Plant Physiology Unit 1 Lecture 4

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Transpiration: Definition
The passive movement of water through the plant body. It allows for the movement of solutes and is responsible for the loss of water vapor in plants. The only active part of this process may be the use of water by the plants through ion movement.
What were the early (and valid) ideas of how Transpiration occurs?
The early concepts of Transpiration centered on root pressure and capillary action. Capillary action is a dipole moment. Water takes advantage of the charges of the cellulose and moves it upwards. Capillary pressure, however, is too insufficient to send the water all the way up to the leaf (can only account for approx. 1m) Turgor pressure is very important for young seedlings. (Is only able to move water approx. 10 m)
What is the system that helps move ions through the plant?
Vascular System
How does water increase cell size?
The way cells increase size is by increasing solute potential. Water enters the cells and pushes them out like a water balloon.
Anatomy of Transpiration pt. 1
Water enters through root hair. Moves through permeable epidermis and moves up through the Xylem. Xylem is compromised of two very specific cell types: -Vessels: main conducting cells, they are dead when mature (can still function). Involved in secondary growth (wood) and flowering plants. -Tracheids: conducting cells, are dead when mature (can still function). Instead of being tubes, they are pointy at the ends. They still have pits and are involved in pines and ferns. These cell types are really the same thing, but are delegated to different plants.
Anatomy of Transpiration pt. 2
Water travels up the Xylem due to the tension created by cohesion and enters the leaf that branches off the Xylem. Water will be distributed throughout the leaf. 1% of water will be cuticular transpiration, 99% of water loss will go through stomatal transpiration. Movement of sugar will reverse water directional arrow. Will drive it back down into the system where it can condense. This condensation will create a starch that can be used by the plant.
Energetics of Transpiration (basic)
One of the factors that contributes to transpiration is the maintenance of a increasingly negative water potential from the soil through the plant to the air. Water potential in the leaf will be significantly more negative than the water potential in the root. The uphill movement of water is energetically favored! (will become more negative as you move up the plant/tree) This is the process that drives evaporation.
Bundle Sheath, Apoplast, Mesophyll
The bundle sheath in a leaf is a layer of compactly arranged parenchyma surrounding the vasculature. It forms a protective covering on leaf vein. Has chloroplasts The apoplast comprises the intercellular space, the cell walls, and the xylem. The mesophyll is a leaf's inner tissue (parenchyma) containing many chloroplasts. Mesophyll chloroplasts are randomly distributed along cell walls, whereas bundle sheath chloroplasts are located close to the vascular tissues or mesophyll cells depending on the plant species.
How do plants welt?
During the day, over a period of days, plant evaporation occurs. Soil loses water potential, which directly affects the plant water potential. Eventually this water potential becomes so negative that it loses turgidity, so it starts to welt.
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Soil and Water potential
Water potential in the soil is always higher than the plant. It tends to be very close to zero. It's only slightly negative in dry soil. Solute potential is almost negligible due to the electrostatic interaction in the soil. It contains clay particles, organic matter, ect. Root hair is always going to have a lower water potential than solution, so water flows into roots.
Soil and Water potential (equation)
Pressure potential is greatly impacted by the surface tension of the water. The radius is impacted by soil particle size. When soil is dry, it creates a meniscus between particles that will generate some level of negative pressure that creates suction. Water starts to flow in the wrong direction. Soil will become a sponge. This is believed to become one of the many causes of the shallow root systems of cacti.
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How does water get taken up by the root?
Water is taken up by the root via osmosis at the root tips mostly. Aquaporins are key for allowing water to move up into plant. The primary function of most aquaporins is to transport water across cell membranes in response to osmotic gradients created by active solute transport.
Explain how water travels apoplastically and symplastically through the root. pt 1
Water travels through the root cortex both apoplastically and symplastically via bulk flow. There are two entrances: the apoplast pathway and transmembrane pathway. For water that travels apoplastically, it is non-cytoplasmic. The water molecule has a capacity to slide around. Apoplastic movement is going to go through cell walls through the epidermis and cortex, until it reaches the endodermis.
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Explain how water travels apoplastically and symplistically through the root. pt 2
Endodermal cell has Casparian Strip. It is a form of filtering. When water molecule runs into the strip, it has no choice but to move down through the cell wall into the cytoplasm. The apoplast has to pass around this barrier, so it must travel symplastically. The water molecule then travels to Xylem vessel. The symplastic pathway stays symplastic the whole time, and enters the Xylem without having to pass through the strip.
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Why does apoplastic movement contribute to the positive pressure potential of cells?
Water is always going to be flowing toward solutes. Apoplastic movement increases pressure potential and makes the cells turgid. Osmotic pressure is small because there are fewer dissolved solutes in the apoplastic water.
Are water flow and sucrose movement in equilibrium?
No cell or apoplastic fluid is in equilibrium. This allows flow from cell to cell to continuous. Sucrose movement goes against water flow to maintain the gradients of water potential!
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What happens to pressure potential when water reaches Xylem?
Once water has moved through the endodermis and is in the stele it can enter the Xylem. Solute loading in the Xylem will cause Pressure Potential to increase. If evaporation is low, the Water potential of the whole plant increases and guttation can occur.
What is Guttation? How does it compare to Transpiration?
Guttation helps plants to dispose of unwanted solutes. Pressure potential becomes so high it moves water out of the plant. In comparison to transpiration, guttation loses water as liquid water, while transpiration loses water as water vapor. Guttation eliminates sugars, salts, and amino acids. Transpiration only evaporates pure water.
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What do pit membranes do?
Pines and ferns have pit membranes up the Xylem that help reduce potential air movements.
The Stem
Water is transported to the Xylem via bulk flow -Cohesion and adhesion are very important The stem is one of the places where the plant has the greatest potential to suffer interference in water movement via cavitation.
The Leaf
Perhaps the most important organ; it is where the driving force for transpiration is generated. Evaporation occurs here. Water leaves through stomate. Adding sugar decreases water potential.
The Leaf and Cohesion Tension Theory
This theory is similar to soil. In leaf. the radius of curvature in the mesophyll meniscus impacts the solute potential. If the leaf dries out, the radius of the curve decreases. Water potential decreases. Drier leaves pull water toward them.
Describe the relationship between negative water potential and the leaf.
The negative solute potential in the leaf creates tension in the water column of the xylem, and coupled with increasingly negative energy potential in the system, the plant keeps water potential negative enough to favor the movement of water upwards.
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How does the physical Structure of the cell wall enhance evaporation?
What does water do in response to increasing solutes? (leaf)
Water will follow the addition of solutes. Will rush in.
How does the leaf keep balance between solute and solvent?
The leaf makes glucose through photosynthesis. From adding solutes, this forces water to rush into cells. This will, in turn, increase the pressure potential. This potential should get so high that instead of pushing back, the leaf lets water in because it is continuously leaving. The glucose will combine together to make starch. The solutes bind up into a polymer and will no longer effect the water potential. (solutes reduce water potential). This keeps water from rushing in and allows time for it to move through the plant and evaporate.
What occurs in the mesophyll?
Stomates (general)
-The location where the bulk of water vapor loss occurs. -Found on all above ground parts of primary plant body. -There are fewer of these for desert plants Important for transpiration and plant breathing! Stomates have.. -Guard cells: the only epidermal cells that have chloroplasts -Subsidiary cells: help guard cells do their job.
Stomates and Guard cell sensitivity
Sensitive to many external and internal factors, such as atmospheric CO2 and O2, light, cytoplasmic acidity, and water status. Additionally, they are highly sensitive to presence of phytohormone ABA.
What is the mechanism for stomatal opening?
Mechanisms mainly involved in turgor pressure due to potassium accumulation in the vacuole. To get the guard cell to open, potassium (K+) rushes in and water follows. This pressure will trigger its opening. The accumulation of potassium alters the osmotic potential of the guard cell. Solute potential becomes negative. Red channel is calcium gated. The channel requires an ion to plug into an active site.
What are the different sides of the leaf?
On top of life is adaxial side. On the bottom is the abaxial side.
What are the mass majority of guard cells on the abaxial side of the leaf?
High light intensity increases the rate of evaporation, which is why guard cells are underneath If leaf is sitting on the water, stomates would be located on the adaxial. They need gases, water isn’t typically good for allowing gases to diffuse. For plants that live in deserts, they are abaxial surface. The leaf forms invaginations, the guard cells are on top of invaginations.
Evaporation at the Stomate
Water evaporates at the surface of the cells in the sub-stomatal cavity and from the guard cell margins.
What is the driving force of transpiration?
Evaporation at the stomate and peristomatal surfaces drives transpiration. Water movement from the leaf to the air are strongly favored.
How does the presence of water vapor in air affect Transpiration?
In apoplastic areas of the leaf, relative humidity is very saturated. Water potential of cells are much greater closer inside. Massive energy drop, so water flows this way. This is what drives evaporation. What determines the RATE of this is the difference in water vapor pressure.
How can we measure rate of water loss?
We can measure the rate of water loss at the leaf using this equation.
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What causes vapor pressure?
Vapor pressure is caused by the movement and escape of the water molecules in solution. In a closed system the system can reach a state of equilibrium.
Factors Influencing Transpiration
Increasing energy of water in the surface effects hydrogen bonds and speeds up evaporation. Light intensity causes stomates to open, can cause increases in Transpiration. Wind makes R get smaller in equation. T therefore, must go up Water potential of soil, effects of cells that move the water, ect.