Capillarity

• Xylem vessels are very thin.
• Water is attracted to the lignin in the walls.
• Water has a tendency to rise up the tube.
• This accounts for water moving up by about 50mm up the xylem.

Root pressure (hydrostatic pressure)

• Ions actively pumped into the Endodermal cells.
• When it comes to IBDP Biology, This creates a water potential gradient.
• Water enters the endodermis from the cortex by osmosis.
• It then moves into the xylem.
• Accumulation of water in the xylem in the roots causes root pressure.
• This contributes to movement of the water up plants, but cannot fully account for water movement up tall trees.

Cohesion-tension theory

• When it comes to IBDP Biology, Cohesion is the force of attraction between like molecules.
• Hydrogen bonds exist between water molecules.
• As water leaves the vessels a tension is created in the columns of water.
• The columns do not break because of cohesion.
• As water is lost from cells in the leaf, it reduces the water potential inside these cells.
• Therefore, pressure at top of xylem is less than at bottom.
• This stretches the column of water, creating a tension
• This pulls the water up the xylem.
• This process can account for water movement up tall trees.
• Lignin in the xylem walls prevents them collapsing.

Capillarity:

• Dip a narrow glass tube in water.
• Water rises up the tube.

Root pressure:

• Cut the shoot of a plant off close to the ground.
• Sap exudes from the xylem.

Passive evaporation from the leaves:

• When it comes toIBDP Biology,Remove leaves from a plant
• Transpiration is reduced

Cohesive tension:

• When it comes toIBDP Biology,The diameter of tree trunk decreases during the day.
• It reaches its minimum size in the afternoon and maximum early in the morning.
• The diameter decreases when transpiration rate is highest.
• This is because of the massive tension in the xylem vessels.

Water moves through the xylem lumen:

• Dyes (eg eosin) can be introduced to the roots.
• These are later found in xylem, not phloem.
• Allow a plant to draw up a fatty solution.
• When it comes toIBDP Biology,This blocks the xylem lumens.
• This prevented further uptake of water.
• The plant wilts.

Water can move sideways:

• When stems bend in the wind, xylem vessels can be stretched.
• When it comes toIBDP Biology,Air bubble form.
• These block individual xylem vessels.
• Water still moves upwards, by moving into adjacent xylem vessels through pits.

That's the end of this topic.

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