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Cells Essay, Research Paper

Cells are composed of oxygen, hydrogen, carbon, and nitrogen, which contain most important organic compounds in a cell are proteins, nucleic acids, lipids, and carbohydrates,Water makes up 60 to 65 percent of the cell.Some cells are complete organisms, as the unicellular bacteria and protozoa; others, such as nerve, liver, and muscle cells, are components of multicellular organisms,Cells range in size from the smallest bacteria like mycoplasmas, which are 0.1 micrometer in diameter, to a egg yolks of ostriches, which are about 8 cm in diameter.tehj shape and althought they may look different they insides are all the same, all cells have a surrounding membrane and an internal, water-rich substance called the cytoplasmCells are of two distinctly different types, prokaryotes and eukaryotes; thus, the living world is divided into two broad categories (see Classification). The DNA of prokaryotes is a single molecule in direct contact with the cell cytoplasm, whereas the DNA of eukaryotes is much greater in amount and diversity and is contained within a nucleus separated from the cell cytoplasm by a membranous nuclear envelope. Many eukaryotic cells are further divided into compartments by internal membranes in addition to the nuclear envelope, whereas prokaryotic cells never contain completely internal membranes. The prokaryotes include the mycoplasmas, bacteria, and cyanobacteria (formerly known as blue-green algae). The eukaryotes comprise all plant and animal cells. In general, plant cells differ from animal cells in that they have a rigid cell wall exterior to the plasma membrane; a large vacuole, or fluid-filled pouch; and chloroplasts that convert light energy to chemical energy for the synthesis of glucose.

Structure and Function

Cells are composed primarily of oxygen, hydrogen, carbon, and nitrogen, the elements that make up the majority of organic compounds. The most important organic compounds in a cell are proteins, nucleic acids, lipids, and polysaccharides (carbohydrates). The “solid” structures of the cell are complex combinations of these large molecules. Water makes up 60 to 65 percent of the cell, because water is a favorable environment for biochemical reactions.

All cells are dynamic at some stage of their life cycle, in the sense that they use energy to perform a variety of cell functions: movement, growth, maintenance and repair of cell structure, reproduction of the cell, and manufacture of specialized cell products such as enzymes and hormones. These functions are also the result of interactions of organic molecules.

Plasma Membrane

The plasma membrane (PM), a continuous double layer of phospholipid molecules 75 to 100 angstroms thick, constitutes the boundary between the cell and its external environment. In addition to lipids, the PM has protein components (polypeptides) that are associated with either the outer or inner surfaces of its layers or are buried within them. The structure as a whole is selectively permeable, or semipermeable; that is, it permits the exchange of water and selected atoms and molecules between the cell exterior and interior. This is vital to the cell because while the PM helps maintain high local concentrations of organic molecules within the cell, it also allows interaction between the cell and its external environment.

The PM mediates such interactions in various ways. The exchange of mineral ions and small nutrient molecules is controlled by PM proteins that act as pumps, carriers, and channels. The PM also participates in the exchange of larger molecules through phagocytosis, the engulfing of large food particles; endocytosis, the intake of fluids and membrane components; and exocytosis, the expulsion of cell products or cell waste. (The PM of some cells, such as those of the human intestine, is convoluted to enhance the surface area for these exchanges.) In addition, the PM contains receptors that selectively receive nerve and hormone signals and transmit them to the interior of the cell. Finally, direct cell-to-cell interactions can occur through specialized regions of the PM known as junctions. Organs such as the skin and the small intestine consist of cells held together by tight junctions and local thickenings, or desmosomes, which constitute another type of junction. Cells can communicate electrically through a third type of junction, called a gap junction, that consists of tiny protein “tunnels” between two cells, through which tiny “message” molecules and ions may be passed. When the PMs of two cells are continuous, an actual bridge of cytoplasm forms between them; in plants these bridges are called plasmodesmata.

Cell Walls

Exterior to the PM of most plant cells and bacteria is a cell wall, a cell product made largely of complex polysaccharides. In higher plants the polysaccharide is cellulose. The presence of a cell wall makes these cells rigid and sturdy, but it poses special problems for the transport of substances into and out of the cell.

Cytoplasm

The cytoplasm is the water-rich matrix within a cell that contains and surrounds the other cellular contents. It is more like a viscous gel than a watery solution, but it liquefies when shaken or stirred. Such gel-to-sol transitions are thought by some cell biologists to play a role in the movement of a cell’s components from place to place within the cell. Rapid movement of cell components is called either streaming or cyclosis, depending on whether it occurs linearly or circularly.

Through an electron microscope the cytoplasmic gel appears as a three-dimensional lattice of slender, protein-rich strands in a continuous water-rich phase. Because the latticework is reminiscent of the internal structure of spongy bone, which is composed of many struts, or trabeculae, it is called the microtrabecular lattice (MTL). The MTL appears to

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