📚 IB OCR Biology: Cell Structure Essentials | IB OCR 生物：细胞结构 考点精讲
Cells are the fundamental units of life, and understanding their structure is essential for success in IB and OCR Biology. This article breaks down the key concepts, from the differences between prokaryotic and eukaryotic cells to the detailed functions of organelles, as well as the specialised structures found in plant and animal cells. We will also explore microscopy techniques, the endosymbiotic theory, and how cell ultrastructure relates to function. Each section provides paired English and Chinese explanations to help bilingual learners master the content with confidence.
细胞是生命的基本单位，理解其结构是在IB和OCR生物中取得成功的基础。本文逐一剖析核心概念，从原核细胞与真核细胞的差异、细胞器的详细功能，到植物与动物细胞的特化结构。我们还将探讨显微技术、内共生学说，以及细胞超微结构与功能的关联。每个小节都提供英文与中文的成对讲解，帮助双语学习者自信地掌握这些内容。
1. Prokaryotes vs Eukaryotes | 原核细胞与真核细胞的比较
Prokaryotic cells are smaller and simpler, lacking a true nucleus and membrane-bound organelles. Their DNA is circular and floats freely in the cytoplasm. Eukaryotic cells, on the other hand, have a distinct nucleus enclosed by a nuclear envelope, and contain numerous membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus. Both cell types share a plasma membrane, cytoplasm, and ribosomes, but the size and complexity set them apart.
原核细胞较小且结构简单，没有真正的细胞核和膜包被的细胞器，其DNA为环状，游离于细胞质中。真核细胞则具有由核膜包裹的清晰细胞核，并含有许多膜包被的细胞器，例如线粒体、内质网和高尔基体。两种细胞都具备细胞膜、细胞质和核糖体，但大小和复杂程度将它们区分开来。
Prokaryotes include bacteria and archaea, which often possess a cell wall, capsule, and flagella for movement. Eukaryotes encompass animal cells, plant cells, fungi, and protists. In OCR specifications, you must be able to identify and draw the ultrastructure of both cell types, highlighting features such as pili, plasmids, and the 70S ribosomes in prokaryotes.
原核生物包括细菌和古菌，它们通常具有细胞壁、荚膜和用于运动的鞭毛。真核生物则涵盖动物细胞、植物细胞、真菌和原生生物。在OCR考纲中，你必须能够识别并绘制两种细胞的超微结构，强调原核细胞中的菌毛、质粒和70S核糖体等特征。
| Feature | 特征 | Prokaryotic Cell | 原核细胞 | Eukaryotic Cell | 真核细胞 |
|---|---|---|
| Nucleus | 细胞核 | Absent | 无 | Present | 有 |
| DNA | DNA | Circular, naked | 环状，裸露 | Linear, histone-bound | 线状，结合组蛋白 |
| Ribosomes | 核糖体 | 70S | 70S | 80S | 80S |
| Membrane-bound organelles | 膜包被的细胞器 | Absent | 无 | Present | 有 |
2. Animal Cell Ultrastructure | 动物细胞超微结构
Animal cells contain a nucleus, mitochondria, rough and smooth endoplasmic reticulum (ER), Golgi apparatus, lysosomes, ribosomes, and a plasma membrane. The nucleus stores genetic information and is the site of transcription. The rough ER is studded with ribosomes and synthesises proteins, while the smooth ER is involved in lipid synthesis and detoxification. Mitochondria are the powerhouses, generating ATP through aerobic respiration.
动物细胞包含细胞核、线粒体、粗面和滑面内质网、高尔基体、溶酶体、核糖体和细胞膜。细胞核储存遗传信息，是转录发生的场所。粗面内质网上附有核糖体，负责蛋白质的合成，而滑面内质网则参与脂质合成及解毒作用。线粒体是细胞的能量工厂，通过有氧呼吸产生ATP。
The Golgi apparatus modifies, sorts, and packages proteins into vesicles for secretion or delivery to other organelles. Lysosomes contain digestive enzymes and break down waste materials. Centrioles, which are absent in most plant cells, play a role in cell division by organising the spindle fibres. The plasma membrane, composed of a phospholipid bilayer with embedded proteins, controls the movement of substances in and out of the cell.
高尔基体对蛋白质进行修饰、分类并包装成囊泡，以便分泌或输送至其他细胞器。溶酶体含有消化酶，可分解废物。中心粒在大多数植物细胞中不存在，它通过组织纺锤丝参与细胞分裂。由磷脂双分子层和嵌入蛋白质组成的细胞膜，控制着物质进出细胞。
3. Plant Cell Ultrastructure | 植物细胞超微结构
Plant cells have all the organelles found in animal cells except centrioles, but they also possess a cell wall, chloroplasts, and a large central vacuole. The rigid cell wall made of cellulose provides structural support and protection. Plasmodesmata are channels that connect adjacent plant cells, allowing communication and transport. The central vacuole stores water, ions, and pigments, and maintains turgor pressure against the cell wall.
植物细胞除中心粒外拥有动物细胞中的所有细胞器，但还具备细胞壁、叶绿体和一个大的中央液泡。由纤维素构成的坚韧细胞壁提供结构支撑和保护。胞间连丝是连接相邻植物细胞的通道，允许通讯和运输。中央液泡储存水分、离子和色素，并对抗细胞壁维持膨压。
Chloroplasts are the site of photosynthesis, containing the green pigment chlorophyll embedded in the thylakoid membranes. They have their own DNA and 70S ribosomes, suggesting an evolutionary origin via endosymbiosis. Starch grains often accumulate within chloroplasts as a storage product. In OCR exams, you must be able to label and describe the functions of these plant-specific organelles.
叶绿体是光合作用的场所，其类囊体膜上嵌有绿色色素叶绿素。它们拥有自己的DNA和70S核糖体，提示其通过内共生的进化起源。淀粉粒常作为储存产物在叶绿体内积累。在OCR考试中，你必须能够标注并描述这些植物特有细胞器的功能。
4. Membrane Structure and Function | 膜结构与功能
The fluid mosaic model explains the structure of cell membranes. A phospholipid bilayer forms the fundamental matrix, with hydrophobic fatty acid tails facing inward and hydrophilic phosphate heads facing outward. Proteins are scattered throughout the membrane, some embedded (integral) and others attached to the surface (peripheral). Cholesterol molecules modulate membrane fluidity, and glycolipids and glycoproteins form the glycocalyx, which aids in cell recognition.
流动镶嵌模型解释了细胞膜的结构。磷脂双分子层构成基本骨架，疏水的脂肪酸尾部朝向内侧，亲水的磷酸头部朝向外侧。蛋白质散布于整个膜中，有些嵌入其中（整合蛋白），有些附着在表面（外周蛋白）。胆固醇分子调节膜的流动性，糖脂和糖蛋白形成糖萼，有助于细胞识别。
Cell membranes are selectively permeable, allowing only certain substances to pass. Small non-polar molecules like O₂ and CO₂ can diffuse freely, while ions and large polar molecules require transport proteins. Active transport uses ATP to move substances against their concentration gradient. Endocytosis and exocytosis are processes for bulk transport across the membrane.
细胞膜具有选择透过性，仅允许特定物质通过。如O₂和CO₂这样的小型非极性分子可以自由扩散，而离子和大型极性分子则需要转运蛋白。主动运输利用ATP使物质逆浓度梯度移动。胞吞和胞吐则是跨膜的大型运输过程。
5. The Nucleus and Genetic Material | 细胞核与遗传物质
The nucleus is the control centre of the eukaryotic cell, containing the cell’s genetic blueprint in the form of chromatin (DNA wound around histone proteins). The nuclear envelope is a double membrane punctuated with nuclear pores, allowing macromolecules and ions to pass between the nucleus and cytoplasm. The nucleolus is a dense region where ribosomal RNA is synthesised and ribosome subunits are assembled.
细胞核是真核细胞的控制中心，以染色质（DNA缠绕组蛋白）的形式储存细胞的遗传蓝图。核膜是双层膜，其上分布着核孔，允许大分子和离子在核与细胞质之间通过。核仁是一个致密区域，核糖体RNA在此合成，核糖体亚基在此组装。
During cell division, chromatin condenses into visible chromosomes. The nucleus coordinates gene expression, DNA replication, and the cell cycle. In comparison, prokaryotes lack a nucleus and have a nucleoid region where the circular DNA is concentrated.
在细胞分裂期间，染色质凝聚成可见的染色体。细胞核协调基因表达、DNA复制和细胞周期。相比之下，原核生物没有细胞核，而是有一个拟核区域，环状DNA集中于此。
6. Ribosomes and Protein Synthesis | 核糖体与蛋白质合成
Ribosomes are the molecular machines that translate messenger RNA (mRNA) into polypeptide chains. Eukaryotic ribosomes (80S) consist of a 40S small subunit and a 60S large subunit. Free ribosomes in the cytoplasm synthesise proteins intended for use within the cell, while those attached to the rough ER produce secretory and membrane proteins. Prokaryotic ribosomes are smaller (70S) and are a target for antibiotics.
核糖体是将信使RNA（mRNA）翻译为多肽链的分子机器。真核核糖体（80S）由40S小亚基和60S大亚基组成。细胞质中游离的核糖体合成供细胞内部使用的蛋白质，而附着在粗面内质网上的核糖体则生产分泌蛋白和膜蛋白。原核核糖体较小（70S），是抗生素的作用靶点。
The process involves initiation, elongation, and termination stages, with transfer RNA (tRNA) bringing amino acids to the ribosome according to the codon sequence on the mRNA. The ribosome’s peptidyl transferase activity (provided by ribosomal RNA in the large subunit) forms peptide bonds. This central mechanism is conserved across all domains of life.
该过程包括起始、延伸和终止阶段，转运RNA（tRNA）根据mRNA上的密码子序列将氨基酸带到核糖体。核糖体的肽基转移酶活性（由大亚基中的核糖体RNA提供）形成肽键。这一核心机制在所有生命域中都保守。
7. Endomembrane System | 内膜系统
The endomembrane system consists of the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and plasma membrane. These organelles work together to synthesise, modify, package, and transport proteins and lipids. The rough ER receives polypeptide chains from ribosomes and folds them, often adding carbohydrate groups to form glycoproteins. The smooth ER synthesises lipids and detoxifies certain chemicals.
内膜系统由核膜、内质网、高尔基体、溶酶体、囊泡和细胞膜组成。这些细胞器协同工作，合成、修饰、包装并运输蛋白质和脂质。粗面内质网从核糖体接收多肽链并进行折叠，通常还会添加糖基形成糖蛋白。滑面内质网合成脂质并解毒某些化学物质。
Vesicles bud off from the ER and travel to the Golgi apparatus, where further modification occurs, such as the trimming of sugar groups and phosphorylation. The Golgi then dispatches vesicles to the membrane for secretion or to other destinations. Lysosomes, which bud from the Golgi, contain hydrolytic enzymes and are involved in autophagy and the digestion of engulfed material.
囊泡从内质网出芽并移至高尔基体，在那里进行进一步修饰，例如糖基的修剪和磷酸化。高尔基体随后将囊泡派送至细胞膜以进行分泌，或派送至其他目的地。由高尔基体出芽形成的溶酶体含有水解酶，参与自噬和被吞噬物质的消化。
8. Mitochondria and Energy Conversion | 线粒体与能量转化
Mitochondria are double-membrane organelles where the inner membrane is highly folded into cristae, increasing surface area for electron transport chains and ATP synthase. The matrix contains enzymes for the Krebs cycle, mitochondrial DNA, and 70S ribosomes. These features support the endosymbiotic theory, which proposes that mitochondria originated from aerobic bacteria engulfed by ancestral eukaryotic cells.
线粒体是双层膜的细胞器，其内膜高度折叠成嵴，增大了电子传递链和ATP合酶的表面积。基质中含有克雷布斯循环的酶、线粒体DNA和70S核糖体。这些特征支持内共生学说，该学说认为线粒体起源于被祖先真核细胞吞噬的好氧细菌。
The primary function of mitochondria is to carry out aerobic respiration, producing up to 38 ATP molecules per glucose molecule. Key stages include glycolysis (in the cytoplasm), the link reaction, Krebs cycle, and oxidative phosphorylation. The inner membrane is impermeable to most ions, allowing the establishment of a proton gradient that drives chemiosmosis.
线粒体的主要功能是进行有氧呼吸，每分子葡萄糖最多可产生38个ATP分子。关键阶段包括糖酵解（在细胞质中）、连接反应、克雷布斯循环和氧化磷酸化。内膜对大多数离子不通透，因此能够建立质子梯度，驱动化学渗透。
9. Chloroplasts and Photosynthesis | 叶绿体与光合作用
Chloroplasts belong to the plastid family and are responsible for photosynthesis. Their double membrane encloses the stroma, which contains enzymes for the Calvin cycle, as well as DNA and 70S ribosomes. The internal membrane system forms thylakoids, stacked into grana, where the light-dependent reactions take place. Pigments such as chlorophyll a and b absorb light energy and convert it into chemical energy.
叶绿体属于质体家族，负责光合作用。其双层膜包裹着基质，基质中含有卡尔文循环的酶、DNA和70S核糖体。内膜系统形成类囊体，堆叠成基粒，光依赖反应在此发生。叶绿素a和b等色素吸收光能并将其转化为化学能。
The overall equation for photosynthesis is:
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
In OCR exams, you should be able to identify the structures visible in electron micrographs and explain how the arrangement of thylakoids maximises light capture. Comparing mitochondria and chloroplasts will also reinforce understanding of energy transduction in cells.
光合作用的总反应式为：
6CO₂ + 6H₂O + 光能 → C₆H₁₂O₆ + 6O₂
在OCR考试中，你应能识别电子显微镜照片中的结构，并解释类囊体的排列如何最大化捕获光能。比较线粒体和叶绿体也有助于加深对细胞能量转换的理解。
10. Cytoskeleton and Cell Motility | 细胞骨架与细胞运动
The cytoskeleton is a network of protein fibres that maintains cell shape, anchors organelles, and enables movement. It consists of three main components: microfilaments (actin filaments), intermediate filaments, and microtubules. Microfilaments are involved in muscle contraction, cell division (cleavage furrow), and amoeboid movement. Intermediate filaments provide tensile strength, while microtubules form the track for organelle transport and the mitotic spindle.
细胞骨架是维持细胞形状、锚定细胞器并实现运动的蛋白质纤维网络。它由三个主要成分组成：微丝（肌动蛋白丝）、中间纤维和微管。微丝参与肌肉收缩、细胞分裂（分裂沟）和变形虫运动。中间纤维提供抗张强度，而微管则为细胞器运输和有丝分裂纺锤体形成轨道。
Cilia and flagella are microtubule-based projections that extend from the cell surface. They have a 9+2 arrangement of microtubule doublets, driven by dynein motor proteins. Centrioles, found in the centrosome of animal cells, also consist of a 9+0 triplet arrangement and help organise spindle fibres during mitosis.
纤毛和鞭毛是伸出细胞表面的基于微管的突起。它们具有9+2排列的微管二联体，由动力蛋白驱动。动物细胞中心体中的中心粒也具有9+0的三联体排列，并在有丝分裂期间帮助组织纺锤丝。
11. Microscopy and Cell Observation | 显微镜技术与细胞观察
Light microscopes use visible light and glass lenses to magnify specimens up to about 1000–1500 times. They can observe living cells and basic structures such as the nucleus, cytoplasm, and large organelles like chloroplasts. Electron microscopes, on the other hand, use a beam of electrons and offer much higher resolution, allowing visualisation of ultrastructure, such as ribosomes, membranes, and virus particles.
光学显微镜使用可见光和玻璃透镜，可将标本放大至约1000至1500倍。它能观察活细胞和基本结构，例如细胞核、细胞质和叶绿体等大型细胞器。电子显微镜则使用电子束，分辨率高得多，可观察超微结构，如核糖体、膜和病毒颗粒。
Transmission electron microscopy (TEM) provides cross-sectional images of thin specimens, while scanning electron microscopy (SEM) gives detailed 3D views of surfaces. In the lab, you may be required to prepare slides using staining techniques (e.g., iodine for plant cells, methylene blue for animal cells) and measure cell dimensions using an eyepiece graticule. Remember: magnification = image size ÷ actual size.
透射电子显微镜（TEM）提供薄标本的截面图像，而扫描电子显微镜（SEM）则展示表面的详细三维视图。在实验室中，你可能需要运用染色技术（如碘液用于植物细胞，亚甲蓝用于动物细胞）制备玻片，并使用目镜测微尺测量细胞尺寸。记住：放大倍数 = 图像大小 ÷ 实际大小。
12. The Endosymbiotic Theory | 内共生学说
The endosymbiotic theory states that certain organelles of eukaryotic cells, specifically mitochondria and chloroplasts, originated from free-living prokaryotes that were engulfed by a larger host cell. Evidence includes the presence of their own circular DNA, 70S ribosomes, double membranes, and the ability to divide by binary fission independently of the host cell cycle.
内共生学说指出，真核细胞的某些细胞器，特别是线粒体和叶绿体，起源于被较大的宿主细胞吞噬的独立生活的原核生物。证据包括它们拥有自己的环状DNA、70S核糖体、双层膜，以及能够独立于宿主细胞周期通过二分裂进行分裂的能力。
This theory is a crucial concept in cell biology and evolution. Mitochondria are thought to have evolved from aerobic bacteria, while chloroplasts originated from photosynthetic cyanobacteria. The nuclear genome of eukaryotes also contains genes of endosymbiotic origin, transferred over evolutionary time.
这一理论是细胞生物学和进化中的重要概念。人们认为线粒体从好氧细菌进化而来，而叶绿体则起源于光合蓝藻。真核生物的核基因组中也含有来自内共生的基因，是在进化过程中转移而来的。
Published by TutorHao | IB OCR Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导