📚 IB OCR Biology: Organelles – Key Points for Exams | IB OCR 生物：细胞器考点精讲
Understanding the structure and function of cellular organelles is fundamental in both IB and OCR A Level Biology. This article provides a focused revision guide covering all essential organelles, highlighting their roles, key exam points, and common comparisons. Mastery of this topic is crucial for success in Paper 1, Paper 2 and practical assessments.
掌握细胞器的结构和功能是 IB 和 OCR A Level 生物的基础。本文提供重点复习指南，涵盖所有必需细胞器，突出其作用、关键考点和常见比较。掌握此主题对于考试及实验评估至关重要。
1. Overview: Eukaryotic vs. Prokaryotic Cells | 概述：真核与原核细胞
Eukaryotic cells possess a true nucleus and membrane-bound organelles, whereas prokaryotic cells lack these structures. The compartmentalisation provided by organelles allows incompatible biochemical reactions to occur simultaneously in different regions of the cell, a key concept in both IB and OCR specifications.
真核细胞具有真正的细胞核和膜结合的细胞器，而原核细胞则没有。细胞器提供的分区化使得不相容的生化反应能够在细胞的不同区域同时进行，这是 IB 和 OCR 考纲中的核心概念。
Typical eukaryotic cells range from 10–100 µm in diameter, much larger than prokaryotes (1–5 µm). The presence of internal membranes increases the total membrane surface area for metabolic processes. Electron micrographs reveal the ultrastructure of these organelles, and students must be able to identify them from images.
典型的真核细胞直径约 10–100 µm，远大于原核细胞 (1–5 µm)。内膜系统的存在增加了代谢过程所需的膜总表面积。电子显微照片揭示了这些细胞器的超微结构，学生必须能够从图片中识别它们。
Key differences also include the arrangement of genetic material: a linear DNA–histone complex inside a nuclear envelope in eukaryotes, versus a naked circular DNA molecule concentrated in a nucleoid region in prokaryotes. Ribosome size also differs – 80S in eukaryotes, 70S in prokaryotes.
主要区别还包括遗传物质的排列方式：真核生物中线性 DNA–组蛋白复合体位于核膜内，而原核生物中裸露的环状 DNA 集中在拟核区。核糖体大小也不同——真核生物为 80S，原核生物为 70S。
2. Nucleus: The Control Centre | 细胞核：控制中心
The nucleus is the largest organelle in eukaryotic cells, typically 5–10 µm in diameter. It is enclosed by a double membrane called the nuclear envelope, perforated by nuclear pores that regulate the passage of mRNA, ribosomal subunits, and proteins. The outer membrane is continuous with the rough endoplasmic reticulum.
细胞核是真核细胞中最大的细胞器，直径通常为 5–10 µm。它由称为核被膜的双层膜包围，上面布满核孔，调控 mRNA、核糖体亚基和蛋白质的进出。核被膜外膜与粗面内质网相连。
Inside the nucleus, chromatin (DNA associated with histones) exists as euchromatin or heterochromatin. During cell division, chromatin condenses into visible chromosomes. The nucleolus, a dense region within the nucleus, is the site of ribosomal RNA (rRNA) synthesis and ribosome subunit assembly. IB exams often ask for an annotated drawing of the nucleus under an electron microscope.
核内，染色质（与组蛋白结合的 DNA）以常染色质或异染色质形式存在。在细胞分裂期间，染色质浓缩成可见的染色体。核仁是核内的致密区域，是核糖体 RNA (rRNA) 的合成和核糖体亚基组装的场所。IB 考试常要求在电子显微镜照片下对细胞核进行标注绘图。
Functions stored in the nucleus include genetic control of cellular activities, DNA replication, and transcription of genes into mRNA. The nucleus therefore orchestrates protein synthesis indirectly, since mRNA travels through pores to ribosomes in the cytoplasm.
细胞核储存的功能包括对细胞活动的遗传控制、DNA 复制以及将基因转录为 mRNA。因此，细胞核通过调控蛋白质合成来间接指挥细胞活动，因为 mRNA 经由核孔进入细胞质中的核糖体。
3. Mitochondria: Powerhouse and Endosymbiosis | 线粒体：动力工厂与内共生
Mitochondria are organelles with an outer smooth membrane and a highly folded inner membrane forming cristae. The inner membrane encloses the matrix, which contains mitochondrial DNA, ribosomes (70S), and enzymes for the Krebs cycle. The cristae greatly increase the surface area for oxidative phosphorylation and ATP synthesis via the electron transport chain and chemiosmosis.
线粒体是一种具有光滑外膜和高度折叠形成嵴的内膜的细胞器。内膜包裹着基质，基质中含有线粒体 DNA、核糖体 (70S) 以及三羧酸循环所需的酶。嵴极大地增加了氧化磷酸化通过电子传递链和化学渗透合成 ATP 的表面积。
Aerobic respiration is summarised by the equation: glucose + oxygen → carbon dioxide + water + ATP. In mitochondria, the link reaction, Krebs cycle, and the electron transport chain convert the potential energy in glucose into ATP. Cells with high energy demands, such as muscle cells and sperm, contain numerous mitochondria.
有氧呼吸的总反应式可概括为：葡萄糖 + 氧 → 二氧化碳 + 水 + ATP。在线粒体中，连接反应、三羧酸循环以及电子传递链将葡萄糖中的势能转化为 ATP。高能量需求的细胞，如肌细胞和精子，含有大量线粒体。
Evidence for the endosymbiotic origin of mitochondria includes their double membrane, circular DNA without histones, 70S ribosomes, and their method of division (binary fission), resembling bacteria. According to the theory, an ancestral aerobic prokaryote was engulfed by an ancestral eukaryotic cell and established a mutualistic relationship.
支持线粒体内共生起源的证据包括：具双层膜、无组蛋白的环状 DNA、70S 核糖体及其类似于细菌的分裂方式（二分裂）。根据该理论，一种远古的好氧原核生物被远古真核细胞吞噬，并建立了互利关系。
4. Chloroplasts: Photosynthesis and Plant Cells | 叶绿体：光合作用与植物细胞
Chloroplasts are found in plant cells and some protists. They also have a double membrane and contain internal membrane-bound sacs called thylakoids, which stack to form grana. The fluid matrix surrounding the grana is the stroma. Thylakoid membranes house chlorophyll and other photosynthetic pigments, as well as the ATP synthase enzyme.
叶绿体存在于植物细胞和一些原生生物中。它们也具有双层膜，内含称为类囊体的膜囊，类囊体堆叠形成基粒。基粒周围的液态基质称为叶绿体基质。类囊体膜上含有叶绿素和其他光合色素，以及 ATP 合酶。
The overall equation for photosynthesis is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The light-dependent reactions occur on thylakoid membranes, producing ATP and NADPH, while the light-independent reactions (Calvin cycle) take place in the stroma, using ATP and NADPH to fix CO₂ into sugars.
光合作用的总反应式为：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。光反应发生在类囊体膜上，产生 ATP 和 NADPH；而暗反应（卡尔文循环）在基质中进行，利用 ATP 和 NADPH 将 CO₂ 固定成糖类。
Like mitochondria, chloroplasts are believed to have evolved via endosymbiosis. They contain their own circular DNA, 70S ribosomes, and divide by binary fission. In vascular plants, chloroplasts are especially abundant in palisade mesophyll cells, which are the primary photosynthetic tissue.
与线粒体类似，叶绿体也被认为是通过内共生进化而来。它们含有自己的环状 DNA 和 70S 核糖体，并通过二分裂繁殖。在维管植物中，叶绿体在栅栏叶肉细胞中特别丰富，这些是主要的光合组织。
5. Ribosomes: Protein Factories | 核糖体：蛋白质工厂
Ribosomes are non–membrane-bound complexes composed of rRNA and proteins. They exist as two subunits (large and small) and are the sites of protein synthesis (translation) in all cells. Eukaryotes possess 80S ribosomes in the cytoplasm and on the rough ER, while mitochondria and chloroplasts contain 70S ribosomes similar to those of prokaryotes.
核糖体是由 rRNA 和蛋白质组成的无膜结合的复合体。它们以大、小两个亚基存在，是所有细胞中蛋白质合成（翻译）的场所。真核生物细胞质和粗面内质网上具有 80S 核糖体，而线粒体和叶绿体含有与原核生物相似的 70S 核糖体。
Free ribosomes in the cytosol synthesise proteins destined for intracellular use, whereas ribosomes attached to the RER produce proteins for secretion, membrane insertion, or lysosomal delivery. The binding of a ribosome to the ER is mediated by the signal peptide at the N-terminus of the nascent polypeptide.
细胞质基质中的游离核糖体合作用于细胞内使用的蛋白质，而附着在粗面内质网上的核糖体则产生用于分泌、插入膜或递送至溶酶体的蛋白质。核糖体与内质网的结合由新生多肽链 N 端的信号肽介导。
In electron micrographs, ribosomes appear as small dark dots. IB and OCR questions may ask you to calculate relative sizes or interpret images showing polysomes (clusters of ribosomes translating the same mRNA).
在电子显微照片中，核糖体呈小黑点。IB 和 OCR 考题可能要求计算相对大小，或解释显示多聚核糖体（翻译同一 mRNA 的核糖体簇）的图像。
6. Endomembrane System: ER, Golgi, Vesicles | 内膜系统：内质网、高尔基体与囊泡
The endomembrane system consists of the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, vesicles, lysosomes, and the plasma membrane. These components work together to synthesise, modify, package, and transport proteins and lipids.
内膜系统由核被膜、内质网、高尔基体、囊泡、溶酶体和细胞膜组成。这些组分协同工作，合成、修饰、包装并运输蛋白质和脂类。
The rough endoplasmic reticulum (RER) is studded with ribosomes and is involved in folding and glycosylating polypeptide chains. The smooth endoplasmic reticulum (SER) lacks ribosomes and synthesises lipids, phospholipids, and steroids; it also detoxifies drugs and poisons in liver cells. Cells active in secretion, such as pancreatic acinar cells, have extensive RER.
粗面内质网 (RER) 附着有核糖体，参与多肽链的折叠和糖基化。滑面内质网 (SER) 无核糖体，合成脂类、磷脂和类固醇，并在肝细胞中解毒药物和毒物。分泌活动旺盛的细胞，如胰腺腺泡细胞，具有丰富的粗面内质网。
The Golgi apparatus is a stack of flattened, membrane-bound cisternae. It receives transport vesicles from the ER at its cis face, further modifies proteins (e.g. adding carbohydrate groups), sorts them, and packages them into secretory vesicles at the trans face. Vesicles fuse with the plasma membrane to release their contents via exocytosis, or become lysosomes.
高尔基体是一叠扁平、膜包被的潴泡。它在顺面接收来自内质网的运输小泡，进一步修饰蛋白质（如添加碳水化合物基团），分拣后在反面将其包入分泌小泡中。小泡与细胞膜融合，通过胞吐作用释放内含物，或形成溶酶体。
7. Lysosomes and Autophagy | 溶酶体与自噬
Lysosomes are membrane-bound vesicles containing hydrolytic enzymes (e.g. proteases, lipases, nucleases) that are active at acidic pH. They function in intracellular digestion, breaking down worn-out organelles (autophagy), engulfed food particles in phagocytes, and even the entire cell during programmed cell death (apoptosis).
溶酶体是包被单层膜、内含酸性 pH 下具有活性的水解酶（如蛋白酶、脂酶、核酸酶）的囊泡。它们承担胞内消化功能，分解衰老细胞器（自噬）、吞噬细胞摄取的食物颗粒，乃至在程序性细胞死亡（凋亡）时降解整个细胞。
The interior of a lysosome is maintained at around pH 5 by proton pumps in the membrane. This compartmentalisation prevents dangerous digestive enzymes from damaging the cytoplasm. The enzymes are synthesised in the RER, processed in the Golgi, and then targeted to lysosomes via a mannose-6-phosphate tag.
溶酶体内部通过膜上的质子泵维持 pH 在 5 左右。这种区室化防止危险消化酶损害细胞质。这些酶在粗面内质网合成，经高尔基体加工，随后通过甘露糖-6-磷酸标签定向到溶酶体。
In OCR and IB exams, lysosomes are frequently linked to phagocytosis by white blood cells and the turnover of cellular components. Autophagy is a survival mechanism during nutrient starvation.
在 OCR 和 IB 考试中，溶酶体常与白细胞的吞噬作用以及细胞组分的更新相关联。自噬是营养缺乏时的一种生存机制。
8. Vacuoles: Storage and Turgor | 液泡：储存与膨压
Vacuoles are large, membrane-bound sacs that are prominent in plant cells. The central vacuole of a mature plant cell can occupy up to 90% of the cell volume. It is surrounded by a single membrane called the tonoplast and contains cell sap – a solution of water, ions, sugars, pigments, and waste products.
液泡是大型的膜结合囊泡，在植物细胞中尤为突出。成熟植物细胞的中央液泡可占据细胞体积的 90%。它被称作液泡膜的单层膜包围，内含细胞液——水、离子、糖类、色素和废物的混合液。
The vacuole maintains turgor pressure against the cell wall, which is essential for structural support in herbaceous plants. When the vacuole loses water, the cell becomes flaccid and the plant wilts. Vacuoles also sequester harmful substances and can contain pigments such as anthocyanins that attract pollinators.
液泡保持对细胞壁的膨压，对草本植物的结构支撑至关重要。当液泡失水，细胞变得松弛，植株萎蔫。液泡还能隔离有害物质，并可含有花青素等色素，用以吸引传粉者。
In animal cells, vacuoles are generally smaller and more temporary, often involved in endocytosis and exocytosis, such as food vacuoles and contractile vacuoles in some protists. However, they are not a defining feature of animal cells.
在动物细胞中，液泡通常较小且更短暂，往往参与胞吞和胞吐，例如食物泡以及某些原生生物的伸缩泡。然而，它们并非动物细胞的决定性特征。
9. Peroxisomes: Detoxification and Metabolism | 过氧化物酶体：解毒与代谢
Peroxisomes are small, spherical organelles containing oxidative enzymes, particularly catalase and oxidases. They catalyse reactions in which hydrogen peroxide (H₂O₂) is both produced and degraded. Catalase converts toxic H₂O₂ into water and oxygen, protecting the cell from oxidative damage.
过氧化物酶体是含有氧化酶（尤其是过氧化氢酶和氧化酶）的小型球状细胞器。它们催化产生和分解过氧化氢 (H₂O₂) 的反应。过氧化氢酶将有毒的 H₂O₂ 转化为水和氧气，保护细胞免受氧化损伤。
A major metabolic role of peroxisomes is the β-oxidation of very long-chain fatty acids, which then enter mitochondria for further ATP production. In plant seeds, specialised peroxisomes called glyoxysomes convert stored fats into sugars via the glyoxylate cycle during germination.
过氧化物酶体的一个主要代谢作用是对极长链脂肪酸进行 β-氧化，生成较短链的脂肪酸后进入线粒体进一步产生 ATP。在植物种子中，称为乙醛酸循环体的特化过氧化物酶体在萌发期间通过乙醛酸循环将储存的脂肪转化为糖类。
Unlike mitochondria and chloroplasts, peroxisomes do not contain their own DNA or ribosomes; all their proteins are imported from the cytosol. They replicate by fission.
与线粒体和叶绿体不同，过氧化物酶体不含自身的 DNA 或核糖体；其所有蛋白质均从细胞质基质输入。它们通过分裂进行增殖。
10. Centrioles, Cilia and Flagella | 中心粒、纤毛与鞭毛
Centrioles are cylindrical structures composed of nine triplets of microtubules (9+0 arrangement). They are found in the centrosome of animal cells and are involved in organising the spindle fibres during cell division. Plant cells lack centrioles but still form mitotic spindles.
中心粒是由九组三联体微管（9+0 排列）构成的圆柱形结构。它们位于动物细胞的中心体内，参与细胞分裂时纺锤体的组织。植物细胞缺少中心粒，但仍能形成有丝分裂纺锤体。
Cilia and flagella are hair-like projections from the cell surface that contain a 9+2 arrangement of microtubules (nine outer doublets and two central singlets). They are anchored in the cell by basal bodies, which are structurally identical to centrioles. Cilia often move fluids across a cell surface, e.g. in the trachea, while flagella enable cell locomotion, as in sperm cells.
纤毛和鞭毛是细胞表面伸出的毛状结构，内含 9+2 微管排列（九对外周双联管和两根中央单管）。它们通过基体固定在细胞上，基体结构与中心粒相同。纤毛通常推动液体流过细胞表面，例如气管上皮的纤毛；而鞭毛则驱动细胞运动，如精子。
Dynein motor proteins slide adjacent microtubule doublets, causing bending. OCR and IB may test the link between centriole duplication and the cell cycle, as well as the role of the cytoskeleton in intracellular transport and maintaining cell shape.
动力蛋白驱动相邻微管双联管滑动，引起弯曲。OCR 和 IB 可能会考查中心粒复制与细胞周期的关联，以及细胞骨架在胞内运输和维持细胞形状中的作用。
11. Cell Wall and Extracellular Structures | 细胞壁与胞外结构
While not organelles, cell walls are critical extracellular structures in plants, fungi, and prokaryotes. The plant cell wall is composed primarily of cellulose microfibrils, hemicellulose, and pectin. It provides mechanical strength, prevents osmotic bursting, and maintains cell shape.
细胞壁虽非细胞器，却是植物、真菌和原核生物的关键胞外结构。植物细胞壁主要由纤维素微纤丝、半纤维素和果胶组成。它提供机械强度，防止渗透破裂，并维持细胞形状。
The middle lamella, rich in pectins, cements adjacent plant cells together. Plasmodesmata are channels through cell walls that allow cytoplasmic continuity and communication between neighboring cells. In IB, the structure of the cell wall is linked to turgor pressure and water transport.
胞间层富含果胶，将相邻植物细胞粘合在一起。胞间连丝是穿过细胞壁的通道，使相邻细胞之间能够进行细胞质连续和通讯。在 IB 中，细胞壁的结构与膨压和水分运输相关联。
Fungal cell walls contain chitin, while bacterial cell walls are made of peptidoglycan. These differences are exploited by antibiotics such as penicillin, which target peptidoglycan synthesis. OCR often asks for comparisons between these cell wall types.
真菌细胞壁含有几丁质，而细菌细胞壁由肽聚糖构成。青霉素等抗生素正是利用了这些差异，靶向肽聚糖的合成。OCR 常要求比较这些细胞壁类型。
12. Plant vs. Animal Cell Comparison | 植物与动物细胞比较
A classic exam question requires tabulating differences between plant and animal cells. Both are eukaryotic and share organelles such as the nucleus, mitochondria, ER
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