[7.7] transport of water in the xylem

Question 1

what does the xylem transport?

Answer 1

• water
• mineral ions

Question 2

what does a mature xylem vessel look like?

Answer 2

• a stack of cell walls on top of each other
• there are no internal structure so there is an unimpeded stream of water flowing through it

Question 3

how is the xylem vessel adapted for it function?

Answer 3

• there is lignin in the walls of xylem vessels
• this is extremely difficult to break down and waterproofs it so water doesn’t leak out
• spiralised thickening gives plant extra strength so pressure inside doesn’t cause the xylem vessel to collapse inwards due to negative pressure

Question 4

what is the role of the xylem?

Answer 4

to transport water through the stem in a process called transpiration, until it diffuses through the stomata

Question 5

describe the movement of water through the stomata

Answer 5

• higher water potential in atmosphere than in air spaces next to stomata
• water potential gradient from the air spaces through the stomata to the air
• provided the stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air
• water lost by diffusion from the air spaces is replaced by water evaporating from the cell walls of the surrounding mesophyll cells
• by changing the size of the stomata, plants can control their rate of transpiration

Question 6

how can water travel through the plant?

Answer 6

• apoplast pathway (cell wall pathway)
• symplast pathway (cytoplasm pathway)

Question 7

how does water enter the root hair cell from the soil?

Answer 7

• through osmosis, from low to high water potential
• sometimes, active transport of mineral ions is needed to manipulate the water potential for their own benefit, but mineral ions are also beneficial

Question 8

what are mineral ions needed for?

Answer 8

• potassium: for sodium-potassium pump in co-transport mechanism
• magnesium: to make chlorophyll, which absorb sunlight for photosynthesis
• nitrate: for amino acids, which form proteins
• phosphate: for nucleic acids (structure) and the phospholipid bilayer

Question 9

due to what force is water effectively drawn up the transpiration stream?

Answer 9

• cohesive forces between water molecules
• this is due to hydrogen bonds between water molecules

Question 10

why is water needed?

Answer 10

• metabolite (eg. photosynthesis)
• keeps mesophyll turgid so it is upright so maximum sunlight can be absorbed for photosynthesis
• cooling. reduces heat generated from plant’s metabolic reactions