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Questions and Answers
What happens to a plant cell when the water potential ($
abla$) is less than the pressure potential ($
abla$p)?
What happens to a plant cell when the water potential ($ abla$) is less than the pressure potential ($ abla$p)?
What is the approximate water potential ($
abla$) value of fully hydrated plant cells?
What is the approximate water potential ($ abla$) value of fully hydrated plant cells?
Which type of transpiration accounts for the majority of water loss in most plants?
Which type of transpiration accounts for the majority of water loss in most plants?
What is the cause of transpiration from leaves?
What is the cause of transpiration from leaves?
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At what water potential ($
abla$) range are water-stressed plants typically found?
At what water potential ($ abla$) range are water-stressed plants typically found?
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What is the primary role of water in plant cells?
What is the primary role of water in plant cells?
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What happens to the majority of water absorbed by well-watered plants?
What happens to the majority of water absorbed by well-watered plants?
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Which structure is primarily responsible for water loss in plants?
Which structure is primarily responsible for water loss in plants?
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How do grassland plants conserve water during high temperatures?
How do grassland plants conserve water during high temperatures?
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What is the main process that drives cell expansion in plants?
What is the main process that drives cell expansion in plants?
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What is the result of a plant having insufficient water in its cells?
What is the result of a plant having insufficient water in its cells?
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What contributes to the regulation of temperature in plants through water loss?
What contributes to the regulation of temperature in plants through water loss?
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What is the relevance of vacuoles in plant water relations?
What is the relevance of vacuoles in plant water relations?
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What is the primary method for calculating the water content of a tissue?
What is the primary method for calculating the water content of a tissue?
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Which membranes in plant cells are described as selectively permeable?
Which membranes in plant cells are described as selectively permeable?
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What is the role of aquaporins in plant cells?
What is the role of aquaporins in plant cells?
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What drives the movement of water in and out of cells?
What drives the movement of water in and out of cells?
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What is true about the cell wall in plant cells?
What is true about the cell wall in plant cells?
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What two main factors influence water potential ($
abla$)?
What two main factors influence water potential ($ abla$)?
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How does water typically move through plant cells?
How does water typically move through plant cells?
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Which of the following is NOT a function of the vacuole in plant cells?
Which of the following is NOT a function of the vacuole in plant cells?
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What primarily causes transpiration from a leaf?
What primarily causes transpiration from a leaf?
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What is the role of guard cells in relation to stomata?
What is the role of guard cells in relation to stomata?
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How does water evaporate from the substomatal space?
How does water evaporate from the substomatal space?
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What does vapor pressure difference determine in the context of transpiration?
What does vapor pressure difference determine in the context of transpiration?
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What is one method to control water loss in a simulation of transpiration?
What is one method to control water loss in a simulation of transpiration?
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From where is water replaced when it evaporates from the substomatal space?
From where is water replaced when it evaporates from the substomatal space?
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What feature of leaf structure primarily helps reduce water loss?
What feature of leaf structure primarily helps reduce water loss?
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In liverworts, what is notable about their pores?
In liverworts, what is notable about their pores?
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Where does CO2 fixation occur in C3 plants?
Where does CO2 fixation occur in C3 plants?
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Which enzyme is responsible for CO2 fixation in C4 plants?
Which enzyme is responsible for CO2 fixation in C4 plants?
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What is a key anatomical feature of C4 plants?
What is a key anatomical feature of C4 plants?
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Which statement is true about the chloroplasts in mesophyll cells of C4 plants?
Which statement is true about the chloroplasts in mesophyll cells of C4 plants?
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How does malate contribute to CO2 fixation in C4 plants?
How does malate contribute to CO2 fixation in C4 plants?
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What is the primary output of the Calvin (C3) cycle?
What is the primary output of the Calvin (C3) cycle?
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What effect does an increase in leaf temperature have on transpiration?
What effect does an increase in leaf temperature have on transpiration?
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Which statement about RuBisCO is true?
Which statement about RuBisCO is true?
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What is a consequence of increased oxygenase activity in RuBisCO?
What is a consequence of increased oxygenase activity in RuBisCO?
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Why do plants need to open their stomata?
Why do plants need to open their stomata?
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How does photorespiration relate to water use efficiency?
How does photorespiration relate to water use efficiency?
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Which environmental factor can cause stomata to remain open?
Which environmental factor can cause stomata to remain open?
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What effect does a trade-off between the speed and substrate specificity of RuBisCO have?
What effect does a trade-off between the speed and substrate specificity of RuBisCO have?
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Study Notes
BIO203: Lecture 6 - Water Relations 1
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Overview: Plant cell water relations, water loss (transpiration), C4 and CAM water use efficiency.
-
Water & Plants: Why Care? Lab 4 discussion. Human civilization relies on 6 inches of soil and rain.
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Water Relations: Key Concepts
- Water relations (at the cellular level)
- Water potential (Ψ)
- Movement: Water uptake (roots), transport (xylem), and loss (stomata).
- Plants grow where water is available.
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Why is water important to plants?
- Cell contents are mostly water (80-95%).
- Water is a solvent, crucial for biochemical reactions (photolysis, hydrolysis).
- Necessary for cell expansion and growth.
- Over 99% of water entering a well-watered plant is transpired directly.
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Water: Plant shape & growth
- Water loss via transpiration regulates temperature (e.g., cool grass).
- Adaptations for water conservation include narrow leaves, thick cuticles, and sunken stomata in grasslands.
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Water: Cell elongation & growth
- Water movement into cells and vacuoles drives cell expansion, loosening cell walls.
- Vacuoles are mostly water. Cell expansion requires minimal extra cytoplasm.
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Water: Calculating water content
- Most cells contain 90%+ water; Fresh cut wood is over 65% water.
- FW (fresh weight) = water + cell wall + cell contents
- DW (dry weight) = cell wall + cell contents
- Water content = FW - DW.
- Oven temperature is approximately 110°C.
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Water relations of a single cell
- Selective membrane permeability (plasmalemma and tonoplast).
- Osmosis = water movement across membranes based on relative solute concentrations.
- Plant cells have cell walls and vacuoles.
- Cell walls maintain plant cell integrity.
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Selective Permeability
- Water passes freely through membranes (aided by aquaporins).
- Salts and other solutes pass through selective channels and transporters.
- Differing concentrations inside/outside cells/organelles create gradients for water movement, membrane potentials and signals.
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Key Points: Water relations of a cell
- Membranes are selectively permeable (not semi-permeable).
- Water moves down a water potential gradient (high to low).
- Relationship between the vacuole and the surrounding medium.
- Cell walls provide rigidity.
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Water Potential (Ψ)
- "Drives" water movement in/out of cells/organelles.
- Ψ is influenced by osmotic potential (solute concentration) and pressure potential (physical pressure on the water, e.g., turgor pressure).
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Osmotic potential (Ψπ or Ψs)
- Pure water has a water potential of zero.
- Solutes decrease water potential.
- Difference between pure water potential and water with solute equals Ψπ
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Pressure potential (Ψp)
- Pure water has a water potential of zero.
- Water gains energy under pressure, becomes more positive.
- Difference between pure water potential and water under pressure = Ψp
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Osmometer
- The principle of osmosis. Water moves from lower to higher solute concentrations across a semipermeable membrane.
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The cell as an osmometer
- Ψπ of liquid (water + sugars/salts) in vacuoles, more negative than surrounding solutions.
- Water flows into cell.
- If water is drawn from the surrounding cells, the cell wall will eventually become rigid and support the cell shape (as turgor builds up).
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Water Loss
- Cuticular transpiration.
- Lenticular transpiration.
- Stomatal transpiration (Most of water loss).
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Transpiration (evapotranspiration)
- Vapor pressure difference between inside leaf and atmosphere.
- Simulation: water loss from a dish.
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Stomata
- Stomatal opening/closing controlled by guard cells.
- Size of stomatal opening is controlled by guard cells.
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Water loss from stomata
- Water is lost by evaporation.
- Vapor pressure difference determines evaporation rate.
- Water movement from surrounding leaf cells replaces lost water.
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Stomata Opening and Closing
- Proton pumps move H+ out of guard cells.
- K+ and Cl− ions move into guard cells, resulting in water entry and stomata opening.
- Conversely, K+ and Cl− ions moving out causing stomata to close
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Factors affecting stomatal activity
- Opening: Light, Low CO2, Overheating.
- Closing: Water loss, Abscisic acid (ABA), Root to shoot signal
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Factors affecting transpiration
- Air Movement: Increased air movement increases transpiration rate.
- Temperature: Higher temperature increases transpiration rate.
- Pollutants: SO2 can cause stomata to remain open, dust can block stomata
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Photosynthesis II
- Calvin Cycle overview: Ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco), steps in C3 fixation (CO2 addition to 5-C RuBP), outputs (G3P)
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RuBisCO
- RuBisCO is also an oxygenase, leading to photorespiration when O2 is high; this is a wasteful process.
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Photorespiration costs
- RuBisCO is most efficient at low O2 : high CO2.
- Increased oxygenase activity reduces the efficiency of photosynthesis in C3 plants.
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RuBisCO & Water-Use Efficiency
- Plants open stomata to take in CO2, leading to water loss.
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C3 Plants
- CO2 is fixed in mesophyll chloroplasts by Rubisco.
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C4 Plants
- Light reactions and the Calvin cycle are physically separated.
- CO2 is fixed in mesophyll cells using PEP carboxylase.
- C4 pathway generates malate and releases CO2 in bundle sheath cells for the Calvin cycle.
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Comparison of C3, C4, and CAM plants (Summary table in page 55).
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C4: Adaptation to environment
- Higher percentage of C4 in tropics.
- C3 plants are more common in temperate environments
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C4 Water Use Efficiency
- C4 typically uses less water to produce the same amount of dry matter compared to C3.
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Photosynthetic adaptations to drought & arid environments
- Open stomata at night to collect CO2 and fixing it in the form of malate, then releasing CO2 during the day for use in the Calvin cycle.
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Crassulacean Acid Metabolism (CAM)
- CAM plants import CO2 at night, with stomata open.
- Malate is stored in the vacuole.
- Released CO2 during day for Calvin cycle.
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CAM: Costs & Benefits
- Dramatically reduced water loss.
- Energetically expensive.
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Coping with dry conditions
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Metabolic approaches: C4 and CAM.
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Morphological and developmental approaches: Trichomes, stomata position, thickened epidermal cells, thickened cuticle, bulliform cells.
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Reducing surface area for water loss
- Reduction in the number of leaves, succulents, and cacti; less permeable cuticles; thicker epidermis; bulliform cells.
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Key points summary water loss
- Types of water loss (cuticular, lenticular, stomatal)
- Stomatal regulation of water loss
- Water loss effects at a cellular level.
- Adaptations to reduce water loss.
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Explore the critical role of water in plant cell relations with this quiz. Understand how water potential, transpiration, and adaptations for water conservation influence plant growth and efficiency. Delve into the reasons why water is vital for both plants and human civilization.
