10.2 Transport in the Xylem

Question 1

How do roots take in water

Answer 1

• Water is taken from the soil
  
  • Through the roots
  • and transported to the leaves
  • where it maintains turgiditity
  • and is a reactant in photosynthesis
• Some roots have hairs, that increase their surface area, which increases the uptake of water.
• Soil water contains has a dilute solution of mineral salts and has a high water potential
• The vacuole and cytoplasm of the root hair has a lower water potential
• Water therefore passes into the root hair by osmosis down the water potential gradient

Question 2

How is water lost from a plant

Answer 2

• Water is lost through the somtata by a process known as transpiration

Question 3

What is transpiration

Answer 3

• Transpiration is the process by which water is lost via the stomata

Question 4

What percentage of water is lost via evaporation?

Answer 4

• The waxy cuticle,
• which is secreted via the epidermis
• helps to restrict the amout of water lost through evaporation to 5%

Question 5

3 things - Xylem, 3 pathways, water potential gradient

How does water move through the root to the rest of the plant

Answer 5

• Water must first move into the xylem to be distributed throughout the rest of the plant
• To reach the Xylem the water has to cross the root cortex via 3 pathways.
  
  • Water enters the cell wall via the apoplast pathway
  • Water moves through the cytoplasm and plasmadesmata via the symplast pathway
  • Water moves from one vacuole to another via the vacuolar pathway
• The water potential in the root hair is high and low in the Xylem. Water is therefore pushed down the the water potential gradient into the Xylem

Question 6

What is the endodermis

Answer 6

• A single layer of cells
  
  • around the pericycle and vascular tissue of the root
• Each cell in the endodermis has an impermeable waterproof barrier in its cell wall

Question 7

What is the casparian strip

Answer 7

• The casparian strip
  
  • is an impermeable band of suberin in the cell walls of endodermal cells
  • which blocks the movement of water in the apoplast
  • driving it into the cytoplasm

Question 8

What is the pericycle

Answer 8

• The pericycle is
  
  • the single layer of cells
  • around the vascular tissue in a root,
  • from which lateral roots arise
• The pericycle is surrounded by a single layer of cells the endodermis

Question 9

what is the structure of the endodermis

Answer 9

• The pericycle is surrounded by a single layer of cells known as the endodermis
• The cell walls of the endodermis are coated with a waxy material called suberin
• Forming a distinctive band called the casparian strip
• The casparian strip prevents water moving further into the apoplast and drives it into the cytoplasm

Question 10

How does water reach the Xylem

Answer 10

• Water cannot enter the xylem from the apoplast pathway
  
  • Because of the waterproof lignin found in the cell walls
• Water can therefore only pass into the Xylem via the symplast and vacuolar pathways
• This is achieved by osmosis,
• For this to be efficient the water potential in the Xylam has to be less than that found in the endodermal cells
• This is achieved in 2 ways
  
  1. Water potential in the endodermal cells is high owing to the amount of water being driven into the Caspian strip
  2. Water potential in the Xylem is decreased by the active transport of mineral salts i.e. NA⁺ from the endodermis and pericycle into the Xylem

Question 11

How does water enter the Xylem

Answer 11

• Water cannot enter the xylem from the apoplast pathway
  
  • Because of the waterproof lignin found in the cell walls
• Water can therefore only pass into the Xylem via the symplast or vacuolar pathways

Question 12

What is the process by which water enters the Xylem via the symplast and vacuolar pathwat

Answer 12

• This is achieved by osmosis, down a water potential gradient, between the the endodermal membrane and the Xylem
• For this to be efficient the water potential in the Xylam has to be less than that found in the endodermal cells
• This is achieved in 2 ways

1. Water potential in the endodermal cells is high owing to the amount of water being driven into the Caspian strip
2. Water potential in the Xylem is decreased by the active transport of mineral salts i.e. NA+ from the endodermis and pericycle into the Xylem

Question 13

Why is oxygen important for roots

Answer 13

• It allows cells to produce ATP in aerobic respiration whichb provides the energy needed for active transport

Question 14

What is the process by which minerals enter the Xylem

Answer 14

• The concentration of minerals found in soil water is less than the concentration found in root hair cells
• Minerals are therefore absorbed into the cytoplasm, via active transport against a concentration gradient
• Mineral ions travel along the apoplast pathway
• When it reaches the endodermis, the casparian strip prevents further movement in the cell walls
• The minerals then enter the cytoplasm by active transport
• From where they diffuse or are transported into the Xylem

Question 15

How does water move from the roots to the leaves?

Answer 15

• Water moves down a water potential gradient.
  
  • Air has a very low water potential whereas
  • Soil water has a very high water potential
• So water moves from the soil into the plant into the air.
• There are 3 mechanisms which allow this to happen:
  
  1. Capilliarity
  2. The pull of transpiration (Most significant)
  3. and Root pressure

Question 16

What is capilliarity

Answer 16

• The movement of water up narrow tubes, in this case the Xylem, by capilliarity action
• It operates over short distances only e.g. 1metre
• It may have a role in mosses but only plays a small role in water movement in very small plants.

Question 17

How does transpiration influence the movement of water in plants

Answer 17

• Water is drawn upwards
• via a transpiration stream
• owing to
• the cohesive forces between the water molecules
• and
• the adhesive forces betweeen the water molecules and the hydrophobic lining of the Xylem vessels

Question 18

How does root pressure influence the movememt of water

Answer 18

• Water moves through the root via osmosis down a water potential gradient
• This pushes water up into the Xylem.

Question 19

What is cohesion

Answer 19

• The attraction of water molecules for each other. seen as hydrogen bonds,
• resulting from the dipole structure of the water molecule

Question 20

What is cohesion tension

Answer 20

• Water vapour evaporates from leaves, via the stomata
• This pulls water through the leaf via the apoplast, symplast and vacuolar pathways from the Xylem
• As the water leaves the Xylem they pull other water molecules behind them because of cohesion