Mass transport in plants Flashcards
(50 cards)
How does water enter a plant cell?
It moves down a water potential gradient across the partially permeable membrane
By osmosis
What is an advantage of transpiration?
The constant stream of water also transports ions around the plant
The evaporation has a cooling effect, stopping the enzymes being denatured if the plant overheats
Why is there cohesion between water molecules?
They are polar molecules
Hydrogen bonding
How are organic substances moved through a plant?
By translocation
In the phloem
Describe the phloem
Elongated cells joined end to end
Holes in end walls - sieve plates
In bundles in the steam
Very few organelles/cytoplasm
Why is there a higher rate of respiration when there is a high concentration of sucrose at the source?
Sucrose –> glucose
Glucose can be respired
Why do phloem cells need companion cells?
They have lost many of their organelles e.g nucleus when they are specialised
It helps them repair and maintain themselves
Produces ATP for active transport of H+
What is a sink?
A part of the plant that uses organic molecules (sucrose) e.g. respiring tissue
What is a source?
A part of the plant that produces organic molecules (Sucrose) e.g. leaf
Translocation moves organic molecules from… to…
Translocation moves organic molecules from source to sink
What does ‘ there is positive pressure in the phloem’ mean?
The organic molecules flow from a high pressure to a low pressure
Describe mass flow in plants
- Sucrose actively transported into phloem at source, by companion cells
- Lowers water potential of the phloem
- Water moves from xylem into phloem by osmosis
- Volume and pressure increase
- Sucrose unloaded from phloem at sink where cells are respiring/ it is stored
- Lowers water potential of sink cells
- Water moves out of phloem by osmosis
Describe how sucrose enters the phloem
- Sucrose moves by facilitated diffusion with H+ into the companion cell from the source cell
- H+ actively transported out
- Sucrose moves into phloem sieve element by facilitated diffusion
How do experiments with aphids support the mass flow hypothesis?
Aphids allowed to feed on plants (phloem)
Mouthparts removed from aphids
Sap flows OUT
Supports idea of positive pressure
What are the 3 pieces of evidence which do not support the mass flow hypothesis?
- Sieve plates don’t make sense (reduce rate of translocation)
- Not always to sink with lowest water potential
- Organic molecules move at different speeds
How do metabolic inhibitors support the mass flow hypothesis?
Inhibit respiration
Translocation does not occur
Shows that it required ATP and active transport
How do ringing experiments support the mass flow hypothesis?
Ring of bark removed from tree (removes phloem NOT xylem)
Sucrose cannot be transported so collects
Lowers water potential
Water moves out of the xylem and bulges
Shows phloem important in transport of organic molecules
What are the 4 pieces of evidence which support the mass flow hypothesis?
- Ringing experiments
- Aphids - mouthparts removed
- Radioactive tracers
- Metabolic inhibitors
How do we know there is a concentration gradient of sucrose from source to sink?
The sap from the source has a higher concentration thank from the sink
How do we know photosynthesis is needed for translocation?
A plant virus will travel up the plant in the light and not the dark
What is the xylem?
A hollow tube of dead cells
What is the function of the xylem?
Transport water and mineral ions in solution
Where does the xylem transport water and mineral ions?
Through the stem and leaves of plants
On a graph of rate of water movement against time, why would the curve for upper branches of a tree differ from the trunk?
Rate of movement increases in the upper branches
Then increases in the trunk
Water moving in branches creates tension/pull
