Cotransport is one means that living organisms use to link energy reactions to the biochemical reactions that take place within their cells. Cotransport is when active transport of solutes can be indirectly driven by a single ATP-powered pump that transports a specific solute. Substances that have been pumped across a membrane can then do work as they diffuse back across the membrane. One example of this is the proton pump and the sucrose-H+ transporter often used by plants. The ATP-powered proton pump transports hydrogen ions outside of the cell, creating a hydrogen gradient, thus storing potential energy that can be used for active transport. The sucrose-H+ cotransporter then uses the diffusion of hydrogen ions down their electrochemical gradient to drive the uptake of sucrose. This cotransport is often used to transfer sucrose produced by photosynthesis into specialized cells in the veins of leaves, and the sugar is then distributed by the vascular tissue to the plant's nonphotosynthetic organs such as roots.