the transpiration stream

• Water movement in Root

  outermost layer of cells (epidermis) of a root contains root hair cells

  • these cells absorb mineral ions + water from soil
  • water then moves across the root cortex down the water potential gradient to endodermis of vascular bundles

  →water can also go through apoplast pathway as far as endodermis, then symplast pathway as apoplast is blocked by Casparian Strip
• Water movement in Root 2

  Role of endodermis

  • movement of water across root is driven by an active process that occurs that occurs at endodermis

  →the endodermis is a layer of cells surrounding the medulla + xylem (also called starch sheath as it contains granules of starch - sign that energy is being used)

  • Casparian strip blocks apoplast pathway between cortex + medulla

  → this ensures water or dissolved minerals have to pass into cell cytoplasm through plasma membrane
• Water movement in Root 2

  Role of endodermis 2

  • plasma membranes contain transporter proteins, that actively pump mineral ions from cytoplasm of cortex cells into medulla + xylem

  → makes water potential of cortex + medulla more negative, so water moves cortex cells into medulla + xylem by osmosis

  • once water has entered medulla, it cannot pass back into cortex, an apoplast pathway of endodermal cells are blocked by Casparian strip
• Movement of water in Stem

  water moves up through xylem by mass flow ( mineral ions + water going same direction)

  3 processes that help water up stem :

  1. root pressure
  2. transpiration pull
  3. capillary action
• Movement of water in Stem

  root pressure

  • action of endodermis moving minerals into medulla + xylem by active transport draws water into medulla by osmosis
  • pressure in root medulla builds up + forces water into xylem, pushing water up xylem

  → root pressure can push water a few metres up a stem, but cannot account for water getting up tall
• Movement of water in Stem

  transpiration pull

  loss of water by evaporating leaves must be replaced by water coming up xylem

  • water molecules attract each other by cohesion forces; strong enough to hold molecules in a long chain

  → as molecules are lost at top of column, water comes up as one chain/column of water molecules

  • pull from above creates tension - reason why xylem is strengthened by lignin
• Movement of water in Stem

  capillary action

  same forces also attract water molecules to the side of the walls, called adhesion

  → because xylem vessels are very narrow, these forces of attraction can pull water up sides of vessel
• Movement of water in Leaf

  most water that leaves leaf exists as vapour through stomata + only tiny amounts leave through waxy cuticle

  • water evaporates from cells lining the cavity immediately above guard cells

  → lowers water potential in these cells, causing water to enter them by osmosis from neighbouring cells

  • in turn, water is drawn from xylem in leaf by osmosis; water may also reach cells by apoplast pathway from xylem