Question

Glucose Transporters A cell biologist working with cultured cells from intestinal epithelium finds that the cells take up glucose from the growth medium 10 times faster when the glucose concentration is \(5 \mathrm{~mm}\) than when it is \(0.2\) mo. She also finds that glucose uptake requires \(\mathrm{Na}^{+}\)in the growth medium. What can you say about the glucose transporter in these cells?

Short answer

The glucose transporter is a sodium-glucose symporter (SGLT).

Step-by-step solution

Understanding the Problem

The biologist notices that glucose uptake is faster at a higher concentration of glucose and is dependent on the presence of sodium ions (\(\mathrm{Na}^{+}\)) in the medium. We need to determine the type of glucose transporter involved.

Identify Transporter Type

Considering that glucose uptake is faster at higher concentrations and requires \(\mathrm{Na}^{+}\), these characteristics suggest the involvement of a secondary active transporter, specifically a symporter. This symporter uses the energy from the \(\mathrm{Na}^{+}\) gradient to facilitate glucose uptake against its concentration gradient.

Analyze Dependency on Na+

The requirement of \(\mathrm{Na}^{+}\) indicates that this transporter uses the electrochemical gradient of \(\mathrm{Na}^{+}\) ions to move glucose into the cell, against its own concentration gradient. This is typical of the sodium-glucose symporter (SGLT).

Conclusion

The glucose transporter in these intestinal epithelium cells is a sodium-glucose symporter, which facilitates active transport of glucose into the cells, enabled by the \(\mathrm{Na}^{+}\) gradient.

Key concepts

Sodium-Glucose Symporter

The sodium-glucose symporter is a type of transporter protein found in the cells lining the intestine. These proteins help transport glucose into cells against its concentration gradient. This means they move glucose from areas with lower concentration to areas with higher concentration.

A symporter does this by simultaneously moving sodium ions (\(\mathrm{Na}^+\)) into the cell. By coupling the movement of glucose with sodium ions, the symporter takes advantage of the sodium ion gradient across the cell membrane to perform its function.

• The sodium-glucose symporter is crucial for nutrient absorption in the intestine.
• It’s especially important after meals, when glucose from digested food needs to be absorbed efficiently.

Active Transport

Active transport is a process where molecules are moved across cell membranes against their concentration gradient. This requires energy, often derived from the breakdown of ATP, the cell’s energy currency.

In the case of glucose transport via the sodium-glucose symporter, active transport is achieved through the energy stored in the sodium ion gradient. This is known as coupling transport, where the movement of one ion (sodium) powers the transport of another molecule (glucose).

• Unlike passive transport, active transport can move substances from areas of low concentration to high concentration.
• Active transport is essential for maintaining cellular homeostasis and can rapidly uptake needed nutrients, even at low external concentrations.

Na+ Dependency

The sodium-glucose symporter’s functionality deeply relies on \(\mathrm{Na}^+\) availability. This dependency is because the symporter uses the downhill movement of \(\mathrm{Na}^+\) (from higher to lower concentration) to power the uphill movement of glucose.

Without \(\mathrm{Na}^+\), the symporter cannot operate effectively. This direct reliance on a sodium gradient is typical of symporters involved in active transport.

• Environments devoid of sufficient \(\mathrm{Na}^+\) result in reduced or halted glucose uptake.
• The \(\mathrm{Na}^+\) gradient is often maintained by other active processes, using ATP energy to pump sodium out of cells.

Secondary Active Transport

Secondary active transport is a form of active transport that does not use ATP directly. Instead, it uses the energy from a pre-existing gradient established by primary active transport. In this case, the sodium-glucose symporter uses the gradient of \(\mathrm{Na}^+\) ions to transport glucose into the intestinal cells.

Primary active transport usually creates this \(\mathrm{Na}^+\) gradient by pumping sodium out of the cell using ATP. The energy stored in this gradient drives the secondary active transport process, making it a highly efficient system for nutrient absorption.

• Secondary active transport is less energy-intensive than primary transport, as it harnesses existing ion gradients.
• This mechanism supports the absorptive capacity of intestinal cells, even when glucose concentrations are low in the gut.