Water Transport In Plants

Question 1

State 4 need for water in plants

Answer 1

• cooling affect by transpiration
• mineral ions and sugars are transported in aqueous solution
• water is a raw material of photosynthesis
• turgor pressure - hydrostatic skeleton

Question 2

What is water potential ?

Answer 2

• the tendency of water molecules to move within and between cells
• net movement of water will occur from one region to another as a result of a difference in water potential
• water will move from a region of higher water potential to a region of lower water potential

Question 3

Is the uptake of water a passive or active transport ?

Answer 3

• passive process
• occurs by osmosis
• the uptake of minerals can be passive or active and occurs by diffusion or active transport respectively

Question 4

How would the water move inside the plant if the water potential inside the cell is low ?

Answer 4

Move in via osmosis

Question 5

How does the water move out of the cell if the water potential inside the cell is high ?

Answer 5

Water will move out by osmosis

Question 6

What occurs in low external water potential ?
and how can plants survive this ?

Answer 6

• water moves out of the cell
• plants can survive this or short periods as they can shrink the cell membrane away from the cell water
• the cell is said to be plasmolysed

Question 7

State some adaptations of the root hair cell

Answer 7

• very thin cellulose walls that are readily permeable to water and dissolved mineral ions
• microscopic in size
• Large SA:V ratio
• conc of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell

Question 8

What are the 3 ways that water moves through the root cells and into the xylem tube by ?

Answer 8

• symplast pathway ( through cytoplasm )
• vacuole pathway ( through vacuoles )
• apoplast pathway ( through cell walls )
• these allow the plant to get water in as fast as possible *

Question 9

How do plants lose water ?

Answer 9

From their leaves via transpiration

Question 10

Describe what the symplast pathway is

Answer 10

• this is the movement of water through the living spaces of the cell - cytoplasm
• enters cells through the plasmodesmata
• each cell further away from the roots has a lower water potential so water is drawn through the plant

Question 11

Describe the pathway of water through the symplast pathway

Answer 11

1. Water enters the cytoplasm across the partially permeable plasma membrane
2. Water can move into the sap in the vacuole through the tonoplast
3. Water may move from cell to cell through the plasmodesmata
4. Water may move from the cell to cell through adjacent plasma membranes and cell walls

Question 12

Which pathway is slower the apoplastic or symplastic pathway ?

Answer 12

Symplastic pathway

Question 13

What is the vacuolar pathway ?

Answer 13

• is the same as the symplast pathway when the water moves through the cells vacuoles in addition to the cytoplasm
• this is the slowest route

Question 14

Describe the movement of water through the vacuolar pathway

Answer 14

1. Water enters the cytoplasm across the partially permeable plasma membrane
2. Water can move into the sap in the vacuole through the tonoplast
3. Water may move from cell to cell through the plasmodesmata
4. Water may move from cell to cell through adjacent plasma membranes and cell walls

Question 15

What is the apoplast pathway ?

Answer 15

• the movement of water through the cell wall and intracellular spaces
• cohesive and tension forces acting on the cell walls pulls the water up the plant
• this is the fastest movement of water

Question 16

Describe the movement of water through the apoplast pathway

Answer 16

• most water travels via the apoplast pathway which is the series of spaces running through the cellulose walls , dead cells and the hollow tubes of the xylem
• water moves by diffusion as its not crossing a partially permeable membrane
• water can move from cell wall to cell wall directly or though the intercellular spaces
• the movement of water through the apoplastic pathway occurs more rapidly than the symplastic pathway
• when water reaches the endodermis the presence of a thick , waterproof, waxy band of SUBERIN within the cell wall blocks the apoplastic pathway
• this band is called the CASPARIAN STRIP and forms an impassable barrier for the water
• when the water and dissolve minerals reach the casparian strip they must take the symplastic pathway - the presence of this strip is not fully understood but it is thought that this may help the plant control which mineral ions reach the xylem and generate root pressure
• as the plant ages the casparian strip thickens as more Suberin is deposited except in cells called the passage cells allowing for further control of the mineral ions

Question 17

What is the casparian strip ?

Answer 17

A impermeable layer of Suberin - a waxy material

Question 18

As a result of Suberin what happens ?

Answer 18

All water in the apoplast pathway is forced into symplast pathways

Question 19

Where is the casparian strip located ?

Answer 19

The endodermis

Question 20

What is the endodermis ?

Answer 20

• water moves across the roots in the apoplast, symplast and vacuolar pathways until it reaches the endodermis
• this is a continuous cylinder of endodermal cells which surrounds the central vascular tissue ( xylem and phloem )

Question 21

Explain what the casparian strip is and what role it plays

Answer 21

• when water reaches the endodermis of the root it’s path is blocked
• the endodermis has a waterproof, impenetrable layer called the casparian strip in its walls —> this is because of the waxy layer of Suberin in the walls of endodermal cells
• in order to cross the endodermis the water that has been moving through the cell walls in the apoplast pathway must now move through the cell surface membrane & into the cytoplasm forced into the symplast pathway
• in this way the selectively permeable plasma membrane of the cells can control what enters the xylem tissue
• this is important as the cell surface membrane can remove any toxic solutes from the soil and only allow necessary water molecules and mineral ions to enter

Question 22

Why is there a casparian strip ?

Answer 22

• helps to control which substances reach the xylem vessels eg. Which mineral ions or toxic solutes
• plays a part in increasing root pressure

Question 23

What are 4 evidence factors for active transport in root pressure ?

Answer 23

1. Affect of cyanide - cyanide stops the mitochondria from working —> root pressure decreases
2. Affect of temperature - root pressure increases as temperature increases and decreases and temperature decreases suggesting of an enzyme controlled chemical reaction
3. Reactant availability - if oxygen levels or respiratory substrate levels drops —> root pressure decreases
4. Guttation - sap and water will move out of cut stems suggesting they are actively pumped out not drawn up by transpiration

Question 24

Explain how water is transported up the stem to the leaves

Answer 24

• the movement of water from roots to leaves is called the transpiration stream
• water evaporates from spongy mesophyll cells into the air spaces of the leaf
• water Vapor then diffuses/ evaporates out of the leaf via the stomata
• this loss of water creates a low hydrostatic pressure at the top of the xylem
• water moves up the xylem by mass flow from huger hydrostatic pressure to lower hydrostatic pressure down a hydrostatic pressure gradient

Question 25

How do water molecules move against gravity ?

Answer 25

• loss of water is replaced into the roots via apoplast/symplast/vacuolar pathways down a water potential gradient
• water is drawn into the xylem
• pressure gradient is created
• this creates tension in the xylem which pulls up water in a continuous column
• within the xylem vessels in the columns of water are held together by cohesion and by adhesion ( the attraction between a water molecule and the walls of the xylem vessels )
• column of water is pulled up by tension
• xylem vessels are very narrow
• movement of these columns of water is known as the transpiration stream

Question 26

Why does the water move into the xylem ?

Answer 26

• the solute concentration in the cytoplasm of the endodermal cells is relatively dilute compared to the cells in the xylem
• endodermal cells move mineral ions into the xylem by active transport
  = the water potential of the xylem cells is much lower than the water potential id the endodermal cells
• this increases the rate of water moving Into the xylem by osmosis

Question 27

What are cohesion and tension ?

Answer 27

• cohesion and tension help water move up the plants from roots to leaves against the force of gravity
• water evaporates from the leaves at the top of the xylem
• this creates a tension which pulls more water into the leaf
• water molecules are cohesive so when some are pulled into the lead others follow
  This means the whole column of water in the xylem from the leaves down to the roots moves upwards
• water enters the stem through the root cortex cells

Question 28

What is adhesion ?

Answer 28

• partially responsible for the movement of water
• as well as being attracted to each other water molecules are attracted to the walls of the xylem vessels
• this helps water to rise up through the xylem vessel

Question 29

What are the factors affecting transpiration rate ?

Answer 29

• light intensity
• temperature
• humidity
• wind
• soil
• water availability