📚 Ultrastructure of a Plant Cell | 植物细胞超微结构
Ultrastructure refers to the fine detail of a cell that is visible only with an electron microscope. In a plant cell, this includes membrane-bound organelles, the cellulose cell wall, and structures such as plasmodesmata that connect neighbouring cells.
超微结构是指只有在电子显微镜下才能看到的细胞精细结构。在植物细胞中，这包括有膜包被的细胞器、纤维素细胞壁，以及连接相邻细胞的胞间连丝等结构。
1. What Is Ultrastructure? | 什么是超微结构
The light microscope can show the nucleus, cell wall, and large vacuole, but it cannot resolve membranes, ribosomes, or the internal structure of chloroplasts and mitochondria.
光学显微镜可以观察到细胞核、细胞壁和大液泡，但无法分辨膜结构、核糖体以及叶绿体和线粒体的内部结构。
Electron microscopy reveals the ultrastructure of these organelles, including the phospholipid bilayer of membranes and the stacked thylakoids inside chloroplasts.
电子显微镜能够揭示这些细胞器的超微结构，包括膜的磷脂双分子层和叶绿体内部堆叠的类囊体。
2. Cell Wall: Structural Support | 细胞壁：结构支撑
The plant cell wall is a rigid layer outside the plasma membrane. It is composed mainly of cellulose microfibrils embedded in a matrix of hemicelluloses, pectins, and structural proteins.
植物细胞壁是细胞膜外的一层刚性结构。它主要由纤维素微纤丝组成，并嵌在由半纤维素、果胶和结构蛋白构成的基质中。
The middle lamella lies between adjacent cells and is rich in pectins, which cement cell walls together. Plasmodesmata are narrow cytoplasmic channels that pass through the cell wall and allow transport between cells.
胞间层位于相邻细胞之间，富含果胶，能将细胞壁黏合在一起。胞间连丝是穿过细胞壁的狭窄细胞质通道，可在细胞之间进行物质运输。
The cell wall provides mechanical strength, prevents osmotic bursting, and determines cell shape.
细胞壁提供机械强度，防止渗透性破裂，并决定细胞形状。
3. Plasma Membrane: Selective Barrier | 细胞膜：选择屏障
The plasma membrane surrounds the cytoplasm and controls the movement of substances into and out of the cell. It has a fluid mosaic structure made of a phospholipid bilayer with embedded proteins, glycoproteins, and sterols.
细胞膜包围细胞质，控制物质进出细胞。它具有流动镶嵌结构，由磷脂双分子层和嵌入其中的蛋白质、糖蛋白及固醇组成。
Transport proteins in the membrane allow ions, sugars, and amino acids to cross. In plants, the plasma membrane also participates in cellulose synthesis and cell wall assembly.
细胞膜上的转运蛋白使离子、糖类和氨基酸得以通过。在植物中，细胞膜还参与纤维素合成和细胞壁组装。
4. Nucleus: Control Centre | 细胞核：控制中心
The nucleus contains most of the genetic information of a plant cell in the form of chromatin. It is bounded by a double membrane called the nuclear envelope, which has nuclear pores.
细胞核以染色质的形式储存植物细胞的大部分遗传信息。它由称为核被膜的双层膜包围，核被膜上有核孔。
Nuclear pores regulate the movement of mRNA and ribosomal subunits between the nucleus and cytoplasm. The nucleolus is the region where ribosomal RNA is transcribed and ribosome subunits are assembled.
核孔调节 mRNA 和核糖体亚基在细胞核与细胞质之间的运输。核仁是转录核糖体 RNA 并组装核糖体亚基的区域。
5. Mitochondria: ATP Production | 线粒体：ATP 生产
Mitochondria are the sites of aerobic respiration. They have a smooth outer membrane and a highly folded inner membrane; the folds are called cristae.
线粒体是有氧呼吸的场所。它们具有光滑的外膜和高度折叠的内膜；这些折叠称为嵴。
The inner membrane encloses the matrix, which contains enzymes for the link reaction and the Krebs cycle. Stalked particles on the cristae contain ATP synthase for oxidative phosphorylation.
内膜包围基质，基质中含有连接反应和克雷布斯循环所需的酶。嵴上的球状颗粒含有用于氧化磷酸化的 ATP 合酶。
Mitochondria also contain circular DNA and 70S ribosomes, which support the endosymbiotic origin of these organelles.
线粒体还含有环状 DNA 和 70S 核糖体，这支持了线粒体起源于内共生的观点。
6. Chloroplasts: Photosynthesis | 叶绿体：光合作用
Chloroplasts are the site of photosynthesis. They are surrounded by a double membrane, or chloroplast envelope, and contain an internal membrane system of thylakoids.
叶绿体是光合作用的场所。它们由双层膜（即叶绿体被膜）包围，内部含有类囊体膜系统。
Thylakoids stack to form grana, where the light-dependent reactions occur. The stroma is the fluid-filled matrix containing enzymes for the Calvin cycle, starch grains, lipid droplets, and circular DNA.
类囊体堆叠形成基粒，光依赖反应就在这里发生。基质是充满液体的内部环境，含有卡尔文循环所需的酶、淀粉粒、脂滴和环状 DNA。
Like mitochondria, chloroplasts have their own 70S ribosomes and DNA, which is evidence for endosymbiosis.
与线粒体一样，叶绿体也有自身的 70S 核糖体和 DNA，这是内共生起源的证据。
7. Endoplasmic Reticulum: Synthesis and Transport | 内质网：合成与运输
The endoplasmic reticulum is a network of flattened membrane sacs called cisternae. Rough ER is studded with ribosomes and is involved in protein synthesis and transport.
内质网是由称为扁囊的扁平膜囊组成的网络。粗面内质网上附着核糖体，参与蛋白质合成和运输。
Smooth ER lacks ribosomes and is involved in lipid synthesis, carbohydrate metabolism, and detoxification. In some plant cells it also stores calcium ions.
光面内质网没有核糖体，参与脂质合成、碳水化合物代谢和解毒。在某些植物细胞中，它还储存钙离子。
8. Golgi Apparatus: Modification and Packaging | 高尔基体：修饰与包装
The Golgi apparatus consists of a stack of flattened membrane-bound sacs called cisternae. It modifies proteins received from the rough ER, for example by adding carbohydrate groups to form glycoproteins.
高尔基体由一叠扁平的膜包被囊泡（称为潴泡）组成。它修饰来自粗面内质网的蛋白质，例如通过添加糖基形成糖蛋白。
In plant cells, the Golgi apparatus is especially important because it synthesizes pectins and hemicelluloses for the cell wall and packages them into secretory vesicles.
在植物细胞中，高尔基体尤其重要，因为它合成果胶和半纤维素用于细胞壁，并将它们包装成分泌小泡。
These vesicles move to the cell surface, fuse with the plasma membrane, and release their contents by exocytosis.
这些小泡移动到细胞表面，与细胞膜融合，并通过胞吐作用释放其内容物。
9. Ribosomes: Protein Synthesis | 核糖体：蛋白质合成
Ribosomes are not membrane-bound. Each ribosome is made of a large subunit and a small subunit, both assembled from ribosomal RNA and protein.
核糖体没有膜包被。每个核糖体由大亚基和小亚基组成，二者由核糖体 RNA 和蛋白质组装而成。
Eukaryotic plant cells contain 80S ribosomes in the cytoplasm and on rough ER, while mitochondria and chloroplasts contain smaller 70S ribosomes.
真核植物细胞的细胞质和粗面内质网上含有 80S 核糖体，而线粒体和叶绿体含有较小的 70S 核糖体。
Ribosomes are the sites of translation, where the sequence of codons in mRNA is used to assemble amino acids into a polypeptide chain.
核糖体是翻译的场所，在这里根据 mRNA 中的密码子序列将氨基酸组装成多肽链。
10. Large Central Vacuole and Tonoplast | 大中央液泡与液泡膜
A mature plant cell typically has a large central vacuole surrounded by a single membrane called the tonoplast. The vacuole contains cell sap, which is a solution of water, ions, sugars, and organic acids.
成熟的植物细胞通常有一个大的中央液泡，由称为液泡膜的单层膜包围。液泡含有细胞液，即水、离子、糖类和有机酸组成的溶液。
The vacuole maintains turgor pressure against the cell wall, which keeps the plant firm. It also stores pigments, such as anthocyanins, and can contain hydrolytic enzymes.
液泡对细胞壁产生膨压，使植物保持坚挺。它还储存花青素等色素，并可能含有水解酶。
11. Peroxisomes and Glyoxysomes | 过氧化物酶体与乙醛酸循环体
Peroxisomes are small, single-membrane organelles that contain enzymes for oxidative reactions. They break down fatty acids and detoxify hydrogen peroxide using catalase.
过氧化物酶体是单层膜包被的小细胞器，含有催化氧化反应的酶。它们分解脂肪酸，并利用过氧化氢酶对过氧化氢进行解毒。
In photosynthetic leaf cells, peroxisomes play a key role in photorespiration by working with chloroplasts and mitochondria to recycle phosphoglycolate.
在光合叶肉细胞中，过氧化物酶体在光呼吸中起关键作用，与叶绿体和线粒体共同回收磷酸乙醇酸。
Some plant storage tissues contain glyoxysomes, specialised peroxisomes that convert stored lipids into carbohydrates during seed germination.
一些植物贮藏组织含有乙醛酸循环体，这是一种特化的过氧化物酶体，在种子萌发时将储存的脂质转化为碳水化合物。
12. Plasmodesmata and Cytoskeleton | 胞间连丝与细胞骨架
Plasmodesmata are cytoplasmic bridges through the cell wall that connect the cytoplasm of adjacent plant cells, forming a continuous symplast. They allow water, ions, sugars, and signalling molecules to move between cells.
胞间连丝是穿过细胞壁的细胞质桥，连接相邻植物细胞的细胞质，形成连续共质体。它们使水、离子、糖类和信号分子能够在细胞间移动。
The cytoskeleton is a network of microtubules and microfilaments. Microtubules guide the movement of vesicles and help orient cellulose microfibrils during cell wall formation.
细胞骨架是由微管和微丝组成的网络。微管引导小泡运动，并在细胞壁形成过程中帮助确定纤维素微纤丝的排列方向。
Microfilaments are involved in cytoplasmic streaming, which moves chloroplasts and other organelles around the plant cell.
微丝参与细胞质环流，使叶绿体和其他细胞器在植物细胞内移动。
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