Mass flow in plants Flashcards
What is the xylem?
The tissue that transports water and ions in stems and leaves
What are the features of the xylem ?
Dead cells
Cell walls contain lignin which water adheres to, provides strength to xylem
No cytoplasm or organelles for no impeded flow
No end walls which forms a continuous column for water transport
Pits
What is adhesion ?
Forces between the water molecules and hydrophilic lining of the cell wall
What is cohesion ?
Force between water molecules forming an unbroken column in the xylem
What is tension ?
Pull generated by water evaporation
What is transpiration ?
Evaporation of water as water vapour from the leaves and shoots of the plant
What is the cohesion tension theory ?
Diffusion of water vapour through stomata lowers the water potential of leaf cells and water moves from a higher water potential in xylem to leaf cells by osmosis
Water is drawn up the xylem which creates tension
Water molecules cohere together to form a continuous column of water
Water molecules also stick to the xylem cell walls by adhesion
Water reenters the roots by osmosis
Factors affecting the rate of transpiration
Temp- more KE means more evaporation from mesophyll cells more diffusion of water vapour through stomata lowers
Humidity- air inside leaf is saturated external air has a lower water potential
Air movement- wind removes layer of saturated air just outside the stomata which reduces the water potential transpiration increases
Light intensity- opens stomata more carbon dioxide for photosynthesis so transpiration increases
What are the two cell types in phloem ?
Sieve tube elements- no nucleus, a few organelles, less resistance to flow and end walls have perforations called sieve plates
Companion cells are very active cells next to the sieve tubes and are connected by plasmodesmata, it has lots of mitochondria to produce atp for the active movement of sucrose
Mass flow hypothesis
Source produces glucose which is converted to sucrose
Sucrose solution actively transported into the phloem
Water potential in the phloem decreases and water moves in from xylem by osmosis
Increases volume in the phloem decreases
Hydrostatic pressure builds up forcing sucrose solution along the phloem by mass flow to respiring cells
At the sink sucrose moves from the phloem to the sink by active transport and this lowers the sinks water potential so water moves down a gradient
Water reenters the xylem by osmosis and sucrose can be stored or used in respiration
What conditions need to be met for the mass flow hypothesis to be 100% correct
Downward unidirectional flow
Higher to lower pressure
Sucrose moves from source to sink
Process is active
Ringing experiment and link to mass flow
Bark containing the phloem can be removed from stems to prevent the movement of organic substances
A bulge forms above the ring and has a higher concentration of sugars than below indicating a gradient
Swelling is caused by the build of sugar solution
Can’t flow any further because sugar solution which has been transported to the phloem which has been removed
Bulge is above the ring due to downward flow
Bulging occurs on one side of the ring so flow of solutes only happens in one direction
Use of aphids
Aphids pierce the phloem top and bottom of stems
Then remove the aphid just leaving the mouthpart
Sap flows out and flows out quicker at the top indicating a pressure gradient
Hydrostatic pressure in the phloem causing the sap to ooze out
Shows that there is a high hydrostatic pressure in the phloem which is needed for mass flow
Auto radiography
The radioactive tree can be used to track organic substances using carbon dioxide which is converted into sugars detect using photographic film
The radioactive carbon in carbon dioxide is incorporated into sucrose and this occurs at the leaf
From here the sucrose is transported towards the roots which is the sink
Proves movement from source to sink
Metabolic inihibitors
If put in phloem then translocation stops indicating atp is needed for the loading of organic molecules into the phloem by active transport
Evidence contradicting mass flow hypothesis
Different organic substances travel at different rates in the phloem if it was just mass flow all molecules would be forced to flow at the same rate as the fluid is pushed from higher to lower pressure
Different substances move in opposite directions in the same sieve tube showing bidirectional flow
Sieve plates would create a barrier to mass flow a lot of pressure needed for solutes to get through at a reasonable rate
