AS Biology: Movement of Water

The Casparian strip

The Casparian strip blocks the apoplast pathway to ensure that water and dissolved nitrate ions have to cross the cell membrane which is done by the transporter proteins Nitrate cane be actively transported into the xylem which lowers the water potential of the xylem and water follows by osmosis.

• The endodermis around the xylem is aka starch sheath which contains starch and uses it as its source of energy.
• The endodermis consists of special cells that have waterproof strip in their walls. - The Casparian strip.
• This strip block the apoplast pathway and ergo water is forced into the symplast pathway.
• The endodermal cells moves minerals by active transport from the cortex to the xylem. - decreases water potential in the xylem by osmosis.
• This reduces the water potential in the cells just outside the epidermis.
• This sets up a water potential gradient across the whole cortex.
• Ergo, water is moved along the symplast pathway from the root hair cells across the cortex and into the xylem - at the same time water an move through the apoplast pathway across the cortex

Movement up the stem

The force that pulls water up the stem of a plant is the evaporation of water from leaves - a process called transpiration. Water molecule evaporates from the leaves, hough the tiny openings called stomata on the surface of a leaf.

• As water move into the xylem by osmosis this pushes the water already present up the xylem. Root pressure can push water a few metres up a stem, but cannot account for movements over great distances.
• Capillary action - the same forces that hold water molecules together also attract the molecules of the side of the xylem vessel - adhesion. These forces can pull up the sides of a vessel.
• Transpiration pull -  water evaporates from leaves as a result of transpiration. Water molecules form hydrogen bonds between one another so they stick together aka cohesion.. Water forms a continuous, unbroken pathway across the mesophyll cells in the leaf and down the xylem. As water evaporates from the mesophyll cells into he leaf  into the air spaces beneath the stomata, more molecules of water draws up as a result of cohesion. Water is then pulled up the xylem as a result of transipation pull. This puts xylem under tension (cohesion tension theory) The lignified xylem vessels prevent collapse under pressure. 

AS Biology: Plant cells and water

Water potential

• Pure water has a water potential of 0 however cells have negative water potential when there are more solute particles than water molecules therefore cells have higher water potential when there are less solute particles and more water molecules
• The plant cells becomes TURGID when water enters by osmosis, vacuole sweels and pushes against the cell wall. Plant cell is FLACCID when the water is lost from the cell and the vacuole shrinks hence the cell loses its shape

Water can travel via three different pathways:

1. Apoplast pathway - water moves through the cell wall

• The cellulose cell wall has many water filled spaces between the cellulose molecule, water can move through these spaces and between cells. The water does not pass through any plasma membranes.

2. Symplast pathway - water moves through the plasma membrane into the cytoplasm

• Water enters the cytoplasm via the plasma membrane. The plasmodesmata allows the movement of water from one cell to the next.

3. Vacuolar pathway - water moves through the vacuole

• This is similar to symplast pathway but the water can now enter and pas through the vacuoles as well.

Water uptake from soil

• The roots have root hair cells, to increase the surface area to absorb minerals by active transport.
• This lowers the water potential in the roots so water moves into the root hair cells by osmosis. 
• Water enter root hair cell by osmosis - minerals are actively transported into xylem. (water moves into xylem my osmosis.) 

Solute can enter the xylem by going through cell membrances - the Casparian strip blocks apoplastic route (outside cells) - water cannot pass between the cell or through cell walls - it must pass into cytoplasm or into symplast pathway

Water moves up the stem due to root pressure, transpirational pull, capillary action

• Transpiration pull - as water molecules are removed from the xylem, more water molecule are pulled to replaced them aka transpirational pull.

Mass flow of water also relies on the properties of water

• Cohesion - the water molecules tend to stick together
• Adhesion - the water molecules also stick the the inside of the xylem vessel
• the drawing of continuous column of water up to xylem vessel is aka cohesion-tension theory