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Differences in water potential govern the osmotic flow of water from the soil into the plant stele; this is a passive process. Uptake of minerals from the soil that occurs along an electrochemical gradient is an active process requiring energy and membrane transport proteins. Water and minerals can move into the root vascular tissue through either the apoplast or the symplast, but must use the symplast pathway to reach the xylem.
learning outcomes
You should be able to:
Describe or illustrate how solute potential and pressure potential affect the movement of water across the plant cell membrane.
Describe how aquaporins, ion channels, and ion pumps affect the movement of water and solutes across the plant cell membrane.
Discuss the roles of the apoplast and the symplast in moving water from the soil to the xylem.
What is the role of the cell wall in determining the direction of water movement and plant form?
Plant cells swell when they take up water. The cell wall resists this swelling, causing an increase in pressure within the cell. This results in more rigid forms of the plant organs and also reduces the tendency of the plant cells to take in more water.
A major component of root growth is osmotically driven cell expansion, which is achieved by the formation and swelling of the central vacuole. What role would you predict the expressions of aquaporins be in this cell expansion?
Aquaporins are proteins that form membrane channels, allowing the passage of water via osmosis. They occur in the cell membrane and vacuolar membrane. Their presence and numbers affect the rate of water movement across the membrane, from a region of higher water potential to a region of lower water potential. One would expect increased expression of aquaporins in the vacuolar membrane as the cell expands, allowing increased uptake of water and turgor pressure.
What are the differences between the apoplast and the symplast?
The apoplast lies outside the cell membrane and consists of the cell wall and intercellular spaces. The symplast is the cytoplasm of cells that can be considered as a continuous compartment if cells are connected by plasmodesmata. Water moves rapidly through the apoplast and more slowly through the symplast.
So far we’ve described the movement of water and minerals into plant roots and their entry into the root xylem. How does the xylem sap move once it is in the xylem?