7.7 - 7.9 Mass Transport in Plants Flashcards
How is water transported in plants?
Xylem
Define transpiration
The force that pulls water through the xylem vessel in a plant due to evaporation of water from leaves
Describe the movement of water out through the stomata
- 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
Describe the movement of water across the cells of a leaf
- 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
Describe the movement of water up the stem in the xylem
- 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
Describe the evidence to support cohesion-tension theory
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
Describe some adaptations of the xylem for its function
- 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)
What is the process by which organic molecules and some mineral ions are transported from one part of a plant to another?
Translocation through the phloem
Describe the structure of the phloem
- Sieve plates with pores
- Cellulose cell wall
- Companion cells containing organelles
- Sieve tube elements
Describe the factors that affect the rate of transpiration
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:
Define assimilates
Sugar and other chemicals
What is the main assimilate?
Sucrose
Define source
Where assimilates are produced, often at a high concentration e.g leaves, photosynthesising cells
Define sink
Where assimilates are used up e.g cells doing lots of respiration, root hair cells
What is mass flow theory?
The current theory for how translocation is achieved
What are the 3 main phases of mass flow?
- Transfer of sucrose from photosynthesising tissue to sieve elements
- Mass flow of sucrose through sieve tube elements
- Transfer of sucrose from sieve tube elements to sink cells
Describe the transfer of sucrose from photosynthesising tissue to sieve elements in mass flow
- 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
Describe the mass flow of sucrose through sieve tube elements
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
Describe the transfer of sucrose from sieve tube elements to sink cells
Sucrose move from the sieve tube to the sink cells via active transport by companion cells
Water from sieve tube to xylem via osmosis
Give the evidence supporting the mass flow hypothesis
- 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
What are the three experiments that can be used to prove mass flow hypothesis?
- Ringing experiments
- Radioactive tracers
- Aphids
Describe the ringing experiment and how it proves mass flow hypothesis
- 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
Describe the aphid experiment and how it proves mass flow hypothesis
- 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)
Describe the radioisotopes experiment and how it proves mass flow hypothesis
- 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
Give the evidence against mass flow hypothesis
- 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)
Describe simplified mass flow hypothesis (4 marks)
- 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
Describe cohesion-tension theory of water movement through the xylem
- Water lost from leaf because of evaporation of water
- Lowers water potential of leaf cells
- Water pulled up xylem (creating tension)
- Water molecules cohere together by hydrogen bonds
- Forming continuous water column
- Adhesion of water molecules to walls of xylem
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.
- Plants have roots
- Xylem cells may be very narrow
