Biochemists working for a national endocrinology institute are investigating the specifics underlying glucose transport across adipose cell membranes. One of their experiments shows that, in the presence of insulin, D-glucose transport across the plasma membrane of adipocytes is much faster than L-glucose transport. Which of the following transport processes best describes the mechanism for glucose entry into these cells?

Question

Biochemists working for a national endocrinology institute are investigating the specifics underlying glucose transport across adipose cell membranes.  One of their experiments shows that, in the presence of insulin, D-glucose transport across the plasma membrane of adipocytes is much faster than L-glucose transport.  Which of the following transport processes best describes the mechanism for glucose entry into these cells?

Quizplus · Accepted Answer

11ec160b_3696_dab1_9249_fd3426dcf328_MD0001_00 Glucose is the major source of energy for all cells of the body. In the majority of tissues, glucose transport occurs along its concentration gradient, from higher concentrations outside the cell toward lower concentrations inside the cell. However, glucose cannot passively diffuse across the cell membrane in any significant amount and requires carrier proteins to aid its crossing. Transport across the cell membrane by carrier proteins (which undergo conformational changes as the substrate is transported, unlike channel proteins) is termed carrier-mediated transport. Transport that is facilitated by transmembrane proteins without the expenditure of energy is called facilitated diffusion. Transmembrane carrier proteins that belong to the GLUT family transport glucose by facilitated diffusion. These proteins are stereoselective and preferentially catalyze the entrance of D-glucose rather than L-glucose into cells. GLUT4 is the insulin-sensitive transporter found in skeletal muscle cells and adipocytes. In these cells, the GLUT4 protein is stored in cytoplasmic vesicles. Under the influence of insulin, the transporter protein is incorporated into the cell membrane. An increased number of transporters in the membrane leads to an increased rate of glucose uptake by the cells. Another important glucose transporter is GLUT2. It facilitates export of glucose from the liver, small intestine, and kidneys into the circulation and also helps to control insulin secretion in the pancreas. (Choice A) Simple diffusion refers to the movement of particles along their concentration gradient directly through the cell membrane. Transport of gases (O₂ and CO₂) occurs via simple diffusion, as the membrane is very permeable to small, nonpolar molecules. (Choice B) Endocytosis allows for cellular uptake through the formation of membrane-bound, typically clathrin-coated, vesicles. Uptake of cholesterol by cells occurs by means of receptor-mediated endocytosis (mediated by the LDL receptor). (Choices D and E) Active transport refers to the movement of a substance against its concentration gradient. This process requires energy as well as transport proteins. In primary active transport, the energy required for transport against the concentration gradient is provided by the hydrolysis of ATP. In secondary active transport, this energy is generated by co-transport of a separate substance down its concentration gradient. Transport of glucose against the concentration gradient occurs via the Na/glucose symporter, which is found in the intestinal and renal tubular epithelium and is used to transfer glucose intracellularly from the lumen. Educational objective: Transport of glucose into the cells of most tissues occurs by means of facilitated diffusion. Glucose moves from areas of high concentration to areas of low concentration with the help of transmembrane glucose transporter proteins (GLUT). These carrier proteins are stereoselective and have preference for D-glucose.

Biochemists Working for a National Endocrinology Institute Are Investigating the Specifics