Question

In some cells glucose can be taken up by two different transport systems: facilitated diffusion and glucose/proton symporter.
a) Why is glucose not taken up by passive diffusion but by means of permeases? Describe the composition and structure of the permeases.
b) Distinguish between facilitated diffusion and carbohydrate/proton symporter.
The energy used to promote the active transport is the sum of the concentration-dependent energy (R∙T∙lnK) and the electrical work (z∙F∙ΔV). This process is usually linked directly or indirectly to ATP hydrolysis. The hydrolysis of ATP to ADP and Pi is -12 kcal/mol, and the reduction potential across the membrane is -60mV.
c) Calculate the maximal concentration gradient ([Glucose]out/[Glucose]inner) that ATP can sustain. It is assumed that one ATP is used to transport one molecule of glucose across the membrane.
d) Describe the mechanism for regulating the transport of glucose across cell membranes.
A high-energy containing compound is formed in the substrate level phosphorylation of glucose. This is seen for example in the transformation of 3-phosphoglycerate via 2-phosphoglycerate and phosphoenolpyruvate to pyruvate. Standard Gibbs free energy change (ΔG°) for the transfer of the phosphate group in 3-phosphoglycerate to 2-phosphoglycerate and the consecutive cleavage of water is 0.4 kCal/mol, while the hydrolysis of the phosphate group in phosphoenolpyruvate is -14.8 kCal/mol. Furthermore, it is known that ΔG° for the dephosphorylation of the energetic compound is -7.3 kCal/mol.
e) Calculate the standard Gibbs free energy for the transformation of i) 3-phosphoglycerate to phosphoenolpyruvate and ii) phosphoenolpyruvate to pyruvate.

Answer 1

a) Whether a cell uses facilitated diffusion or active transport depends on the specific needs of the cell. For example, the sugar glucose is transported by active transport from the gut into intestinal epithelial cells, but by facilitated diffusion across the membrane of red blood cells. The permeases are membrane transport proteins, a class of multipass transmembrane proteins that allow the diffusion of a specific molecule in or out of the cell in the direction of a concentration gradient, a form of facilitated diffusion.