Question

a part of a membrane at rest showing the transport of glucose using the glucose-Na+ symport protein. We know that the average mammalian cell has a cytosol chemical concentration of roughly 15mM of Na+ as compared to its extracellular concentration of 154 mM. Overall, the electrical gradient in cells at rest are more negative inside as compared to outside of the cell.

1.There is strong electrochemical gradient for Na+, explain why this is true (hint, think about what makes up the electrochemical gradient).

2.The ion, chloride (Cl-) is more concentrated on the outside (110 mM) than the inside of the cell (15 mM). Do you expect the overall electrochemical gradient for chloride to be as large as the Na+ electrochemical gradient? Explain why or why not (hint, think about what makes up the electrochemical gradient).

3.If the concentration of Na+ increased on the inside of the cell so that it was equal on both sides, would we see an increase or decrease in the overall electrochemical gradient of Na+ as compared to the resting cell? Explain your answer

4.The glucose-Na+ symport protein uses the electrochemical gradient of Na+ to move glucose against its concentration gradient. Predict whether the transport of glucose from the extracellular side into the cytosol would increase or decrease under the following scenarios:

a.The Na+ electrochemical gradient is greatly reduced.

b. A ligand binds to the symport protein so that the site which binds Na+ no longer has a strong affinity for Na+.

the Na+ K+ ATPase pump, also known as the Na+ pump. This pump moves both Na+ and K+ against their concentration gradients through the hydrolysis of ATP (here is another way, ATP, the “currency of the cell” is used).

5. Compare and contrast the properties and the functioning of these two types of transporters.

Answer 1

.1 There is strong electrochemical gradient for Na⁺ -

Electrochemical gradient for Na⁺ is due to action of Na⁺ pump. This pump using ATP actively transports 3 Na⁺ outside of cell cytoplasm and takes two K⁺ ions inside of cell cytoplasm. Because of which Na⁺ ions get accumulated outside of cell (154mM) and inside cell concentration of Na⁺ ions is low (15mM). So extracellular environment of cell is more positive as compare to cytoplasm. This difference of charge as well as chemical concentration creates gradient. It is responsible for generating membrane potential of -60mV for animal cell and -100mv for plant cell. This makes cytoplasm negatively charged.

Na pump acts against electrochemical gradient. i.e. from low concentration of Na ions (15mM) to high concentration of Na ions (154mM). So it needs ATP to drive the transport.

Q.3 If concentration of Na+ ions increased inside of cell and made it same as outside of cell- then as concentration of Na+ ions on both sides is same there will be no gradient of Na+ ions. As there is no difference of electrical charge and chemical inside and outside of cell, there will be sharp decrease in gradient. Even Resting cells have its membrane potential value of -60mV so as compare to resting cell electrochemical gradient, there will be decrease of electrochemical gradient when Na+ ions concentration of inside and outside of cell will be equal.

Q.4a when electrochemical gradient is reduced transport of glucose inside of cell will also be reduced.

Because- Na+ glucose symport is secondary active transport of Na+ ion and glucose molecule in same direction(inside cytoplasm from outside). This type of transport uses gradient established by Na+ pump. Using the energy of gradient 2 Na+ ions along with one glucose molecule is transported. But when Na ion gradient is decreased, there will be decrease in energy available for transport. This transport is totally Non spontaneous, it requires energy. As energy is less available glucose transport inside of cell will be reduced.

Q.4 b when ligand bind to Na- glucose symporter and reduces its affinity for Na+ ions it will reduce the transport of glucose inside cell. Because - molecular structure of transporter is responsible for the transport. When Na+ ions bind to symporter, it induces conformational change in symporter which results in increase of affinity towards glucose. Hence Glucose is cotransported inside cell. But when Na+ ion cannot bind , there will be no conformational change and no affinity for glucose molecule so there will not be transport of glucose inside of cell.