Question

Transport Types You have just discovered a new Lalsnine transporter in liver cells (hepatocytes). Poisoning hepatocytes with cyanide (which blocks ATP synthesis) reduces alanine transport by 909. Tenfold reduction in extracellular [Na \(^{+}\)] has no immediate effect on alanine transport. How would you use these observations to decide whether the alanine transporter is passive or active, primary or secondary?

Short answer

The alanine transporter is likely a primary active transporter.

Step-by-step solution

Analyze the Effect of Cyanide Poisoning

Cyanide blocks ATP synthesis, which is crucial for active transport. The observation that cyanide poisoning reduces alanine transport by 90% suggests that the transport mechanism for alanine requires energy, indicating active transport involvement. This step is crucial to determine if ATP-dependent transport is involved.

Assess the Impact of Sodium Ion Concentration

The lack of immediate effect when [Na^{+}] is reduced tenfold suggests a transport mechanism not immediately dependent on sodium ion gradients. This rules out sodium gradient-coupled transport, meaning it is unlikely to be secondary active transport reliant on Na^+.

Conclusion: Determine the Type of Transporter

Based on the effects analyzed, the transporter likely involves ATP usage due to the cyanide sensitivity, indicating primary active transport. The absence of an immediate response to Na^+ concentration changes confirms it does not rely on Na^+-gradient coupling.

Key concepts

Active Transport

Active transport is a fascinating and essential mechanism for moving molecules across cell membranes. Unlike passive transport, which does not require energy and moves substances down their concentration gradient, active transport needs cellular energy, typically in the form of ATP. This energy usage allows cells to move molecules against their concentration gradients, from areas of lower concentration to areas of higher concentration.
Active transport plays a vital role in maintaining cellular functions like ion homeostasis and nutrient uptake. It can be further divided into primary and secondary active transport, which we'll discuss in another section. This mode of transport is key in settings where concentrations need to be controlled actively, despite natural diffusion trends.

Alanine Transporter

The alanine transporter specifically moves alanine, an amino acid, across cell membranes in processes like protein synthesis and cellular metabolism.
Alanine transport can happen through either passive or active means, depending on the conditions and transporter type. In the scenario described, alanine transport in liver cells is affected by cyanide, indicating an active, energy-dependent process. This means that the transporter likely uses ATP to function.
Understanding the type of transport involved is crucial because it influences how cells handle high demand for amino acids and maintain metabolic activities efficiently.

Cyanide Poisoning Effect

Cyanide poisoning profoundly affects cellular processes by inhibiting ATP synthesis—a critical component for active transport. Since many vital biochemical processes rely on energy supplied by ATP, cyanide poisoning can halt these processes.
In our example, a 90% reduction in alanine transport due to cyanide points to active transport, which requires ATP. Cyanide's blockage of ATP formation disrupts pathways that depend on this energy, revealing whether a transporter’s function is energy-dependent. Without ATP, substances that rely on active transport cannot move as effectively across membranes.

Sodium Ion Concentration

Sodium ions ( Na^{+} ) are essential in various transport mechanisms within the body. Their gradients across membranes frequently drive secondary active transport, where the movement of another substance, like glucose or amino acids, is coupled with the movement of sodium ions.
In the provided scenario, reducing sodium ion concentration did not immediately affect alanine transport, implying that the mechanism is not sodium-dependent. This suggests that the alanine transporter does not rely on Na^{+} gradients and hence is not an example of secondary active transport based on sodium movement.

Primary and Secondary Active Transport

Primary active transport and secondary active transport are two different mechanisms cells use to move molecules against their concentration gradients.
Primary active transport directly uses ATP to power transport proteins that move ions or molecules across the membrane. A classic example is the sodium-potassium pump, which maintains cell potential and volume by moving Na^{+} and K^{+} ions against their gradients using direct ATP energy.
Secondary active transport, on the other hand, does not directly use ATP. Instead, it utilizes the energy stored in gradients created by primary active transporters. For instance, the sodium-glucose symporter uses the sodium gradient generated by the sodium-potassium pump to transport glucose into cells.
In our case of alanine transport, the lack of impact from sodium concentration changes, combined with the cyanide sensitivity, suggests the process is an example of primary active transport.