How water passes up the stem

1
How water passes up the stem

• Movement of water and mineral ions through the
  xylem

2
How water passes up the stem

• The tallest trees are over 60m high and may need
  as much as 250 dm3 of water an hour to be
  transported upwards.

3
How water passes up the stem

• In leaves
• Water evaporates from spongy mesophyll cells so
  the air spaces become saturated with water.

4
How water passes up the stem

• The air outside the stoma is not saturated so
  water diffuses out from an area of high water
  potential to region of lower water potential.

5
How water passes up the stem

• As water evaporates from mesophyll walls more is
  drawn into cells to replace it.
• Water can pass from the xylem to the leaf cells
  by both the apoplast and symplast pathway.

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Water
7
Water is drawn up the xylem
Water
8
cytoplasm
Water is drawn up the xylem
Water
9
cytoplasm
cell wall
Water is drawn up the xylem
Water
10
cytoplasm
cell wall
plasmodesmata
Water is drawn up the xylem
Water
11
cytoplasm
cell wall
plasmodesmata
Water is drawn up the xylem
Water
stoma
12
cytoplasm
cell wall
plasmodesmata
Water is drawn up the xylem
Water
vacuole
stoma
13
cytoplasm
cell wall
plasmodesmata
Water is drawn up the xylem
Water
vacuole
epidermal cell
stoma
14
cytoplasm
cell wall
plasmodesmata
xylem vessel
Water is drawn up the xylem
Water
vacuole
epidermal cell
stoma
15
cytoplasm
cell wall
plasmodesmata
xylem vessel
Water is drawn up the xylem
Water
air space
vacuole
epidermal cell
stoma
16
cytoplasm
cell wall
plasmodesmata
xylem vessel
Water is drawn up the xylem
Water
air space
vacuole
epidermal cell
stoma
17
cytoplasm
cell wall
plasmodesmata
xylem vessel
Water is drawn up the xylem
Water
air space
vacuole
epidermal cell
stoma
18
cytoplasm
cell wall
plasmodesmata
xylem vessel
Water is drawn up the xylem
Water
air space
vacuole
Apoplast pathway (cell wall)
epidermal cell
stoma
19
cytoplasm
cell wall
plasmodesmata
xylem vessel
Water is drawn up the xylem
Water
air space
vacuole
Apoplast pathway (cell wall)
epidermal cell
stoma
20
cytoplasm
cell wall
plasmodesmata
xylem vessel
Water is drawn up the xylem
Water
air space
vacuole
Apoplast pathway (cell wall)
epidermal cell
stoma
21
cytoplasm
cell wall
plasmodesmata
xylem vessel
Water is drawn up the xylem
Water
air space
vacuole
Apoplast pathway (cell wall)
epidermal cell
stoma
22
cytoplasm
cell wall
plasmodesmata
xylem vessel
Water is drawn up the xylem
Water
air space
vacuole
Apoplast pathway (cell wall)
epidermal cell
stoma
Symplast pathway (cytoplasm)
23
cytoplasm
cell wall
plasmodesmata
xylem vessel
Water is drawn up the xylem
Water
air space
Evaporation of water vapour
vacuole
Apoplast pathway (cell wall)
epidermal cell
stoma
Symplast pathway (cytoplasm)
24
How water passes up the stem

• Cohesion-tension theory
• As water is removed from xylem more water
  molecules are pulled up to replace them this
  pulling force is known as transpiration pull (a
  bit like sucking on a straw).

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How water passes up the stem

• This mass flow of water relies upon two
  properties of water.
• cohesion water molecules tend to stick together
• adhesion the molecules also stick to the inside
  of the xylem vessels.

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How water passes up the stem

• Drawing up a column of water through the xylem in
  this way is called the cohesion-tension theory.

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How water passes up the stem

• It requires a large force to break such a water
  column the xylem vessels would collapse under
  such tension if they were not thickened
• The cohesion-tension theory is considered the
  main way water reaches the leaves from the roots.

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How water passes up the stem

• Root pressure also contributes to flow of water
  up the plant the roots push water into the
  xylem and help to push up the water column.
• (Evidence if you cut a plants stem at soil level
  water will continue to come out of the cut stem
  for some time)

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How water passes up the stem

• Capillarity is a third force contributing to the
  movement of water. Water will move up narrow
  tubes by capillary action as water molecules will
  stick to the walls of the xylem vessels.
• This force is important in small plants but of
  little significance in large trees.