3.3.4 Mass Transport in Plants Flashcards
(Mass Transport in Plants) What does phloem tissue transport?
Organic solutes (mainly sugars like sucrose) around plants.
(Mass Transport in Plants) What are phloem tissues formed from?
Cell arranged in tubes.
(Mass Transport in Plants) Describe the structure of sieve tube elements, regarding phloem tissue.
Living cells that form a tube for transporting solutes. They have no nucleus and few organelles, so there is a companion cell for each sieve tube element.
(Mass Transport in Plants) Describe the function of sieve tube elements, regarding phloem tissue.
They carry out living functions for sieve cells, e.g. providing the energy needed for active transport of solutes.
(Mass Transport in Plants) What is translocation?
The movement of solutes (e.g. amino acids and sugars) to where they are needed in the plant.
This is an energy-requiring process.
(Mass Transport in Plants) Where does translocation occur?
In the phloem.
(Mass Transport in Plants) What are solutes sometimes called?
Assimilates.
(Mass Transport in Plants) Translocation moves solutes from ‘sources’ to ‘sinks’. Describe what is meant by these terms.
‘Source’ is where assimilates are produced (high concentration).
‘Sink’ is where assimilates are used up (lower concentration).
(Mass Transport in Plants) What is the function of enzymes regarding the phloem, and was are the effects of this?
Maintain a concentration gradient from the source to the sink by changing the solutes at the sink.
Makes sure there is always a lower concentration at the sink than the source
(Mass Transport in Plants) Describe how to use a potometer to measure the rate of transpiration. (6)
- Cut a leafy shoot underwater to avoid air passing into the xylem vessels.
- Cut at an angle to avoid crushing the xylem.
- Fill the potometer ensuring there are no air bubbles.
- Use vaseline to create a tight seal with the stem and potometer.
- Introduce an air bubble to the capillary tubing.
- Ensure leaves are dry so stomata are not blocked.
(Mass Transport in Plants) Describe the ‘source’ section of the mass flow hypothesis. (4)
- Companion cells actively transport solutes (e.g. sucrose) into the adjacent phloem.
- Lowers water potential in phloem’ sieve cells.
- Water diffuses in by osmosis.
- Increases the turgor pressure.
(Mass Transport in Plants) Describe the ‘flow’ section of the mass flow hypothesis.
Mass flow (translocation) of solutes in the phloem, down a pressure gradient, from SOURCE to SINK.
The higher the concentration of sucrose at the source, the higher the rate of translocation.
(Mass Transport in Plants) Describe the ‘sink’ section of the mass flow hypothesis. (4)
- Solutes (e.g. sucrose) are removed from the phloem.
- Raises water potential in phloem sieve cells.
- Water diffuses out by osmosis.
- Decreases the turgor pressure.
(Mass Transport in Plants) What does xylem tissue transport?
Water and mineral ions in solution.
(Mass Transport in Plants) Describe the structure of the xylem.
Very long, tube-like structures formed from dead cells (vessel elements) joined end to end. No end walls on these cells, making an uninterrupted tube that allows water to pass through the middle easily.
(Mass Transport in Plants) Describe structures xylem as the tissue that transports water in the stem and leaves of plants.
- Facilitates the ‘transpiration stream’.
- Dead, hollow, open ended vessel elements.
- Narrow.
- Lignified, structurally strong.
- Water.
(Mass Transport in Plants) What is transpiration?
The evaporation of water from the leaf (stomata) down a water potential gradient.
(Mass Transport in Plants) What 4 factors affect transpiration rate?
Light intensity, temperature, humidity, and wind.
(Mass Transport in Plants) Explain the cohesion-tension theory of water transport in the xylem. (5)
- Transpiration - water evaporates from leaves via the open stomata (down a water potential gradient).
- Creates a negative pressure in the leaves which pulls water particles forward from the xylem.
- Water particles are polar (slightly charged) and cohesive (hydrogen bonding).
- This “pull” or tension acts on the entire continuous column of water in the xylem.
- Water is also adhesive and attracted to the xylem walls resulting in upward capillary action.
(Mass Transport in Plants) Outline the use of tracers and ringing experiments to investigate transport in plants as evidence for / against the mass flow hypothesis.
Ringing experiments that cut phloem in photosynthesising plants result in a build up of sucrose above the ring cut and stem swelling yet an absence of sucrose below the ring.
Plants exposed to radioactively labelled CO2 show the presence of radioactive organic compounds present in the phloem as evidenced by exposure of photographic film over the transverse section of a stem.
(Mass Transport in Plants) Describe the purpose of the xylem’s structure of ‘facilitates the ‘transpiration stream’’.
One way mass flow of water from the roots to the leaves to the air (down a water potential gradient).
(Mass Transport in Plants) Describe the purpose of the xylem’s structure of ‘dead, hollow, open ended vessel elements’.
Allows the continuous flow of water.
(Mass Transport in Plants) Describe the purpose of the xylem’s structure of ‘narrow’.
Aids capillary action.
(Mass Transport in Plants) Describe the purpose of the xylem’s structure of ‘lignified, structurally strong’.
Resist the high negative pressure created in the leaf by transpiration (evaporation).
(Mass Transport in Plants) Describe the purpose of the xylem’s structure of ‘water’. (3)
1) Used as a metabolite in photosynthesis
2) Maintains cell turgidity
3) Inorganic ions in the soil e.g. nitrates, phosphates are transported to the leaves
