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| Human Cell |
It is estimated that an adult human body consists of about 75 trillion cells. These cells have much in common, yet those in different tissues van, in a number of ways. For example, they vary considerably in size. Cell sizes are measured in units called micrometers (microns). A micrometer equals 1/1000th of a millimeter and is symbolized Mm. Measured in micrometers, a human egg cell is about 140 microns meter in diameter and is just barely visible to an unaided eye. This is large when compared to a red blood cell, which is about 7.5 microns meter in diameter, or the most common white blood cells, which van from 10-12 microns meter in diameter. On the other hand, smooth muscle cells can be between 20-500 microns meter long.
Cells also vary in shape, and typically their shapes are closely related to their functions. For instance, nerve cells often have long, threadlike extensions that transmit nerve impulses from one part of the body to another. The epithelial cells that line the inside of the mouth serve to shield underlying cells. These protective cells are thin, flattened, and tightly packed, somewhat like the tiles of a floor Muscle cells, which function to pull parts closer together, are slender and rod-like, with their ends attached to the parts they move.
Composite Cell
Since cells vary so greatly in size, shape, and function, it is not possible to describe a typical cell. For purposes of discussion, however, it is convenient to imagine that one exists. Such a composite cell would contain parts observed in many kinds of cells, even though some of these cells lack parts included in the imagined structure.
Commonly a cell consists of two major parts, one within the other and each surrounded by a thin membrane. The inner portion is called the cell nucleus and it is enclosed by a nuclear membrane. A mass of fluid called cytoplasm surrounds the nucleus and is encircled by a cytoplasmic or cell membrane. Within the cytoplasm are other membranes that separate it into small subdivisions. These include networks of membranes and membranes that mark off tiny. distinct parts called cytoplasmic or ganelles. These organelles perform specific metabolic functions necessary for cell survival. The nucleus, on the other hand, directs the overall activities of the cell.
Cell Membrane
The cell membrane is the outermost limit of the cell, but it is more than a simple envelope surrounding the cellular contents. It is an actively functioning part of the living material and many important metabolic reactions actually take place on its surface. The membrane is extremely thin—visible only with the aid of an electron microscope—but is flexible and somewhat elastic It typically has a complex surface with many outpourings and infoldings that provide extra surface area. The membrane quick seals minute breaks, but if it is extensively damaged, the cell contents escape, and the cell dies.
In addition to maintaining the wholeness of the cell, the membrane serves as a "gateway" through which chemicals enter and leave. This gate acts in a special manner; it allows some substances to pass and excludes others. When a membrane functions in this way, it is called selectively permeable. A permeable membrane, on the other hand, is one that allows all materials to pass through. The mechanism by which the membrane accomplishes its selective function is not well understood. It is known, however, that the mechanism involves the chemical nature of the membrane— which is about 55% protein and 42% lipid (usually phospholipid and cholesterol) with a small amount of carbohydrate The lipids, for example, are largely insoluble in water. They form a boundary layer that separates the water)- contents of the cell from its watery environment, and prevent many water soluble substances from passing through. The way molecules are positioned within the cell membrane is not well understood either However, evidence indicates that the structural qualities of the membrane are due primarily to the molecules of phospholipids, which are arranged in two layers (bi layer).
Although there appear to be onh a fetty pes of lipid molecules in the membrane, there are many kinds of proteins. Some of these protein molecules are quite large and seem to extend across the lipid layer, other kinds are located on the outer surface of the lipid layer and project outward from it, and still other kinds reside on the cytoplasmic side of the membrane. The phospholipid molecules and the protein molecules embedded among them can more sideways in any- direction, so that instead of being a rigid structure, the membrane is flexible and acts like a thin film of liquid—a liquid of phospholipid molecules with protein molecules floating in it.
The carbohydrates of the membrane all seem to be associated with the outer surface, where some of them are combined with lipids (glycolipids) and others are combined with proteins (glycoproteins).
Since different kinds of molecules are located on the inner and outer surfaces of the membrane, it is not surprising that the two surfaces have functional differences For instance, various molecules on the outer surface function as receptor sites that can combine with specific chemicals, such as hormones. Many of the proteins of the inner surface function as enzymes that speed chemical reactions, such as those that help vital substances pass through the membrane.
Since the membrane is largely phospholipid, molecules that are soluble in lipids can pass through it easily. On the other hand, molecules of substances like water, which do not dissolve in lipids, cannot penetrate the phospholipid layers. However, water and certain other small molecules can pass through some of the regions where large protein molecules span the thickness of the membrane, creating minute passageways or "pores." Other pores serve as selective channels that allow only particular substances to pass through For example some channels control the movements of sodium and potassium ions and play important roles in the functions of muscle and nerve cells.Many cells, such as blood cells, are not in direct contact with their neighbors because fluid filled space (extracellular space) separates them. In other tissues, the cells are tightly packed, and the membranes of these cells are commonly connected by inter cellular junctions. One type of specialized junction, a desmosome. serves to rivet or "spot weld" adjacent skin cells so they form a reinforced structural unit. The membranes of certain other cells, such as those in heart muscle, are interconnected by gap junctions in the form of tubular channels. These channels link the cytoplasm of adjacent cells. They allow ions and nutrients, such as sugars, amino acids, and nucleotide, and certain other relatively small molecules to be exchanged between them.
