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Structure and Function of Inter Cell Parts

(2010-06-21 17:05:45)

Inside the Living Cell: Structure and Function of Inter Cell Parts


1.   Cytoplasm:The Dynamic Mobile


Most of the properities we associate with life are properities of the cytoplasm . Much of the mass of a cell consists of this semifuluid substance, which is bounded on the outside by the plasma membrance. Organelles are suspended within it , supported by the filamentous network of the cytoskeleton. Dessolved in the cytoplasmic fluid are nutrients, ions , soluble proteins, and other materials needed for cell functioning.


2.  The Nucleus: Information Central


The eukaryotic cell nucleus is the largest organelle and houses the geneticmaterial(DNA) on chromosome. (In prokaryotes athe hereditary material is found in the nucleoid.)The nucleus also contains one or two organelles-the nuleoli-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 via the pores.


3.   Organelles: Specialized Work Units


All eukaryotic cells contain most of the various kinds of organelles, 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.


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 process. A complete ribosome is composed of one larger and one smaller subunit. During protein synthesis the two subunits move along a strand of mRNA, “reading”the genetic sequence coded in it and tanslating 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 proteins and membrane proteins are usually made in association with the endoplasmic reticulum.


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 seems to be the source of the nuclear envelope after a cell divides. SER lacks polysomes; it is active in the synthesis of fats and steroids and in the oxidation of toxic subtances in the cell. Both types of endoplasmic reticulum serve as compartments wihiin the cell where specific products can be isolated and subsequently shunted to particular areas in or outside the cell.


Transport vesicles may carry exportable molecules from the endoplasmic reticulum to another membranous organelle, the Golgi complex. Whithin the Golgi complex molecules are modified and packaged for export out of the cell or for delivery else where in the cytoplasm.


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 storage sites for sugars and other molecules. Vacuoles in animal cells carry out phagocytosis (the intake of particulate matter )and pinocytosis (vacuolar drinking )


A subset of vacuoles is the organelles known as lysosomes, which contain digestive enzymes (packaged in lysosome in the Golgi complex )that can break down most biological macromolecules. They act to digest food particles and to degrade damaged cell parts.


Mitochodria are the sites of energy-yielding chemical reactions 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 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.


There are two types of plastids: leucoplasts, which lack pigments and serve as storage sites for starch, proteins, and oils: and chromoplasts, which contain pigments. The most important chromoplasts are chloroplasts-organelles that contain the chorophyll used in photosythesis. The internal structure of chloroplasts includes stacks of membranes called grana, which are embedded ,in a matrix called the stroma.


4.       The Cytoskeleton

All eukaryotic cells have a ytoskeleton, which is a convoluted latticework of filaments and tubules that apears to fill all available space in the cell and provides support for various other organelles. A large portion of the cytoskeleton consists of threadlike icrofilaments composed mainly of the contractile protein actin. They are involed of intracellular movements in plant and animal cells. A second protein , myosin , is involve in the contraction of the globular protein tubulin and of the cytoskeleton consists of microtubules, which are composed of the globulr protein tubulin and together act as scaffolding that provides a stable cellshape. Cyotoskeletal intermediate filaments appear to impart tensile strength to the cell cytoplasm. Mechanoenzymes such as yosin, dynein, and kinesin interact with the cytoskeletal filaments and tubules to generate forces that cause movements.


所有的原核细胞都含有一个细胞骨架,细胞骨架是纤丝和细管缠绕在一起的格子结构。似乎充满着细胞中所有可利用的空间,  为各种各样的其他细胞提供支持。细胞骨架的大部分主要是由可收缩的蛋白肌动蛋白组成的细长的微丝。细胞骨架的另一个主要结构成分是由球状蛋白微管蛋白组成组很的微管,并一起作用作为支架提供一个稳定的细胞形态。细胞骨架的中间丝为细胞质传递张力。像肌球蛋白,动力蛋白和驱动蛋白这样的机械酶,与细胞骨架纤丝和细管作用产生动力,引起细胞运动。

5.       Cellular Movements

Although the cytoskeleton provides some stability to cells, its microtubules and filaments and their associated proteins enable cells to move by creeping or gliding. Such movements require a solid substrate to which the cell can adhere and be guided by the geomery of the surface. Some cells also exhibit chemotaxis, the ability to move toward or away from the source of a diffusing chemical.



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 cilum or flagellum, and two more imcrotubules run down the center of the ring. Every cilium or flagellun 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.


Nutrients, proteins, and other materials within most plant cells are moved about via cytoplasmic streaming. The process occurs as myosin proteins attached to organelles push against microfilaments arrayed throughout the cell. Microsfilaments and microtubules are responsible for almost all major cytoplasmic movements. During cell division, microbubules of the spindle-assembled form tubulin subunits near organelles called centrioles-move the chromosomes.



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