TRANSPORT IN THE XYLEM

Question 1

transpiration

Answer 1

water taken up from the soil, through the roots to the leaves where it’s lost via the stomata

Question 2

how do plants avoid dehydrating

Answer 2

water is constantly replaced

Question 3

where and why is water uptake highest (3)

Answer 3

• root hair zone
• surface areas is increased by hairs
• thin cell walls enhance uptake

Question 4

how does water pass into the root hair cells (3)

Answer 4

• soil is a very dilute solution of minerals and has a high water potential
• vacuole and cytoplasm contain very concentrated solution of solutes therefore have a very negative water potential
• water passes into the cell via osmosis, down a concentration gradient

Question 5

two pathways water moves through the root (2)

Answer 5

• apoplast pathway - moving through the cell walls, cellulose fibres seperated by spaces where the water moves
• symplast pathway - water moves through cytoplasm and plasmodesmata, so a continual path across root cortex

Question 6

plasmodesmata

Answer 6

strands of cytoplasm joining adjacent cells

Question 7

why can’t water enter the xylem straight from the apoplast pathway? (2)

Answer 7

• because water can only pass into the xylem via symplast/vacuole pathways
• so it must move out of the apoplast to get into the xylem

Question 8

pericycle

Answer 8

• region surrounding vascular tissue

Question 9

endodermis

Answer 9

• single layer of cells around the pericycle

Question 10

casparian strip (2)

Answer 10

• endodermis cell walls are made of suberin, a waxy, waterproof substance
• forms a distinctive band and drives water from the apoplast pathway into the cytoplasm

Question 11

how does water move efficiently from the root endodermis to the xylem? (3)

Answer 11

• water potential of endodermis is raised by water being pushed out of the apoplast by the casparian strip
• water potential of the xylem decreased by active transport of mineral salts (Na)
• so water moves down potential gradient in the xylem

Question 12

root pressure

Answer 12

water entering the xylem generates upward push on water already in the xylem

Question 13

how are minerals transported from the soil

Answer 13

• actively (against gradient) because they are dilute in the soil
• can move along the apoplast pathway until casp. strip then enter cyoplasm by active transport then into xylem

Question 14

three ways water moves up a plant (3)

Answer 14

• cohesion-tension - combination of adhesion (hydrophilic lining of vessels causing attraction) and cohesion (water molecules attracting other water molecules)
• capillarity/capillary action - movement of water up narrow tubes (only makes a small contribution)
• root pressure - water entering via osmosis pushes water already present up (short distances)

Question 15

how does water in a transpiration stream move up? (2)

Answer 15

• cohesive forces between water molecules

- adhesive forces between water molecules and hydrophilic lining of xylem vessels