Cell Structure and Function – A-Level Biology Revision | 细胞结构与功能详解

📚 Cell Structure and Function | 细胞结构与功能详解

Cells are the fundamental units of life, and understanding their structure and the functions of each component is a cornerstone of A-Level Biology. This revision guide breaks down the eukaryotic and prokaryotic cell architecture, organelle functions, and key comparisons you need to master for your exams.

细胞是生命的基本单位，理解细胞的结构及各组成部分的功能是A-Level生物学的基石。本复习指南将详细解析真核细胞与原核细胞的结构、细胞器的功能以及你需要在考试中掌握的关键比较。

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1. Prokaryotic vs Eukaryotic Cells | 原核细胞与真核细胞

All living organisms are classified into two major groups based on their cellular organisation: prokaryotes (bacteria and archaea) and eukaryotes (plants, animals, fungi, and protists). The presence or absence of a membrane-bound nucleus is the defining difference.

所有生物根据其细胞组织形式分为两大类：原核生物（细菌和古菌）和真核生物（植物、动物、真菌和原生生物）。是否具有膜包被的细胞核是决定性差异。

Prokaryotic cells are generally smaller (0.5–5 μm), lack membrane-bound organelles, and have circular DNA free in the cytoplasm. Eukaryotic cells are larger (10–100 μm), possess a true nucleus, and contain numerous specialised membrane-bound organelles.

原核细胞通常较小（0.5–5微米），没有膜包被的细胞器，其环状DNA游离于细胞质中。真核细胞较大（10–100微米），具有真正的细胞核，并含有众多特化的膜包被细胞器。

Key structural differences are summarised below:

关键结构差异总结如下：

Feature | 特征	Prokaryotic | 原核	Eukaryotic | 真核
Nucleus | 细胞核	Absent (nucleoid region) | 无（拟核区）	Present, membrane-bound | 有，膜包被
DNA | 遗传物质	Circular, naked | 环状、裸露	Linear, associated with histones | 线性、与组蛋白结合
Ribosomes | 核糖体	70S (smaller) | 70S（较小）	80S (larger) | 80S（较大）
Membrane-bound organelles | 膜包被细胞器	Absent | 无	Present | 有
Cell wall | 细胞壁	Peptidoglycan (if present) | 肽聚糖（如有）	Cellulose (plants) / chitin (fungi) | 纤维素（植物）/几丁质（真菌）

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2. The Plasma Membrane | 质膜（细胞膜）

The plasma membrane surrounds all cells, acting as a selectively permeable barrier. According to the fluid mosaic model, the membrane consists of a phospholipid bilayer with embedded proteins, cholesterol (in animal cells), and glycoproteins/glycolipids on the outer surface.

质膜包围着所有细胞，充当选择性通透屏障。根据流动镶嵌模型，细胞膜由磷脂双分子层构成，其中镶嵌着蛋白质、胆固醇（动物细胞中）以及分布于外表面的糖蛋白和糖脂。

Phospholipids have a hydrophilic phosphate head and two hydrophobic fatty acid tails, so they naturally arrange into a bilayer in aqueous environments. This arrangement allows lipid-soluble substances to pass through easily while preventing the passage of water-soluble molecules.

磷脂具有亲水的磷酸头部和两条疏水的脂肪酸尾部，因此在水性环境中自然排列成双分子层。这种排列使得脂溶性物质容易通过，同时阻止水溶性分子的穿过。

Membrane proteins perform a variety of functions:

膜蛋白执行多种功能：

• Channel proteins – allow specific ions and small molecules to diffuse through | 通道蛋白——允许特定离子和小分子扩散通过

• Carrier proteins – transport molecules across via active transport or facilitated diffusion | 载体蛋白——通过主动运输或易化扩散转运分子

• Receptor proteins – bind hormones and signalling molecules | 受体蛋白——结合激素和信号分子

• Enzymes – catalyse reactions at the membrane surface | 酶——在膜表面催化反应

• Glycoproteins – cell recognition and cell adhesion | 糖蛋白——细胞识别和细胞黏附

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3. The Nucleus | 细胞核

The nucleus is the control centre of the eukaryotic cell, typically the largest organelle. It is enclosed by a double membrane called the nuclear envelope, which contains nuclear pores that allow mRNA and ribosomes to pass through while preventing the free movement of larger molecules.

细胞核是真核细胞的控制中心，通常是最大的细胞器。它由称为核膜的双层膜包围，核膜上具有核孔，允许mRNA和核糖体通过，同时阻止较大分子的自由移动。

The nucleus contains chromatin – DNA wrapped around histone proteins. 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 assembly.

细胞核内含有染色质——即缠绕在组蛋白上的DNA。在细胞分裂期间，染色质浓缩为可见的染色体。核仁是细胞核内的致密区域，是核糖体RNA（rRNA）合成和核糖体组装的场所。

Functions of the nucleus include:

细胞核的功能包括：

• Storing genetic information as DNA | 以DNA形式储存遗传信息

• Controlling gene expression and protein synthesis | 控制基因表达和蛋白质合成

• Coordinating cell division | 协调细胞分裂

• Producing ribosome subunits in the nucleolus | 在核仁中产生核糖体亚基

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4. Ribosomes | 核糖体

Ribosomes are small, dense organelles composed of rRNA and proteins, responsible for protein synthesis. They are not membrane-bound and can be found free in the cytoplasm or attached to the rough endoplasmic reticulum (RER).

核糖体是由rRNA和蛋白质组成的小型致密细胞器，负责蛋白质合成。它们没有膜包被，可游离于细胞质中，或附着于粗面内质网上。

Prokaryotic ribosomes are 70S (made of 50S and 30S subunits), while eukaryotic ribosomes are 80S (made of 60S and 40S subunits). The ‘S’ (Svedberg unit) refers to the sedimentation rate during ultracentrifugation, which reflects size and density.

原核核糖体为70S（由50S和30S亚基组成），而真核核糖体为80S（由60S和40S亚基组成）。“S”代表超速离心时的沉降系数，反映核糖体的大小和密度。

Free ribosomes synthesise proteins that remain within the cytoplasm, whereas ribosomes attached to the RER synthesise proteins destined for secretion, incorporation into membranes, or delivery to lysosomes.

游离核糖体合成留在细胞质内的蛋白质，而附着于粗面内质网上的核糖体合成用于分泌、嵌入膜或运送到溶酶体的蛋白质。

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5. Endoplasmic Reticulum | 内质网

The endoplasmic reticulum (ER) is an extensive network of membrane-bound tubes and flattened sacs called cisternae. There are two types: rough ER and smooth ER.

内质网是由膜包被的管状结构和扁平囊状结构（称为潴泡）组成的广泛网络。内质网分为两种类型：粗面内质网和滑面内质网。

Rough ER (RER) is studded with ribosomes on its cytoplasmic surface. Its primary function is the folding and processing of newly synthesised proteins. Proteins are translocated into the lumen of the RER, where they undergo post-translational modifications such as glycosylation, before being packaged into vesicles and transported to the Golgi apparatus.

粗面内质网（RER）在其细胞质侧表面密布核糖体。其主要功能是折叠和加工新合成的蛋白质。蛋白质被转运到RER腔内，在那里进行翻译后修饰（如糖基化），然后被包装进囊泡并运送到高尔基体。

Smooth ER (SER) lacks ribosomes. It is involved in the synthesis of lipids and steroids, the detoxification of drugs and toxins (especially in liver cells), and the storage of calcium ions in muscle cells.

滑面内质网（SER）没有核糖体。它参与脂质和类固醇的合成、药物和毒素的解毒（尤其在肝细胞中），以及肌细胞中钙离子的储存。

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6. Golgi Apparatus | 高尔基体

The Golgi apparatus (or Golgi body/complex) consists of a stack of flattened, curved membrane sacs (cisternae) with associated vesicles. It has a distinct polarity: the cis face receives vesicles from the ER, and the trans face releases modified proteins.

高尔基体由一叠扁平、弯曲的膜状潴泡及其相关囊泡组成。它具有明确的极性：顺面接收来自内质网的囊泡，反面释放加工后的蛋白质。

Functions of the Golgi apparatus include:

高尔基体的功能包括：

• Modifying proteins (e.g., adding carbohydrate groups to form glycoproteins) | 修饰蛋白质（例如添加糖基形成糖蛋白）

• Sorting and packaging proteins into vesicles for secretion or intracellular transport | 分选并将蛋白质包装入囊泡，用于分泌或胞内运输

• Forming lysosomes | 形成溶酶体

• Synthesising certain polysaccharides (e.g., pectin in plant cells) | 合成某些多糖（如植物细胞中的果胶）

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7. Mitochondria | 线粒体

Mitochondria are the sites of aerobic respiration and are responsible for ATP production. They are double-membrane organelles: the outer membrane is smooth, while the inner membrane is folded into cristae, which greatly increases the surface area for oxidative phosphorylation.

线粒体是有氧呼吸的场所，负责ATP的产生。它们是双层膜细胞器：外膜光滑，内膜折叠形成嵴，极大地增加了氧化磷酸化的表面积。

The fluid inside the inner membrane is called the matrix. It contains enzymes for the Krebs cycle, mitochondrial DNA (circular, similar to prokaryotic DNA), ribosomes (70S), and large stores of calcium phosphate.

内膜内部的液体称为基质。基质中含有三羧酸循环所需的酶、线粒体DNA（环状，类似原核DNA）、核糖体（70S）以及大量磷酸钙储备。

Cells with high energy demands – such as muscle cells, liver cells, and sperm cells – contain large numbers of mitochondria. During aerobic respiration, the key stages involving mitochondria are:

能量需求高的细胞——如肌细胞、肝细胞和精子细胞——含有大量线粒体。在有氧呼吸期间，涉及线粒体的关键阶段包括：

Link reaction: Pyruvate → Acetyl-CoA
Krebs cycle: Acetyl-CoA → CO₂ + NADH + FADH₂
Oxidative phosphorylation (on cristae): NADH/FADH₂ → ATP + H₂O

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8. Chloroplasts | 叶绿体

Chloroplasts are organelles found in plant cells and some protists, responsible for photosynthesis. Like mitochondria, they are double-membrane-bound and contain their own DNA and ribosomes.

叶绿体存在于植物细胞和某些原生生物中，负责光合作用。与线粒体类似，它们也是双层膜包被的，含有自身的DNA和核糖体。

Inside the chloroplast, there is a system of thylakoid membranes – flattened sacs. Thylakoids are stacked into structures called grana (singular: granum). The fluid surrounding the grana is called the stroma.

叶绿体内部有一套类囊体膜系统——扁平的囊状结构。类囊体堆叠成称为基粒的结构。围绕基粒的流体称为基质。

The light-dependent reactions of photosynthesis occur on the thylakoid membranes, which contain chlorophyll and the electron transport chain. The light-independent reactions (Calvin cycle) occur in the stroma, which contains enzymes such as RuBisCO.

光合作用的光依赖反应发生在含有叶绿素和电子传递链的类囊体膜上。光不依赖反应（卡尔文循环）则发生在含有RuBisCO等酶的基质中。

Overall: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Overall: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

A-level exam tip: Remember the structural adaptation – the extensive thylakoid membrane system maximises light absorption, while the fluid stroma provides enzymes for carbon fixation.

A-level考试提示：记住结构适应——广泛的类囊体膜系统最大化光吸收，而液态基质提供碳固定所需的酶。

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9. Lysosomes | 溶酶体

Lysosomes are membrane-bound vesicles containing hydrolytic enzymes (lysozymes) capable of breaking down macromolecules such as proteins, nucleic acids, lipids, and carbohydrates. They are formed by the Golgi apparatus and are abundant in phagocytic cells such as macrophages.

溶酶体是含有水解酶（溶菌酶）的膜包被囊泡，能够分解蛋白质、核酸、脂质和碳水化合物等大分子。它们由高尔基体形成，在巨噬细胞等吞噬细胞中含量丰富。

Functions of lysosomes:

溶酶体的功能：

• Intracellular digestion – breaking down ingested material via phagocytosis | 胞内消化——通过吞噬作用分解摄入的物质

• Autophagy – recycling damaged organelles or cellular components | 自噬——回收受损的细胞器或细胞组分

• Apoptosis – programmed cell death, where lysosomal enzymes are released to destroy the cell | 细胞凋亡——程序性细胞死亡，溶酶体酶释放以摧毁细胞

• Extracellular digestion – releasing enzymes outside the cell to break down substances (e.g., sperm cells use this to penetrate the egg) | 胞外消化——将酶释放到细胞外分解物质（如精子细胞借此穿透卵细胞）

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10. Cytoskeleton and Other Structural Components | 细胞骨架及其他结构组分

The cytoskeleton is a network of protein filaments extending throughout the cytoplasm. It provides mechanical support, maintains cell shape, and enables intracellular transport and cell motility. The three main components are microfilaments, microtubules, and intermediate filaments.

细胞骨架是贯穿整个细胞质的蛋白质纤维网络。它提供机械支持、维持细胞形状，并实现胞内运输和细胞运动。三大主要组分是微丝、微管和中间丝。

Microfilaments (actin filaments) are involved in muscle contraction, cell division (cleavage furrow), and cell movement. Microtubules form the mitotic spindle during cell division and serve as tracks for motor proteins (kinesin and dynein) to transport vesicles and organelles. Intermediate filaments provide mechanical strength to cells and tissues.

微丝（肌动蛋白丝）参与肌肉收缩、细胞分裂（卵裂沟）和细胞运动。微管在细胞分裂期间形成纺锤体，并作为马达蛋白（驱动蛋白和动力蛋白）运输囊泡和细胞器的轨道。中间丝为细胞和组织提供机械强度。

Additional structural components include: centrioles (paired organelles involved in spindle formation in animal cells), cilia and flagella (motile structures composed of microtubules), and the cell wall in plants, fungi, and prokaryotes (provides rigidity and protection).

其他结构组分包括：中心粒（参与动物细胞纺锤体形成的成对细胞器）、纤毛和鞭毛（由微管组成的运动结构），以及植物、真菌和原核生物中的细胞壁（提供刚性和保护）。

Centrioles have a 9 + 2 arrangement of microtubules in cilia and flagella, while each centriole itself consists of nine triplets of microtubules arranged in a cylinder.

纤毛和鞭毛中的微管呈“9 + 2”排列，而每个中心粒本身由九组三联微管排列成圆柱体。

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