Proton facilitated diffusion drives both cotransports

Steps:

• Both nicotine and sucrose cotransports are secondary active transports using proton gradients.
• Protons move down their electrochemical gradient into the cell, powering uphill solute movement.
• This proton influx occurs via facilitated diffusion through cotransporter proteins.
• Option B matches this shared proton mechanism.

Why B is correct:

• Cotransport definition: Protons enter cells by facilitated diffusion down their gradient, coupling to solute uptake against its gradient.

Why the others are wrong:

• A: Cotransporters use existing proton gradients; they do not generate them (that's ATP-driven proton pumps).
• C: Incomplete, but cotransporters move protons and solutes in the same direction, not opposite.
• D: Cotransporters rely on proton gradients, not direct ATP hydrolysis for proton or solute transport.

Final answer: B