[Music] hi and welcome back to free slice lessons by the end of this video you should be able to describe how root hair cells are adapted for the absorption of water and minerals you should then be able to describe how water passes through the roots to the xylem including the simplast and apoplast pathways okay I'm showing you here the root grown from a Seedling as you can see these roots are covered with very fine root hairs root hairs grow from cells in the epidermis of the root in other words the outer layer and we can see that under the microscope now water moves into the root Hair by osmosis and root hair cells are adapted so that as Moses takes place rapidly firstly the densely packed root hairs massively increase the surface level to volume ratio of the root secondly the surface of the router consists only of the cell wall and the cell membrane this makes the surface extremely thin increasing the rate of osmosis finally the root hair cells use another mechanism to increase the rate of osmosis the water in the soil contains dissolved mineral ions for example magnesium which plants use to make chlorophyll however the concentration of these mineral ions is lower in the soil than in the root hair so root hair cells use active transport to move these metal ions into the cell the root hair cell also contains other dissolved compounds such as sugars so because of this the water potential inside the root hair cell is lower than in the soil so water now moves into the root hair cell by osmosis down the water potential gradient now the water must move from the root hair cells through the root cortex to the xylem water can move through the cortex by two pathways in the simplus pathway water moves from the cytoplasm of one cell to the cytoplasm of an adjacent cell to do this the water moves through the plasma does mortar linking the cells remember that plasma does mortar on microscopic channels through the cell wall connecting the cytoplasm of cells the simplest pathway is driven by The Walker potential gradient between the root hair cells and the xylem water continually moves into the root hair cells by osmosis from the soil this makes the water potential of the root hair cells greater than the cortex cells in the xylem the water potential is relatively low so water moves by osmosis across the cortex down the water potential gradient now I should point out that the simplest pathway is relatively slow that's because the pathway for water in the cytoplasm is obstructed by the organelles water can also move by the upperplus pathway in this case water moves within the cell walls and the spaces between the cells the cellulocell walls have a relatively open structure allowing water to move easily between the cellulose fibers remember that water molecules are attracted to each other and scientists call this attraction cohesion this is due to the fact that water molecules can form hydrogen bonds to each other as water moves into the xylem and is carried away more water moves along the upperplus pathway due to cohesion the Upper Plus pathway offers much less resistance to water flow than this simplest pathway now before the water can pass into the xylem it must pass through a lay of cells called the endodermis the cells in the endodermis have an unusual feature a band of waterproof material called subarin runs around the cell wall and scientists call this bomb the casparian strip because of the kasparian strip water can no longer move through the upper plast pathway instead the water now passes through the cell membrane and into the cytoplasm becoming part of the simplus pathway by force and all water through the cytoplasm this allows the cell membrane to control which substances can enter the xylem now cells in the endodermis use active transport to put mineral ions into the xylem this lowers the water potential of the xylem triggering water to move into the xylem vessels by osmosis in scientists call this effect root pressure now you need to remember that root pressure is an active process requiring respiration if we inhibit respiration by using a metabolic poison such as cyanide then root pressure stops root pressure also stops if we prevent our robot respiration by excluding oxygen in the next video we're going to look at transpiration [Music]