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

Answer a) Facilitated diffusion is also known as passive type of transport in which carrier transmembrane protein transport the molecule Across the membrane down the concentration gradient without using the energy . Glucose molecule is Transported through permeases because due to the complex structure . Permeases help in alter the conformation of molecule to facilitate the diffusion.

Permease : permeases are memberane transporter which allow the diffusion of specific molecule transportation.

Membrane permeases have hydrophilic patches as well as hydrophobic patches calculated in hydropathy index . Hydrophilic patches found outside the membrane while hydrophobic found in lipid bilayer membrane .