In apoplastic loading, sucrose leaves the symplast and travels through the apoplast, the cell walls and spaces between cells, before being actively taken into the companion cells. Many herbaceous plants, including many crop plants, load in this way.1. Sucrose made in the mesophyll cells moves through plasmodesmata to cells near the phloem.
2. It then passes out of these cells into the cell walls.
3. It diffuses through the cell walls to the companion cells.
4. It is taken up into the companion cells by active loading, using the proton-sucrose co-transport mechanism described in Phloem Loading: proton pumps use ATP to move hydrogen ions out, and the hydrogen ions diffuse back in through co-transport proteins, carrying sucrose with them.
5. Sucrose then passes from the companion cells into the sieve tube elements through plasmodesmata.
Because step 4 is active, the companion cells can build up sucrose to a much higher concentration than in the mesophyll cells, against its concentration gradient. This produces a high sucrose concentration, and therefore a low water potential, in the sieve tubes, which is needed for mass flow.In many of these plants, the companion cells are transfer cells: their walls have many infoldings, which increase the surface area of the plasma membrane for more co-transport proteins.Apoplastic loading depends on respiration, so it is stopped by respiratory inhibitors.
2. It then passes out of these cells into the cell walls.
3. It diffuses through the cell walls to the companion cells.
4. It is taken up into the companion cells by active loading, using the proton-sucrose co-transport mechanism described in Phloem Loading: proton pumps use ATP to move hydrogen ions out, and the hydrogen ions diffuse back in through co-transport proteins, carrying sucrose with them.
5. Sucrose then passes from the companion cells into the sieve tube elements through plasmodesmata.
Because step 4 is active, the companion cells can build up sucrose to a much higher concentration than in the mesophyll cells, against its concentration gradient. This produces a high sucrose concentration, and therefore a low water potential, in the sieve tubes, which is needed for mass flow.In many of these plants, the companion cells are transfer cells: their walls have many infoldings, which increase the surface area of the plasma membrane for more co-transport proteins.Apoplastic loading depends on respiration, so it is stopped by respiratory inhibitors.