Organisms Exchange Substances with their Environment: Mass Transport in Plants - Xylem

Question 1

What do xylem tissue transport?

Answer 1

• Xylem tissue transport water and mineral ions in solution

- These substances move from the roots to the leaves

Question 2

Describe the structure of the xylem vessel

Answer 2

• Xylem vessels are the part of the xylem tissue that transport water and ions
• Xylem vessels are long, tube-like structures formed from dead hollow cells joined end to end
• These cells have open ends, forming a continuous column water can pass through

Question 3

Describe the structure of xylem tissue

Answer 3

• Epidermis - outermost cell layer of the primary plant body
• Cortex - outer layer of tissue immediately below epidermis of stem/root
• Endodermis - an inner layer of cells in the cortex of a root and of some stems, surrounding a vascular bundle

Question 4

Which cells absorb water from the root?

Answer 4

• Water enters the plant through root hair cells

- They provide a large SA, they are long extensions and occur in thousands on each branch of a root

Question 5

How does water enter root hair cells?

Answer 5

• The soil solution is mostly water and has a high Ψ
• Cells have much lower Ψ due to sugars dissolved inside them
• Water moves into root hair cell down a Ψ gradient by osmosis

Question 6

What are the different pathways water can undergo across a root?

Answer 6

• Apoplast pathway

- Symplast pathway

Question 7

What is the symplast pathway?

Answer 7

• Symplast pathway goes through living parts of cell, cytoplasm, by osmosis
• Cells are connected with tiny openings called plasmodesmata
• They connect the cytoplasm of one cell to the next

Question 8

Describe how changes in water potential play a part in the symplast pathway

Answer 8

• As water enters one cell, the Ψ of that cell will be higher than its neighbouring cell
• The water will move to the neighbouring cell, its Ψ will increase and the process repeats
• A Ψ gradient is set up across the cortex from root hair cell to endodermis
• Cortex - outer layer of tissue immediately below epidermis of stem/root
• Endodermis - an inner layer of cells in the cortex of a root and of some stems, surrounding a vascular bundle

Question 9

What is the apoplast pathway?

Answer 9

• Apoplast pathway goes through non-living parts of cell - spaces between cellulose molecules in cell walls - by diffusion
• Water doesn’t pass through any plasma membranes, so it can carry mineral ions and salt
• But water still undergoes symplast pathway after encountering Casparian strip

Question 10

What happens to water undergoing the apoplast pathway once it reaches the endodermis?

Answer 10

• Once water reaches endodermis, it encounters a Casparian strip
• Strip is impermeable to water and mineral ions, blocking path to centre of root
• Water must now enter symplast pathway to cross endodermis and into xylem vessel

Question 11

How does water move up the stem by root pressure?

Answer 11

• At other side of endodermis are xylem and phloem tissue - vascular tissue
• Endodermal cells actively transport ions/salts into xylem
• Creates lower Ψ in xylem
• Water moves into xylem by osmosis, along Ψ gradient
• Creates force (hydrostatic pressure) helping water move up xylem (down a pressure gradient)

Question 12

How does the cohesion-tension theory help water move up the stem?

Answer 12

• Water transpires from leaves
• This creates tension (suction), pulling more water into leaf
• Water molecules form hydrogen bonds between each other and stick together (cohesion)
• Water molecules form bonds with walls of xylem vessels (adhesion)
• Water molecules in xylem, from leaves to roots, move upwards
• Creates continuous water column