Molecule X has a higher concentration inside the cell. The cell uses to move the molecule across the membrane and into the cell.
diffusion
a protein transporter and the concentration gradient of molecule X
a protein transporter and ATP (ie, primary active transport
a protein channel (ie, facilitated diffusion)
The correct answer and explanation is :
The correct answer is: a protein transporter and ATP (i.e., primary active transport).
Explanation (300 words):
Molecule X has a higher concentration inside the cell, yet the cell continues to move more of it into the cell. This suggests the molecule is moving against its concentration gradient—from an area of lower concentration (outside the cell) to an area of higher concentration (inside the cell). Such movement is not passive and cannot occur via diffusion or facilitated diffusion.
Why not diffusion or facilitated diffusion?
- Simple diffusion moves substances down their concentration gradient—high to low—without energy or assistance. This does not apply to Molecule X.
- Facilitated diffusion, though it involves protein channels or carriers, also only moves substances down the gradient, and still does not require energy.
Why not just a protein transporter and the gradient?
- A protein transporter working with the concentration gradient can assist in passive transport (facilitated diffusion or secondary active transport), but Molecule X is going against the gradient, so this option is incorrect.
The correct mechanism: Primary Active Transport
- In primary active transport, the cell uses ATP (energy) to power a protein transporter (often a pump) to move molecules against their gradient.
- This process is active, meaning it consumes energy, and is vital for maintaining concentration differences across membranes.
- Classic examples include the Na⁺/K⁺ pump, which moves sodium and potassium ions against their gradients using ATP.
Therefore, since Molecule X is being moved into the cell despite already being more concentrated there, it requires a protein transporter and ATP—hallmarks of primary active transport. This mechanism ensures cells can accumulate essential substances even when it goes against natural diffusion tendencies.