Carrier proteins in the cell surface membranes of companion cells take part in moving assimilates into phloem sieve tubes. The diagram shows how two kinds of carrier protein are used in this process. What are the substances labelled X and Y?
- AX: $\text{H}^+$ ions, Y: sucrose
- BX: $\text{H}^+$ ions, Y: glucose
- CX: sucrose, Y: $\text{H}^+$ ions
- DX: glucose, Y: $\text{H}^+$ ions