Now that we've talked about the conditions that lead water to flow, that is, water potential, let's discuss how water gets from the soil into the xylem. Water flows from the soil into the root hairs, and from there it moves into the xylem, which is represented as this orange structure in the image. Water moves into root hairs via osmosis, pressured by factors related to water potential. Solute potential is the major factor in moving water into the root hairs.
Once inside the plant, water can travel through one of three routes to reach the xylem. The first route we'll talk about is the transmembrane route. Aquaporins, which are channels that allow water to pass through, play a crucial role here since they increase the efficiency of water movement through the membrane. A smaller amount also passes directly through the membrane, but the majority goes through the aquaporins.
The second route is the apoplastic route, involving flow outside of the plasma membranes of cells, specifically in the spaces between cells and their porous cell walls. This region outside the plasma membranes is called the apoplast. This route is interrupted by the Casparian strip, a waxy layer consisting of suberin that is secreted by the endodermis to block off unauthorized access to the xylem. The apoplast is often forced to direct water into the endodermal cells, which act as filters regulating ion flow and maintaining concentration gradients.
The third route is the symplastic route, where water flows through the cytosol of cells, interconnected by plasmodesmata. This network of cytoplasm is called the symplast. As water flows through the symplastic route, it bypasses the Casparian strip, allowing cells to manage solute concentration and ion flow effectively.
When water moves through the xylem, it does so without crossing membranes, propelled by differences in pressure potential. This movement of molecules along a pressure gradient is known as bulk flow. Water enters the xylem where pressure potential is higher and moves to regions of lower pressure potential, allowing the efficient transport of xylem sap, which includes dissolved minerals, nutrients, and hormones in addition to water.
Xylem's primary function is to move water upward through the plant, while phloem transports sugars. However, xylem sap also carries other substances as it travels. Consequently, the concept of bulk flow includes the movement of all these molecules influenced by the pressure potential difference. This understanding is crucial for grasping how water and nutrients are circulated within plants.
With this explanation, let's flip the page.