7.7 - 7.9 Mass Transport in Plants

Question 1

How is water transported in plants?

Answer 1

Xylem

Question 2

Define transpiration

Answer 2

The force that pulls water through the xylem vessel in a plant due to evaporation of water from leaves

Question 3

Describe the movement of water out through the stomata

Answer 3

• Water potential gradient between exterior of leaf and air space in the leaf
• Higher water potential inside
• Stomata open
• Water diffuses out through stomata into open air

Question 4

Describe the movement of water across the cells of a leaf

Answer 4

• Mesophyll cells lose water to air space by evaporation due to heat of the sun
• These cells now have lower water potential
• Water enters from neighbouring cells via osmosis
• Loss of water from neighbouring cells lowers their water potential
• They, in turn, take water from neighbouring cells

Question 5

Describe the movement of water up the stem in the xylem

Answer 5

• Water evaporates from mesophyll cells due to heat from the sun
• This leads to transpiration
• Water molecules form hydrogen bonds between one another and this creates cohesion
• Water forms a continuous, unbroken column across the mesophyll cells and down the xylem
• A column of water is pulled up the xylem as a result of transpiration (called transpiration pull)
• Transpiration pull puts the the xylem under pressure (there is negative pressure within)
• This is cohesion-tension theory

Question 6

Describe the evidence to support cohesion-tension theory

Answer 6

Change in the diameter of tree trunks across the day:

• During the day, transpiration is at its greatest
• There is more tension/negative pressure in the xylem
• This pulls the xylem walls inwards
• Diameter of trunk narrows
• Opposite at night

If air enters xylem, tree can no longer draw up water:

• Continuous column of water is broken
• Cohesion chain of water molecules broken

If xylem vessel broken, water does not leak out:

• Xylem under tension
• If vessel broken, air sucked in

Question 7

Describe some adaptations of the xylem for its function

Answer 7

• Xylem cells are dead (do not contain organelles which would disrupt flow of water)
• No end walls (do not disrupt flow of water)
• Cell walls contain lignin (for support and structure)

Question 8

What is the process by which organic molecules and some mineral ions are transported from one part of a plant to another?

Answer 8

Translocation through the phloem

Question 9

Describe the structure of the phloem

Answer 9

• Sieve plates with pores
• Cellulose cell wall
• Companion cells containing organelles
• Sieve tube elements

Question 10

Describe the factors that affect the rate of transpiration

Answer 10

Light intensity:

• More photosynthesis
• More water used in cells
• Increasing water potential gradient
• Faster transpiration

Temperature:

• Water more likely to evaporate out of stomata
• Increasing water potential gradient
• Faster transpiration

Humidity:

Wind:

Question 11

Define assimilates

Answer 11

Sugar and other chemicals

Question 12

What is the main assimilate?

Answer 12

Sucrose

Question 13

Define source

Answer 13

Where assimilates are produced, often at a high concentration e.g leaves, photosynthesising cells

Question 14

Define sink

Answer 14

Where assimilates are used up e.g cells doing lots of respiration, root hair cells

Question 15

What is mass flow theory?

Answer 15

The current theory for how translocation is achieved

Question 16

What are the 3 main phases of mass flow?

Answer 16

1. Transfer of sucrose from photosynthesising tissue to sieve elements
2. Mass flow of sucrose through sieve tube elements
3. Transfer of sucrose from sieve tube elements to sink cells

Question 17

Describe the transfer of sucrose from photosynthesising tissue to sieve elements in mass flow

Answer 17

• Companion cells actively transport H+ into tissue surrounding phloem
• H+ then moves back into companion cells via facilitated diffusion, cotransporting sucrose with it, into the companion cell
• Sucrose then diffuses, down the concentration gradient, out of the companion cells into the sieve tube elements, through plasmodesmata

Question 18

Describe the mass flow of sucrose through sieve tube elements

Answer 18

Water moves from the xylem to the sieve tube via osmosis

This creates high hydrostatic pressure

Mass flow of sucrose and water from source to sink due to hydrostatic pressure gradient and concentration gradient of sucrose

Question 19

Describe the transfer of sucrose from sieve tube elements to sink cells

Answer 19

Sucrose move from the sieve tube to the sink cells via active transport by companion cells

Water from sieve tube to xylem via osmosis

Question 20

Give the evidence supporting the mass flow hypothesis

Answer 20

• There is a pressure within sieve tubes, as shown by sap being released when they are cut
• The concentration of sucrose is higher in leaves (source) than in the roots (sink)
• Downward flow in the phloem during daylight, but cease in shade or at night
• Increase in sucrose levels in the leaf are followed by similar increases in sucrose levels in the phloem
• Metabolic poisons and/or lack of oxygen inhibit translocation of sucrose in the phloem
• Companion cells possess many mitochondria and readily produce ATP

Question 21

What are the three experiments that can be used to prove mass flow hypothesis?

Answer 21

1. Ringing experiments
2. Radioactive tracers
3. Aphids

Question 22

Describe the ringing experiment and how it proves mass flow hypothesis

Answer 22

• Remove a ring of bark (bark contains the phloem but not the xylem)
• Fluid accumulates in the phloem on the source-side, causing swelling
• Tissues below the ring die

Shows transport of organic substance is via phloem, not xylem

Question 23

Describe the aphid experiment and how it proves mass flow hypothesis

Answer 23

• Aphids feed by inserting a needle-like structure into the phloem
• The body of the aphid is then removed
• Sap leaks out of the mouthpart
• When tested, the sap contains organic substances

This proves that the phloem is under positive pressure (leaks out) and transports organic substances (sap analysis)

Question 24

Describe the radioisotopes experiment and how it proves mass flow hypothesis

Answer 24

• Plant photosynthesises using radioactive carbon dioxide
• A cross section of the stem is taken and x-rayed
• Phloem contains radioactive carbon

This shows products of photosynthesis are transported in the phloem

Question 25

Give the evidence against mass flow hypothesis

Answer 25

• Function of sieve plates unclear as they seem to hinder flow
• Not all solutes move at the same speed (as they would if moved by mass flow)
• Sucrose is delivered at roughly the same rate to all regions rather than going to the regions with the lowest sucrose concentration (as they would if moved by mass flow)

Question 26

Describe simplified mass flow hypothesis (4 marks)

Answer 26

• Leaf sugars actively transported into phloem by companion cells
• Lowers water potential of sieve cell so water moves in from xylem via osmosis
• Increase in pressure causes mass movement to sink
• Sugars used in root for respiration or converted for storage

Question 27

Describe cohesion-tension theory of water movement through the xylem

Answer 27

1. Water lost from leaf because of evaporation of water
2. Lowers water potential of leaf cells
3. Water pulled up xylem (creating tension)
4. Water molecules cohere together by hydrogen bonds
5. Forming continuous water column
6. Adhesion of water molecules to walls of xylem

Question 28

The rate of water movement through a shoot in a potometer may not be the same as the rate of water movement through the shoot of a whole plant. Suggest one reason why.

Answer 28

• Plants have roots
• Xylem cells may be very narrow