Tree of Water Potential

The diagram indicates that water moves from higher to lower water potential areas.

Plants acquire their nutrients by the root hairs absorbing soil solute, which includes water molecules and dissolved mineral ions. The soil solution is then drawn into the hydrophilic walls of epidermal cells and travels along the cell walls and the extracellular spaces into the root cortex. The flow amplifies the exposure of the cortex cells to the solution which provides a greater membrane surface area for absorption.

Then, the water and nutrients obtained from the soil solution travels through the endodermis, which is the last layer of cells in the root cortex, and continue onto the xylem by the use of root pressure. The transportation of the xylem sap, or the water and minerals in the xylem, requires the extensive loss of water through transpiration, the loss of water vapor from leaves and other aerial parts of the plant. Now, root pressure causes more water to enter the leaves than the amount of water that is transpired, so that creates guttation or the exudation of water droplets that can be seen in the morning.

There is also the cohesion-tension hypothesis. This theory suggests that transpiration is the force that pulls the xylem sap upwards and cohesion is responsible for water molecules continuous structure through the use of hydrogen bonds. The transpirational pull works as the negative pressure potential causes water to travel through the xylem develops at the surface of mesophyll cell walls in the leaf. The cell wall functions similarly as a very thin capillary network. Water would stick to the cellulose microfibrils and other hydrophilic components of the cell wall. Water would evaporate from the water film that encases the cell wall. Due to the high surface tension of water, there is tension, or negative pressure potential, in the water. So when water evaporates from the cell wall, the air-water interface would increase, causing the pressure of the water to become more negative. Water molecules that are found at the more hydrated parts of the leaf are attracted to the negative area, reducing the amount of tension.

The ever-important role of the negative pressure potential in transpiration is consistent with the water potential equation since tension, or the negative pressure potential, are key factors in lowering water potential. This is significant as osmosis is the movement of water from areas with high water potential to the areas with lower water potential - the more negative pressure potential present at the air-water interface would allow water in the xylem cells to be pulled into the mesophyll cells and diffuse the water out of the leaf by means of stomata.

Root hairs are significantly similar to microvilli in the human gut. Microvilli vastly increase the surface area available for absorption of microscopic or molecular sized particles compared to the same linear inches of surface area if they were not present. The same function is performed by the tiny root hairs from the cells of the roots of plants. The value of the structure is to get a huge increase in the surface area without having to add to the length.