Question

The following questions are related to glucose and its transport into cells.
A. Why do cells need a protein to transport glucose across the cell membrane? i.e.could glucose simply diffuse across the lipid bilayer? (1 point)

B. Even after glucose is transported into the cell, there is effectively no increase in intracellular glucose concentration. This can be attributed to the action of hexokinase, the first enzyme in the glycolytic pathway. Explain. (2 points)

C. GLUT1 is a glucose transporter found in many cells. It facilitates the movement of glucose down its concentration gradient. GLUT1 has a lower KM for glucose (~1 mM) than its family member GLUT2 (~15mM). Except during starvation, the brain uses glucose as its sole metabolic fuel and consumes ~40% of the glucose circulating in the body. Which of the two, GLUT1 or GLUT2, would you expect to find in brain tissue? Explain. (2 points)

Answer 1

A.Small uncharged molecules can diffuse freely through a phospholipid bilayer. However, the bilayer is impermeable to larger polar molecules (such as glucose and amino acids) and to ions.therefore such transport proteinsdetermine the selective permeability ofcell membranes and thus play a critical role in membrane function.

B. hexokinase isan enzyme that phosphorylates hexoses (six-carbon sugars), forming hexose phosphate. In most organisms, glucose is the most important substrate of hexokinases, and glucose-6-phosphate is the most important product.thats why concentration of glucose remains unchanged.

C.In the brain, both types of transporters are present with different function, affinity, capacity, and tissue distribution. GLUT1 occurs in brain in two isoforms. The more glycosylated GLUT1 is produced in brain microvasculature and ensures glucose transport across the blood brain barrier (BBB). The less glycosylated form is localized in astrocytic end-feet and cell bodies and is not present in axons, neuronal synapses or microglia. Glucose transported to astrocytes by GLUT1 is metabolized to lactate serving to neurons as energy source. Proinflammatory cytokine interleukin (IL)-1β upregulates GLUT1 in endothelial cells and astrocytes, whereas it induces neuronal death in neuronal cell culture. GLUT2 is present in hypothalamic neurons and serves as a glucose sensor in regulation of food intake. In neurons of the hippocampus, GLUT2 is supposed to regulate synaptic activity and neurotransmitter release.