The endomembrane system consists of the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, and vesicles (tiny membranous sacs) (Fig. 3.5). These components of the cell work together to produce and secrete a product.
The Endoplasmic Reticulum
The endoplasmic reticulum (ER), a complicated system of membranous channels and saccules (flattened vesicles), is physically continuous with the outer membrane of the nuclear envelope. Rough ER is studded with ribosomes on the side of the membrane that faces the cytoplasm. Here proteins are synthesized and enter the ER interior where processing and modification begin. Some of these proteins are incorporated into membrane, and some are for export. Smooth ER, which is continuous with rough ER, does not have attached ribosomes. Smooth ER synthesizes the phospholipids that occur in membranes and has various other functions, depending on the particular cell. In the testes, it produces testosterone, and in the liver it helps detoxify drugs. Regardless of any specialized function, ER also forms vesicles in which large molecules are transported to other parts of the cell. Often these vesicles are on their way to the plasma membrane or the Golgi apparatus.
The Golgi Apparatus
The Golgi apparatus is named for Camillo Golgi, who discovered its presence in cells in 1898. The Golgi apparatus consists of a stack of three to twenty slightly curved saccules whose appearance can be compared to a stack of pancakes (Fig. 3.5). In animal cells, one side of the stack (the inner face) is directed toward the ER, and the other side of the stack (the outer face) is directed toward the plasma membrane. Vesicles can frequently be seen at the edges of the saccules. The Golgi apparatus receives protein and/or lipid-filled vesicles that bud from the ER. Some biologists believe that these fuse to form a saccule at the inner face and that this saccule remains a part of the Golgi apparatus until the molecules are repackaged in new vesicles at the outer face. Others believe that the vesicles from the ER proceed directly to the outer face of the Golgi apparatus, where processing and packaging occur within its saccules. The Golgi apparatus contains enzymes that modify proteins and lipids. For example, it can add a chain of sugars to proteins and lipids, thereby making them glycoproteins and glycolipids, which are molecules found in the plasma membrane. The vesicles that leave the Golgi apparatus move to other parts of the cell. Some vesicles proceed to the plasma membrane, where they discharge their contents. Because this is secretion, note that the Golgi apparatus is involved in processing, packaging, and secretion. Other vesicles that leave the Golgi apparatus are lysosomes.
Lysosomes, membranous sacs produced by the Golgi apparatus, contain hydrolytic digestive enzymes. Sometimes macromolecules are brought into a cell by vesicle formation at the plasma membrane (Fig. 3.5). When a lysosome fuses with such a vesicle, its contents are digested by lysosomal enzymes into simpler subunits that then enter the cytoplasm. Even parts of a cell are digested by its own lysosomes (called autodigestion). Normal cell rejuvenation most likely takes place in this manner, but autodigestion is also important during development. For example, when a tadpole becomes a frog, lysosomes digest away the cells of the tail. The fingers of a human embryo are at first webbed, but they are freed from one another as a result of lysosomal action. Occasionally, a child is born with Tay-Sachs disease, a metabolic disorder involving a missing or inactive lysosomal enzyme. In these cases, the lysosomes fill to capacity with macromolecules that cannot be broken down. The cells become so full of these lysosomes that the child dies. Someday soon, it may be possible to provide the missing enzyme for these children.
Figure 3.5 The endomembrane system. Vesicles from the ER bring proteins and lipids to the Golgi apparatus where they are modified and repackaged into vesicles. Secretion occurs when vesicles fuse with the plasma membrane. Lysosomesmade at the Golgi apparatus digestmacromolecules after fusing with incoming vesicles.
Although the size and shape of mitochondria (sing., mitochondrion) can vary, all are bounded by a double membrane. The inner membrane is folded to form little shelves called cristae, which project into the matrix, an inner space filled with a gel-like fluid (Fig. 3.6). Mitochondria are the site of ATP (adenosine triphosphate) production involving complex metabolic pathways. As you know, ATP molecules are the common carrier of energy in cells. A shorthand way to indicate the chemical transformation that involves mitochondria is as follows:
Mitochondria are often called the powerhouses of the cell: Just as a powerhouse burns fuel to produce electricity, the mitochondria convert the chemical energy of glucose products into the chemical energy of ATP molecules. In the process, mitochondria use up oxygen and give off carbon dioxide and water. The oxygen you breathe in enters cells and then mitochondria; the carbon dioxide you breathe out is released by mitochondria. Because oxygen is involved, we say that mitochondria carry on cellular respiration. The matrix of a mitochondrion contains enzymes for breaking down glucose products. ATP production then occurs at the cristae. The protein complexes that aid in the conversion of energy are located in an assembly-line fashion on these membranous shelves. Every cell uses a certain amount of ATP energy to synthesize molecules, but many cells use ATP to carry out their specialized functions. For example, muscle cells use ATP for muscle contraction, which produces movement, and nerve cells use it for the conduction of nerve impulses, which make us aware of our environment.
Figure 3.6 Mitochondrion structure. Mitochondria are involved in cellular respiration. a. Electron micrograph of a mitochondrion. b. Generalized drawing in which the outer membrane and portions of the inner membrane have been cut away to reveal the cristae.
Several types of filamentous protein structures form a cytoskeleton that helps maintain the cell’s shape and either anchors the organelles or assists their movement as appropriate. The cytoskeleton includes microtubules, intermediate filaments, and actin filaments (see Fig. 3.1). Microtubules are hollow cylinders whose wall is made up of 13 logitudinal rows of the globular protein tubulin. Remarkably, microtubules can assemble and disassemble. Microtubule assembly is regulated by the centrosome which lies near the nucleus. Microtubules radiate from the centrosome, helping to maintain the shape of the cell and acting as tracks along which organelles move. It is well known that during cell division, microtubules form spindle fibers, which assist the movement of chromosomes. Intermediate filaments differ in structure and function. Actin filaments are long, extremely thin fibers that usually occur in bundles or other groupings. Actin filaments have been isolated from various types of cells, especially those in which movement occurs. Microvilli, which project from certain cells and can shorten and extend, contain actin filaments. Actin filaments, like microtubules, can assemble and disassemble.
Centrioles are short cylinders with a 9 + 0 pattern of microtubules, meaning that there are nine outer microtubule triplets and no center microtubules (see Fig. 3.1). Each cell has a pair of centrioles in the centrosome near the nucleus.
The members of each pair of centrioles are at right angles to one another. Before a cell divides, the centrioles duplicate, and the members of the new pair are also at right angles to one another. During cell division, the pairs of centrioles separate so that each daughter cell gets one centrosome. Centrioles may be involved in the formation of the spindle that functions during cell division. Their exact role in this process is uncertain, however. Centrioles also give rise to basal bodies that direct the formation of cilia and flagella.
Cilia and Flagella
Cilia and flagella (sing., cilium, flagellum) are projections of cells that can move either in an undulating fashion, like a whip, or stiffly, like an oar. Cilia are shorter than flagella (Fig. 3.7). Cells that have these organelles are capable of selfmovement or moving material along the surface of the cell. For example, sperm cells, carrying genetic material to the egg, move by means of flagella. The cells that line our respiratory tract are ciliated. These cilia sweep debris trapped within mucus back up the throat, and this action helps keep the lungs clean. Each cilium and flagellum has a basal body at its base, which lies in the cytoplasm. Basal bodies, like centrioles, have a 9 + 0 pattern of microtubule triplets. They are believed to organize the structure of cilia and flagella even though cilia and flagella have a 9 + 2 pattern of microtubules. In cilia and flagella, nine microtubule doublets surround two central microtubules. This arrangement is believed to be necessary to their ability to move.
Figure 3.7 Cilia and flagella. a. Cilia are common on the surfaces of certain tissues, such as the one that forms the inner lining of the respiratory tract. b. Flagella form the tails of human sperm cells.