Problem 9 - AP Psychology Premium Prep 2021

Question

A cotransporter is something that moves two substances across a membrane, one passively and the other actively. The \(\mathrm{Na}^{+} \mathrm{K}^{+}\) ATPase transports sodium and potassium ions across the plasma membrane against their concentration gradients. This pump is not considered a cotransporter because \((\) A) ATP is produced through this transporter (B) both ions are moved via active transport (C) both ions are moved via passive transport (D) ATP hydrolysis does not occur during transport

Step-by-Step Solution

Answer

The correct reason why the Na+ K+ ATPase pump is not considered a cotransporter is because both ions are moved via active transport (option B). The pump transports sodium and potassium ions against their concentration gradients, which requires energy from ATP hydrolysis, making it an active transport process for both ions.

Step 1: Understand cotransporters and the Na+ K+ ATPase pump

Cotransporters are proteins that move two substances across a membrane, one actively and the other passively. The Na+ K+ ATPase pump transports sodium and potassium ions across plasma membranes against their concentration gradients, which means it is an active transport process. Now, let's analyze each option provided.

Step 2: Analyze option (A)

Option (A) states that ATP is produced through this transporter. This statement is incorrect because the Na+ K+ ATPase actually consumes ATP to transport the ions across the membrane. ATP is not produced but used in this process.

Step 3: Analyze option (B)

Option (B) states that both ions are moved via active transport. This statement is correct because the Na+ K+ ATPase pump moves sodium and potassium ions against their concentration gradients. To do this, it requires the energy provided by the hydrolysis of ATP, making this an active transport process for both ions.

Step 4: Analyze option (C)

Option (C) states that both ions are moved via passive transport. This statement is incorrect because, as mentioned before, the Na+ K+ ATPase pump moves sodium and potassium ions against their concentration gradients, needing energy from ATP hydrolysis. This is an active transport process, not a passive one.

Step 5: Analyze option (D)

Option (D) states that ATP hydrolysis does not occur during transport. This statement is incorrect because ATP hydrolysis (breaking down of ATP into ADP and a phosphate group) is the source of energy for the Na+ K+ ATPase pump to transport sodium and potassium ions against their concentration gradients.

Step 6: Determine the correct answer

Based on our analysis, option (B) is the most accurate reason why the Na+ K+ ATPase pump is not considered a cotransporter: both ions are moved via active transport. The other options contain incorrect statements about the pump or ATP production/hydrolysis.

Key Concepts

Active Transport

Active transport is the process by which molecules move across a cell membrane against their concentration gradient. This means molecules move from an area of low concentration to an area of high concentration. This process requires energy because it involves moving substances in a direction that naturally goes against the concentration gradient.

Key Characteristics of Active Transport:

• Energy Requirement: Since molecules are moving uphill against the gradient, energy is needed. Usually, this energy comes from ATP.
• Specificity: Active transport often involves proteins that are specific for the transported molecule, ensuring that cells can control what gets in and out.
• Carrier Proteins: These proteins, like Na+ K+ ATPase, play a crucial role in facilitating active transport.

Active transport is essential for a variety of cellular functions, including maintaining cell potential and concentration differences in various ions and molecules.

Sodium-Potassium Pump

The sodium-potassium pump, also known as Na+ K+ ATPase, is a vital cell membrane protein that plays a critical role in active transport. It helps maintain the necessary concentration gradients of sodium and potassium across the cell membrane, which are essential for proper cellular function.

How It Works:

• The pump actively transports three sodium ions out of the cell while moving two potassium ions into the cell.
• Both actions occur against their respective concentration gradients, meaning they require energy input.
• The energy needed for this process is derived from ATP hydrolysis.

This pump is not classified as a cotransporter since it does not transport one ion or molecule passively while another is transported actively. Instead, both ions, sodium and potassium, are moved actively requiring energy.

ATP Hydrolysis

ATP hydrolysis is the chemical reaction that involves the breakdown of ATP (adenosine triphosphate) into ADP (adenosine diphosphate) and a free phosphate group. This reaction releases energy, which can then be used by the cell for various biological processes, including active transport.

Role of ATP in Active Transport:

• ATP is the primary energy currency in cells, and its hydrolysis provides the energy necessary for many transport mechanisms.
• In the Na+ K+ ATPase pump, ATP binds to the pump, provides energy through its hydrolysis, and allows the pump to alter its shape to transport ions across the membrane.
• This process enables the active transport of ions against their gradients, which is crucial in maintaining cellular homeostasis.

Without ATP hydrolysis, the sodium-potassium pump could not function, highlighting the critical nature of this reaction in cellular activities.

Cotransporters

Cotransporters are proteins that facilitate the movement of two different substances across a membrane simultaneously. One substance typically moves down its concentration gradient (passively), while the other moves against its concentration gradient (actively). This feature distinguishes cotransporters from pumps like the Na+ K+ ATPase pump, where both substances are moved actively.

Types of Cotransporters:

• Symporters: Transport two substances in the same direction across a membrane. For example, sodium-glucose symporters transport both sodium and glucose into the cell.
• Antiporters: Move two substances in opposite directions. An example is the sodium-calcium exchanger that removes calcium from cells while bringing in sodium ions.

Cotransporters leverage the energy stored in one substance's gradient to drive the active transport of another, making them efficient in cellular transport.