Essential Functions and Structure of a Cell

Cell structure and functions
The cells of the body perform specific functions, and therefore, their structures vary greatly. Even so, because all cells have the same basic organization, we can begin the study of cell structure by examining a generalized cell. Knowledge of the generalized animal cell was obtained by using the light microscope and the electron microscope. The light microscope, which utilizes light to view the object, does not show much detail, but the electron microscope, which uses electrons to view the object, allows cell biologists to make out cell structure in great detail. The plasma membrane has also been examined using the electron microscope.
The Plasma Membrane 
Our cells are surrounded by an outer plasma membrane. The plasma membrane is the boundary between the inside of the cell, termed the cytoplasm, and the outside of the cell. Plasma membrane integrity is necessary to the life of the cell.
The plasma membrane is a phospholipid bilayer with attached or embedded proteins. The phospholipid mole­cule has a polar head and nonpolar tails. Be­cause the polar heads are charged, they are hydrophilic (water-loving) and face outward, where they are likely to encounter a watery environment. The nonpolar tails are hy­drophobic (water-fearing) and face inward, where there is no water. When phospholipids are placed in water, they natu­rally form a spherical bilayer because of the chemical prop­erties of the heads and the tails.
At body temperature, the phospholipid bilayer is a liq­uid; it has the consistency of olive oil, and the proteins are able to change their positions by moving laterally. The fluid-mosaic model, a working description of membrane structure, suggests that the protein molecules have a chang­ing pattern (form a mosaic) within the fluid phospholipid bilayer. Our plasma membranes also contain a substantial number of cholesterol molecules. These mole­cules lend stability to the phospholipid bilayer and prevent a drastic decrease in fluidity at low temperatures.
Short chains of sugars are attached to the outer surfaces of some protein and lipid molecules (called glycoproteins and glycolipids, respectively). These carbohydrate chains, specific to each cell, mark the cell as belonging to a partic­ular individual and account for such characteristics as blood type or why a patient's system sometimes rejects an organ transplant. Some glycoproteins have a special config­uration that allows them to act as a receptor for a chemical messenger like a hormone. Some plasma membrane pro­teins form channels through which certain substances can enter cells, while others are carriers involved in the passage of molecules through the membrane.
The Nucleus 
The nucleus (nu'kle-us) is enclosed by a nuclear envelope that is continuous with the endoplasmic reticulum (ER), another part of the cell. Pores, or openings, in this nuclear envelope allow the passage oflarge molecules from the nucleoplasm, the fluid portion of the nucleus, to the cytoplasm.
The nucleus is the control center that oversees the cell's metabolic functioning and ultimately determines the cell's characteristics. Within the nucleus are masses of threads called chromatin (kro'mah-tin), so named because they take up stains and become colored. Chromatin is indistinct in the nondividing cell, but it condenses to rodlike struc­tures called chromosomes (kro'mo-somz) just prior to cell division.
Chromosomes contain DNA, which makes up the genes. DNA is double-stranded; each strand carries a particular sequence of nitrogen bases. These serve as a genetic code that is passed on to a type of RNA called messenger RNA (mRNA) for the purpose of di­recting protein synthesis in the cell. Another type of RNA, called transfer RNA (tRNA) , is also made in the nucleus and functions in protein synthesis. Some of the synthe­sized proteins have a structural role, and some are enzymes involved in metabolism, which is all the chemical reac­tions that occur in the cell. In this way, DNA controls cell structure and function.
Occasionally, the sequence of bases in DNA changes, and this mistake, called a mutation, is copied by mRNA and may result in a faulty protein. The individual in which this occurs is said to have a genetic disease because the cells cannot function properly, and the result is a notice­able illness.
The nucleolus (nu-kle' o-lus) is a spherical body found in the nucleus. Here, another type of RNA, called ribosomal RNA (rRNA), is formed and contributes to the manufacture of small granules called ribosomes (ri'bo-somz). After their formation in the nucleolus, ribo­somes are transported from the nucleus to the cytoplasm, where they function in protein synthesis.
The nucleus contains chromatin, which condenses into chromosomes just prior to cell division. Genes, composed of DNA. are on the chromosomes, and they code for the production of proteins in the cytoplasm. The nucleolus is involved in ribosome formation.

Next: Cytoplasm and Ribosomes                                                   Continue>>

Posted in Human Anatomy and Physiology