3.3.4.2 Mass Transport in plants Flashcards
What is xylem and what does it transport?
Vascular tissue
Transports water and dissolved nutrients up from the roots to the rest of the plant.
What does phloem transport and in what direction?
Sugars
All around the plant
What is translocation?
Translocation is the transport of organic substances in the phloem from a source (leaf cell) to a sink (root cell)
why do some leaves have thick waxy cuticle?
increases diffusion distance
reduces evaporation/transpiration of water and so reduces water loss
why might the rate of water uptake by a plant NOT be the same as the rate of transpiration?
- Water used for support/turgidity;
- Water used in photosynthesis;
- Water used in hydrolysis;
- Water produced during respiration;
The mass movement of sugars around the plant is called…?
translocation
Describe how sugars (sucrose) are transported in the plant.
- Sucrose is actively transported from the source into the sieve tubes
- Via companion cells using ATP
- This lowers water potential in sieve tube causing water to diffuse from xylem and into sieve tubes by osmosis.
- Creates high hydrostatic pressure inside the phloem
- Therefore sucrose is translocated down the sieve tubes.
- At sink, sucrose is actively unloaded into root cell via companion cells
- Raising the water potential in the phloem and so water diffuses out of phloem and into xylem by osmosis.
Describe cohesion-tension theory in the xylem
Water is polar so hydrogen bonds form between molecules
Causing them to stick to each other (cohesion)
Surface tension of water adds to the sticking effect
Therefore as water is lost through transpiration, more can be drawn up the stem from the roots
The presence of an air bubble in a xylem vessel in the stem blocked the movement of water through that vessel. Use the cohesion tension theory to explain why
Water evaporates from the leaves
So the water in the xylem is under tension causing the water molecules to stick together
So the water is stuck together in a single column, the air bubble breaks the column which prevents cohesion
What is transpiration?
The evaporation of water from the stomata and into the atmosphere down a water potential gradient, i.e from high (e.g 0) to low (e.g -247)
Give 3 features of xylem
- Long continuous columns of dead tissue with nothing inside, allowing water to travel without any barriers in the way.
- Contains lignin which thickens the vessel, giving structural support
- Contains pits located on both sides of the vessel allowing water to diffuse out sideways
Give 3 features of phloem
- Contains sieve plates which are end walls. They are present along the whole phloem vessel and gives strength to it.
- Sieve plates have perforations allowing transport of sugar molecules in BOTH directions
- Phloem connected to companion cells via plasmodesmata which provide ATP for active transport of sugars. Phloem contains no organelles and so can’t make its own ATP
How can ringing experiments be used to investigate transport in plants?
- Bark and phloem of a tree are removed as a ring
- Leaving behind xylem only
- Eventually tissues above the missing ring swell as sugars accumulate
- And the tissues below begin to die as no sugar is being delivered below
- Therefore sugar mist be transported by phloem.
How can tracing experiments be used to investigate transport in plants?
- Plants are grown in radioactive carbon dioxide which will get incorporated in plant sugars
- Autoradiography will display the areas on plant exposed to radiation which corresponds to where the sugars translocated and therefore where the phloem is.
What causes the sugars in the phloem to translocate?
Hydrostatic pressure
