L2 - water transport in multicellular plants

• The transpiration stream:

  -water moves from soil to root hair cell by osmosis, then passes from cell to cell inside roots by osmosis until it reaches xylem

  -water moves up xylem by capillary action

  -water is used by the leaf cells (for photosynthesis) or evaporates and diffuses out of the leaf
• Capillary action: pushed up from the water below and pulled up by the water molecules above by cohesion
• A plant cell is plasmolysed if all the water has diffused out
• A plant cell is turgid if lots of water has entered the cell
• Importance of water in plants:

  -water reacts with carbon dioxide in photosynthesis to produce carbohydrates

  -water contains dissolved minerals (such as nitrates) and glucose made during photosynthesis

  -water cools the plant as it evaporates from the aerial parts using heat energy

  -water enters the cell by osmosis and the increased hydrostatic pressure pushes the membrane against the cell wall
• Transport of water into the plant:

  -apoplast

  -symplast
• Apoplast: water moves between the water filled spaces in the cellulose cell walls, it doesn’t pass through the plasma membrane itself
• Symplast: water passes through the plasma membrane into the cell cytoplasm. water diffuses from a cell with higher water potential to a cell with lower water potential, through the plasmodesmata, by osmosis
• Water transport into the plant:

  
• The endodermis is a ring of cells between the cortex of the root and the area which houses the xylem and phloem. It contains cells containing starch and the casparian strip which contains a waxy, waterproof material called suberin
• The casparian strip (in the endodermis) stops the flow of the apoplast pathway so water must pass through the symplast pathway
• Transport of water into the plant (1-root):

  -starting in root hair cells

  -active transport of minerals into root hair cells

  -lowers water potential in cells

  -so water moves in by osmosis
• Transport of water into the plant (2-root cortex):

  -the water passes through cells by apoplast and symplast paths

  -casparian strip is waterproof, so water is forced to enter the cytoplasm of the endodermis cell
• Transport of water into the plant (3-xylem):

  -at the endodermis there is active transport of minerals into xylem from the cortex cells

  -transporter proteins in plasma membrane actively transport nitrates/mineral ions into xylem

  -water potential in the xylem is lowered

  -water diffuses by osmosis into xylem from cortex

  -water cannot move back as casparian strip blocks the apoplast pathway
• Root pressure: occurs when the endodermis moves minerals into xylem by active transport. water then moves in by osmosis and this force pushes water up stem

  -a possible way for water to move from the root to the leaves
• Water moves through the plant from roots to leaves and is lost to the air as water vapour:

  -this is known as the transpiration vapour

  -about 98% of water entering a plant is lost to the air through transpiration

  -remaining 2% is used for photosynthesis
• How water is lost from a plant by transpiration:

  -water enters the leaf from the xylem by osmosis

  -water evaporates from the spongy mesophyll cells forming water vapour

  -water vapour collects in the air space above the leaf

  -the water vapour potential rises in the leaf

  -when the water vapour potential is higher in the leaf than outside, water vapour diffuses out of the leaf via the stomata
• Water loss in plants:

  -stomata in the leaves, account for about 90% of water loss

  -cuticle (the waxy layer), some water is lost by evaporation through the epidermis and the cuticle
• Stomatal opening:

  -guard cells get distended by turgor pressure

  -guard cells expand

  -tough inner walls become convex

  -stoma open
• Stomatal closing:

  -turgor pressure in the guard cells decrease

  -guard cells sag

  -inner cell walls come closer

  -stoma close
• Properties of water that enable transpiration:

  -transpiration pull

  -capillary action

  -cohesion

  -adhesion
• Transpiration pull: evaporation of water from the leaves pulls water up the xylem by cohesion of water molecules in a column. This is the cohesion-tension theory
• Capillary action: water molecules are attracted to the sides of xylem, this is called adhesion. Pulls water molecules up sides of xylem