Organisms Exchange Substances with their Environment: Mass Transport in Plants - Phloem Flashcards
1
Q
What do phloem transport?
A
- Phloem transport organic compounds (mainly sugar like sucrose) in plants
2
Q
Describe the structure of phloem
A
- Phloem is formed from cells arranged in tubes
- Sieve tube elements are living cells that form the tube for transporting solutes - no nucleus and few organelles
- So, there’s a companion cell for each sieve tube element to carry out living functions of sieve cells e.g provide energy for active transport of solutes
- Sieve plates have holes and join the cytoplasm of one cell to the next
3
Q
What is translocation?
A
- Translocation is the movement of solutes (e.g amino acids, sucrose) to where they’re needed in a plant
- Energy-requiring process that occurs in phloem
4
Q
Where are solutes transported to during translocation?
A
- Translocation moves solutes from ‘sources’ to ‘sinks’
• Source
- Source is where assimilates (solutes) are produced
- They’re at high concentration there
- E.g leaves
• Sink
- Sink is where assimilates are used up
- So they’re at a lower concentration there
5
Q
Do scientists know how solutes are transported during translocation?
A
- Scientists aren’t exactly sure how solutes are transported from source to sink by translocation
- Best supported theory is the mass flow hypothesis
6
Q
Describe the mass flow hypothesis
A
• Source
- Active transport used to load solutes from companion cells into sieve tubes of phloem at source
- This lowers Ψ inside sieve tubes so water enters tubes by osmosis from xylem and companion cells
- This creates a high hydrostatic pressure inside sieve tubes at source end of phloem
• Sink
- At sink end, solutes are removed from phloem to be used up
- This increases Ψ inside sieve tubes, so water also leaves tubes by osmosis
- This lowers hydrostatic pressure inside sieve tubes
• Flow
- Result is pressure gradient and mass flow from source to sink
- When solutes reach sink the solutes will be used or stored
- The higher the concentration of sucrose at the source, the higher the rate of translocation
7
Q
List examples of supporting evidence of the mass flow hypothesis
A
- Ring of bark
- Pressure
- Radioactive tracer
- Metabolic inhibitor
8
Q
How does a ring of bark show evidence for mass flow hypothesis?
A
- When ring of bark that includes phloem, not xylem is removed from stem, a bulge forms above ring
- Fluid from bulge has higher concentration of sugars than fluid below
- Because sugars can’t move past area where bark is removed
- Evidence that there can be a downward flow of sugars
9
Q
How does pressure show evidence for mass flow hypothesis?
A
- When a stem of a plant is cut, the sap flows out quicker nearer the leaves than further down the stem
- Evidence that there’s a pressure gradient
10
Q
How do metabolic inhibitors show evidence for mass flow hypothesis?
A
- Metabolic inhibitor stops ATP production
- When metabolic inhibitor is put in phloem, translocation stops
- Evidence that active transport is involved
11
Q
Describe how radioactive tracers show evidence for mass flow hypothesis?
A
- Translocation of solutes can be followed with use of radioactive tracers
- Leaf is supplied with carbon dioxide containing radioactive isotope ^14C
- Radioactive carbon will be incorporated into organic substances made by leaf (sugars) which will be moved around plant by translocation
- Movement of these substances is tracked using autoradiography
- The plant is killed and placed onto photographic film
- Wherever film turns black, radioactive substance is present
- Over time results show translocation of substances from source to sink over time
12
Q
Describe evidence against mass flow hypothesis
A
- Sugar travels to many different sinks, not just to one with highest Ψ, as model would suggest
- Sieve plates would create a barrier to mass flow - a lot of pressure would be needed for solutes to get through at reasonable rate
