water transport Flashcards
what is osmosis
movement of water from a solution of a high concentration to low concentration across the partially permeable membrane
what is water potential
measure of tendency of water molecules to diffuse from one region to another (high to low)
why does the casparian strip block endodermis
-ensures the water and dissolved mineral ions have to pass into the cell cytoplasm through the plasma membrane
-the plasma membranes contains transporter proteins
-The water potential of medulla and xylem become negative so water moves from cortex into medulla and xylem by osmosis
-water enters medulla, it cannot pass back into cortex as apoplast pathway is blocked
what happens when there is low external water potential
-water moves out the cell
-plants can survive this for short periods as they can shrink the cell membrane away from the cell wall
-the cell is said to be plasmolysed
what happens in the symplast route in plants
-Assimilates are stored in the vacuoles of cells.
-The assimilates are moved through the cytoplasm of mesophyll cells into the sieve tubes across connecting plasmodesmata
-This is a largely passive process
-The assimilates are moved by changes in water potential of cells
what happens in the apoplast route in plants
-Assimilates diffuse through the cell wall and intermembrane spaces of mesophyll cells - apoplast
-Energy is needed to actively transport hydrogen ions out of the companion cells.
-This increases their concentration gradient outside the cells and decreases their concentration inside the companion cells.
-As a result, a concentration gradient is created.
-The hydrogen ions diffuse back into the companion cells though cotransporter proteins. The proteins only allow movement of hydrogen ions if they are accompanied by assimilates.
where does water move if the water potential inside the cell is low
water will move in by osmosis
where does water move if the water potential inside the cell is high
water will move out by osmosis
when is a cell plasmolysed
-in low external water potential, water moves out of cell
-plants shrink the cell membrane away from the cell wall
what happens in the vacuoles pathway
same as the symplast pathway but water moves through the cell vacuoles in addition to the cytoplasm
this is the slowest route
what is the endodermis
single layer of cells surrounding the vascular tissue in the roots
what is the casuarina strip
-impermeable layer of suberin
function of casparian strip
-makes water pas through the cell instead of around the outside
-water has to travel in symplast pathway after it reaches casparian strip
-controls substances travelling through a cell membrane
-stops flow in apoplast pathway so water must pass through symplast pathway
how does water enter a plants root hair cell
osmosis
what is a potometer used for
to measure the rate of transpiration
why should cutting the shoot for potometer be done underwater
to prevent air from entering the xylem
why is it important to keep the capillary tube end of the potometer submerged throughout the expriment
to maintain an airtight system
what should be done before recording the rate of bubble movement in a potometer
gives the shoot time to acclimatise
