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Transport in Plants
Chapter 38
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Transport Mechanisms
Water first enters the roots and then moves to the xylem, the innermost vascular tissue
-Water rises through the xylem because of a combination of factors
-Some of that water exits through the stomata in the leaves
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Transport Mechanisms
Short-distance movement
-Movement of water at the cellular level plays a major role in bulk water transport
-Water can diffuse through cell membranes
-However, ions and organic compounds rely on membrane-bound transporters
-Active or passive mechanisms
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Transport Mechanisms
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Transport Mechanisms
Long-distance movement
-Some “pushing” from the pressure of water entering the roots is involved
-However, most of the force is “pulling”
-Caused by transpiration – evaporation from thin films of water in the stomata
-Occurs because water molecules stick to each other (cohesion) and to the
walls of the vessels (adhesion)
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Water Potential
Potentials are a way to represent free energy
Water potential (w) is used to predict which way water will move
-Measured in units of pressure called megapascals (MPa)
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Water Potential
Diffusion of water across a semi-permeable membrane is termed osmosis
If a plant cell is placed in a solution with high water potential (low osmotic concentration)
-It will become swollen or turgid
If a plant cell is placed in a solution with low water potential (high osmotic concentration)
-It will exhibit shrinkage or plasmolysis
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Water Potential
Pressure potential (p): Turgor pressure against the cell wall
-As turgor pressure increases, p increases
Solute potential (s): Pressure arising from presence of solute in a solution
-As solute concentration increases, s decreases (< 0 MPa)
The total potential energy of water in the cell
w = p + s
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Water Potential
When a cell is placed in pure water, water moves into the cell because the water potential of the cell is relatively negative
When a cell is placed in a solution with a different s, water moves in the direction that eventually result in equilibrium
-Both cell and solution have the same w
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Water Potential
Aquaporins are water channels that exist in vacuole and cell membranes
-They speed up osmosis, without changing the direction of water movement
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Water Potential
Water potential regulates movement of water through the whole plant as well
-Water moves from the soil into the roots only if the soil’s water potential is greater
-It then moves along gradients of successively more negative water potentials in the stems, leaves and air
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Water Potential
Evaporation of water in a leaf creates negative pressure or tension in the xylem
-This “negative water potential” literally pulls water up the stem from the roots
The driving force for transpiration is the gradient in vapor pressure
-From 100% relative humidity inside the leaf, to much less than 100% outside the stomata
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Water and Mineral Absorption
Most of the water absorbed by plants comes in through root hairs
-Collectively provide enormous surface area
-Almost always turgid because their water potential is greater than that of soil
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Water and Mineral Absorption
An expenditure of energy is required for ions to accumulate in root cells
-Once in the roots, the ions are transported via the xylem throughout the plant
Surface area for water and mineral absorption is further increased by mycorrhizal fungi
-Particularly helpful in phosphorus uptake
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Water and Mineral Absorption
Three transport routes exist through cells
-Apoplast route = Movement through the cell walls and the space between cells
-Symplast route = A cytoplasm continuum between cells connected by plasmodesmata
-Transmembrane route = Membrane transport between cells and across the membranes of vacuoles within cells
-Permits the greatest control
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Water and Mineral Absorption
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Water and Mineral Absorption
Eventually on their journey inward, molecules reach the endodermis
-Any further passage through the cell walls is blocked by the Casparian strips
-Molecules must pass through the cell membranes and protoplasts of the
endodermal cells to reach the xylem
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Xylem Transport
Root pressure is caused by the continuous accumulation of ions in the roots
-Causes water to move into plant and up the xylem despite the absence of transpiration
Guttation (production of dew) is loss of water from leaves when root pressure is high
Root pressure alone, however, is insufficient to explain xylem transport
-Transpiration provides the main force
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Xylem Transport
Water has an inherent tensile strength that arises from the cohesion of its molecules
-The tensile strength of a water column varies inversely with its diameter
-Because tracheids and vessels are tiny in diameter, they have strong cohesive
water forces
-The long column of water is further stabilized by adhesive forces
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Xylem Transport
An air bubble can break the tensile strength of a water column
-A gas-filled bubble can expand and block the tracheid or vessel, causing embolism or cavitation
-The damage can be minimized by anatomical adaptations
-Connections among tracheids provide alternative pathways
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Xylem Transport
Tracheids and vessels are essential for the bulk transport of minerals
-Ultimately the minerals are relocated through the xylem from the roots to other metabolically active parts of the plant
-Phosphorus, potassium, nitrogen, iron, and calcium
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The Rate of Transpiration
Over 90% of the water taken in by the plant’s roots is ultimately lost to the atmosphere
-At the same time photosynthesis requires a CO2 supply from the atmosphere
Closing the stomata can control water loss on a short-term basis
-However, the stomata must be open at least part of the time to allow CO2 entry
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The Rate of Transpiration
Guard cells have thicker cell walls on the inside and thinner cell walls elsewhere
-This allows them to bulge and bow outward when they become turgid
-Causing the stomata to open
Turgor in guard cells results from the active uptake of potassium (K+), chloride (Cl–), and malate
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The Rate of Transpiration
Transpiration rates increase with temperature and wind velocity because water molecules evaporate more quickly
Several pathways regulate stomatal opening and closing
-Abscisic acid (ABA) initiates a signaling pathway to close stomata in drought stress
-Opens K+, Cl–, and malate channels
-Water loss follows
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The Rate of Transpiration
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The Rate of Transpiration
The stomata close at high temperatures or when CO2 concentrations increase
They open when blue wavelengths of light promote uptake of K+ by the guard cells
Alternative photosynthetic pathways, such as Crassulacean acid metabolism (CAM), reduce transpiration
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Water Stress Responses
Many morphological adaptations allow plants to limit water loss in drought conditions
-Dormancy
-Loss of leaves
-Covering leaves with cuticle and wooly trichomes
-Reducing the number of stomata
-Having stomata in pits on the leaf surface
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Water Stress Responses
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Water Stress Responses
Plants have adapted to flooding conditions which deplete available oxygen
-Form larger lenticels and adventitious roots
Plants have also adapted to life in fresh water
-Form aerenchyma, which is loose parenchymal tissue with large air spaces
-Collect oxygen and transport it to submerged parts of the plant
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Water Stress Responses
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Water Stress Responses
Plants, such as mangroves, that grow in salt water produce pneumatophores
-Long, spongy, air-filled roots, that emerge above the mud
-Have large lenticels through which oxygen enters
-These plants also secrete large quantities of salt
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Water Stress Responses
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Water Stress Responses
Plants called halophytes live in saline soil
-Produce high concentrations of organic molecules in their roots
-This decreases the water potential enhancing water uptake from the soil
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Phloem Transport
Most carbohydrates produced in leaves are distributed through phloem to rest of plant
-This translocation provides building blocks for actively growing regions of the plant
The carbohydrate- and nutrient-rich fluid moved through the body is called sap
Sucrose is transported up & down the phloem
-Hormones and mRNA as well
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Phloem Transport
Pressure-flow theory is a model describing the movement of carbohydrates in phloem
-Dissolved carbohydrates flow from a source and are released in sink
-Sources include photosynthetic tissues
-Sinks include growing root and stem tips as well as developing fruits
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Phloem Transport
A process called phloem-loading occurs at the source
-Active transport of sugars into the phloem causes a reduction in water potential
-As water moves into the phloem, turgor pressure drives the contents to the sink
-Sugar is actively transported into cells
-Water diffuses back into the xylem to be reused or lost through transpiration
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