3.3.4.2 Mass transport in plants Flashcards
1
Q
Explain the relation between stomatal opening and photosynthesis
A
- stomata allow uptake of carbon dioxide
- carbon dioxide required for photosynthesis
2
Q
How is xylem tissue adapted for its function?
A
- long cells with no end wall
- continuous water columns
- no cytoplasm and no organelles
- to obstruct flow for easier flow
- walls thickened with lignin
- to support xylem to withstanding tension and waterproof so keeps water inside cells
- pits and wells
- to allow lateral (sideways) movement of water around blocked vessels
3
Q
Why does xylem tissue contain hollow tubes?
A
Unrestricted water flow in a continuous column so it maintains the transpiration stream
4
Q
Why is lignin present in xylem cell walls?
A
- Provides support and strength
- prevents water loss
- maintains column of water due to adhesion
5
Q
Explain the cohesion tension theory
A
- Water evaporates into air spaces and diffuses out of stomata of a leaf
- it transpires from leaves
- this reduces water potential in leaf cells/mesophyll
- So water drawn out of xylem by osmosis
- this creates tension (negative pressure) on water in xylem
- So water pulled up as a continuous column
- because of cohesive forces, due to hydrogen bonding, between molecules
- adhesion of water molecules to walls of xylem
6
Q
Describe the ringing experiments for phloem
A
- ring of protective layer and phloem removed from stem (just xylem left)
- sucrose in phloem build up above the ring - swelling
- dead tissue below region (interrupted sucrose flow)
- if xylem was used in transpiration this wouldn’t have happened
7
Q
Describe the tracer experiment for phloem
A
- Radioactive isotope 14C to radioactively label 14CO2
- isotope 14C incorporated into sucrose
- So the sucrose can be identified and traced as they move through plant using autoradiography
- stem cross section placed on x ray film
- turns black if exposed to 14C
- only phloem turned black
8
Q
How does water travel across the leaf?
A
- Water moves through cells through air spaces and out through stomata
- by diffusion
- down a water potential gradient
9
Q
Describe and explain how water in the mesophyll cells passes out of the leaf
A
- Water leaving the mesophyll cells
- reduces the water potential in and across cells
- So water is drawn out of xylem by osmosis
10
Q
Describe the mass flow hypothesis
A
- souce cells (leaf) produce sugars/sucrose from photosynthesis
- in source sucrose is actively transported into the sieve tube element of phloem by companion cells
- this lowers water potential of sieve tube element so water enters by osmosis from xylem
- this increases hydrostatic pressure and causes a mass flow movement in the phloem (from source to sink)
- sucrose is actively transported out of phloem by companion cells into sink cells
- sucrose used for respiration or converted into starch for storage
11
Q
What evidence supports mass flow?
A
- when cut, sap is released meaning it is under pressure
- concentration of sucrose is higher in leaves than roots
- downwards flow only takes place in the light
- if sucrose concentration increases in leaves, a similar increase is involved in phloem shortly after
- a lack of oxygen or inhibiting metabolism stops translocation of sucrose
- companion cells have a high number of mitochondria
12
Q
What evidence doesn’t support mass flow?
A
- sieve plates seem to hinder mass flow
- solutes move at different speeds, this should be the same if they are the same flow
- sucrose delivered at the same speed everywhere - mass flow would suggest that areas needing more sucrose have a lower hydrostatic pressure and the flow there would be greater
