Water transport stomata Flashcards
Explain the evolution of vascular plants
*To limit water loss plants developed a waxy cuticle and stomata to allow CO2 entry for photosynthesis.
*A vascular system supplies non-photosynthetic tissues with nutrients and leaves with water and mineral nutrients.
*With a vascular system sugars can be taken evenly across the whole of the plant
*roots need to grow for better anchorage as now plants are growing taller for seed dispersal. Roots need nutrients so there needs to be a system to bring nutrients to them
How does a vascular system supplies non-photosynthetic tissues with nutrients and water.
Sugars are taken from leaves to non-photosynthetic tissues in the phloem. Water is transported by the xylem.
What are the two types of xylem transport cells and describe their structure?
Tracheids (slower water transport) and tracheary vessel elements, they make up the xylem. Only angiosperms have vessel elements.
*In gymnosperm trees, tracheids can account for more than 90% of softwood. vessels make it hard wood.
*A tracheid is a long and tapered lignified cell in the xylem of vascular plants. It is a type of conductive cell called a tracheary element. Going through tiny holes all the time so quite slow water transport
*Faster water transport. Vessel elements are stacked and have perforations at the top and bottom. Xylem vessels can be several centimeters long. One vessel includes a few vessel elements.
Both types have pits in their cell walls and are dead when in use for transport. cells that make up the xylem are secondary cell walls that contain lignin and are waterproof but contain holes.
What are the two types of phloem conducting cells?
Sieve cells and sieve tube members (only in angiosperms). Both types are live cells with dense areas of channels between cells (sieve areas) and have a thick cell wall.
Sieve tube members are stacked and have a sieve plate on each extremity, they are larger and shorter than sieve cells but when they stack they become longer in total.
They lose their nucleus and most of their organelles so they are mostly cell walls and a plasma membrane which can sustain pressure. The phloem is really important in transporting sugars.
Principles of water movement: water potential
What does water potential represent?
Explain each sign and meaning;
ψ
ψπ
ψp
ψm
ψs
What is pure water?
Water potential represents the energy available for water movement.
ψ: water potential
ψπ: osmotic potential (solutes)
ψp: pressure potential
ψm: matric potential (solid inside of cells is negligible in cell wall you do, apoplasts)
ψs: gravimetric potential
ψ = ψπ + ψp (+ ψm )
you can mostly expect psi m (ψm) to be 0 as this doesn’t change during when the cell experiencing osmosis
Water moves from areas of high potential to areas of low potential.
*Osmotic potential decreases with solute concentration.
Ψπ = 0 for pure water at 1 atm
*In plant cells, pressure potential often caused by the cell wall.
*Gravimetric potential is only significant in tall trees if it is not a tree be careful that the s in ψs doesn’t stand for sucrose
*Temperature increases water potential.
What are the units for water potential?
megapascals or how many atmospheres
Explain gravimetric potential
If water rises up gravimetric potential become more negative and normal levels are 0
Describe turgor and wall pressure
Water inside the cell can exert turgor pressure on the cell wall which in return the wall exerts pressure on the water.
How much water does this maize plant transpire each day?
2-4 Litres
Guard cells
They are apart of the epidermis cells
What do stomata do?
Stomata close and open depending on environmental conditions (temperature, water availability, wind, light and humidity as well as time of day.
Influx of ions –> water influx –> increase in guard cell turgor
more turgor pressure =open
relaxed, flaccid=closed
Influx of ions in guard cells so the followed by a water influx which increases in guard cell turgor pressure and opens it
if we start with a closed stomata the guard cells are going to import ions (potassium) from adjacent epidermal cells which is going to decrease water potential in guard cells and increase water potential in adjacent cells.This creates a gradient making the water want to go into the guard cells which increases the turgor on the cell changing its shape so it opens allowing for more transpiration.
90-98% of water is going to be lost through transpiration
CO2 also comes in through the stomata for photosynthesis
What happens after water is absorbed in the roots?
After entering the roots, mostly through root hairs, water passes through the cortex to reach the xylem.
What are aquaporins and what are their roles?
Aquaporins are protein channels forming pores inside the plasma membrane through which water can flow.
Small modifications to aquaporins allow for closure under certain conditions (e.g. drought)
Aquaporins and motor cells
Motor cells, found e.g. in the sensitive plant Mimosa pudica, rely on a rapid transfer of water through a high density of aquaporins.
Mimosa pudica
The plant’s unusually quick response to touch is due to rapid water release from specialized cells located at the bases of leaflet and leaf stalks. The leaves reopen in several minutes, and it is thought that this adaptation is a defense against browsing herbivores who may be startled by the movement.
What is the apoplast?
Extracellular space within tissue
Transpiration and cohesion-tension
In the xylem, water molecules form a narrow column unbroken by transpiration-driven tension, thanks to:
-Cohesion provided by hydrogen bonds between H2O molecules
-Adhesion to the cell wall of the xylem (also hydrogen bonds) cell wall also has water molecules so this causes adhesion and allows for the xylem to be filled with water groups not just single water molecules attached = they fill the space.
The failure of cohesion-tension: cavitation
What is cavitation and when does it occur?
Cavitation is the result of the breakage of the water column in the xylem: a bubble (embolism) blocks water movement in the vessel.
Cavitation occurs under conditions of low water availability (e.g. drought and frozen soil).
once embolism happens the water column is broken however there are many other vessels and ways around the air bubbles to carry on with water movement
explain root pressure
Higher concentration of ions in the root xylem than in the soil solution creates a difference in water potential driving root pressure (osmotic push from the roots).
Root pressure occurs only under high water availability and in the absence of transpiration
Root pressure can lead to guttation
Water movement in the phloem
source= doing photosynthesis
Phloem conduction is driven by differences in sugar
concentration between source and sink:
Sugar is loaded into phloem sections near photosynthetic tissue, attracting water by osmosis, increasing turgor pressure.
In phloem sections near sugar sinks, turgor pressure is reduced.
Because of mass flow, water carries solutes along the sieve tube.
The “Tension-Cohesion” theory is used to explain the transpiration pull;
Transpiration is the passive process where water evaporates out of the leaf, through the stomata, causing more water to be drawn from the soil.
1) Heat from the Sun causes water to evaporate from mesophyll cells and into the air spaces of the leaf.
2) This decreases the water potential of the cells; as a result, water is absorbed from neighbouring cells by osmosis, creating a chain of events until mesophyll cells right next to the xylem absorb water directly from the xylem vessels, creating tension.
3) Water molecules are very cohesive due to the strong hydrogen bonds between water molecules.
4) As a result, a continuous, unbroken column of water is created across the mesophyll cells and down the xylem vessels.
5) As more water evaporates out of the leaf through the stomata, e.g. due to an increase in temperature, more water is absorbed from the soil by root hair cells and transported up the xylem.
Explain translocation
Translocation is the process where sugars and dissolved solutes are transported throughout the plant to wherever they are needed. Phloem vessels consist of sieve tube elements - narrow, tube-like structures that consist of living cells, arranged end-to-end. The end walls of the sieve tube elements are perforated to form sieve tube plates. Associated with the sieve tube elements are companion cells.
Explain the “Mass Flow” hypothesis is used to explain translocation:
1) Solutes, e.g. sucrose produced by the source diffuses into the companion cells by facilitated diffusion. In here, sucrose is actively transported into the sieve tube elements using ATP.
2) This decreases the water potential of the sieve tubes, causing water from the companion cells and the xylem to diffuse into the phloem by osmosis, increasing the hydrostatic pressure.
3) Near the sink, sucrose is either used up or converted to starch for storage. Either way, this decreases the water potential of the sink.
4) As a result, water diffuses from the sieve tubes into the sink by osmosis, decreasing the hydrostatic pressure in the phloem.
5) As a result, there is a high hydrostatic pressure gradient as there’s a high hydrostatic pressure near the companion cells and a low hydrostatic pressure near the sink. Therefore, we have a mass flow of solutes throughout the plant.
