Question

Explain how the rate of glucose transport across cell membranes can be altered without a change in the external glucose concentration at which V1/2 and Vmax are measured. Explain what cellular changes can alter the external glucose concentration at which V1/2 is measured

Answer 1

Glucose transport across the cell membrane can take place by facilitated transport or passive transport. Passive diffusion occurs across the cell membrane when concentration gradient between the extracellular and intracellular environments is larger. In such kind of diffusion no ATP hydrolysis is required. In this case higher is the extracellular concentration, higher the transport rate. As the concentration gradient goes on decreasing, the transport rate also goes on decreasing. In order to increase the transport rate, the extracellular concentration of glucose needs to be increased.

However, the glucose molecule simply cannot diffuse through the cell membrane, because of its larger size. Glucose is therefore, associated with glucose transporter or permease that changes the conformation and allows passage of glucose.

The transport rate of glucose is saturable and at high concentration of glucose, the glucose transporters get occupied thus reaching maximum velocity. The rate of uptake or transport is the measure of affinity of glucose to bind its transporter. This binding is stereospecific. Binding is more specific for D isomer of glucose than L-isomer of glucose.

Thus, the rate of transport can be altered by altering the stereo isomer of the glucose. Also, the glucose transporter availability changes the transport rate of glucose without changing the external glucose concentration.