Sucrose made in the mesophyll cells of a leaf must be loaded into the sieve tubes. There are two routes it can take. In symplastic loading, sucrose travels entirely through the symplast, the continuous cytoplasm of the cells, and never leaves the cytoplasm.1. Sucrose is made in the cytoplasm of mesophyll cells.
2. It passes from cell to cell through plasmodesmata, the strands of cytoplasm that connect neighbouring cells.
3. It reaches the companion cells and then passes through more plasmodesmata into the sieve tube elements.
Sucrose moves along the symplast by diffusion, down its concentration gradient, so this route does not directly use ATP. For diffusion to continue, the concentration of sucrose in the sieve tubes must be kept lower than in the mesophyll cells.In many plants that load symplastically, such as some trees and shrubs, this is helped by polymer trapping. In the companion cells, sucrose is joined to other sugars to make larger sugars such as raffinose. These are too big to diffuse back through the plasmodesmata towards the mesophyll, but they can pass into the sieve tubes through wider plasmodesmata. This keeps the sucrose concentration in the companion cells low, so more sucrose diffuses in.Symplastic loading needs a large number of plasmodesmata between the mesophyll, companion cells and sieve tubes. The alternative route, through the cell walls, is covered in the next lesson.
2. It passes from cell to cell through plasmodesmata, the strands of cytoplasm that connect neighbouring cells.
3. It reaches the companion cells and then passes through more plasmodesmata into the sieve tube elements.
Sucrose moves along the symplast by diffusion, down its concentration gradient, so this route does not directly use ATP. For diffusion to continue, the concentration of sucrose in the sieve tubes must be kept lower than in the mesophyll cells.In many plants that load symplastically, such as some trees and shrubs, this is helped by polymer trapping. In the companion cells, sucrose is joined to other sugars to make larger sugars such as raffinose. These are too big to diffuse back through the plasmodesmata towards the mesophyll, but they can pass into the sieve tubes through wider plasmodesmata. This keeps the sucrose concentration in the companion cells low, so more sucrose diffuses in.Symplastic loading needs a large number of plasmodesmata between the mesophyll, companion cells and sieve tubes. The alternative route, through the cell walls, is covered in the next lesson.