water transport in multicellular plants 9.2

Question 1

Name some ways in which water is key for the structure of plants

Answer 1

-Turgor pressure (hydrostatic pressure) as a result of osmosis in plant cells provides a hydrostatic Skeleton to support the stems and leaves (the turgor pressure in leaf cells is around 1.5 MaP)
-Turgor also drives cell expansion - it is the force that enables plant roots to force their way through tarmac and concrete, helps to drive roots down to properly anchor the plant

Question 2

Name some ways in which water is important for the metabolism of plants

Answer 2

-The loss of water by evaporation helps to keep the plants cool
-Minerals ions and the products of photosynthesis are transported in aqueous solutions
-Water is a raw material for photosynthesis

Question 3

Through which exchange surface is water brought into the plant

Answer 3

Root hair cells are the exchange surface in plants where water is taken into the body of the plant from the soil

Question 4

Describe the basic structure of a root hair cells

Answer 4

A root hair is a long, thin extension from a root hair cell, a specialised epidermal cell found near the growing root tip

Question 5

Name some ways in which root hair cells are adapted to their function

Answer 5

-Their microscopic size means they can penetrate easily between soil particles
-Each microscopic hair has a large SA:V ratio and there are thousands on each growing root tip
-Each hair has a thin surface layer (just the cell wall and cell-surface membrane) through which diffusion and osmosis can take place quickly
-The concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil and water in the cell

Question 6

Explain why water moves into the root hair cell from the soil

Answer 6

Soil water has a very low concentration of dissolved minerals so it has a very high water potential.
The cytoplasm and vacuolar sap of the root hair cells (and the other root cells) contain many different solvents including sugars, mineral ions and amino acids so the water potential in the cell is lower.
As a result water moves into the root hair cells by osmosis

Question 7

Once into the root hair cell where does the water go

Answer 7

Once the water has moved into the root hair cell it continues to move across the root to the xylem

Question 8

Name the 2 pathways in which water moves to the xylem

Answer 8

The symplast pathway
The apoplast pathway

Question 9

what is the symplast

Answer 9

the continuous cytoplasm of the living plant cells that is connected through the plasmodesmata

Question 10

Describe the symplast pathway and how it is maintained

Answer 10

• Water moves through cytoplasm & plasmodesmata by osmosis.
• root hair cell has higher water potential than adjacent cell so water moves from root hair cell
• cell next to that has lower water potential
• process continues until water reaches xylem
• as water leaves root hair cell water potential drops so conc gradient maintained

Question 11

what is the apoplast

Answer 11

space between cell membrane and cell wall in plant cells

Question 12

Describe the apoplast pathway

Answer 12

This is the movement of water through the apoplast.
Water fills the spaces between the loose, open network of fibres in the cellulose cell wall.
As water molecules move into the xylem, more water molecules are pulled through the apoplast behind them due to the cohesive forces between the water molecules. The pull from water moving into the xylem and up the plant along with the cohesive forces between the water molecules creates a tension that means there is a continuous flow of water through the open structure of the cellulose wall, which offers little or no resistance.

Question 13

what is an endodermis

Answer 13

The layer of cells surrounding the vascular tissue of the roots

Question 14

what is the casparian strip

Answer 14

The casparian stip is a band of waxy material called Suberin that runs around each of the endodermal cells forming a waterproof layer

Question 15

Describe how water gets into the xylem

After the pathways

Answer 15

Water moves across the root in the apoplast and symplast pathways until it reaches the endodermis.
At this point, water in the apoplast pathway can go no further and it is forced into the cytoplasm of the cell, joining the water in the symplast pathway.
The water then passes through the a selectively permeable cell-surface membrane.
The solute concentration in the cytoplasm of the endodermal cells is relatively dilute compared to the cells in the xylem.
In addition, endodermal cells move mineral ions into the xylem by active transport.
As a result the water potential of the xylem cells is much lower than the water potential of the endodermal cells.
This increases the rate of water moving into the xylem by osmosis down a water potential gradient from the endodermis through the symplast pathway.
Once inside the vascular bundle, water returns to the apoplast pathway to enter the xylem itself and move up the plant.
Root pressure gives water a push up the xylem, but it is not the major factor in the movement of water up from the roots to the leaves.

Question 16

why is it important that during the movement of water from the apoplast to symplast pathway that it passes a selectively permeable cell-surface membrane

Answer 16

It removes any potentially-toxic solutes in the soil water from reaching living tissues, as the membranes would have no carrier proteins to admit them

Question 17

What produces root pressure

Answer 17

The active pumping of minerals into the xylem to produce movement of water by osmosis results in root pressure and it is independent of any effects of transpiration

Question 18

what is the function of the casparian strip

Answer 18

it forces water from the apoplast pathway to the symplast pathway

Question 19

What evidence is there for the role of active transport in root pressure

Answer 19

• Some poisons such as cyanide, affect the mitochondria and prevent the production of ATP. If cyanide is applied to root cells so there is no energy supply, the root pressure disappears
• Root pressure increases with a rise in temperature and falls with a fall in temperature, suggesting chemical reactions are involved
• if levels of oxygen or respiratory substrates fall, root pressure falls
• Xylem sap may exude from the cut end of stems at certain times. In the natural world, xylem sap is forced out of special pores as the ends of leaves in some conditions - for example overnight, when transpiration is low. This is known as guttation.