Topic 4.4 Transport in plants

Question 1

What substances are transported by the phloem?

Answer 1

water and sucrose

Question 2

What substances are transported by the xylem?

Answer 2

water and mineral ions

Question 3

Describe the differences between the structure of xylem and phloem

Answer 3

• xylem cell walls contain cellulose and lignin, phloem cell walls only contain cellulose
• xylem is hollow, phloem has cell contents and sieve plates
• xylem has pits but phloem does not
• xylem is dead, phloem is alive and has companion cells

Question 4

Define the sclerenchyma

Answer 4

small circles surrounding the phloem, described as strengthening tissue. they are plant cells that have very thick lignified walls and an empty lumen with no living contents

Question 5

Define the cambium

Answer 5

a layer of unspecialised cells that divide, giving rise to more unspecialised cells that form the xylem or phloem

Question 6

What is the metaxylem?

Answer 6

mature xylem vessels made up lignified tissue

Question 7

What is the protoxylem?

Answer 7

the first xylem formed that can stretch and row because its walls are not lignified

Question 8

What are the phloem sieve tubes?

Answer 8

main transport vessel, made up of living cells joined together to make very long tubes which run from the highest shoots to the end of the roots

Question 9

What are companion cells?

Answer 9

very active cells closely associated with the sieve tube elements that supply the phloem vessels with everything they need to actively load sucrose into the phloem

Question 10

What direction does the phloem transport substances?

Answer 10

up and down

Question 11

What direction does the xylem transport substances?

Answer 11

only up

Question 12

What are pits?

Answer 12

specialised holes in the walls of the xylem vessels which water moves out of into the surrounding cells

Question 13

What is the xylem?

Answer 13

the main tissue transporting water around a plant

Question 14

What is the phloem?

Answer 14

the main tissue transporting dissolved solutes around the plant

Question 15

What is the symplast pathway?

Answer 15

the route by which substances can move by diffusion through the interconnected cytoplasm in the plasmodesmata connecting cells in a plant

Question 16

What’s the apoplast pathway?

Answer 16

the route by which substances can move by the attraction between the molecules across adjacent cell walls in a plant

Question 17

What is the Casparian strip?

Answer 17

the waterproof layer of waxy tissue in the walls of cells in the endothermis

Question 18

Define cohesion

Answer 18

the attraction between like molecules

Question 19

Define adhesion

Answer 19

the attraction between unlike molecules

Question 20

What is transpiration?

Answer 20

the loss of water vapour from the surface of the plant that has evaporated from the surface of the spongy mesophyll cells mainly within the leaves

Question 21

What is translocation?

Answer 21

the movement of substances around a plant

Question 22

What is the cohesion-tension theory?

Answer 22

theory of transpiration based on the idea that a stream of water molecules stick together by cohesive forces, so that when a molecule of water is lost by transpiration it puts a tension on the column and another molecule of water is pulled up to replace it

Question 23

Summarise the mass-flow hypothesis of translocation

Answer 23

• sugar loaded into sieve tubes by active transport
• lowers water potential, causing water to move in from the xylem
• hydrostatic pressure causes sugars to move