Question

Which of these is not an example of cotransport? a. Movement of glucose and \(\mathrm{Na}^{+}\)through the apical epithelial membrane in the intestinal epithelium b. Movement of \(\mathrm{Na}^{+}\)and \(\mathrm{K}^{+}\)through the action of the \(\mathrm{Na}^{+} / \mathrm{K}^{+}\)pumps c. Movement of \(\mathrm{Na}^{+}\)and glucose across the kidney tubules d. Movement of \(\mathrm{Na}^{+}\)into a cell while \(\mathrm{Ca}^{2+}\) moves out

Short answer

The short answer is: Option b, the movement of \(\mathrm{Na}^{+}\) and \(\mathrm{K}^{+}\) through the action of the \(\mathrm{Na}^{+} / \mathrm{K}^{+}\) pumps, is not an example of cotransport as it involves an antiporter where ions move in opposite directions.

Step-by-step solution

Option A: Movement of glucose and \(\mathrm{Na}^{+}\) through the apical epithelial membrane in the intestinal epithelium

In the intestinal epithelium, glucose and sodium ions are co-transported across the apical epithelial membrane to be absorbed into the body. This is an example of cotransport because glucose and sodium ions move in the same direction, with the movement of sodium ions driving the uptake of glucose.

Option B: Movement of \(\mathrm{Na}^{+}\) and \(\mathrm{K}^{+}\) through the action of the \(\mathrm{Na}^{+} / \mathrm{K}^{+}\) pumps

The \(\mathrm{Na}^{+} / \mathrm{K}^{+}\) pump is an example of an antiporter, which means it moves ions in opposite directions and not in the same direction. The pump moves 3 sodium ions out of the cell and 2 potassium ions into the cell. Therefore, this is not an example of cotransport.

Option C: Movement of \(\mathrm{Na}^{+}\) and glucose across the kidney tubules

In the kidney tubules, glucose and sodium ions are also co-transported. The movement of sodium ions down their concentration gradient drives the uptake of glucose against its concentration gradient. This process reabsorbs glucose back into the body from the filtrate in the nephron. Therefore, this is an example of cotransport.

Option D: Movement of \(\mathrm{Na}^{+}\) into a cell while \(\mathrm{Ca}^{2+}\) moves out

In this option, sodium ions move into the cell, and calcium ions move out of the cell. The movement of these ions is coupled, but they move in opposite directions. So, this is an example of an antiporter, not cotransport. Based on the analysis above, the correct answer is: b. Movement of \(\mathrm{Na}^{+}\) and \(\mathrm{K}^{+}\) through the action of the \(\mathrm{Na}^{+} / \mathrm{K}^{+}\) pumps

Key concepts

Sodium-Glucose Cotransport

Think of the sodium-glucose cotransport system like a tiny, efficient elevator in your body's cells, specifically designed for glucose and sodium ions. In places like the intestines and the nephrons of kidneys, this mechanism acts as a crucial nutrient and electrolyte uptake system.

Here, glucose is like a passenger who doesn't have a key card but needs to ride the elevator. Sodium ions, on the other hand, are the VIPs with full access. As sodium flows into cells, following its concentration gradient created by the Na+/K+ pump, it brings glucose along for the ride against its own concentration gradient. This coupling is essential for proper nutrient absorption, energy production, and overall homeostasis in the body. The cotransport is a symporter system, meaning both particles move in the same direction across the cell membrane.

Na+/K+ Pump

At the heart of cellular activity lies the Na+/K+ pump—a diligent worker maintaining the necessary gradients for other processes. This pump is a type of membrane transport protein that's crucial for cell life.

For every ATP molecule used, the Na+/K+ pump moves 3 sodium ions out and brings 2 potassium ions in. This creates a gradient where there is a high concentration of sodium outside the cell and a high concentration of potassium inside. This difference in concentration is not just important for cell volume control and electrical signaling but also sets the stage for other transport mechanisms like cotransport systems to function properly. By moving ions in the opposite directions, this pump works as an antiporter, strictly different from the cotransport mechanism where substances travel together.

Antiporter Systems

The concept of antiporter systems can be compared to a revolving door that allows people to enter and exit a building simultaneously, but always in opposite directions. Imagine sodium and calcium ions as people using this door, but while one goes in, the other must exit.

In cells, antiporters are integral membrane proteins that transport two or more ions or molecules across the phospholipid bilayer in opposite directions. This is evident in options b and d of the exercise, showcasing this reciprocal trade. Antiporters can also play a role in regulating pH, calcium levels, and other cellular concentrations, making them fundamental to cell function and survival.

Membrane Transport Proteins

Exploring cell membranes without mentioning membrane transport proteins is like trying to bake a cake without a pan—the essential structure would be missing. These proteins provide specific pathways for substances to travel in and out of cells.

Types include channels, carriers, and pumps, each with unique roles. Channels are like tunnels—they let specific molecules pass through by diffusion. Carriers, such as those used in sodium-glucose cotransport, change shape to shuttle molecules across the membrane. Pumps, which include the Na+/K+ type, use energy to move substances against their gradients. Working in concert, these proteins control the cell’s internal environment, impacting everything from nutrient uptake to nerve impulses.