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Review Questions Plasma Membrane Review Questions Plasma Membrane 1. What is the function of the plasma membrane? The plasma membrane forms the outer boundary of all cells. Described as semi-permeable, the membrane regulates the passage of atoms and molecules in and out of the cell. All membrane-bound organelles are also built of plasma membrane. 2. Draw a phospholipid bilayer and label the polar heads and the non-polar tails. The plasma membrane is made of a phospholipid bilayer. Phospholipids have a glycerol backbone attached to two fatty acid chains (one is unsaturated) and one phosphate group. The phosphate has a negative charge. The rest of the lipid is nonpolar. Water is attracted to the phosphate group and repelled by the rest of the molecule, so a phospholipid has a dual nature: a hydrophilic region (“head”) and a hydrophobic region (“tails”). When placed in water, the phospholipids form a bilayer. The heads face outward and the tails stay inside. The bilayer is semi-permeable barrier. Some substances can freely cross the membrane whereas others are stopped. 3. Why is the plasma membrane described as a “fluid mosaic”? The plasma membrane is a fluid because the phospholipid molecule is unsaturated. At room temperature, this causes the bilayer to have the consistency of salad oil. The term “mosaic” refers to the variety of embedded transmembrane proteins scattered throughout the membrane. 4. Why is cholesterol an important component of animal cell membranes? Cholesterol is a component of the cell membranes of animals. Cholesterol makes the membrane less fluid and therefore more impermeable to biological molecules. In a sense, cholesterol creates a more solid membrane and a more restrictive membrane. At low temperatures, however, cholesterol prevents the close packing of the phospholipids and keeps the membrane fluid. 5. Name 7 specialized proteins that are embedded in the phospholipid bilayer. Describe the function of each of the 7 proteins. Transmembrane Structure Function Protein Glycoprotein Small carbohydrate chain projects Cell recognition (a chemical out from the cell nametag) identify self vs. nonself. Successful organ transplants require matching glycoproteins Channel protein Protein has a small tunnel through Allows ions passage the center through cell membrane; highly specific Carrier protein Protein is shaped to grasp Transports molecules substance and shuttle it across across membrane (e.g. glucose, amino acids, etc) Receptor protein Protein has a small binding site that sensory input, fits the shape of a specific atom or communication, initiates molecule (e.g. glycoprotein, tastant, metabolic change in target neurotransmitter, hormone, etc.) cell (signal transduction) Enzymatic protein Enzymes with active sites Metabolism (build and embedded in membrane; degrade molecules) organelles with a lot of membrane contain loads of enzymatic proteins (e.g. cristae of mitochondria, thylakoids of chloroplasts) Anchoring proteins Attach membrane to cytoskeleton Supports cytoskeleton Intercellular Joining Membrane proteins of adjacent Holds cells to each other cells hook together 6. What kinds of substances can pass easily through a plasma membrane (be specific)? What kinds of substance cannot pass through a plasma membrane (be specific)? Can pass through Cannot pass through 1. Small and relatively uncharged 1. Large Examples: Examples: O2, CO2, H2O monosaccharides, amino acids, nucleotides, biological monomers and polymers 2. Lipid soluble 2. Charged Examples: Examples: Steroids, Hydrocarbons, Ions (Na+, K+, H+, Ca2+, Cl-, - - - Alcohol, Anesthetics PO4 , HCO3 , OH , etc.) 7. What is diffusion? Give an example of diffusion in the human body. Diffusion is the net movement of solutes (or solvents) to regions of lower concentration as a result of random spontaneous motions. Diffusion produces a uniform distribution of particles. The atoms of any piece of matter above absolute zero are in motion. In liquids and gases, atoms are colliding and bouncing off each other randomly. A glass of water may look still but inside those water molecules are in constant motion. Add dye molecules and the dye will spread out until it is equally dispersed. The random motion of the water molecules and the dye mixed them into a homogenous solution. Diffusion is a free service for cells. The cell does not have to spend any energy (ATP) for diffusion to occur. All it needs is a concentration gradient. In our lungs, if there is a higher concentration of oxygen in the air sacs than the blood, oxygen gas will automatically diffuse into the blood. If there is a higher carbon dioxide level in the blood than the air sacs, then carbon dioxide will diffuse out of the blood and into the lungs. The shape of all life is dictated by diffusion. Every cell depends on diffusion for survival. Diffusion is the process cells use to exchange respiratory gases, absorb nutrients, and eliminate wastes. 8. Why are cells so small? Most cells are invisible to the unaided eye. Cells rely on diffusion for their survival. Since diffusion occurs across cell membranes, a cell’s surface area has to be large enough to accommodate sufficient exchange to sustain life. A cell cannot afford a diffusion bottleneck. An interesting geometrical paradox kicks into gear when a cell increases in volume. As volume increases surface area also increases but not at the same rate. Let me give you an example. A box with a volume of 1 cm3 (1 cm length x 1 cm height x 1 cm depth), has a surface area of 6 cm2 (6 sides x 1 cm length x 1 cm height). The surface area-to-volume ratio is 6:1. Let’s double the volume. A box with a volume of 8 cm3 (2 cm length x 2 cm height x 2 cm depth) has a surface area of 24 cm2 (6 sides x 2 cm length x 2 cm height). The surface area-to-volume ratio is now only 3:1. Let’s triple the volume. A box with a volume of 27 cm3 (3 cm length x 3 cm height x 3 cm depth) has a surface area of 54 cm2 (6 sides x 3 cm length x 3 cm height). The surface area-to-volume ratio has dropped to 2:1. In fact, every time volume triples, surface area only doubles. If a cell is too large, the amount of surface area for diffusion is not enough to support life. So cells are limited in size by the dictates of diffusion. It is much better for a growing organism not to make its cells bigger but simply to make more of them. 9. Describe osmosis. If you cut a potato into strips and submerge one strip into a bowl of tap water and likewise submerge one strip into a bowl of salt water, after an hour, the tap water strip will be firm, stiff, and crisp. The salt water strip, on the other hand, will be limp and flexible. What happened? This is an example of a special kind of diffusion called osmosis. The potato strip placed in tap water gained water. Water flowed from the bowl into the cells of the potato making it stiff and crisp. Salt water had the opposite effect on the potato strip. Water flowed out of the cells and into the salt water. What made the water flow differently? Osmosis is the diffusion of water across a semi-permeable membrane. Just like any liquid or gas, water will diffuse from a region of high concentration to a region of low concentration until the concentrations are equal. In our example, each potato cell has a cell membrane that is selectively-permeable. Water can cross the membrane without any problem, but the solutes inside the potato cannot. Since water will follow its concentration gradient, we can predict where it will flow, depending on the solute concentration on either side of the membrane. The cells of the potato placed in tap water have a higher concentration of solutes than what is in the tap water. This means that there is actually a higher concentration of water outside the cell than inside (the water has to share space with the solutes inside the cell so there is less). So water flows into the cell until the concentration of water versus solute is equal on both sides. Conversely, the salt water has a higher concentration of solute relative to water than the cell. Since there is more water inside the cell than out is the salt water. Water follows the concentration gradient and will flow out of the cells. When cells are placed in an aqueous solution that has a lower solute concentration than the cells, we call this a “hypotonic” solution, water flows into the cells. Cells placed in an aqueous solution with a higher solute concentration, known as a “hypertonic” solution, water flows out of the cells. If cells are placed into a solution with the same solute concentration as the cell, no flow occurs. We call this solution “isotonic”. All cells, not just plant cells, react the very same way but with a few subtle differences. An animal cell submerged in a hypotonic solution will swell in size. If the concentration gradient is big enough, the animal cell with burst. Traditionally, osmosis in animal cells is demonstrated using red blood cells. The bursting of red blood cells in a hypotonic solution is called “hemolysis”. In other kinds of animal cells, we just say “lysis”. An animal cell submerged in a hypertonic solution will shrivel up and get smaller. “Crenation” is the term we use to describe this in red blood cells. For all other animal cells, we just called it “shriveling”. Cells with cell walls react slightly differently in hypotonic solutions. The cell walls prevent the cell from bursting. The cell swells and firms up. We say the cell is turgid.
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