📚 Cell Structure | 细胞结构
Cells are the basic structural and functional units of all living organisms. In Cambridge A-Level Biology, understanding cell structure involves comparing prokaryotic and eukaryotic cells, recognising organelles from electron micrographs, and explaining how ultrastructure relates to function.
细胞是所有生物体基本的结构和功能单位。在剑桥 A-Level 生物中，理解细胞结构包括比较原核细胞和真核细胞、从电子显微照片识别细胞器，以及解释超微结构如何与功能相适应。
1. Cell Theory and Microscopy | 细胞理论与显微镜
The cell theory states that all living things are composed of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells.
细胞理论指出，所有生物都由细胞组成，细胞是生命的基本单位，所有细胞都来自已有的细胞。
Light microscopes use visible light and glass lenses to magnify specimens up to about 1500 times, allowing observation of nuclei, chloroplasts and cell walls in live or stained cells.
光学显微镜利用可见光和玻璃透镜将样品放大到约1500倍，能观察活细胞或染色细胞中的细胞核、叶绿体和细胞壁。
Electron microscopes use beams of electrons and electromagnetic lenses to achieve much higher resolution, revealing ultrastructure such as ribosomes, membranes and internal organelles.
电子显微镜使用电子束和电磁透镜获得高得多的分辨率，能揭示核糖体、膜和内部细胞器等超微结构。
Transmission electron microscopy gives two-dimensional images of thin sections, while scanning electron microscopy gives three-dimensional surface views.
透射电子显微镜提供薄切片的二维图像，扫描电子显微镜提供三维表面视图。
2. Magnification and Resolution | 放大率与分辨率
Magnification is how many times larger an image is than the actual object; resolution is the minimum distance at which two points can be seen as separate.
放大率是图像比实际物体放大的倍数；分辨率是能区分两个点的最小距离。
Magnification is calculated using the equation:
放大率用以下公式计算：
magnification = image size ÷ actual size
For example, if a mitochondrion measures 20 mm in an electron micrograph and its actual length is 2 μm, the magnification is 20 000 μm ÷ 2 μm = 10 000×.
例如，如果电子显微照片中线粒体长 20 mm，实际长度为 2 μm，则放大率为 20 000 μm ÷ 2 μm = 10 000×。
Remember to convert all measurements to the same unit; common conversions include 1 mm = 1000 μm and 1 μm = 1000 nm.
记住把所有测量值换算成相同单位；常用换算包括 1 mm = 1000 μm、1 μm = 1000 nm。
3. Eukaryotic Cell Ultrastructure | 真核细胞超微结构
Eukaryotic cells possess a true nucleus bounded by a nuclear envelope, membrane-bound organelles, and 80S ribosomes in the cytoplasm.
真核细胞具有由核膜包围的真正细胞核、具膜细胞器和细胞质中的 80S 核糖体。
Animal and plant cells share organelles such as the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes and ribosomes.
动物细胞和植物细胞共有细胞核、线粒体、内质网、高尔基体、溶酶体和核糖体等细胞器。
Plant cells additionally have a cellulose cell wall, a large permanent vacuole and chloroplasts, while animal cells have centrioles and store carbohydrates as glycogen.
植物细胞还具有纤维素细胞壁、大型永久液泡和叶绿体，而动物细胞有中心粒并以糖原形式储存糖类。
The ultrastructure of an organelle refers to its detailed arrangement of membranes and macromolecules as seen under an electron microscope.
细胞器的超微结构是指在电子显微镜下观察到的膜和大分子的精细排列。
4. Nucleus and Ribosomes | 细胞核与核糖体
The nucleus is surrounded by a double membrane called the nuclear envelope, which contains nuclear pores for mRNA and ribosomal subunits to pass through.
细胞核由称为核膜的双层膜包围，核膜上有核孔，供 mRNA 和核糖体亚基通过。
Inside the nucleus, chromatin consists of DNA wrapped around histone proteins; during cell division it condenses into visible chromosomes.
细胞核内，染色质由缠绕在组蛋白上的 DNA 组成；细胞分裂时它凝缩成可见的染色体。
The nucleolus is the site of ribosomal RNA synthesis and ribosome subunit assembly.
核仁是核糖体 RNA 合成和核糖体亚基装配的场所。
Ribosomes are small non-membrane-bound particles made of rRNA and protein; 80S ribosomes occur in eukaryotic cytoplasm, while 70S ribosomes occur in prokaryotes, mitochondria and chloroplasts.
核糖体是由 rRNA 和蛋白质组成的小型无膜颗粒；80S 核糖体存在于真核细胞质中，70S 核糖体存在于原核生物、线粒体和叶绿体中。
Ribosomes are the sites of translation, where amino acids are joined together into polypeptide chains.
核糖体是翻译的场所，氨基酸在此连接成多肽链。
5. Endomembrane System: ER and Golgi | 内膜系统：内质网与高尔基体
Rough endoplasmic reticulum is studded with ribosomes and is involved in protein synthesis and initial folding of proteins destined for secretion or membrane insertion.
粗糙内质网表面附着核糖体，参与蛋白质合成以及待分泌或插入膜中的蛋白质的初步折叠。
Smooth endoplasmic reticulum lacks ribosomes and synthesises lipids, steroid hormones and detoxifies harmful substances in liver cells.
光滑内质网没有核糖体，合成脂质、类固醇激素，并在肝细胞中解毒有害物质。
The Golgi apparatus consists of flattened membrane sacs called cisternae; it modifies, packages and sorts proteins received from the rough ER.
高尔基体由称为扁平膜囊的膜囊组成；它对来自粗糙内质网的蛋白质进行修饰、包装和分拣。
Vesicles bud off the Golgi and either fuse with the plasma membrane for exocytosis or become lysosomes.
囊泡从高尔基体出芽，或与质膜融合进行胞吐，或成为溶酶体。
This endomembrane system provides a continuous pathway for protein processing and transport within the eukaryotic cell.
内膜系统为真核细胞内蛋白质加工和运输提供了连续的途径。
6. Mitochondria and Chloroplasts | 线粒体与叶绿体
Mitochondria are the sites of aerobic respiration, producing ATP through oxidative phosphorylation.
线粒体是有氧呼吸的场所，通过氧化磷酸化产生 ATP。
They have a double membrane: the inner membrane is highly folded into cristae, increasing surface area for ATP synthase and electron carriers.
它们具有双层膜：内膜高度折叠成嵴，增加了 ATP 合酶和电子载体的表面积。
The mitochondrial matrix contains enzymes for the link reaction and Krebs cycle, as well as circular DNA and 70S ribosomes.
线粒体基质含有连接反应和克雷布斯循环的酶，还含有环状 DNA 和 70S 核糖体。
Chloroplasts carry out photosynthesis; they contain thylakoid membranes stacked into grana where light-dependent reactions occur, and a stroma for the Calvin cycle.
叶绿体进行光合作用；它们含有堆叠成基粒的类囊体膜，光依赖反应在此发生，以及用于卡尔文循环的基质。
Like mitochondria, chloroplasts have circular DNA and 70S ribosomes, supporting the endosymbiotic theory of their evolutionary origin.
与线粒体一样，叶绿体含有环状 DNA 和 70S 核糖体，支持其进化起源的内共生理论。
7. Lysosomes, Vacuoles and Centrioles | 溶酶体、液泡与中心粒
Lysosomes are membrane-bound sacs containing hydrolytic enzymes that digest worn-out organelles, engulfed pathogens and autophagic material.
溶酶体是含有水解酶的具膜囊泡，能消化衰老细胞器、被吞噬的病原体和自噬物质。
Their enzymes work best at acidic pH and are kept separate from the cytoplasm, preventing damage to the cell.
溶酶体酶在酸性 pH 下活性最佳，并与细胞质分离，防止对细胞造成损伤。
Plant cells usually have a large central vacuole surrounded by a tonoplast; it stores cell sap, maintains turgor pressure and can contain pigments.
植物细胞通常有一个由液泡膜包围的大型中央液泡；它储存细胞液、维持膨压并可含有色素。
Centrioles are paired cylindrical structures made of microtubules, located near the nucleus in animal cells; they organise the spindle during cell division.
中心粒是由微管组成的成对圆柱状结构，位于动物细胞细胞核附近；它们在细胞分裂时组织纺锤体。
8. Plant Cell Wall and Cell Surface Membrane | 植物细胞壁与细胞表面膜
The plant cell wall is made mainly of cellulose microfibrils embedded in a matrix of hemicellulose and pectin; it provides mechanical strength and prevents osmotic lysis.
植物细胞壁主要由嵌入半纤维素和果胶基质中的纤维素微纤丝组成；它提供机械强度并防止渗透裂解。
The cell surface membrane, or plasma membrane, is a phospholipid bilayer with embedded proteins; it controls the movement of substances into and out of the cell.
细胞表面膜或质膜是嵌有蛋白质的磷脂双分子层；它控制物质进出细胞。
The fluid mosaic model describes the membrane as a fluid structure because phospholipids and proteins can move laterally within the bilayer.
流动镶嵌模型将膜描述为流动结构，因为磷脂和蛋白质可以在双分子层内横向移动。
Cholesterol in animal cell membranes regulates fluidity and stability, while glycolipids and glycoproteins are involved in cell recognition.
动物细胞膜中的胆固醇调节流动性和稳定性，糖脂和糖蛋白参与细胞识别。
9. Prokaryotic Cells | 原核细胞
Prokaryotic cells, such as bacteria, lack a true nucleus; their circular DNA lies free in the cytoplasm in a region called the nucleoid.
原核细胞（如细菌）没有真正的细胞核；其环状 DNA 游离在细胞质中称为拟核的区域。
Bacteria have a cell wall made of peptidoglycan, a plasma membrane, 70S ribosomes, and may possess plasmids, flagella, pili and a capsule.
细菌具有由肽聚糖组成的细胞壁、质膜、70S 核糖体，并可能具有质粒、鞭毛、菌毛和荚膜。
Prokaryotic cells are smaller than eukaryotic cells and divide by binary fission rather than mitosis.
原核细胞比真核细胞小，通过二分裂而非有丝分裂进行分裂。
A comparison table can help summarise differences between prokaryotic and eukaryotic cells.
比较表有助于总结原核细胞与真核细胞的差异。
10. Viruses and Cell Fractionation | 病毒与细胞分级分离
Viruses are not considered living cells because they lack cellular organisation, metabolism and ribosomes; they consist of nucleic acid enclosed in a protein coat called a capsid, sometimes with a lipid envelope.
病毒不被视为活细胞，因为它们缺乏细胞组织、代谢和核糖体；它们由包裹在蛋白质衣壳中的核酸组成，有时还有脂质包膜。
Viruses can only replicate by infecting a host cell and using its ribosomes and enzymes to produce new virus particles.
病毒只能通过感染宿主细胞并利用其核糖体和酶来复制产生新的病毒颗粒。
Cell fractionation separates organelles by differential centrifugation after homogenisation in a cold, isotonic, buffered solution.
细胞分级分离是在冷、等渗、缓冲溶液中均质化后，通过差速离心分离细胞器。
The homogenate is centrifuged at increasing speeds, so nuclei sediment first, followed by mitochondria and chloroplasts, then microsomes and ribosomes.
匀浆以不断提高的速度离心，因此细胞核首先沉淀，然后是线粒体和叶绿体，最后是微粒体和核糖体。
This technique allows scientists to study organelle functions in isolation and is often followed by density gradient centrifugation for greater purity.
该技术使科学家能够分离研究细胞器功能，之后常用密度梯度离心以获得更高纯度。
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