Organelles

Smooth ER

Although the smooth ER communicates with the rough variety, it plays no role in protein synthesis. Instead it functions in lipid metabolism (cholesterol and fat synthesis and breakdown), and detoxification of drugs and pesticides. Hence it is not surprising that the liver cells are chock-full of smooth ER. So too are body cells that produce steroid-based hormones-for instance, cells of the male testes that manufacture testosterone.

Cytoskeleton

An elaborate network of protein structures extends throughout the cytoplasm. This network, or cytoskeleton, acts as a cell's "bones and muscles" by furnishing an internal framework that determines cell shape, supports other organelles, and provide the machinery for intracellular transport and various types of cellular movements. From its largest to its smallest elements. the cytoskeleton is made up of microtubules, intermediate filaments, and microfilaments. Although there is some overlap , in roles, generally speaking the strong, stable, rope like intermediate filaments help form desmosomes and provide internal g wires to resist pulling forces on the cell. Microfilaments (such as actin and mysoin) are most involved in cell motility and in producing changes in cell shape. The tube like mircotubules determine the overall shape of a cell and the distribution of organelles. They are very important during cell division.

Cilia

Cilia are whip like cellular extensions that move substances along he cell surface. For example, the ciliated cells of the respiratory system lining move mucus up and away from the lungs. Where cilia appear, there are usually many of them projecting from the exposed cell surface. When a cell is about to make cilia, its centrioles multiply and then line up beneath the plasma membrane at the free cell surface. Mircotubules the begin to "sprout" from the centrioles and put pressure on the membrane, forming the projections.

Flagella

If the projections formed by the centrioles are substantially longer, they are called flagella. The only example of a flagellated cell in the human body is the sperm, which has a single propulsive flagellum called its tail.

Lysosomes

Lysosomes or breakdown bodies, which appear in different sizes, are membranous "bags" containing powerful digestive enzymes. Because lysosomal enzymes are capable of digesting worn-out or nonusable cell structures and most foreign substance that enter the cell, lysosomes function as the cell's demolition sites. Lysosomes are especially abundant in phagocytes, the cells that dispose o bacteria and cell debris. As described above,the enzymes they contain formed by ribosomes and packaged by the Golgi apparatus. The lysosomal membrane is ordinarily quite stable, but it becomes fragile when the cell is injured or deprived of oxygen and when excessive amount of vitamin A are present. When lysosomes rupture, the cell self-digests.

Microvilli

Microvilli are tiny, finger like extensions of the plasma membrane that project from an exposed cell surface. they increase the cell's surface are tremendously and so are usually found on the surface of cells active in absorption like intestinal and kidney tubule cells. Microvilli have a core of actin filaments that extend into the internal cytoskeleton of the cell and stiffen the microvillus.

Mitochondria (mi'to-kon'dre-ah)

Mitochondria are usually depicted as tiny, lozenge-like or sausage-shaped organelles (see Figure 3.4), but in living cells, they squirm, lengthen, and change shape almost continuously. The mitochondrial wall consists of a double membrane, equal to two plasma membranes, placed side by side. The outer membrane is smooth and featureless, but the inner membrane has shelflike protrusions called cristae (kris'te; "crests"). Enzymes dissolved in the fluid within the mitochondria, as well as enzymes that form part of the cristae membranes, carry out the reactions in which oxygen is used to break down foods. As the foods are broken down, energy is released. Much of this energy escapes as heat, but some are captured and used to form ATP molecules. ATP provides the energy for all cellular work, and every living cell requires a constant supply of ATP for its many activities. Because the mitochondria supply most of this ATP, they are the "powerhouse" of the cell. Metabolically "busy" cells, like liver and muscle cells, use huge amounts of ATP and have hundreds of mitochondria. By contrast, cells that are relatively inactive (an unfertilized egg, for instance) have just a few.

Peroxisomes

Peroxisomes are membranous sacs containing powerful oxidase enzymes that us e molecular oxygen (O2) to detoxify a number of harmful or poisonous substances, including alcohol and formaldehyde. However, their most important function is to "disarm" dangerous free radicals. Free radicals are highly reactive chemicals with unpaired electrons that can scramble the structure of proteins and nucleic acids. Free radicals are normal by-products of cellular metabolism, but if allowed to accumulate, they can have devastating effects on cells. Peroxisomes convert free radicals t hydrogen peroxide (H2O2), a function indicated in their naming (peroxisomes = "peroxide bodies"). The enzyme catalase then converts excess hydrogen peroxide to water. Peroxsomes are especially numerous in liver and kidney cells, which are very active in detoxification. Although peroxisomes look like small lysosomes, they do not arise by budding from the Golgi apparatus. Instead, they appear to replicate themselves by simply pinching in half, as do mitochondria.

Cytoplasm

The cytoplasm is the cellular material outside the nucleus and inside the plasma membrane. It is the site of most cellular activities, so you might think of the cytoplasm as the "factory area" of the cell. Although early scientists believed that the cytoplasm was a structureless gel, the electron microscope has revealed that it has three major elements: the cytosol, organelles, and inclusions.

Cytosol

The cytosol is semitransparent fluid that suspends the other elements. Dissolved in the cytosol, which is largely water, are nutrients and a variety of other solutes (dissolved substances).

Endoplasmic Reticulum (en"do-plas'mik rě-tik'u-lum;

The endoplasmic reticulum (ER) is a system of fluidfilled cisterns (tubules, or canals) that coil and twist through the cytoplasm. It accounts for about half of a cell's membranes. It serves as a minicirculatory system for the cell because it provides a network of channels for carrying substances (primarily proteins) from one part of the cell to another. There are two forms of ER; a particular cell have both forms or only one, depending may on its specific functions.

Organelles

The organelles are the metabolic machinery of the cell. Each type of organelle is specialized to carry out a specific function for the cell as a whole. Some synthesize proteins, other s package those proteins, and so on.

Centrioles

The paired centrioles lie close to the nucleus. They are rod-shaped bodies that lie at right angles to each other, internally they are made up of a pinwheel array of fine microtubules. Centrioles are best known for their role in generating microtubules, and during cell division, the centrioles direct the formation of the mitotic spindle.

Rough ER

The rough ER is so called because it is studded with ribosomes. Because essentially all of the building materials of cellular membranes are formed either in it or on it, you can think of the rough ER as the cell's membrane factory. The proteins made on its ribosomes migrate into the tubules of the rough ER, where they fold into their functional three-dimensional shapes and then are dispatched to other areas of the cell in transport vesicles. Rough ER is especially abun- dant in cells that make and export proteins-for example, pancreas cells, which produce digestive enzymes to be delivered to the small intestine. The enzymes that catalyze the synthesis of membrane lipids reside on the external face of the rough ER, where the needed building blocks are readily available.

Inclusions

are chemical substances that may or may not be present, depending on the specific cell type. Most inclusions are stored nutrients of cell products. They include the lipid droplets common in fat cells, glycogen granules abundant in liver and muscle cells, pigments such as melanin in skin and hair cells, mucus and other secretory products, and various kinds of crystals.

Ribosomes (ri'bo-somz)

are tiny, bilobed, dark bodies made of proteins and one variety of RNA called ribosomal RNA. Ribosomes are the actual sites of protein synthesis in the cell. Some ribosomes float free in the cytoplasm, where they manufacture proteins that function in the cytoplasm. Others attach to membranes, and the whole ribosome-membrane combination is called the rough endoplasmic reticulum.

Golgi Apparatus

the Golgi Apparatus appears as a stack of flattened membranous sacs, associated with swarms of tin vesicles. It is generally found close to the nucleus and is the principal "traffic director" for cellular proteins. Its major function is to modify and package proteins (sent to it by the rough ER via transport vesicles) in specific ways, depending on their final destination. As proteins "tagged" for export accumulate in the Golgi apparatus, the sacs swell. Then their swollen ends, filled with protein, pinch off and form secretory vesicles, which travels to the plasma membrane. When the vesicles reach the plasma membrane, they fuse with it, the membrane ruptures, and the contents of the sac are ejected to the outside of the cell. Mucus is packaged this way, as are digestive enzymes made by the pancreas cells. In addition to its packaging-for-release functions, the Golgi Apparatus pinches off sacs containing proteins and phospholipids destined for a "home" in the plasma membrane or other cellular membranes. IT also packages hydrolytic enzymes into membranous sacs calle lysosomes that remain in the cell.