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Sample essay topic, essay writing: The Cell - 1602 words

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The cell is the fundamental structural unit of all living organisms. Some cells are complete organisms, such as the unicellular bacteria and protozoa; others, such as nerve, liver, and muscle cells, are specialized components of multi-cellular organisms. Cells range in size from the smallest bacteria-like mycoplasmas, which are 0.1 micrometer in diameter, to the egg yolks of ostriches, which are about 8 cm (about 3 in) in diameter. Although they may differ widely in appearance and function, all cells have a surrounding membrane and an internal, water-rich substance called the cytoplasm, the composition of which differs significantly from the external environment of the cell. Within the cell is genetic material, deoxyribonucleic acid (DNA), containing coded instructions for the behavior and reproduction of the cell and also the chemical machinery for the translation of these instructions into the manufacture of proteins. Viruses are not considered cells because they lack this translation machinery; they must parasitize cells in order to translate their own genetic code and reproduce themselves.Cells are of two distinctly different types, prokaryotes and eukaryotes; thus, the living world is divided into two broad categories.

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 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

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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, 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 plasma membrane 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 plasma membrane helps maintain high local concentrations of organic molecules within the cell, it also allows interaction between the cell and its external environment.The plasma membrane mediates such interactions in various ways. The exchange of mineral ions and small nutrient molecules is controlled by plasma membrane proteins that act as pumps, carriers, and channels. The plasma membrane 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. In addition, the plasma membrane 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 plasma membrane 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 plasma membranes 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 plasma membrane 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 interconnect and support the other 'solid' structures of the cell.

The composition and function of the MTL are as yet still unknown, but it is thought to control the spatial arrangement of cell components within the cytoplasm.Cytoskeleton The so-called cytoskeleton influences the shape of the cell in much the same way tent poles determine the shape of a tent. Without the cytoskeleton a cell tends to become spherical. The cytoskeleton probably gives direction to the movement of components within the cytoplasm as well and participates in movement of the cell itself. The cytoskeleton is composed of three main filament types: the microtubules, microfilaments, and intermediate-sized filaments that are supported and distributed within the MTL. Microtubules are long rigid cylinders that act as the bones of the cell. They also may act as tracks along which intracellular components are transported. The walls of the cylinders are composed of two proteins, alpha- and beta-tubulin. Microfilaments are composed of actin, a major protein of muscle.

They often occur in long bundles called stress fibers and may act as the muscles of the cell. The intermediate-sized filaments are a heterogeneous class of proteins whose function is largely unknown.Nucleus The membrane-bounded structures contained within the cytoplasm of eukaryotes are referred to as organelles. The nucleus is the most easily recognizable of these. DNA, combined with protein, is organized inside the nucleus into structural units called chromosomes, which usually occur in identical pairs. The DNA in each chromosome is a single, very long, highly coiled molecule subdivided into functional subunits called genes. Genes contain the coded instructions for the assembly of polypeptides and larger proteins. Together the chromosomes contain all the information needed to build an identical functioning copy of the cell.The nucleus is surrounded by an envelope of two concentric membranes.

Interaction between the nuclear contents and the surrounding cytoplasm is permitted through holes, called nuclear pores, in this envelope. The nucleus also contains a specialized region, the nucleolus, where nucleoprotein particles are assembled. These particles migrate through the nuclear pores into the cytoplasm, where they are modified to become ribosomes.Ribosomes Ribosomes are the 'factories' where the instructions encoded in the DNA of the nucleus are translated to make proteins. The instructions are carried from the DNA to the ribosomes by long nucleic-acid molecules called messenger ribonucleic acids (RNAs).Endoplasmic Reticulum (Er) Among the other membranous structures within the eukaryotic cell are extensive membrane systems that make up the smooth and the rough endoplasmic reticulum. The smooth endoplasmic reticulum often takes the form of branching tubes The rough endoplasmic reticulum is made up of sheet-like flattened sacs, which often are stacked one on top of the other; the term rough refers to the numerous ribosomes that dot the cytoplasmic surfaces of the sacs.

The rough endoplasmic reticulum is one of the sites of protein synthesis in the cytoplasm. Proteins are synthesized on the cytoplasmic surface and pass through the membrane to become sequestered within the sacs. These packaged proteins are destined for secretion to the outside of the cell. Other proteins, synthesized on ribosomes that are not attached to membranes, are not secreted and remain as structural proteins or metabolic enzymes.Golgi Apparatus Similar in appearance to and perhaps continuous with the ER is a region of smooth, stacked membranous sacs known as the Golgi apparatus. Cell biologists think that the apparatus modifies proteins, after they are synthesized and packaged on the rough endoplasmic reticulum, by linking them with sugars or other molecules.Lysosomes Lysosomes are membrane-bounded bags, or vesicles, containing digestive enzymes. Their normal function is digestion of complex nutrients and broken-down organelles.

In disease fighting, the lysosomes of white blood cells aid in the digestion of engulfed bacteria and other foreign or toxic materials.Mitochondria and Chloroplasts (Plastids) Mitochondria are the powerhouses of the animal cell, where the products of the enzymatic breakdown, or metabolism, of nutrients such as glucose are converted into energy in the form of the molecule adenosine triphosphate ...

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