Question

part ii:active transport is used to generate electrochemical gradients & membrane potential
active transport via a uniport...
...coupled to...
cotransport (indirect active transport) via a symport
how does the energetics of this system work?

Explanation:

Step1: Identify energy - source

The proton pump uses ATP as an energy source. ATP hydrolysis provides the energy to actively transport hydrogen ions ($H^+$) out of the cytoplasm, creating an electrochemical gradient.

Step2: Analyze cotransport

The electrochemical gradient of $H^+$ created by the proton - pump is then used in the cotransport process. The $H^+$ ions diffuse back into the cell down their electrochemical gradient through the sucrose - $H^+$ cotransporter. This diffusion of $H^+$ provides the energy to transport sucrose into the cell against its concentration gradient.

Answer:

ATP hydrolysis by the proton pump provides energy for $H^+$ active transport out of the cell, creating an electrochemical gradient. The diffusion of $H^+$ back into the cell down this gradient through the sucrose - $H^+$ cotransporter provides the energy to transport sucrose into the cell.