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1、 Most of the properties we associate with life are properties of the cytoplasm. Much of the mass of a cell consists of this semifluid substance, which is bounded on the outside by the plasma membrane. Organelles are suspended within it, supported by t. of the cytoskeleton. Dissolved inluid are nutri
2、ents, ions, soluble proteins, and other materials needed for cell functioning.生命的大局部特征表现在细胞质的特征上。细胞质大局部由半流体物质组成，并由细胞膜包被。细胞器悬浮在其中，并由丝状的细胞骨架支撑。细胞质中溶解了大量的营养物质、离子、可溶蛋白以与维持细胞生理需求的其它物质。The eukaryotic cell nucleus is the largest organelle and houses the genetic material (DNA) on chromosomes. (In prokaryote
3、s the hereditary material is found in the nucleoid.) The nucleus also contains one or two organelles-the nucleoli-that play a role in cell division. A pore-perforated sac called the nuclear envelope separates the nucleus and its contents from the cytoplasm. Small molecules can pass through the nucle
4、ar envelope, but larger molecules such as mRNA and ribosomes must enter and exit via the pores.真核细胞的细胞核是最大的细胞器，为染色体上遗传物质DNA提供空间原核生物遗传物质存在于拟核中。细胞核含有一或二个核仁，在细胞分裂中发挥作用。一个穿孔的囊叫核膜，将细胞核和其中含物与细胞质分开，小分子物质可以自由通过核膜，但大分子物质如mRNA 和核糖体必须通过核孔进出核膜。All eukaryotic cells contain most of the various kinds of organelles
5、, and each organelle performs a specialized function in the cell. Organelles described in this section include ribosomes, the endoplasmic reticulum, the Golgi complex, vacuoles, lysosomes, mitochondria, and the plastids of plant cells.所有的真核细胞都含有多种细胞器，每种细胞器在细胞中行使一项专门功能。本节主要介绍核糖体、质网、高尔基体系、液泡、溶酶体、线粒体和植
6、物细胞中的质体。The number of ribosomes within a cell may range from a few hundred to many thousands. This quantity reflects the fact that, ribosomes are the sites at which amino acids are assembled into proteins for export or for use in cell processes. A complete ribosome is composed of one larger and one
7、smaller subunit. During protein synthesis the two subunits move along a strand of mRNA, reading the genetic sequence coded in it and translating that sequence into protein. Several ribosomes may become attached to a single mRNA strand; such a combination is called a polysome. Most cellular proteins
8、are manufactured on ribosomes in the cytoplasm. Exportable proteins and membrane proteins are usually made in association with the endoplasmic reticulum.细胞中核糖体的数量变化从几百到几千，核糖体数量如此之多反映了核糖体是氨基酸组装成蛋白质并被运出或在细胞过程中使用的位点这样一个事实。完整的核糖体由大亚基和小亚基组成。在蛋白质合成中两个亚基沿mRNA 链移动，阅读其中编码的遗传序列，将序列翻译成蛋白质。一条mRNA上可能有多个核糖体，这种组合称
9、多聚核糖体。大多数细胞中的蛋白质由细胞质中的核糖体游离核糖体制造。运输蛋白和膜蛋白通常由附着于质网上的核糖体制造。 The endoplasmic reticulum, a lacy array of membranous sacs, tubules, and vesicles, may be either rough (RER) or smooth (SER). Both types play roles in the synthesis and transport of proteins. The RER, which is studded with polysomes, also see
10、ms to be the source of the nuclear envelope after a cell divides.质网是由膜状囊、管和小泡等排列在一起，有粗面质网和滑面质网两种类型。两种类型都与蛋白质的合成和运输有关。粗面质网上散布着多聚核糖体，似乎也是细胞分裂后形成核膜的来源。SER lacks polysomes; it is active in the synthesis of fats and steroids and in the oxidation of toxic substances in the cell. Both types of endoplasmic
11、reticulum serve as compartments within the cell where specific products can be isolated and subsequently shunted to particular areas in or outside the cell.光滑质网上无核糖体/光滑质网上缺乏多聚核糖体，主要作用是脂肪和类固醇的合成以与细胞有毒物质的氧化。两种质网合成的产物在其中进展分流或运输到细胞外/这两种类型的质网都担当细胞中的隔断，致使特殊产物能够别离并最终逃避到细胞外的特定区域。Transport vesicles may carry
12、 exportable molecules from the endoplasmic reticulum to another membranous organelle, the Golgi complex. Within the Golgi complex molecules are modified and packaged for export out of the cell or for delivery else where in the cytoplasm.运输小泡能够将可运输分子从质网运输到高尔基复合体上。在高尔基复合体中修饰，包装后输出细胞或传递到细胞质中的其他场所。Vacuo
13、les in cells appear to be hollow sacs but are actually filled with fluid and soluble molecules. The most prominent vacuoles appear in plant cells and serve as water reservoirs and storage sites for sugars and other molecules. Vacuoles in animal cells carry out phagocytosis (the intake of particulate
14、 matter) and pinocytosis (vacuolar drinking).细胞中的液泡好象是中空的，但实际上充满了液体和可溶性分子。最典型的液泡存在于植物细胞中，储藏水，糖以与其它分子。动物中的液泡起吞噬颗粒物的吸收和胞饮作用。 A subset of vacuoles are the organelles known as lysosomes, which contain digestive e(packaged in lysosomes in the Golgi complex) that can break down most biological macromolecu
15、les. They act to digest food particles and to degrade damaged cell parts.溶酶体是液泡亚单位，含有消化酶在高尔基体中的溶酶体中包装而成，能够降解大局部生物大分子。消化食物微粒和降解损伤的细胞残片/行使消化食物颗粒和分解损坏的细胞组件的作用。Mitochondria are the sites of energy-yielding chemical reactions in all cells. In addition, plant cells contain plastids that utilize light ener
16、gy to manufacture carbohydrates in the process of photosynthesis. It is on the large surface area provided by the inner cristae of mitochondria that ATP-generating enzymes are located. Mitochondria are self-replicating, and probably they are the evolutionary descendants of what were once free-living prokaryotes.线粒体是所有细胞中发生产能化学反响的场所。另外，植物细胞中的质体在光合作用中利用光能产生碳水化合物，线粒体嵴上提供了很大的外表积并分布着产ATP酶。线粒体自我复制，并且可能是自由生活的原核生物在进化中形成的后代。Th
