Water uptake into the plant is by osmosis into root hair cells - the presence of these cells massively increases the surface area of the root for absorption to occur over. Mineral elements exist in the form of ions. Uptake is greatest in region of the root hairs. Uptake occurs mainly by active transport into the root hair cells. The water and mineral ions then move across the root to the xylem vessels.
|Figure 6 : Movement of water across the root
Figure 6 shows the movement of water across the root. There is a water potential gradient across the root which means the water is actually transported by osmosis. The two main pathways are the apoplast and the symplast. Basically the apoplast is through the cell walls - these are freely permeable to all small molecules, and the plant has no control over the movement of substances through them (they can also be seen as non-living). The symplast can also have the subdivision of the vacuolar pathway. It is important to note that the Casparian strip effectively blocks the apoplast pathway at the endodermis, therefore all water enters the stele through the symplast - this prevents leakage of water from xylem vessels and aids the development of root pressure (an upwards force pushing water up the stem).
There are three factors which cause water to move upwards through the xylem from the roots. These are:
- Root pressure (-minor)
- Capillarity (-minor)
- Cohesion tension (-major)
Root pressure is caused by the mineral ions which are actively transported into xylem vessels in the root by endodermal cells. This makes the water potential of the xylem more negative and causes water to enter the xylem by osmosis.
Capillarity is responsible for some water creeping up the xylem vessels simply because they are very narrow.
Cohesion tension Water evaporating from the leaves drives the movement of water from the roots. The evaporation creates a WP gradient and the properties of water and the xylem vessels allow this pressure to be transmitted all the way to the roots even in the tallest trees. This is known as the cohesion tension theory of water transport. Conditions always allow evaporation of water from the leaves. When water evaporates from a leaf mesophyll cell, this cell's WP will become more negative. Water from an adjacent cell will then move into this cell by osmosis as a result of the WP difference between them. This 'chain' of WP differences continues back to the xylem sap. Water moves smoothly and continuously along this WP gradient. The continual movement of water from the roots to the leaves is often called the Transpiration Stream. This can be thought of as the movement of water from a less negative to more negative water potential. Water moves from less negative in soil to the more negative air surrounding leaves. This provides a very steep gradient from soil to air, and is one of the driving forces of the transpiration stream.