Cell Membrane Dynamics and Transport Mechanisms

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Cell Membrane Dynamics

Plasma membrane phospholipids are labeled with a fluorescent tag and then the phospholipids in one area are bleached with a laser beam to eliminate the fluorescent signal. What would one observe after the cell is incubated for a few minutes? The bleached molecules would diffuse laterally through the membrane and intermix with unbleached molecules. Cellular membranes are mosaic and fluid. Glycosylated proteins and lipids within the plasma membrane are important for cell recognition and protection.

Passive Diffusion

Passive diffusion is a spontaneous process. Water diffuses through certain organs such as the kidneys and bladder much faster than would occur by passive diffusion through a lipid bilayer alone. What accounts for this more rapid rate of water transport in these organs? Presence of aquaporin channels for facilitated diffusion of water.

Transport Proteins

The movement of sucrose and H+ into the cell by the same membrane protein reflects the actions of what type of transporter? Symporter. Antiporters bind two or more ions or molecules and transport them in opposite directions across a membrane. The plasma membrane Na+/K+-ATPase pump is used to generate sodium and potassium gradients across the membrane.

Enzyme Catalysis

Plant cell membranes don’t contain cholesterol. A researcher is studying the integral membrane protein 'mysteriase' and chooses to perform an experiment like that performed by Frye and Edidin. The researcher fuses a mouse cell expressing mysteriase with a human cell. The fused cell is incubated at 37°C prior to being cooled to 0°C and labeled with a fluorescent antibody against mysteriase. When observed with a fluorescent microscope, mysteriase is observed to remain on one side of the fused cell. What is the most likely explanation for this surprising observation? Mysteriase is bound to the cytoskeletal filament. A chemical reaction that has a positive ΔG is correctly described as endergonic. In ALL exergonic reactions, the reaction proceeds with a net release of free energy. If an enzyme catalyzes the reaction A + B → C, then the enzyme undergoes a conformational change when bound to A and B. Altering the three-dimensional structure of an enzyme might prevent the substrate from binding the enzyme's active site.

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