Movement of water in plants

Question 1

What does the xylem do

Answer 1

Water and dissolved minerals are transported from the soil to the rest of the plant in the xylem

Question 2

How is the xylem specialised and how does this help

Answer 2

• Hollow with no end walls = continuous flow of water
• No organelles = No obstruction of water flow
• Lignin
  . provides waterproofing = prevents the loss of water by evaporation
  . Strengthens the xylem = reduces breakages

Question 3

What is the process of transpiration

Answer 3

• Water diffuses (due to there being no membrane) out of the leaf via the stomata
• Lowers water potential in air spaces
• Water in the mesophyll cells evaporates/osmosis into the leaf air spaces
• Water potential lowered in the mesophylls
• Water is drawn from the xylem through pits (no lignin) into mesophyll cells via osmosis
• Water moves up the xylem in a continuous stream to replace this water loss (transpiration stream)

Question 4

What is the theory called when water moves up a plant

Answer 4

Cohesion tension theory

Question 5

What is the cohesion tension theory

Answer 5

• Water transpires from the leaf
• This lowers water potential
• Water is then drawn out of the xylem creating tension
• The cohesion of the water and the adhesion of the water to the walls of the xylem causes a constant upwards movement of water in the plant maintaining the water column

Question 6

How does cohesion and adhesion form the water stream

Answer 6

• cohesion = due to hydrogen bonds between water molecules
  . upward force creates tension
• adhesion = They hydrogen bonds bind to other molecules (e.g. cellulose in xylem walls)
  . Can pull the Xylem walls inwards creating a continuous flow of water

Question 7

Why is transpiration important

Answer 7

• Cools the plant via evaporative cooling
• Uptake of dissolved mineral ions
• Turgor pressure provides support to leaves = bigger surface area

Question 8

What is the role of the stomata

Answer 8

Transpiration is controlled by guard cells
- Open when turgid guard cells = more transpiration
- Close when they lose water = less transpiration
- typically open during the day

Question 9

What is the Apoplastic and Symplastic pathway

Answer 9

Apoplatic - Transpiration pulls water through the mesophyll cell wall and out

Symplastic - Transpiration pulls water through cytoplasm then cell wall then out