Components of Eukaryotic Cells and Their Functions

Components of Eukaryotic Cells and Their Functions
Typically, eukaryotic cells are enclosed within a thin, selectively permeable cell membrane (Figure 3-4). The most prominent organelle is the spherical or ovoid nucleus, enclosed within two membranes to form the double-layered nuclear envelope (Figure 3-4). The region outside the nucleus is regarded as cytoplasm. Within the cytoplasm are many organelles, such as mitochondria, Golgi complexes, centrioles, and endoplasmic reticulum. Plant cells typically contain plastids, some of which are photosynthetic organelles, and plant cells bear a cell wall containing cellulose outside the cell membrane.

The fluid-mosaic model is the currently accepted concept of cell membranes. By electron microscopy, the cell membrane appears as two dark lines, each approximately 3 nm thick, at each side of a light zone (Figure 3-5). The entire membrane is 8 to 10 nm thick. This image is the result of a phospholipid bilayer, two layers of phospholipid molecules, all oriented with their water-soluble ends toward the outside and their fat-soluble portions toward the inside of the membrane (Figure 3-6). An important characteristic of the phospholipid bilayer is that it is liquid, giving the membrane flexibility and allowing the phospholipid molecules to move sideways freely within their own monolayer. Molecules of cholesterol are interspersed in the lipid portion of the bilayer (Figure 3-6). They make the membrane even less permeable and decrease its flexibility.

Figure 3-5 Plasma membranes of two adjacent cells. Each membrane (between arrows) shows a typical dark-light-dark staining pattern. (×325,000)

Figure 3-6 Diagram illustrating fluid-mosaic model of a cell membrane

Glycoproteins (proteins with carbohydrates attached) are essential components of cell membranes. Some of these proteins catalyze the transport of substances such as negatively charged ions across the membrane. Others act as specific receptors for various molecules or as highly specific markings. For example, the self/nonself recognition that enables the immune system to react to invaders (Immunity) is based on proteins of this type. Some aggregations of protein molecules form pores through which small polar molecules may enter. Like the phospholipid molecules, most of the glycoproteins can move laterally in the membrane, although more slowly.

Nuclear envelopes contain less cholesterol than cell membranes, and pores in the envelope (Figure 3-7) allow molecules to move between nucleus and cytoplasm. Nuclei contain chromatin, a complex of DNA, basic proteins called histones, and nonhistone protein. Chromatin carries the genetic information, the code that results in most of the components characteristic of the cell after transcription and translation (see Principles of Genetics:A Review). Nucleoli are specialized parts of certain chromosomes that stain in a characteristically dark manner. They carry multiple copies of the DNA information to synthesize ribosomal RNA. After transcription from DNA, ribosomal RNA combines with protein to form a ribosome, detaches from the nucleolus, and passes to the cytoplasm through pores in the nuclear envelope.

The outer membrane of the nuclear envelope is continuous with extensive membranous elements in the cytoplasm called endoplasmic reticulum (ER) (Figures 3-7 and 3-8). The space between the membranes of the nuclear envelope communicates with channels (cisternae) in the ER. The ER is a complex of membranes that separates some of the products of the cell from the synthetic machinery that produces them, apparently functioning as routes for transport of proteins within the cell. Membranes of the ER may be covered on their outer surfaces with ribosomes and are thus designated rough ER, or they may lack ribosomal covering and be called smooth ER. Smooth ER functions in synthesis of lipids and phospholipids. Protein synthesized by ribosomes on rough ER enters the cisternae and from there is transported to the Golgi apparatus or complex.

The Golgi complex (Figures 3-9 and 3-10) is composed of a stack of membranous vesicles that function in storage, modification, and packaging of protein products, especially secretory products. The vesicles do not synthesize protein but may add complex carbohydrates to the molecules. Small vesicles of ER containing protein pinch off and then fuse with sacs on the “forming face” of a Golgi complex. After modification, the proteins bud off vesicles on the “maturing face” of the complex (Figure 3-10). The contents of some of these vesicles may be expelled to the outside of the cell, as secretory products destined to be exported from a glandular cell. Others may contain digestive enzymes that remain in the same cell that produces them. Such vesicles are called lysosomes (literally “loosening body,” a body capable of causing lysis, or disintegration). Enzymes that they contain are involved in the breakdown of foreign material, including bacteria engulfed by the cell. Lysosomes also are capable of breaking down injured or diseased cells and worn-out cellular components. Their enzymes are so powerful that they kill the cell that formed them if the lysosome membrane ruptures. In normal cells the enzymes remain safely enclosed within the protective membrane. Lysosomal vesicles may pour their enzymes into a larger membrane-bound body containing an ingested food particle, the food vacuole or phagosome. Other vacuoles, such as contractile vacuoles of some single-celled organisms, may contain only fluid and function to regulate ions and water.

Figure 3-8 Endoplasmic reticulum. A, Endoplasmic reticulum is continuous with the nuclear envelope. It may have associated ribosomes (rough endoplasmic reticulum) or not (smooth endoplasmic reticulum). B, Electron micrograph showing rough endoplasmic reticulum. (× 28,000)

Figure 3-9 Golgi complex (= Golgi body, Golgi apparatus). A, The smooth cisternae of the Golgi complex have enzymes that modify proteins synthesized by the rough endoplasmic reticulum. B, Electron micrograph of a Golgi complex. (×46,000).

Mitochondria (sing., mitochondrion) (Figure 3-11) are conspicuous organelles present in nearly all eukaryotic cells. They are diverse in size, number, and shape; some are rodlike, and others are more or less spherical. They may be scattered uniformly through the cytoplasm, or they may be localized near cell surfaces and other regions where there is high metabolic activity. A mitochondrion is composed of a double membrane. The outer membrane is smooth, whereas the inner membrane is folded into numerous platelike or fingerlike projections called cristae (Figure 3-11), which increases internal surface area where chemical reactions take place. These characteristic features make mitochondria easy to identify among the organelles. Mitochondria are often called “powerhouses of the cell,” because enzymes located on the cristae carry out the energy-yielding steps of aerobic metabolism. ATP (adenosine triphosphate), the most important energytransfer molecule of all cells, is produced in this organelle. Mitochondria are self-replicating. They have a tiny, circular genome, much like the genomes of prokaryotes except that it is much smaller. It contains DNA that specifies some, but not all, of the proteins of the mitochondrion.



Eukaryotic cells characteristically have a system of tubules and filaments that form the cytoskeleton (Figures 3-12 and 3-13). These provide support and maintain the form of cells, and in many cells, they provide a means of locomotion and translocation of organelles within the cell. Microfilaments are thin, linear structures, first observed distinctly in muscle cells, where they are responsible for the ability of the cell to contract. They are made of a protein called actin. Several dozen other proteins are known that bind with actin and determine its configuration and behavior in particular cells. One of these is myosin, whose interaction with actin causes contraction in muscle and other cells. Actin microfilaments also provide a means for moving messenger RNA from the nucleus to particular positions within the cell. Microtubules, somewhat larger than microfilaments, are tubular structures composed of a protein called tubulin (Figure 3-13). They play a vital role in moving the chromosomes toward the daughter cells during cell division as will be seen later, and they are important in intracellular architecture, organization, and transport. In addition, microtubules form essential parts of the structures of cilia and flagella. Microtubules radiate out from a microtubule organizing center, the centrosome, near the nucleus. Centrosomes are not membrane bound. Within centrosomes are found a pair of centrioles (Figures 3-4 and 3-14), which are themselves composed of microtubules. Microtubules radiating from the centrioles form the aster. Each centriole of a pair lies at right angles to the other and is a short cylinder of nine triplets of microtubules. They replicate before cell division. Although cells of higher plants do not have centrioles, a microtubule organizing center is present. Intermediate filaments are larger than microfilaments but smaller than microtubules. There are five biochemically distinct types of intermediate filaments, and their composition and arrangement depend on the cell type in which they are found.