Cell Structure | 细胞结构

📚 Cell Structure | 细胞结构

Cells are the fundamental units of life. In A-Level Biology, a thorough understanding of cell structure is essential, as it links to topics such as membrane transport, enzyme localisation, protein synthesis, and genetics. This article covers the key structural features of prokaryotic and eukaryotic cells, details the organelles found in animal and plant cells, and explains how techniques like microscopy and cell fractionation are used to study cells.

细胞是生命的基本单位。在 A-Level 生物学中，透彻理解细胞结构至关重要，因为它与膜运输、酶的定位、蛋白质合成和遗传学等主题紧密相连。本文涵盖原核与真核细胞的关键结构特征，详细介绍动、植物细胞中的细胞器，并说明如何利用显微技术和细胞分级分离等方法研究细胞。

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1. The Cell Theory | 细胞学说

Cell theory states that all living organisms are composed of one or more cells, the cell is the basic unit of structure and organisation, and all cells arise from pre-existing cells. This principle underpins the study of biology and highlights the continuity of life.

细胞学说指出，所有生物体都由一个或多个细胞组成，细胞是结构和组织的基本单位，并且所有细胞都来自已有的细胞。这一原理是生物学研究的基础，体现了生命的连续性。

Exceptions to cell theory include the first cell that originated from non-living material, and viruses, which are not cellular but still possess genetic material. However, for examination purposes, the cell remains the smallest unit that can carry out all life processes.

细胞学说的例外情况包括第一个起源于非生命物质的细胞，以及病毒——它们没有细胞结构但拥有遗传物质。然而，就考试而言，细胞仍然是能够执行所有生命过程的最小单位。

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2. Prokaryotic Cells | 原核细胞

Prokaryotic cells are generally smaller (0.1–5.0 μm) and structurally simpler than eukaryotic cells. They lack a true nucleus; instead, their DNA is circular and located in a region called the nucleoid. No membrane-bound organelles are present.

原核细胞通常比真核细胞更小（0.1–5.0 μm）、结构更简单。它们没有真正的细胞核；其 DNA 呈环状，位于称为拟核的区域。不存在膜包围的细胞器。

Typical features include a cell wall made of peptidoglycan (in bacteria), a plasma membrane, 70S ribosomes, plasmids – small loops of DNA, a capsule for protection, and sometimes flagella for movement. An example is Escherichia coli.

典型特征包括由肽聚糖（细菌中）构成的细胞壁、质膜、70S 核糖体、质粒（小型环状 DNA）、用于保护的荚膜，有时还有用于运动的鞭毛。例如大肠杆菌（Escherichia coli）。

The smaller ribosomes (70S) differ from the larger 80S ribosomes found in eukaryotic cytoplasm, making prokaryotic ribosomes a target for certain antibiotics.

较小的核糖体（70S）与真核细胞质中的 80S 核糖体不同，这使得原核核糖体成为某些抗生素的作用靶点。

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3. Eukaryotic Cells: An Overview | 真核细胞概述

Eukaryotic cells are typically 10–100 μm in diameter and are characterised by a distinct nucleus enclosed by a nuclear envelope. They contain numerous membrane-bound organelles, each with specific functions, which allows compartmentalisation of metabolic pathways.

真核细胞直径通常为 10–100 μm，其特征是具有由核膜包裹的明显细胞核。它们含有许多膜包围的细胞器，每个细胞器都有特定功能，从而使代谢途径得以区隔化。

The cytoplasm includes cytosol, organelles, and a network of protein fibres known as the cytoskeleton. The cytoskeleton provides mechanical support, aids in cell division, and facilitates intracellular transport.

细胞质包括胞质溶胶、细胞器以及称为细胞骨架的蛋白质纤维网络。细胞骨架提供机械支持，协助细胞分裂，并促进细胞内运输。

A key difference between animal and plant cells is that plant cells possess a cellulose cell wall, a large permanent vacuole, and chloroplasts, whereas animal cells have centrioles and store carbohydrates as glycogen rather than starch.

动、植物细胞之间的一个关键区别是，植物细胞具有纤维素细胞壁、一个大中央液泡和叶绿体，而动物细胞具有中心粒，并以糖原而非淀粉的形式储存碳水化合物。

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4. Nucleus: Control Centre | 细胞核：控制中心

The nucleus is the largest organelle in most eukaryotic cells. It is surrounded by a double membrane called the nuclear envelope, which contains nuclear pores that allow the passage of mRNA and ribosomes.

细胞核是大多数真核细胞中最大的细胞器。它被双层膜（核膜）包围，核膜上有核孔，允许 mRNA 和核糖体通过。

Inside, chromatin (DNA associated with histones) is present, and during cell division it condenses into chromosomes. The nucleolus is a dense region where ribosomal RNA (rRNA) is synthesised and ribosome subunits are assembled.

核内存在染色质（与组蛋白结合的 DNA），在细胞分裂期间它凝缩成染色体。核仁是致密区域，在此合成核糖体 RNA（rRNA）并组装核糖体亚基。

The nucleus controls cellular activities by regulating gene expression and is the site of DNA replication and transcription.

细胞核通过调控基因表达来控制细胞活动，是 DNA 复制和转录的场所。

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5. Ribosomes and Protein Synthesis | 核糖体与蛋白质合成

Ribosomes are small, non-membrane-bound organelles composed of rRNA and protein. They exist as two subunits (large and small) and are the sites of translation, where mRNA is decoded to synthesise polypeptides.

核糖体是小型、无膜包裹的细胞器，由 rRNA 和蛋白质组成。它们以两个亚基（大亚基和小亚基）存在，是翻译的场所，在此解码 mRNA 以合成多肽。

Eukaryotic ribosomes are 80S in size, while those in mitochondria and chloroplasts are 70S, similar to prokaryotic ribosomes. Ribosomes can be free in the cytosol or bound to the rough endoplasmic reticulum.

真核生物的核糖体大小为 80S，而线粒体和叶绿体中的核糖体为 70S，类似于原核核糖体。核糖体可以游离在胞质溶胶中，或附着在粗面内质网上。

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6. Endoplasmic Reticulum and Golgi Apparatus | 内质网与高尔基体

The endoplasmic reticulum (ER) is a network of membranous sacs and tubules. Rough ER is studded with ribosomes and is involved in synthesising and modifying proteins destined for secretion or for use in membranes. Smooth ER lacks ribosomes and synthesises lipids, including phospholipids and steroids.

内质网（ER）是由膜囊和管状结构组成的网络。粗面内质网上附有核糖体，参与合成和修饰将要分泌或用于膜的蛋白质。滑面内质网无核糖体，合成脂类，包括磷脂和类固醇。

The Golgi apparatus consists of flattened, stacked membrane sacs (cisternae). It receives proteins from the ER via transport vesicles, modifies them (e.g., by adding sugar groups), and packages them into vesicles for transport to the plasma membrane or to lysosomes.

高尔基体由扁平的层叠膜囊（潴泡）组成。它通过运输囊泡接收来自内质网的蛋白质，对其进行修饰（如添加糖基），并包装入囊泡中，以运往质膜或溶酶体。

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7. Lysosomes and Cellular Digestion | 溶酶体与细胞消化

Lysosomes are spherical, membrane-bound organelles containing hydrolytic enzymes (e.g., proteases, lipases) that break down macromolecules, old organelles, and engulfed pathogens. They maintain an acidic internal pH (about 5.0) to optimise enzyme activity.

溶酶体是球形的膜包被细胞器，含有水解酶（如蛋白酶、脂肪酶），可分解大分子、衰老细胞器和被吞噬的病原体。它们保持内部酸性 pH（约 5.0），以优化酶活性。

They are formed by the Golgi apparatus and play a crucial role in autophagy (digesting worn-out cell components) and in programmed cell death (apoptosis).

它们由高尔基体形成，在自噬（消化废旧细胞组分）和程序性细胞死亡（凋亡）中发挥关键作用。

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8. Mitochondria: Energy Production | 线粒体：能量生产

Mitochondria are the ‘powerhouses’ of the cell. Each mitochondrion has a double membrane: the inner membrane is highly folded into cristae, which increases surface area for oxidative phosphorylation, while the outer membrane is smooth and permeable.

线粒体是细胞的“动力工厂”。每个线粒体具双层膜：内膜高度折叠成嵴，增大了氧化磷酸化的表面积，而外膜光滑且通透。

The matrix enclosed by the inner membrane contains enzymes for the Krebs cycle and mitochondrial DNA, along with 70S ribosomes. ATP is produced during aerobic respiration via the electron transport chain embedded in the inner membrane.

内膜所围的基质含有三羧酸循环所需的酶、线粒体 DNA 以及 70S 核糖体。ATP 在有氧呼吸过程中通过嵌在内膜上的电子传递链产生。

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9. Chloroplasts and Photosynthesis | 叶绿体与光合作用

Chloroplasts are found in plant cells and some algae. They are surrounded by a double membrane and contain an internal system of flattened membrane sacs called thylakoids, stacked to form grana (singular: granum).

叶绿体存在于植物细胞和某些藻类中。它们被双层膜包围，内部具有称为类囊体的扁平膜囊系统，堆叠形成基粒（单数：granum）。

The thylakoid membranes house chlorophyll and other photosynthetic pigments, as well as enzymes for the light-dependent reactions. The stroma, a fluid-filled space, contains enzymes for the Calvin cycle, starch grains, and its own circular DNA and 70S ribosomes.

类囊体膜上含有叶绿素及其他光合色素，以及光反应所需的酶。基质是充满液体的空间，含有卡尔文循环所需的酶、淀粉粒、自身的环状 DNA 和 70S 核糖体。

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10. Cell Surface Membrane | 细胞表面膜

All cells are bounded by a cell surface membrane (plasma membrane), described by the fluid mosaic model. It consists of a phospholipid bilayer with embedded proteins, cholesterol (in animal cells), and glycolipids/glycoproteins.

所有细胞都由细胞表面膜（质膜）界定，其结构可用流动镶嵌模型描述。它由磷脂双分子层和嵌入其中的蛋白质、胆固醇（动物细胞中）以及糖脂/糖蛋白组成。

The membrane is selectively permeable, controlling the movement of substances into and out of the cell. Transport can be passive (diffusion, facilitated diffusion, osmosis) or active (requiring ATP, e.g., sodium–potassium pump).

膜具有选择通透性，控制物质进出细胞。运输方式可以是被动的（扩散、协助扩散、渗透）或主动的（需要 ATP，如钠钾泵）。

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11. Plant Cell Wall and Vacuole | 植物细胞壁与液泡

The plant cell wall is a rigid, fully permeable structure primarily composed of cellulose fibres embedded in a matrix of hemicellulose and pectin. It provides mechanical strength, maintains cell shape, and prevents osmotic lysis.

植物细胞壁是坚硬、全通透的结构，主要由嵌在半纤维素和果胶基质中的纤维素纤维构成。它提供机械强度，维持细胞形状，并防止渗透裂解。

Adjacent plant cells are connected through plasmodesmata, which are channels that traverse cell walls, allowing communication and transport of substances between cells.

相邻植物细胞通过胞间连丝相连，胞间连丝是贯穿细胞壁的通道，可进行细胞间通讯和物质运输。

The large central vacuole of mature plant cells is surrounded by a membrane called the tonoplast. It stores water, ions, sugars, and pigments, and contributes to turgor pressure, which supports the plant when turgid.

成熟植物细胞的大中央液泡由称为液泡膜的膜包围。它储存水、离子、糖类和色素，并产生膨压，使植物在膨胀状态下获得支撑。

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12. Microscopy and Cell Fractionation | 显微镜技术与细胞分级分离

Studying cell structure requires instruments. Light microscopes (resolution ~200 nm) can resolve nuclei and chloroplasts, but electron microscopes (resolution ~0.5 nm for TEM) reveal ultrastructure, such as cristae and ribosomes.

研究细胞结构需要仪器。光学显微镜（分辨率约 200 nm）可分辨细胞核和叶绿体，但电子显微镜（TEM 分辨率约 0.5 nm）能显示超微结构，如嵴和核糖体。

Scanning electron microscopes (SEM) produce three-dimensional images of surfaces. Magnification and resolution must be distinguished: magnification is how much larger an image appears, whereas resolution is the ability to distinguish two separate points.

扫描电子显微镜（SEM）可产生表面三维图像。必须区分放大倍数和分辨率：放大率是图像放大的倍数，而分辨率是区分两个独立点的能力。

Cell fractionation isolates organelles. Cells are homogenised in a cold, isotonic, buffered solution, then subjected to differential centrifugation. At low speeds, nuclei pellet first; increasing speeds successively sediment mitochondria, chloroplasts, and microsomes (ER fragments).

细胞分级分离可分离细胞器。将细胞在冷、等渗、缓冲的溶液中匀浆，然后进行差速离心。低速时细胞核首先沉淀；逐步提高转速可依次沉降线粒体、叶绿体和微粒体（内质网碎片）。

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Published by TutorHao | Biology Revision Series | aleveler.com

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