---|---|---|
| Membrane-bound (膜结合) | Nucleus, mitochondria, chloroplasts, ER, Golgi, lysosomes | Internal chemistry isolated from cytosol (内环境与胞质隔离) |
| Non-membrane-bound (无膜) | Ribosomes, centrioles, microtubules | Assembled from protein/RNA subunits (由蛋白质/RNA亚基组装) |
2. Nucleus | 细胞核
The nucleus is the largest organelle in eukaryotic cells, bounded by a double membrane called the nuclear envelope. The outer membrane is continuous with the rough endoplasmic reticulum, emphasising the functional link between gene expression and protein processing.
细胞核是真核细胞中最大的细胞器，由双层核被膜包裹。外膜与粗面内质网相连，这凸显了基因表达与蛋白质加工之间的功能联系。
Nuclear pores perforate the envelope, acting as selective gates for the passage of messenger RNA (mRNA), transfer RNA (tRNA), ribosomal subunits, and regulatory proteins. Small molecules diffuse freely, but macromolecules require active transport.
核被膜上分布着核孔，它们作为选择性门控，控制信使 RNA (mRNA)、转运 RNA (tRNA)、核糖体亚基和调控蛋白的进出。小分子可自由扩散，但大分子需要主动运输。
Inside, genetic material resides as chromatin – DNA wound around histone proteins. During cell division, chromatin condenses into visible chromosomes. The dark-staining nucleolus synthesises ribosomal RNA (rRNA) and assembles it with ribosomal proteins imported from the cytoplasm.
在核内，遗传物质以染色质形式存在——即 DNA 缠绕在组蛋白上。细胞分裂时，染色质凝缩为可见的染色体。深染的核仁负责合成核糖体 RNA (rRNA)，并与从细胞质输入的核糖体蛋白进行组装。
3. Mitochondria | 线粒体
Mitochondria are the sites of aerobic respiration, generating the majority of a cell’s ATP. They are rod-shaped, 1-10 µm long, and bounded by a smooth outer membrane and a highly folded inner membrane whose folds are called cristae.
线粒体是有氧呼吸的场所，产生细胞中绝大多数的 ATP。它们呈杆状，长 1–10 µm，由光滑的外膜和高度折叠的内膜包围，内膜的折叠称为嵴。
Cristae greatly increase the surface area for the electron transport chain and ATP synthase complexes. The inner membrane encloses the mitochondrial matrix, which contains enzymes for the Krebs cycle, mitochondrial DNA (mtDNA), and 70S ribosomes.
嵴极大地增加了电子传递链和 ATP 合酶复合体所需的内膜表面积。内膜包裹着线粒体基质，其中包含柠檬酸循环所需的酶、线粒体 DNA (mtDNA) 和 70S 核糖体。
The presence of mtDNA and 70S ribosomes enables mitochondria to transcribe and translate some of their own proteins, providing evidence for their endosymbiotic origin. The overall phosphorylation reaction can be summarised as:
mtDNA 和 70S 核糖体的存在使线粒体能够自行转录和翻译部分蛋白质，这为内共生起源提供了证据。整个磷酸化反应可总结为：
ADP + Pi + energy → ATP + H₂O
- Outer membrane contains porins, making it permeable to small molecules. (外膜含有孔蛋白，对小分子可通透。)
- Intermembrane space accumulates H⁺ ions, building a proton motive force. (膜间隙积累 H⁺，建立质子动力势。)
- Matrix hosts pyruvate oxidation and the Krebs cycle. (基质中进行丙酮酸氧化和柠檬酸循环。)
4. Chloroplasts | 叶绿体
Chloroplasts are the photosynthetic organelles of plant and algal cells, converting light energy into chemical energy stored in carbohydrates. They are lens-shaped, 2-10 µm in diameter, and belong to the plastid family.
叶绿体是植物和藻类细胞的光合细胞器，将光能转化为储存在碳水化合物中的化学能。它们呈透镜形，直径 2–10 µm，属于质体家族。
Each chloroplast is enclosed by a double membrane – the chloroplast envelope – and contains an internal membrane system of flattened sacs called thylakoids. A stack of thylakoids is a granum (plural grana), and the fluid-filled stroma surrounds them.
每个叶绿体由双层膜——叶绿体包膜包围，内部含有一系列扁平囊状结构，称为类囊体。一叠类囊体称为基粒，基粒周围是充满液体的基质。
Thylakoid membranes house chlorophyll, accessory pigments, and the photosystems needed for the light-dependent reactions. The stroma contains enzymes for the Calvin cycle, along with its own circular DNA and 70S ribosomes, again hinting at an endosymbiotic origin.
类囊体膜上嵌有叶绿素、辅助色素以及光依赖反应所需的光系统。基质中含有卡尔文循环的酶，还有自身的环状 DNA 和 70S 核糖体，这再次暗示内共生起源。
The overall photosynthetic equation is:
光合作用的总方程式为：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Note: Oxygen released originates from water molecules, not carbon dioxide. (注意：释放的氧气来源于水分子，而非二氧化碳。)
5. Endoplasmic Reticulum (ER) | 内质网
The endoplasmic reticulum is a network of membranous tubules and flattened sacs called cisternae, extending from the nuclear envelope throughout the cytoplasm. It exists in two forms: rough ER (RER) and smooth ER (SER).
内质网是由膜管和扁平囊状结构（腔）构成的网络，从核被膜延伸至整个细胞质。它有两种形式：粗面内质网 (RER) 和滑面内质网 (SER)。
Rough ER is studded with 80S ribosomes on its cytoplasmic surface, making it the primary site of protein synthesis for proteins destined for secretion, membrane insertion, or lysosomal delivery. As polypeptides are synthesised, they enter the ER lumen where chaperones assist folding and enzymes carry out initial glycosylation.
粗面内质网在胞质面布满 80S 核糖体，是分泌蛋白、膜蛋白或溶酶体蛋白合成的主要场所。多肽链合成后进入内质网腔，在此分子伴侣协助折叠，酶进行初步糖基化。
Smooth ER lacks ribosomes and functions in lipid and steroid synthesis, carbohydrate metabolism, detoxification of drugs and poisons (especially in liver cells), and calcium ion storage in muscle cells (sarcoplasmic reticulum).
滑面内质网没有核糖体，其功能包括脂质和类固醇合成、碳水化合物代谢、药物与毒物的解毒（尤其在肝细胞中），以及肌细胞中的钙离子储存（肌质网）。
6. Golgi Apparatus | 高尔基体
The Golgi apparatus consists of stacked, flattened cisternae with a forming face (cis face, toward the ER) and a maturing face (trans face, toward the plasma membrane). It receives transport vesicles from the ER, modifies their protein and lipid cargo, then sorts them into secretory vesicles for delivery.
高尔基体由堆叠的扁平膜囊组成，有形成面（顺面，朝向内质网）和成熟面（反面，朝向质膜）。它接收来自内质网的运输囊泡，对蛋白质和脂质货物进行修饰，然后将其分拣至分泌囊泡中向外递送。
Key post-translational modifications occurring in the Golgi include further glycosylation to produce glycoproteins, sulphation, and proteolytic cleavage of inactive precursors (e.g., proinsulin to insulin). The Golgi also synthesises lysosomes and contributes to cell wall formation in plants by producing pectin and hemicellulose.
高尔基体中发生的关键翻译后修饰包括进一步糖基化生成糖蛋白、硫酸化，以及无活性前体蛋白的水解剪接（如前胰岛素剪接为胰岛素）。高尔基体还合成溶酶体，在植物中通过制造果胶和半纤维素参与细胞壁形成。
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 around 5. They bud from the Golgi apparatus and function as the cell’s digestive system, breaking down worn-out organelles (autophagy), engulfed particles (phagocytosis), and macromolecules.
溶酶体是含有水解酶（如蛋白酶、脂肪酶、核酸酶）的膜包裹细胞器，这些酶在 pH 约 5 的酸性环境中具有活性。它们从高尔基体出芽形成，作为细胞的消化系统，降解衰老细胞器（自噬）、吞噬颗粒（吞噬作用）以及大分子物质。
Peroxisomes are smaller, single-membrane organelles that contain oxidases and catalase. They perform fatty acid β-oxidation and detoxify hydrogen peroxide (H₂O₂) by converting it to water and oxygen via the reaction:
过氧化物酶体是更小的单层膜细胞器，含有氧化酶和过氧化氢酶。它们进行脂肪酸 β-氧化，并通过将过氧化氢 (H₂O₂) 转化为水和氧气的反应来解毒：
2H₂O₂ → 2H₂O + O₂
8. Ribosomes | 核糖体
Ribosomes are non-membrane-bound complexes composed of ribosomal RNA (rRNA) and proteins. They translate messenger RNA (mRNA) into polypeptide chains. Each ribosome consists of a small subunit and a large subunit, which assemble around mRNA during translation.
核糖体是无膜复合体，由核糖体 RNA (rRNA) 和蛋白质组成。它们将信使 RNA (mRNA) 翻译为多肽链。每个核糖体由一个小亚基和一个大亚基构成，翻译时围绕 mRNA 组装。
Eukaryotic ribosomes are 80S (40S + 60S subunits), while prokaryotic ribosomes, as well as those in mitochondria and chloroplasts, are 70S (30S + 50S). Free ribosomes in the cytosol synthesise proteins for intracellular use, whereas ribosomes attached to the rough ER synthesise proteins for secretion or membrane placement.
真核细胞的核糖体为 80S（40S + 60S 亚基），而原核细胞以及线粒体和叶绿体中的核糖体为 70S（30S + 50S）。胞质中的游离核糖体合成供细胞自身使用的蛋白质，附着在粗面内质网上的核糖体则合成用于分泌或定位到膜上的蛋白质。
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 sometimes toxic compounds for defence.
植物细胞通常含有一个大型的、永久性中央液泡，
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