【正文】 它物質(zhì)。Much of the mass of a cell consists of this semifluid substance, which is bounded on the outside by the plasma membrane.Dissolved in the cytoplasmic fluid are nutrients, ions, soluble proteins, and other materials needed for cell functioning.生命的大部分特征表現(xiàn)在細胞質(zhì)的特征上。The Nucleus: Information Central（細胞核：信息中心）The eukaryotic cell nucleus is the largest organelle and houses the genetic material (DNA) on chromosomes. (In prokaryotes the hereditary material is found in the nucleoid.) The nucleus also contains one or two organellesthe nucleolithat play a role in cell division.Small molecules can pass through the nuclear envelope, but larger molecules such as mRNA and ribosomes must enter and exit via the pores.真核細胞的細胞核是最大的細胞器，細胞核對染色體組有保護作用（原核細胞的遺傳物質(zhì)存在于擬核中）。 such a bination is called a polysome.Exportable proteins and membrane proteins are usually made in association with the endoplasmic reticulum.核糖體的數(shù)量變化從幾百到幾千，核糖體是氨基酸組裝成蛋白質(zhì)的重要場所。大多數(shù)細胞蛋白是由細胞質(zhì)中核糖體生產(chǎn)。Both types play roles in the synthesis and transport of proteins.粗糙內(nèi)質(zhì)網(wǎng)上分布許多核糖體，也可能提供細胞分裂后所需的細胞膜。Both types of endoplasmic reticulum serve as partments within the cell where specific products can be isolated and subsequently shunted to particular areas in or outside the cell.光滑內(nèi)質(zhì)網(wǎng)上無核糖體，主要作用是脂肪和類固醇的合成以及細胞內(nèi)有毒物質(zhì)的氧化。Within the Golgi plex molecules are modified and packaged for export out of the cell or for delivery else where in the cytoplasm.運輸小泡能夠?qū)⒖蛇\輸分子從內(nèi)質(zhì)網(wǎng)運輸?shù)礁郀柣鶑秃象w上。The most prominent vacuoles appear in plant cells and serve as water reservoirs and storage sites for sugars and other molecules.動物中的液泡起吞噬和胞飲作用。消化食物微粒和降解損傷的細胞殘片。另外，植物細胞中的質(zhì)體在光合作用中利用光能產(chǎn)生碳水化合物，線粒體內(nèi)嵴上提供了很大的表面積并分布著產(chǎn)ATP酶。葉綠體是最重要的色質(zhì)體，含有與光合作用有關(guān)的葉綠素。A large portion of the cytoskeleton consists of threadlike microfilaments posed mainly of the contractile protein actin.A second protein, myosin, is involve in the contraction of muscle cells.Cytoskeletal intermediate filaments appear to impart tensile strength to the cell cytoplasm.動植物細胞的許多種類型細胞內(nèi)運動與肌動蛋白有關(guān)。機械酶，例如，肌球蛋白，動力蛋白，驅(qū)動蛋白與微絲，微管相互作用產(chǎn)生動力而引起細胞運動。Certain eukaryotic cells can swim freely in liquid environments, propelled by whiplike cilia or flagella. Both cilia and flagella have the same internal structure: nine doublets (pairs of microtubules) are arranged in a ring and extend the length of the cilium or flagellum, and two more microtubules run down the center of the ring. Every cilium or flagellum grows only from the cell surface where a basal body is located. Movement is based on the activities of tiny dynein side arms that extend from one of the microtubules of each doublet.某些真核細胞能在液體液體中自由運動，由纖毛或鞭毛推動。在細胞分裂期間，中心粒周圍的由微管蛋白亞基裝配形成的紡錘體微管移向染色體?？傮w來說，這是一個將光能轉(zhuǎn)化成化學能，并將能量貯存在分子鍵中，從化學和動能學角度來看，它是細胞呼吸作用的對立面。In the first set, called the lightdependent reactions, water molecules are split (oxidized), 02 is released, and ATP and NADPH are formed.In the second set, called lightindependent reactions, CO2 is reduced (via the addition of H atoms) to carbohydrate.此反應需要光能的存在。Most of the enzymes and pigments for the lightdependent reactions are embedded in the thylakoid membrane of chloroplasts.暗反應發(fā)生在基質(zhì)中。光子的能量激活了分子的能量狀態(tài)，使其從穩(wěn)定態(tài)進入不穩(wěn)定的激活態(tài)。大部分光反應細胞器擁有兩套反應中心，P680和P700，每個光系統(tǒng)都含有一個電子受體和電子供體。This packaging takes place through a series of oxidation reduction reactions set in motion when light strikes the P680 reaction center in photosystem Ⅱ.These electrons are accepted first by plastoquinone and then by a series of carriers as they descend an electron transport chain.The last acceptor in the chain is the P700 reaction center of photosystem Ⅰ. At this point ining photons boost the energy of the electrons, and they are accepted by ferredoxin.The ATP generated previously and the NADPH then take part in the light independent reactions.光反應的光系統(tǒng)將光能轉(zhuǎn)化成化學復合物ATP和NADPH。每傳遞4個電子，形成2個ATP。早期產(chǎn)生的ATP和NADPH進入暗反應。 plants also derive additional ATP through cyclic photophosphorylation, in which some electrons are shunted back through the electron transport chain between photosystems Ⅱ and Ⅰ.由電子傳遞鏈偶連產(chǎn)生ATP的過程稱為光合磷酸化。The reactions are also known as the CalvinBenson cycle.The reduction Of C02 to carbohydrate (fructose diphosphate) is pleted via several more steps of the cycle.二磷酸核酮糖固定二氧化碳，由二磷酸核酮糖羧化酶催化。Among C4Thus C4 plants lessen photorespiration by carrying out photosynthesis only in cells that are insulated from high levels of CO2.這是碳固定的一個新機制。這些螺旋和超螺旋形成致密的染色體組結(jié)構(gòu)。非性染色體稱常染色體。Such cycling in effect makes singlecelled organisms immortal.多細胞生物中的許多細胞，包括動物肌肉和神經(jīng)細胞，要么降低循環(huán)速度，要么同時分裂。 S phase, during which normal synthesis of biological molecules continues, DNA is replicated, and histones are synthesized。頭三期包括G1,正常新陳代謝；S期,正常新陳代謝同時，DNA復制，組蛋白合成； G2 期，短期的新陳代謝和少許生長。Mitosis: Partitioning the Hereditary MaterialBiologists divide the mitotic cycle into four phases.As prophase ends and metaphase begins, the condensed chromosomes bee associated with the spindle.Next, during anaphase, the two sister chromatids of each chromosome split, and one from each pair is drawn toward each pole of the cell.在分裂前期后期和分裂中期前期，濃縮的染色體與紡錘體相連，最后以正確的角度排列在赤道板上。在分裂后期，纖維開始變短，染色單體分離。動物細胞與中心粒相連，而在植物和真菌細胞中，紡錘體與微管組織中心的離子相連。在植物細胞中，在赤道板形成新的細胞板。As in mitosis two chromatids exist for each chromosome at the beginning of prophase 1. During this phase the homologous chromosomes undergo synapsis, or pairing, which is brought about by a bridging structure of proteins and RNA called the synaptonemal plex.Unlike the anaphase of mitosis, however, during anaphase I the two chromatids of each chromosome stay joined at the centromere and move together to one of the two poles of the cell.由此而導致4個子細胞染色體數(shù)減半。產(chǎn)生4個單倍體，父母染色體隨機分配。s successful genetic plement, requires little or no specialization of reproductive organs, and is more rapid than sexual reproduction.A prime benefit of sexual reproduction is that it provides genetic variability and a ready mechanism for the elimination of deleterious mutations.但一個簡單災難性事件或疾病都可能摧毀一個細胞群體。 theory of pangenesis and August Weismands germ plasm theory. Based on expe