Verizon LTE 4:38 PM KBack CHAP-3-Journal Questions.docx Chapter 3: Cell Structur
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Verizon LTE 4:38 PM KBack CHAP-3-Journal Questions.docx Chapter 3: Cell Structure and Function 1) Describe the location, basic structure and function of the plasma or cell membrane. 2) Describe the parts of a phospholipid and how they are oriented in the cell membrane. 3) Describe the Fluid mosaic model. 4) Describe the term selectively permeable. 5) Define diffusion and its basic characteristics. 6) Using a source other than your textbook, define solution, solute and solvent. 7) Describe osmosis. Describe the characteristics of an isotonic, hypertonic and hypotonic solution. 8) How do molecules cross the cell membrane using facilitated diffusion? What are the basic features of facilitated diffusion? What disease is associated with this process? 9) How does active transport differ from passive transport? 10) Name and describe two active ways that cell takes things in? Name an active process that lets cells move things out. Previous Next Dashboard Calendar ToDo Notifications InboxExplanation / Answer
Cell Membrane:
The cell membrane (also called the plasma membrane),which envelops the cell, is a thin, pliable, elastic structure only 7.5 to 10 nanometers thick. It is composed almost entirely of proteind and lipids.The approximate composition is proteins,55 percent; phospholipids, 25 percent; cholesterol, 13 percent; other lipids, 4 percent; and carbohydrates, 3 percent.
a)Lipid:
Its basic structure is a lipid bilayer, which is a thin, double-layered film of lipids—each layer only one molecule thick—that is continuous over the entire cell surface. Interspersed in this lipid film are large globular protein molecules. The basic lipid bilayer is composed of phospholipid molecules. One end of each phospholipid molecule is soluble in water; that is, it is hydrophilic. The other end is soluble only in fats; that is, it is hydrophobic. The phosphate end of the phospholipid is hydrophilic, and the fatty acid portion is hydrophobic. Because the hydrophobic portions of the phospholipid molecules are repelled by water but are mutually attracted to one another, they have a natural tendency to attach to one another in the middle of the membrane.
The hydrophilic phosphate portions then constitute the two surfaces of the complete cell membrane, in contact with intracellular water on the inside of the membrane and extracellular water on the outside surface. The lipid layer in the middle of the membrane is impermeable to the usual water-soluble substances, such as ions, glucose, and urea. Conversely, fat-soluble substances, such as oxygen, carbon dioxide, and alcohol, can penetrate this portion of the membrane with ease.The cholesterol molecules in the membrane are also lipid in nature because their steroid nucleus is highly fat soluble. These molecules, in a sense, are dissolved in the bilayer of the membrane. They mainly help determine the degree of permeability (or impermeability) of the bilayer to water-soluble constituents of body fluids. Cholesterol controls much of the fluidity of the membrane as well.
b)Integral and Peripheral Cell Membrane Proteins:
These are membrane proteins, most of which are glycoproteins. There are two types of cell membrane proteins: integral proteins that protrude all the way through the membrane and peripheral proteins that are attached only to one surface of the membrane and do not penetrate all the way through. Many of the integral proteins provide structural channels (or pores) through which water molecules and watersoluble substances, especially ions, can diffuse between the extracellular and intracellular fluids. These protein channels also have selective properties that allow preferential diffusion of some substances over others. Other integral proteins act as carrier proteins for transporting substances that otherwise could not penetrate the lipid bilayer. Sometimes these even transport substances in the direction opposite to their electrochemical gradients for diffusion, which is called “active transport.” Still others act as enzymes. Integral membrane proteins can also serve as receptors for water-soluble chemicals, such as peptide hormones, that do not easily penetrate the cell membrane. Interaction of cell membrane receptors with specific ligands that bind to the receptor causes conformational changes in the receptor protein. This, in turn, enzymatically activates the intracellular part of the protein or induces interactions between the receptor and proteins in the cytoplasm that act as second messengers, thereby relaying the signal from the extracellular part of the receptor to the interior of the cell. In this way, integral proteins spanning the cell membrane provide a means of conveying information about the environment to the cell interior. Peripheral protein molecules are often attached to the integral proteins. These peripheral proteins function almost entirely as enzymes or as controllers of transport of substances through the cell membrane “pores.”
c)Membrane Carbohydrates—The Cell “Glycocalyx.”
Membrane carbohydrates occur almost invariably in combination with proteins or lipids in the form of glycoproteins or glycolipids. In fact, most of the integral proteins are glycoproteins, and about one tenth of the membrane lipid molecules are glycolipids. The “glyco” portions of these molecules almost invariably protrude to the outside of the cell, dangling outward from the cell surface. Many other carbohydrate compounds, called proteoglycans—which are mainly carbohydrate substances bound to small protein cores—are loosely attached to the outer surface of the cell as well. Thus, the entire outside surface of the cell often has a loose carbohydrate coat called the glycocalyx.
The carbohydrate moieties attached to the outer surface of the cell have several important functions: (1) Many of them have a negative electrical charge, which gives most cells an overall negative surface charge that repels other negative objects. (2) The glycocalyx of some cells attaches to the glycocalyx of other cells, thus attaching cells to one another. (3) Many of the carbohydrates act as receptor substances for binding hormones, such as insulin; when bound, this combination activates attached internal proteins that, in turn, activate a cascade of intracellular enzymes.
Functions:
1)The cell membrane surrounds the cytoplasm of living cells, physically separating the intracellular components from the extracellular environment.
2)The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix and other cells to hold them together to form tissues.
3)The cell membrane is selectively permeable able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival.
4)The membrane also maintains the cell potential.
Fluid mosaic model
According to the fluid mosaic model of S. J. Singer and G. L. Nicolson
(1972), which replaced the earlier model of Davson and Danielli, biological membranes can be considered as a two-dimensional liquid in which lipid and protein molecules diffuse more or less easily.Although the lipid bilayers that form the basis of the membranes do indeed form two-dimensional liquids by themselves, the plasma membrane also contains a large quantity of proteins, which provide more structure. Examples of such structures are protein-protein complexes, pickets and fences formed by the actin based cytoskeleton, and potentially lipid rafts.
Semipermeable membrane
is a type of biological or synthetic, polymeric membrane that will allow certain molecules or ions to pass through it by diffusion—or occasionally by more specialized processes of facilitated diffusion, passive transport or active transport.
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