Question

1. If glucose is phosphorylated upon entry, how does it get out of the epithelial cell?

2. Context: referring to the basic K+ and Na+ levels of a cell and how they move in and out of it. Both ions have a positive charge. K+ is more ubundant in the cell and Na+ is more outside. The inside of the cell is more naturally negative.

If K+ is going out while Na+ is coming into the cell, why does the membrane potential(Vm) become more positive?

Answer 1

1.Glucose after entering into the cell via phosphorylation by hexokinase or glucokinase (in liver) forms glucose 6-phosphate.Now the glucose 6-phosphate either gets polymerized into glycogen and stored in the cells or it uses facilitated diffusion transport and comes out of the cell.

2.In Na+, K+ transport there is a transport of 3Na+ and K+ ions via pumps using ATP(energy source).The pump after binding ATP, binds intracellular 3Na+ ions.ATP is hydrolyzed leading to phosphorylation of pump at highly conserved asparate residue and subsequent release of ADP. A confirmational change in the pump exposes the Na+ ions to the outside and released as they have low affinity.The pump binds to 2extracellular K+ ions, this causes dephosphorylation of the pump reversing it to previous confirmation of the pump releasing K+ ions into the cell.

During depolarisation, voltage gated Na and calcium channels open leading to rush of sodium ions into the cell leading to addition of positive charge to the interior makes membrane potential positive.