Two graphs illustrating the transport rate of solutes across the plasma membrane are shown on the slide below. Which of the following best explains the difference in the shape of the curves?

Question

Two graphs illustrating the transport rate of solutes across the plasma membrane are shown on the slide below.  Which of the following best explains the difference in the shape of the curves?

Quizplus · Accepted Answer

11ec160b_3697_76f3_9249_7346399eaea3_MD0001_00 The image above illustrates the difference between the rate of transport of solute across the cell membrane in simple diffusion (line 2) and carrier-mediated transport (line 1). There are two types of diffusion: Carrier proteins are typically transmembrane proteins that possess binding sites for the substrate they transport. Binding is followed by movement of the substrate across the cell membrane to the intracellular space, where it is released into the cytoplasm. Because there is a finite number of carrier proteins in the cell membrane, transporter saturation occurs with facilitated diffusion, and can be seen as a flattening of the curve (maximum diffusion speed), even as solute concentration continues to increase. This maximum rate of transport is referred to as the transport maximum (T_m) and is similar in principle to the V_max in standard enzyme kinetics. (Choices A, B, C, D) These other factors are important for determining the rate of diffusion (ie, slope of the line), but would not explain the flattening of the curve in line 1 (which is best accounted for by saturation of a protein carrier). In general, the rate of diffusion increases with: For molecules that move through the membrane via simple diffusion, the degree of lipophilicity is also important; molecules with a high oil-water partition coefficient are more easily able to cross the membrane and will diffuse faster. Educational objective: Carrier-mediated transport includes facilitated diffusion and active transport. Movement of substrate across the cell membrane by these mechanisms depends on the presence of carrier proteins that can become saturated at high substrate concentrations.

Two Graphs Illustrating the Transport Rate of Solutes Across the Plasma