2-41 Unique structures in eukaryotes

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• Microfilaments and microtubules form a network of fibers that determine cell shape, participate in cell division, move organelles around the cell and form motility structures.
• The endoplasmic reticulum is a network of tube and vesicles involved in membrane synthesis and in export of protein and other substances from the cell.
• The Golgi apparatus is a double membrane structure mainly concerned with the maturation of proteins synthesized in the endoplasmic reticulum and the formation of lysosomes.
• Lysosomes are membrane encircled structures that contain digestive enzymes. Their exact role in the cell depends upon the cell type.

Microfilaments and microtubules

The cytoskeleton is a network of filaments and fibers found in the cytoplasm of many eukaryotic cells. It serves four known roles in cells.

• The cytoskeleton helps to determine the shape of the cell.
• It is involved in cell division, allowing the separation of the chromosomes into the daughter cells.
• The cytoskeleton moves organelles around the cell.
• It is involved in motility either through the use of flagella or by amoeboid movement.

The cytoskeleton components can be divided into three classes based upon the size, distribution and function of the filaments. Microfilaments are the smallest at 4 to 6 µm in diameter and are made of actin. These lie beneath the surface of the cell membrane and are anchored to it, forming a web inside the cell. They dictate the cell's shape and can also be involved in motility by contraction or expansion of the filament. Filaments may also tether organelles to the membrane and help move them around the cell. This movement can be important for modulation of organelle function. Intermediate filaments are 10 µm in diameter and are made of keratin, which is the same protein found in hair and fingernails. These filaments take different forms and are found in many types of cells, but their exact function is unknown. They may play a structural role similar to that of some microfilaments. Figure 2-56 shows some examples of cytoskeletal elements

In addition to the above, there are more complex fibers and structures. Microtubules are hollow cylindrical structures that are 20-25 µm in diameter containing tubulin as the major structural protein. Tubulin polymerizes into a helical cylindrical structure and thirteen of these protofilaments then combine to make a microtubule. Microtubules can form the basis of a number of different structures. Many of these structures form to perform a necessary function and are then disassembled afterwards. Often microtubules form centrioles that contain nine sets of microtubules arranged in a circular matrix. These are 400 µm long and 150 µm wide and are usually found in pairs at right angles to each other. Centrioles are important in proper chromosome segregation and cell division as discussed in the section on the nucleus below. Microtubules are also part of the basal bodies of flagella and cilia.

Cilia and flagella are examples of more permanent structures that contain microtubules. Both cilia and flagella have a similar structure of nine pairs of microtubules arranged in a circular fashion around a tenth pair that runs down the center. Both are attached to the membrane and project into the environment. In a process that requires energy, the microtubules in the outer ring are moved with respect to each other, causing the cilia or flagella to bend and snap back in a whip-like fashion. This bending causes the movement of liquid near the structures such that spent liquid with few nutrients and waste products is moved away from the cell and is replaced by fresh liquid containing nutrients and oxygen. The beating of cilia and flagella can also push the cell through its environment. Cilia are 2 to 10 µm long and 0.5 µm wide and are shorter and typically more numerous than flagella with hundreds of them on some types of cells. An example of a ciliated organism is the unicellular protist Paramecium, which can be found in fresh water ponds. The microbe is a predator of bacteria and motility is vital in this life-style, both for chasing down prey and moving away from danger. Cilia cover the surface of Paramecium and move the organism through the environment by beating in a coordinated fashion. Figure 2-57 shows one example of a protozoan.

Flagella are 50-100 µm in length and there are typically only one or two per cell. Eukaryotic flagella are larger than those found on bacteria or archaea and have a more complex structure. Flagella are found in many unicellular creatures with one example being the dinoflagellates and their primary role is cell motility. These aquatic creatures contain two flagella; one encircling the body of the organism while the other is attached in a perpendicular fashion to the first. Dinoflagellates are often photosynthetic and important as primary producers in the oceans.

Endoplasmic reticulum

The endoplasmic reticulum (ER) is a finely divided system of interconnected membranes, consisting of tubules and vesicles that loop through the cell and are contiguous with the nuclear membrane. A drawing of the ER is shown in Figure 2-58. It functions in the synthesis of membranes and membrane proteins and is also involved in protein secretion. Not surprisingly, the ER is especially prominent in cells doing a large amount of protein secretion. The ER works very closely with the Golgi apparatus (see below) to carry out these functions. There is no structure in bacterial cells that is analogous to the ER, but many of the same functions are carried out on the inside surface of the cellular membrane in bacteria. ER comes in two types: rough ER and smooth ER.

Rough ER gets its appearance from the presence of ribosomes on its surface as seen in the electron microscope, and its function is the production, processing and export of proteins. During translation an appropriate signal guides the ribosome to the ER membrane and causes the protein to be synthesized directly across the membrane into the lumen of the ER. There proteins may be processed or modified by the addition of carbohydrates to form glycoproteins. After processing proteins move slowly through the ER and are packaged into vesicles of ER membrane called transition vesicles. These release from the ends of the ER and move by elements of the cytoskeleton either to the Golgi apparatus or to the plasma membrane. Once contact is made between the transition vesicle and the Golgi or the plasma membrane, the two fuse and release the contents of the vesicle into the target compartment.

Smooth ER does not contain ribosomes and the lumen and membrane of smooth ER contain a variety of enzymes that perform many functions including modification of toxins and synthesis of steroids.

Golgi apparatus

The Golgi apparatus is an organelle containing a double membrane and it is mainly devoted to the processing of proteins synthesized in the ER. A drawing of the Golgi apparatus is shown in Figure 2-59. It is found in many eukaryotic cells, but it lacks a well-formed structure in many fungi and ciliate protozoa. It consists of regions of stacked contiguous membranes containing no ribosomes. Each membrane sac is 15 to 20 µm thick and separated from the next stack by about 30 µm. A complex network of tubes and vesicles extend from the edges of these sacs into the surrounding cytoplasm. The stack of membranes has a definite polarity with those near the ER (the cis face) having a different shape and enzyme content than those at the opposite end (the trans or maturing face). Studies of the Golgi apparatus appear to show material flowing into the cis face from vesicles, through the apparatus and then exiting at the trans face.

The role of the Golgi apparatus is to package material for export, but its exact function varies depending upon the organism. For example, Giardia and Entamoeba utilize Golgi apparatus to form cell walls during cyst formation. It often participates in the synthesis of cell membranes and the final processing of proteins before export. In many cases the Golgi apparatus contains glycosylation enzymes that add sugars to proteins as they move through its lumen. The type of glycosylation that takes place is dependent upon signals contained within the protein sequence. The Golgi also processes enzymes using proteases, which clip at specific amino acids sequences to form mature proteins and hormones. These mature proteins then move to their final destinations, which may be in the membrane, in lysosomes or secreted into the environment.

Lysosomes

One of the most important functions of the Golgi apparatus is the synthesis of lysosomes, an organelle that is found in a variety of eukaryotic cells. Lysosomes are spherical structures enclosed in a single membrane that can vary in size from 50 µm to several µm. They are involved in intracellular digestion and contain enzymes (called hydrolases) that digest many types of macromolecules. Hydrolases function best under acidic conditions (pH 3.5 to 5.0) and the lysosome maintains this pH by membrane proteins that pump protons into its interior. Enzymes bound for lysosomes are synthesized on ribosomes that deposit them in the rough ER. They then move though the smooth ER and Golgi apparatus before being package into lysosomes. In some cases lysosomes can also bud off from the smooth ER.

Lysosomes serve a variety of functions depending upon the cell type. In unicellular eukaryotes they are often digestive structures that take bacteria or other substances from the outside environment and degrade them into usable nutrients. In mammals, lysosomes serve to eliminate unwanted particles, either cell structures that are no longer needed or foreign macromolecules (from viruses or bacteria) that have invaded the cell. In any case the hydrolytic enzymes and low pH typically inactivate and then degrade any particle that enters the lysosome

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