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1、1/21 Lesson OneInside the Living Cell: Structure andFunction of Internal Cell PartsCytoplasm: The Dynamic, Mobile Factory (细胞质：动力工厂)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
2、outside by the plasma membrane. Organelles are suspended within it, supported by the filamentous network of the cytoskeleton. 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现在细胞质的特征上。细胞质大部分由半流体物质组成，并由细胞膜(原生质膜)包被。细 胞器
3、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，可溶蛋白以及维持细胞生理需求的其它物质。The Nucleus: Information Central (细胞核：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 on
4、e 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 nuclear envelope, but larger molecules such as mRNA and ribosomes must enter and exit
5、via the pores.真核细胞的细胞核是最大的细胞器，细胞核对染色体组有保护作用(原核细胞的遗传物质存在于拟核中)。细胞 核含有一或二个核仁，核仁促进细胞分裂。核膜贯穿许多小孔，小分子可以自由通过核膜，而象mRNA 和核糖体等大分子必须通过核孔运输。Organelles: Specialized Work Units(细胞器：特殊的功能单位)All eukaryotic cells contain most of the various kinds of organelles, and each organelle performs a specialized function in t
6、he 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，每个细胞器都有其特定功能。本节主要介绍核糖体，内质网，高尔基体系，液泡，溶酶体，线粒体和植物细胞中的质体。The number of ribosomes within a cell may range from
7、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 smaller subunit. During protein synthesis the two subunits mo
8、ve 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 are manufactured on ribosomes in the cytoplasm. Exportable pro
9、teins and membrane proteins are usually made in association with the endoplasmic reticulum.核糖体的数量变化从几百到几千，核糖体是氨基酸组装成蛋白质的重要场所。完整的核糖体由大亚基和小亚基组 成。核糖体沿着mRNA 移动并阅读遗传密码，翻译成蛋白质。 一条mRNA 上可能有多个核糖体，称多聚核糖体。大多数细胞蛋白是由细胞质中核糖体生产。输出蛋白和膜蛋白通常与内质网有关。The endoplasmic reticulum, a lacy array of membranous sacs, tubules,
10、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 seems to be the source of the nuclear envelope after a cell divides.内质网，带有花边的生物囊，有管状，泡状之分，以及光滑和粗糙面区别。两种都与蛋白质的合成和运输有关。粗糙内质网上分布许多核糖体，
11、也可能提供细胞分裂后所需的细胞膜。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 reticulum serve as compartments within the cell where specific products can be isolated and subsequently shunted to particular area
12、s in or outside the cell.光滑内质网上无核糖体，主要作用是脂肪和类固醇的合成以及细胞内有毒物质的氧化。两种内质网合成的产物在其中进行分流或运输到细胞外。Transport vesicles may carry exportable molecules from the endoplasmic reticulum to another membranous organelle,2/21 the Golgi complex. Within the Golgi complex molecules are modified and packaged for export out
13、 of the cell or for delivery else where in the cytoplasm.运输小泡能够将可运输分子从内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。Vacuoles 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
14、 storage sites for sugars and other molecules. Vacuoles in animal cells carry out phagocytosis (the intake of particulate 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分子。最典型的液泡存在于植物细胞中，储备水，糖以及其它分子。动物中的液泡起吞噬和胞饮作用。A subset of vacuoles are the organelles known as lysosomes, whic
15、h contain digestive enzymes (packaged in lysosomes in the Golgi complex) that can break down most biological macromolecules. 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，降解大部分生物大分子。消化食物微粒和降解损伤的细胞残片。Mitochondria are the sites of energy-yielding chemical reaction
16、s in all cells. In addition, plant cells contain plastids that utilize light energy to manufacture carbohydrates in the process of photosynthesis. It is on the large surface areaprovided by the inner cristae of mitochondria that ATP-generating enzymes are located. Mitochondria are self-replicating,
17、and probably they are the evolutionary descendants of what were once free-living prokaryotes.线粒体是细胞中化学产能的场所。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，线粒体内嵴 上提供了很大的表面积并分布着产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 。There are two types of plastids: leucoplasts, which lack pigments and serve as storage sites for starch
18、, proteins, and oils; and chromoplasts, which contain pigments. The most important chromoplasts are chloroplasts-organelles that contain the chlorophyll used in photosynthesis. The internal structure of chloroplasts includes stacks of membranes called grana, which are embedded in a matrix called the
19、 stroma.质体有两种类型：白色体，缺乏色素，是淀粉，蛋白质和油的储备场所；色质体，含有色素。叶绿体是最重要的 色质体，含有与光合作用有关的叶绿素。叶绿体的内部结构是由多层膜形成的叶绿体基粒，其中包埋在基质中的基粒称子座。The Cytoskeleton (细胞骨架)All eukaryotic cells have a cytoskeleton, which is a convoluted latticework of filaments and tubules that appears to fill all available space in the cell and provid
20、es support for various other organelles. A large portion of the cytoskeleton consists of threadlike microfilaments composed mainly of the contractile protein actin. They are involved in many types of intracellular movements in plant and animal cells. A second protein, myosin, is involve in the contr
21、action of muscle cells. Another main structural component of the cytoskeleton consists of microtubules, which are composed of the globular protein tubulin and together act as scaffolding that provides a stable cell shape. Cytoskeletal intermediate filaments appear to impart tensile strength to the c
22、ell cytoplasm. Mechanoenzymes such as myosin, dynein, and kinesin interact with the cytoskeletal filaments and tubules to generate forces that cause movements.所有的细胞都有细胞骨架，网络结构的纤丝充满了它所能触及的全部空间并且对细胞器提供支持作用。细胞骨架 大部分由微丝组成，微丝主要由可收缩的肌动蛋白组成。动植物细胞的许多种类型细胞内运动与肌动蛋白有关。 第二类蛋白是肌球蛋白，它与肌肉细胞的收缩有关。细胞骨架的另一个主要结构成分是微管，
23、由球状的微管蛋白 组成，象脚手架一般维持细胞的稳定形态。细胞骨架的中间丝提供了细胞质伸缩动力。机械酶，例如，肌球蛋白，动力蛋白，驱动蛋白与微丝，微管相互作用产生动力而引起细胞运动。Cellular Movements (细胞运动Although the cytoskeleton provides some stability 稳固 to cells, its microtubules英 maikr u tju:bju:l and filaments 丝状物 and their associated proteins enable cells to move by creeping 爬行 or
24、gliding 滑动 . Such movements require a solid 固体的 substrate to which the cell can adhere 附 着 and can be guided by the geometry d i mitri 几何形状 of the surface. Some cells also exhibit ig zibit 展览chemotaxis,kem tksis趋药性 the ability to move toward or away from the source of a diffusing 扩 散 chemical.3/21 尽
25、管细胞骨架提供了细胞的某些稳定性，微丝，微管及相关蛋白能使细胞爬行或滑动。这种运动需要固体基质依托并通过表面几何形状的改变而运动。某些细胞具备趋药性，即趋向或逃离扩散开的化学源。Certain eukaryotic cells can swim freely in liquid environments, propelled by whiplike cilia sili 纤毛or flagella 英 fl d el 鞭毛. Both cilia and flagella have the same internal structure: nine doublets (pairs of mic
26、rotubules) 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 基体is located. Movement is based on the activities of tiny dynein 动力蛋白 side arms
27、 that extend from one of the microtubules of each doublet 成对的东西.某些真核细胞能在液体液体中自由运动，由纤毛或鞭毛推动。纤毛和鞭毛具有同样的内部结构：九个双微管环形 排列，纵向延伸，环中心是两个或以上微管组成。纤毛或鞭毛从细胞表面的基体出生长，双微管的动力蛋白臂从一侧延伸到另一侧而引起运动。Nutrients, proteins, and other materials within most plant cells are moved about via cytoplasmic streaming. The process occ
28、urs as myosin mai usin 肌凝蛋白 proteins attached to organelles 细胞器，小器官 push against microfilaments arrayed rei 展示throughout the cell. Microfilaments and microtubules are responsible for almost all major cytoplasmic movements. During cell division, microtubules of the spindle assembled 集合，收集 from tubuti
29、n subunits near organelles called centrioles move the chromosomes.大部分植物细胞的营养，蛋白质和其它物质由细胞质流运输。这个过程是由于依附在细胞器上的肌球蛋白反推排 列在细胞周围的微丝形成的。绝大部分细胞质运动由微丝和微管完成。在细胞分裂期间，中心粒周围的由微管蛋 白亚基装配形成的纺锤体微管移向染色体。Lesson TwoPhotosynthesisPhotosynthesis occurs only in the chlorophyllchlorophyll 叶绿素-containing cells of green plan
30、ts, algae 藻，and certain protists原生生物 and bacteria. Overall, it is a process that converts light energy into chemical energy that is stored in the molecular bonds. From the point of view of chemistry and energetics, it is the opposite of cellular respiration. Whereas 然而 cellular 细胞的 respiration 呼吸is
31、highly exergonic 吸收能量的 and releases energy, photosynthesis 光合作用 requires energy and is highly endergonic.光合作用只发生在含有叶绿素的绿色植物细胞，海藻，某些原生动物和细菌之中。总体来说，这是一个将光能转 化成化学能，并将能量贮存在分子键中，从化学和动能学角度来看，它是细胞呼吸作用的对立面。细胞呼吸作用 是高度放能的，光合作用是需要能量并高吸能的过程。Photosynthesis starts with CO2 and H2O as raw materials and proceeds th
32、rough two sets of partial reactions. In the first set, called the light-dependent reactions, water molecules are split裂开 (oxidized), 02 is released, and ATP and NADPH are formed. These reactions must take place in the presence of 在面前 light energy. In the second set, called light-independent reaction
33、s, CO2 is reduced (via the addition of H atoms) to carbohydrate. These chemical events rely on the electron carrier NADPH and ATP generated by the first set of reactions.光合作用以二氧化碳和水为原材料并经历两步化学反应。第一步，称光反应，水分子分解，氧分子释放， ATP和 NADPH 形成。此反应需要光能的存在。第二步，称暗反应，二氧化碳被还原成碳水化合物，这步反应依赖电子 载体NADPH 以及第一步反应产生的ATP。Both
34、 sets of reactions take place in chloroplasts. Most of the enzymes and pigments 色素 for the lightdependent reactions are embedded 深入的内含的 in the thylakoid类囊体 membrane 膜隔膜 of chloroplasts 叶绿体. Thedark reactions take place in the stroma.基质两步反应都发生在叶绿体中。光反应需要的大部分酶和色素包埋在叶绿体的类囊体膜上。暗反应发生在基质中。How Light Energy
35、 Reaches Photosynthetic Cells (光合细胞如何吸收光能的)The energy in light photons in the visible part of the spectrum can be captured by biological molecules to do constructive work. The pigment chlorophyll in plant cells absorbs photons within a particular absorption spectrums statement of the amount of light
36、 absorbed by chlorophyll at different wavelengths. When light is absorbed it alters the arrangement of electrons in the absorbing molecule. The added energy of the photon boosts the energy4/21 co ndition of the molecule from a stable state to a less-stable excited state. During the light-dependent r
37、eactions of photosynthesis, as the absorbing molecule returns to the ground state, the excess excitation energy is transmitted to other molecules and stored as chemical energy.生物分子能捕获可见光谱中的光能。植物细胞中叶绿素在不同光波下吸收部分吸收光谱。在吸收分子中，光的作用使分子中的电子发生重排。光子的能量激活了分子的能量状态，使其从稳定态进入不稳定的激活态。All photosynthetic organisms c
38、ontain various classes of chlorophylls and one or more carotenoid (accessory) pigments that also contribute to photosynthesis. Groups of pigment molecules called antenna complexes are present on thylakoids. Light striking any one of the pigment molecules is funneled to a special chlorophyll a molecu
39、le, termed a reaction-center chlorophyll, which directly participates in photosynthesis. Most photosynthetic organisms possess two types of reaction-center chlorophylls, P680 and P700, each associated with an electron acceptor molecule and an electron donor. These aggregations are known respectively
40、 as photosystem I (P700) and photosystem (P680).所有的光合作用生物含有不同等级的叶绿素和一个或多个类胡萝卜素(光合作用的辅助色素)。称作天线复合体 的色素分子群存在于类囊体中。激活色素分子的光能进入叶绿素反应中心，其直接参与光合作用。大部分光反应细胞器拥有两套反应中心， P680 和 P700, 每个光系统都含有一个电子受体和电子供体。这些集合体就是大家熟识的光合系统I 和光合系统。The Light-Dependent Reaction: Converting Solar Energy into Chemical-Bond Energy光反应：
41、光能转化成化学键能The photosystems of the light-dependent reactions are responsible for the packaging of light energy in the chemical compounds ATP and NADPH. This packaging takes place through a series of oxidation reduction reactions set in motion when light strikes the P680 reaction center in photosystem
42、I. In this initial event water molecules are cleaved, oxygen is released, and electrons are donated. These electrons are accepted first by plastoquinone and then by a series of carriers as they descend an electron transport chain. For each four electrons that pass down the chain, two ATPs are formed
43、. The last acceptor in the chain is the P700 reaction center of photosystem I.At this point incoming photons boost the energy of the electrons, and they are accepted by ferredoxin. Ferredoxin is then reoxidized, and the coenzyme NADP+ is reduced to the NADPH. The ATP generated previously and the NAD
44、PH then take part in the light independent reactions.光反应的光系统将光能转化成化学复合物 ATP 和 NADPH。 当光激活光系统的光反应中心时，通过一系列的氧 化还原反应实现能量的传递。反应开始时，水被分解，氧被释放并提供电子。电子首先传递给质体醌，然后通过 一系列载体形成的电子传递链。每传递4个电子，形成2个ATP。 最后一个受体存在于光反应系统I 的反应中心里。此处光子激活电子，电子传递给铁氧还蛋白。铁氧还蛋白再氧化，并且辅酶 NADP+ 还原成 NADPH。 早期产生的 ATP 和 NADPH 进入暗反应。The productio
45、n of ATP from the transport of electrons excited by light energy down an electron transport chain is termed photophosphorylation. The one-way flow of electrons through photosystems ll and I is called noncyclic photophosphorylation; plants also derive additional ATP through cyclic photophosphorylatio
46、n, in which some electrons are shunted back through the electron transport chain between photosystems and I.由电子传递链偶连产生ATP 的过程称为光合磷酸化。通过光合系统流经光合系统I 的电子路径称非循环式光合磷酸化；植物通过循环式光合磷酸化获得额外的ATP, 一些电子在光合系统I 和之间的电子传递链中回流。The Light-Independent Reactions: Building Carbohydrates暗反应：碳水化合物的形成In the light-independent
47、 reactions of photosynthesis, which are driven by ATP and NADPH, C02 is converted to carbohydrate. The reactions are also known as the Calvin-Benson cycle. Atmospheric CO2, is fixed as it reacts with ribulosebiphosphate(RuBP), a reaction that is catalyzed by the enzyme ribulosebiphosphate carboxylase. The reduction Of C02 to carbohydrate (fructose diphosphate) is completed via several more steps of the cycle. Finally, RUBP is regenerated so that the cycle may continue.由 ATP 和 NADPH 驱动的暗反应中，二氧化碳转化成碳水化合物。即卡尔文循环。二磷酸核酮糖固定二氧化碳，由二磷酸核酮糖羧化酶催化。Oxygen: An Inhibitor of photosynthesis (氧：光合作用的抑制因子)5/21