File:An Annotated Diagram of the Vertical Cross-Sections for Apoplast and Symplast Pathways Involved in Phloem Loading.svg

English: This is an annotated diagram of the apoplast and symplast pathways which are the two main ways that sucrose is loaded into the phloem. The apoplast pathway is the top diagram and the symplast diagram is the bottom. In the apoplast pathway, sucrose is loaded into the phloem with the help of active transport. A sucrose transporter protein is used to co-transport H+ and sucrose across the cell membrane. A unique aspect that distinguishes the apoplast pathway from the symplast pathway is that the apoplast path utilizes ATP to pump H+ against the concentration gradient. ATP is reduced to ADP+Pi which expels energy that enables H+ to be pushed against the gradient. The H+ proton that is pumped against the gradient is then used by the sucrose transporter protein to move sucrose through the membrane. The sucrose accumulation in the companion cell is able to flow down its concentration gradient via the plasmodesmata and into the phloem. In the symplast pathway, sucrose travels in the plasmodesmata which are connections between cells. Therefore, the sucrose is able to flow down the sucrose concentration gradient into the phloem which has a low concentration of sucrose.