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It contains a pair of centrioles, two structures that lie perpendicular to each other. The centrosome is a region near the nucleus of animal cells that functions as a microtubule-organizing center. In cilia and flagella, the microtubules are organized as a circle of nine double microtubules on the outside and two microtubules in the center. They are also the structural components of flagella and cilia. Microtubules guide organelle movement and are the structures that pull chromosomes to their poles during cell division. These are hollow tubes that can dissolve and reform quickly. Microtubules are the thickest of the cytoskeletal fibers. Keratin, the compound that strengthens hair and nails, forms one type of intermediate filament. Intermediate filaments are of intermediate diameter and have structural functions, such as maintaining the shape of the cell and anchoring organelles. These components are also common in muscle cells and are responsible for muscle cell contraction. They also maintain the structure of microvilli, the extensive folding of the plasma membrane found in cells dedicated to absorption. Microfilaments are the thinnest of the cytoskeletal fibers and function in moving cellular components, for example, during cell division. Figure 3.10 Microfilaments, intermediate filaments, and microtubules compose a cell’s cytoskeleton. There are three types of fibers within the cytoskeleton: microfilaments, also known as actin filaments, intermediate filaments, and microtubules ( Figure 3.10). Collectively, this network of protein fibers is known as the cytoskeleton. Within the cytoplasm, there would still be ions and organic molecules, plus a network of protein fibers that helps to maintain the shape of the cell, secures certain organelles in specific positions, allows cytoplasm and vesicles to move within the cell, and enables unicellular organisms to move independently. If you were to remove all the organelles from a cell, would the plasma membrane and the cytoplasm be the only components left? No. Many metabolic reactions, including protein synthesis, take place in the cytoplasm. Ions of sodium, potassium, calcium, and many other elements are also dissolved in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are found there too. However, proteins are not the only organic molecules found in the cytoplasm. Even though the cytoplasm consists of 70 to 80 percent water, it has a semi-solid consistency, which comes from the proteins within it. It is made up of organelles suspended in the gel-like cytosol, the cytoskeleton, and various chemicals. The cytoplasm comprises the contents of a cell between the plasma membrane and the nuclear envelope (a structure to be discussed shortly). Patients suffering from celiac disease must follow a gluten-free diet. This leads to malnutrition, cramping, and diarrhea. The immune response damages microvilli, and thus, afflicted individuals cannot absorb nutrients. People with celiac disease have an immune response to gluten, which is a protein found in wheat, barley, and rye. This is an excellent example of form matching the function of a structure. Such cells are typically found lining the small intestine, the organ that absorbs nutrients from digested food. This folding increases the surface area of the plasma membrane. The plasma membranes of cells that specialize in absorption are folded into fingerlike projections called microvilli (singular = microvillus). There are other components, such as cholesterol and carbohydrates, which can be found in the membrane in addition to phospholipids and protein. Other compounds move passively across the membrane.įigure 3.9 The plasma membrane is a phospholipid bilayer with embedded proteins. The plasma membrane regulates the passage of some substances, such as organic molecules, ions, and water, preventing the passage of some to maintain internal conditions, while actively bringing in or removing others. A phospholipid is a lipid molecule composed of two fatty acid chains, a glycerol backbone, and a phosphate group. Like prokaryotes, eukaryotic cells have a plasma membrane ( Figure 3.9) made up of a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. Animal cells have lysosomes and centrosomes. What structures does a plant cell have that an animal cell does not have? What structures does an animal cell have that a plant cell does not have? Plant cells have plasmodesmata, a cell wall, a large central vacuole, chloroplasts, and plastids. Figure 3.8 (a) This figure shows a typical animal cell Figure 3.8 (b) This figures shows a typical plant cell.