the transpiration stream

• the outermost layer of cells of the root contains the root hair cells. these are cells with a long extension that increases the surface area of the root. these cells absorb mineral ions and water from the soil. the water then moves across the root cortex down a water potential gradient to the endodermis of the vascular bundle. water may also travel through the apoplast pathway as far as the endodermis then entering the symplast pathway due to blockage by the casparian strip.
• The movement of water across the root is driven by an active process that occurs at the endodermis. the endodermis is a layer of cells surrounding the medulla and xylem. this layer of cells is also known as the starch sheet as it contains granules of starch- this shows that energy is being used,.
• The casparian strip blocks the apoplast pathway fro the cortex and medulla, this ensures that the water and dissolved mienral ions, especially nitrates, have to pass into the cell cytoplasm through the plasma membrane.
• The plasma membrane contains transporter proteins which actively pump mineral ions from the cytoplasm of the correct cells into the medulla and xylem. This makes the water potential of the medulla and xylem more negative so that water moves from the cortex cells into the medulla and xylem via osmosis. Once water has entered the medulla it can't pass back into the cortex as the apoplast pathway of the endodermal cells is blocked by thew casparian strip.
• the movement up the xylem is called mass flow. a flow of water and mineral ions in the same direction .
• what is root pressure?

  The action of the endodermis moving minerals into the medulla and xylem by active transport drawss water into the medulla by osmosis. pressure in the root medulla builds up and forces water into the xylem. root pressure can push water a few metres up a stem but cannot account for water getting to the top of tall trees.
• the loss of water by evaporation from the leaves needs to be replaced by water coming up from the xylem. The water molecules are attracted to each other due to cohesion. the cohesive forces are strong enough to hold water molecules together in a long chain or column. As water molecules are lost at the top of the column, the whole column is pulled up.
• The pull from water molecules above creates tension in the column of water molecules. The walls of xylem are strengthened with lignin to prevent the vessel from collapsing under the water tension.
• The same forces that hold water molecules together also attract water molecules to the sides of a xylem vessel. this is called adhesion. because the xylem vessels are very narrow, these forces of attraction pulls water up the sides of the vessel.