Mass Transport in Plants

Question 1

Cohesion - Tension Theory

Answer 1

• water transpires (evaporates) from leaves
• reduces water potential in surrounding cells
• water potential gradient created across cells
• water is drawn up the xylem due to negative pressure ->tension
• many hydrogen bonds means water is cohesive
• drawn up the xylem in continuous columns

Question 2

Evidence for Cohesion - Tension Theory

Answer 2

Tree trunks change diameter accordingly to the rate of transpiration.
During the day when transpiration is at its greatest, there is more tension (negative pressure) in the xylem and the trunk decreases in diameter.

Question 3

Xylem Structure

Answer 3

• long, dead cells with no end walls
• continuous water columns; no cytoplasm/ organelles to obstruct water flow
• strong lignin to withstand tension