Water and photosynthate transport Flashcards
Is xylem unidirectional?
Yes, will only go from the roots to the leaves
Is phloem unidirectional?
Usually just goes from leaves to roots, but has the ability to go wherever it’s needed
What drives water transport in plants?
Evapotranspiration
How much of the water absorbed by plants is actually used by it?
1%. The other 99% is lost through transpiration
What is the cohesion-tension theory?
Plants evaporate water, which concentrates solutes in the leaves and creates a physical tension from osmosis in the stem, which pulls the water up to the leaves. Because of cohesion, the water molecules stick together and pull each other along
What is the water potential gradient?
The ability of one system to draw water from another system
Where will water move to according to water potential?
High to low
What are 5 ways to move water across a semi-permeable membrane?
- Add more solute to one side
- Add more water to one side or remove water from one side
- Vapour pressure deficiency
- Capillary forces
- Temperature
What is vapour pressure deficiency?
The ability of air to pull water from other systems
What is water potential usually like in plants?
Almost always negative, but less negative at the roots and more negative in the leaves
How does water transport work in leaves?
Water gets pulled out of the cells by the air into the air spaces in the leaves, which then concentrates the solutes in the leaf cells. The concentrated solutes creates osmotic tension in the stem and the leaf cells pull more water out of the xylem. The leaves have a lower water potential than the xylem, so water gets pulled out of there and the water molecules pull each other along because of cohesion
What does resistance to water transport in the stem depend on?
The size of the tube
Why do conifer stems have more resistance to water transport than angiosperm stems?
They don’t have vessel elements, so only have the small tracheids and a smaller tube has more resistance
What is the difference in the water potential in the stem vs in the roots?
More negative in the stem, so water goes from the roots to the stem
What does water do under extreme negative tension?
Creates a column, and the water molecules pull each other along because of cohesion
What are pits? Why are they there?
Physical blockages in the xylem between tracheids. They stop any air that gets into the xylem from getting into the next tracheid
What would happen if there were no pits and air got into the xylem?
The air would explode and break the tension of the xylem, rendering the whole system useless
What system do conifers have to stop air from getting into the next tracheid?
Margo and torus. If there is air in a tracheid, water tension in the next tracheid will pull the torus over the pore and seal it like a plug
Why do tropical trees have fewer pits in their xylem?
There is very little danger of drying out, so they don’t have to worry about the exploding air problem
What are the 3 things water transport in the leaves depends on?
Cohesion, tension, water potential
What are the 3 things water transport in the stem depends on?
Cohesion, tension, water potential
What are the 3 things water transport in the roots depends on?
Cohesion, tension, water potential
Why does water get pulled into the xylem from the soil and root hairs?
Evapotranspiration from the leaves creates the tension in the xylem, and the roots have a negative water pressure relative to the soil. Cohesion causes the water molecules to pull each other along
What are the 3 routes for water to take through the root cells to get into the vasculature?
Apoplastic, symplastic, transmembrane
What is the apoplastic route?
Water moves through the intercellular spaces and cell walls, and never crosses a membrane
What is symplastic route?
Water crosses a cell membrane and moves through the cytoplasm and plasmodesmata
What is the transmembrane route?
Combination of the apoplastic and symplastic routes. Goes in and out of membranes a few times
Why are the Casparian strips there?
Make sure everything coming into the roots crosses a membrane at least once
Which route is water blocked from taking once it hits the Casparian strip?
Apoplastic. All water movement is symplastic after the casparian strip
What is water use efficiency?
The amount of CO2 assimilated per unit of water used. I.E. how many molecules of water does the plant need to pay to get one molecule of CO2
Which way does water go down its concentration gradient between the leaf and the air?
From the leaf to the air
Which way does the CO2 go down its concentration gradient between the leaf and the air?
From the air to the leaf
What will happen to the concentration gradient of water from a plant in a dry location?
The concentration gradient will be steeper and the plant will lose water quicker
What determines the rate of water use efficiency?
The concentration gradients of water and CO2
If a plant is moved to a drier location, will the water concentration gradient or the CO2 concentration gradient change?
The water concentration gradient changes, but the CO2 gradient will stay pretty much the same no matter what
How can plants stop water loss?
Close the stomata
What is one of the main drivers behind the rate of photosynthesis in a plant?
Water use efficiency
How are ecosystem productivity and water use efficiency related?
Water use efficiency determines the rate of photosynthesis, which largely determines ecosystem productivity. Regions where plants pay less water for their CO2 generally have higher rates of photosynthesis and higher growth rates, and the ecosystem has greater productivity
What determines the water use efficiency of a plant in the sense of how well it responds to drought?
Genome
What hormone signals a drought response in the guard cells of the stomata? Where does it come from?
Abscissic acid. Released from the roots
What happens inside the guard cells when abscissic acid binds to its receptor in response to drought?
ABA binding to its receptor causes a high amount of calcium ions to be released inside the cell. The calcium causes activation of Cl- and K+ transport out of the cell, and the water follows through osmosis. The loss of turgor pressure from the water leaving then causes the guard cells to close the stoma
What are the 2 types of plant movements?
Fast and slow
What do all plant movements depend on?
Changes in turgor pressure
What causes slow plant movements?
Differential growth response based on the irreversible deformation of cells
What causes fast plant movements?
Reversible water loss from either the vacuole or cells
How do Venus fly traps do their fast movement of closing the trap?
They use touch activated trichomes that create action potentials. The electrical signals get transmitted between the lobes and the midrib of the trap, and two action potentials in 20 seconds creates an ion influx out of the cells and the trap closes
What two hormones regulate trap closing movement in Venus fly traps?
Jasmonates and ABA
What type of transport do all fast movements use?
Active transport
What are 3 types of slow plant movements?
Growth, opening and closing the leaves, root movement through the soil
How do guard cells open the stomata?
Blue light activates an ATPase proton pump, which transports protons out of the cell. The proton transport produces the energy needed to create an ion influx into the cell, and the water follows through osmosis. The turgor pressure in the cell increases and the guard cells open
How do slow cell movements work?
Cell wall loosening either on one side or both sides of the cell. The elongation of the cells causes the plant to grow if both sides were loosened, or causes a plant to bend towards or away from a stimulus if the loosening was only on one side
How do plants bend in one direction?
The cell wall gets loosened on one side, so only expands on the one side which causes the plant to bend
Are fast plant movements reversible or irreversible? Why?
Reversible. They occur because of changes in water, which can be reversed
Are slow plant movements reversible or irreversible? Why?
Irreversible. They occur due to complete remodelling of the cell wall
What are tropisms?
The bending of a plant towards or away from an external stimulus
What are 3 types of tropisms?
Phototropism, gravitropism, thigmotropism
What is thigmotropism?
Response of a plant to an external touch stimulus
How are phloem cells different than xylem cells?
They are alive at maturity, but strongly reduced, and have very thin secondary cell walls
Does water potential drive the movement of solutes in the phloem like in the xylem?
No, phloem transport is driven by active pressure instead
Can water leave the xylem and go into the phloem?
It can where there is a greater water concentration gradient in the xylem vs the phloem
What is the water potential like in the phloem?
Positive, so the phloem gets pushed to an area of lower pressure instead of being pulled
Is solute transport in the phloem active or passive?
Active
Is water transport in the xylem active or passive?
Passive
Do the phloem cells themselves do the active transport to load the solutes?
No, the companion cells do it
