This preview shows pages 1–3. Sign up to view the full content.
This preview has intentionally blurred sections. Sign up to view the full version.View Full Document
Unformatted text preview: Bio B teus 21.10 Several Features Distinguish Uniport Transport from Passive Diffusion The protein-mediated movement of glucose and other small hydrophilic molecules across a membrane, known as uniport transport, exhibits the following distinguishing properties: 1. The rate of facilitated diffusion by uniporters is far higher than passive diffusion through a pure phospholipid bilayer. 2. Because the transported molecules never enter the hydrophobic core of the phospholipid bilayer, the partition coefficient K is irrelevant. 3. Transport occurs via a limited number of uniporter molecules, rather than throughout the phospholipid bilayer. Consequently, there is a maximum transport rate V max that is achieved when the concentration gradient across the membrane is very large and each uniporter is working at its maximal rate. 4. Transport is specific. Each uniporter transports only a single species of molecule or a single group of closely related molecules. A measure of the affinity of a transporter for its substrate is K m, which is the concentration of substrate at which transport is half-maximal. These properties also apply to transport mediated by the other classes of proteins depicted in Figure 7-2. One of the best-understood uniporters is the glucose transporter GLUT1 found in the plasma membrane of erythrocytes. The properties of GLUT1 and many other transport proteins from mature erythrocytes have been extensively studied. These cells, which have no nucleus or other internal organelles, are essentially bags of hemoglobin containing relatively few other intracellular proteins and a single membrane, the plasma membrane (see Figure 5-3a) . Because the erythrocyte plasma membrane can be isolated in high purity, isolating and purifying a transport protein from mature erythrocytes is a straightforward procedure. Figure 7-3 shows that glucose uptake by erythrocytes and liver cells exhibits kinetics characteristic of a simple enzyme catalyzed reaction involving a single substrate. - The kinetics of transport reactions mediated by other types of proteins are more complicated than for uniporters. 1- Nonetheless , all protein-assisted transport reactions occur faster than allowed by passive diffusion, - are substrate-specific as reflected in lower K m values for some substrates than others, and exhibit a maximal rate ( V max).---------------------------------------------- EXPERIMENTAL FIGURE 7-3 Cellular uptake of glucose mediated by GLUT proteins exhibits simple enzyme kinetics and greatly exceeds the calculated rate of glucose entry solely by passive diffusion. The initial rate of glucose uptake (measured as micromoles per milliliter of cells per hour) - in the first few seconds is plotted against increasing glucose concentration in the extracellular medium. In this experiment, the initial concentration of glucose in the cells is always zero....
View Full Document
This note was uploaded on 05/30/2010 for the course MEDICEN 2100 taught by Professor Saba during the Spring '10 term at Columbia State Community College.
- Spring '10