3.3 Transport in Plants

Question 1

What is the phloem.

Answer 1

Plant vessels specialised for transporting sugars bi-directionally.

Question 2

What is the source?

Answer 2

The part of the plant that loads assimilates into the phloem.

Question 3

What is the sink?

Answer 3

The part of the plant that removes assimilates from the phloem.

Question 4

What are xerophytic plants?

Answer 4

Plants that are specialised to dry habitats where water availability is low.

Question 5

What are some properties of xerophytic plants?

Answer 5

• fewer stomata to avoid water loss
• sunken stomata to trap moist air and therefore lower the water potential gradient with the outside
• hairs to trap water

Question 6

What is the Xylem?

Answer 6

Plant vessels responsible for transporting water and soluble mineral ions up the plant.

Question 7

What does lignin do?

Answer 7

It strengthens the xylem walls and keeps them open (hollow). Lignin cells are dead.

Question 8

What are vessel elements?

Answer 8

Long tubed cells of the xylem.

Question 9

What are end walls?

Answer 9

When vessel elements are stacked on top of each other creating the continuous tube that is the xylem.

Question 10

What are sieve tube elements?

Answer 10

Living cells that stack on top of each other to make the phloem, their cell walls are made of cellulose.

Question 11

What are sieve plates?

Answer 11

Large pores in between the sieve tube elements that allows assimilates to move through the phloem.

Question 12

What are companion cells?

Answer 12

Cells connected to the sieve tube elements by the plasmodesmata. They are connected by the cytoplasms.

Question 13

What is translocation?

Answer 13

The process where sugars produced by photosynthesis in the leaves are transported to other parts of the plant.

Question 14

What is transpiration?

Answer 14

The loss of water vapour from the upper parts of the plant.

Question 15

Describe transpiration.

Answer 15

• the loss of water creates tension in the xylem as hydrogen bonds form with the sides of the xylem
• this creates a transpirational stream that pulls up the water
• water molecules also form hydrogen bonds with each other and pull up the water molecules this is cohesion
• when water is pulled up the stem, it lowers the water potential at the roots so there is more diffusion of water into the roots

Question 16

Describe Translocation.

Answer 16

• sucrose produced in the leaves by photosynthesis actively transports into the companion cells (the energy for this AT is provided by the mitochondria in the companion cells)
• the sucrose then diffuses from the companion cells into the sieve tube elements
• this increase in sucrose lowers the water potential which then allows water to enter the phloem via osmosis
• this creates a hydrostatic pressure gradient in the phloem allowing the assimilates to move down the gradient from high to low (mass flow)
• some of the sucrose assimilates are used up by the surrounding cells and the rest are removed into the sink
• the water potential then increases without the sucrose so water leaves via osmosis and there is no longer a hydrostatic pressure gradient

Question 17

What is the Casparian Strip?

Answer 17

A waxy layer on the endodermis that ensures water has to pass through a membrane. The water is prevented from travelling through the apoplast pathway and it acts as a filter. It also helps to prevent water loss.

Question 18

What are the 3 different pathways water can travel through in transpiration?

Answer 18

• apoplast
• symplast
• vacuolar

Question 19

Describe the apoplast pathway.

Answer 19

When water passes through the spaces and and between cells, moving by mass flow (where there is a hydrostatic pressure gradient). It is fast but unfilitered and is stopped at the casparian strip.

Question 20

Describe the symplast pathway.

Answer 20

Water enters the cytoplasm through the plasma membrane by osmosis.It passes through the plasmodesmatas which connect the cytoplasms of the different cells.

Question 21

Describe the vacuolar pathway.

Answer 21

Water moves via osmosis through the vacuoles of the cells.

Question 22

What is the transpirational pull?

Answer 22

When water is pulled up by a cohesion chain. It creates tension which is why lignin is needed to strengthen the xylem.

Question 23

What is capillary action?

Answer 23

When water is attracted to the side of the xylem due to adhesion. It helps pull the water up the transpirational stream.

Question 24

What is root pressure?

Answer 24

When minerals getting pumped into to the medulla decreases the water potential, which causes water to enter the xylem via osmosis. Hydrostatic pressure builds up and pushes water up the xylem.