The role of the Na+ glucose symporter of intestinal epithelial cells can be desc

Question

The role of the Na+ glucose symporter of intestinal epithelial cells can be described by the following equation:

The free energy of transporting glucose from the outside to the inside of the cell is supplied by coupling it to the transport of Na+ ions from outside to inside of the cell. Note that the [glucose]in >> [glucose]out.

b) Explain in a few sentences how the Na+K+ ATPase pump provides the driving force for the Na+ glucose symporter

The role of the Na+ glucose symporter of intestinal epithelial cells can be described by the following equation:

2Na+(out) + glucose(out) 2Na+(in) + glucose(in)

The free energy of transporting glucose from the outside to the inside of the cell is supplied by coupling it to the transport of Na+ ions from outside to inside of the cell. Note that the [glucose]in >> [glucose]out.

Explanation / Answer

a) Sodium-glucose transport in the small intestine--transport of each molecule of glucose into cells is accompanied by the simultanious movement of one sodium ion in the same direction [Na] is lower in cell than outside...so energy is provided

The favourable conditions are when the system is at equilibrium i.e. the delta G = 0

In the above equation,

i) Free energy change, Delta G, for transporting 1 mol of substance from region where concentration is C1 to region where concentration is C2

If C2 < C1 then Delta G is negative (Favorable)

Here, C2 is in and c1 is out. also the sodium concentration is less inside as compared to the outside

As more is transferred (between two finite compartments) C1 decreases and C2 increases until C2 =C1

ii) Delta psi = membrane potential in volts

Z = charge on ion being considered

If Delta psi is negative going from ouside to in, and Z is positive, then ZF makes a negative contribution to Delta G and transport in of cations is favored

Here the membrane potential is around -70mV (in case of mammalian cells) and charge on the ion i.e sodium is positive, hence the sodium ion transport is favoured

b) The sodium glucose symporter functions by transporting the sodium ions from the outside of the cell to the inside of the cell in flow of the concentration gradient. The sodium ions are provided by the sodium potassium pump which transports the sodium out of the membrane against the concentration gradient. Thus the sodium potassium provides first a concentration gradient with sodium more on the outside than inside of the cell and also it provides the two sodium ions required for the transport of each glucose molecule inside the cell.

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