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 (PM) | Cell Walls
| Cytoplasm | Cytoskeleton
| Nucleus | Ribosomes |
Endoplasmic Reticulum (Er) | Golgi Apparatus |
Lysosomes | Mitochondria
and Chloroplasts (Plastids) | Cilia and Flagella |
Centrioles and Basal Bodies
The plasma membrane (PM) is 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.
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.
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.
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.
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 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).
Among the other membranous structures within the eukaryotic cell are extensive membrane systems that make up the smooth and the rough endoplasmic reticulum (SER and RER). The SER often takes the form of branching tubes. (In skeletal muscle it acts as a reservoir for calcium ions and is called the sacroplasmic reticulum.) The RER is made up of sheetlike 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 RER 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.
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 RER, by linking them with sugars or other molecules.
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 (ATP). This process uses up oxygen and is called aerobic respiration. Plants possess, in addition to mitochondria, similar organelles called chloroplasts. Each chloroplast contains the green pigment chlorophyll, which is used to convert light energy from the sun into ATP. This process is called photosynthesis.
Some cells have flexible, whiplike external appendages called cilia and flagella, which are used for locomotion and for capturing food. Cilia are 3 to 10 micrometers long and are found on protozoa as well as in human oviducts and respiratory tracts. In the respiratory tract they sweep large particles up the trachea and prevent them from passing into the lungs. Flagella, which may be ten times as long, are found on some protozoa and unicellular plants, and they are used for locomotion by the sperm of higher organisms. Eukaryotic cilia and flagella are composed of microtubules covered by a membrane sheath. Prokaryotic flagella are more slender and are composed of the protein flagellin. They propel the cell by rotating like the propeller of a ship rather than by a whipping motion.
All animal and some plant cells contain a pair of centrioles, which are cylindrical structures composed of short microtubules. They are surrounded by a cloud of fuzzy material, the exact function of which is unknown. Centrioles control the arrangement of microtubules in the cell cytoskeleton. Basal bodies, which are similar, are structures that anchor cilia and flagella within the cytoplasm, just inside the plasma membrane. Centrioles and basal bodies both contain DNA and apparently can duplicate themselves independently of duplication of the entire cell.