Resource Acquisition and Transport in Vascular Plants
Adaptations aid in the acquisition of resources, including water, minerals, carbon dioxide, and light
Early nonvascular (no xylem & phloem) land plants lived in shallow water and had aerial shoots
Nothing could live on land until plants colonized it
Xylem: transports water and minerals from roots to shoots
Phloem: transports photosynthetic products from where they are made to where they are needed
Trade-off between growing tall & branching
More energy invested in branching→ less energy available for height growth
Water availability signals leaf growth
Phyllotaxy: arrangement of leaves on a stem; a species-specific trait important for light capture → more leaves
Angle between leaves is 137.5 degrees
Minimizes shading of lower leaves
If there are too many top leaves, will shade bottom leaves
Competition among plants
Self-pruning: shedding of lower shaded leaves
Occurs when they respire more than they photosynthesize
Costs more than it benefits
Community: multiple different species living in the same area & interacting
Leaf-area index: ratio of total upper leaf surface of a plant divided by the surface area of land on which it grows
7→ shading so self-pruning occurs
less leaves→ smaller leaf area index
Roots are less competitive with other roots from the same plant than with roots from different plants
Nitrates necessary for growth
Mycorrhizae: mutualistic associations formed between roots and hyphae of soil fungi (roots/extensions of soil fungi)
ex/ increase surface area for absorbing water and minerals, especially phosphate (necessary for making DNA & phospholipids)
2 major transport pathways
Apoplast: everything external to the plasma membrane
Includes cell walls, extracellular spaces, and interior of dead cells like vessel elements and tracheids (water-conducting cell in xylem)
Symplast: consist of cytosol of all living cells & plasmodesmata
everything inside plasma membrane
Apoplastic route: through cell walls and extracellular spaces
Symplastic route: where water and solutes cross a plasma membrane once and then travel through cytosol
Transmembrane route: water and solutes repeatedly cross membranes when they pass from cell to cell
Can also occur in cells with the same plasmodesmata (some nutrients may not want to travel through the cytosol)
Active transport: need ATP
Proton pumps establish membrane potential by pumping H+ and establishing a proton gradient (plants)
Membrane potential is established by pumping Na+ by sodium-potassium pumps (animals)
Plant cell membranes have ion channels that only allow certain ions to pass
Osmosis: the diffusion of water into our out of a cell that is affected by solute concentration and pressure
Water potential: quantity that includes effects of solute concentration and physical pressure
Determines direction of movement of water
Flows from regions of higher water potential to regions of lower water potential
Potential→ refers to water’s capacity to perform work
Unit of pressure is a megapascal
Diffusion: high to low concentration
Solute potential: osmotic potential; directly proportional to molarity
Pressure potential: physical pressure on a solution (can be positive or negative)
Turgor pressure: positive pressure exerted by plasma membrane against cell wall & cell wall against protoplast
Protoplast: living part of cell, includes plasma membrane
Turgor loss results in wilting, which can be reversed by watering the plant
Flaccid: limp/deflated cell
If flaccid cell placed in environment with higher solute concentration, cell will lose water and undergo plasmolysis
Plasmolysis: protoplast shrinks and pulls away from cell wall
If flaccid cell placed in solution with lower solute concentration, cell will gain water and become turgid (plump, lots of water)
Aquaporins: transport proteins in cell membrane that facilitate passage of water
Opening & closing of aquaporins affect rate of osmotic water movement across membrane
Hydrophilic head & hydrophobic tail
Bulk-flow: the movement of a fluid driven by a pressure gradient (long-distance transport)
Water and solutes move through tracheids and vessel elements (in xylem, tube-like components) of xylem & sieve-tube elements (tube-like element of phloem) of phloem
Enhanced by structural adaptations of xylem and phloem
Mature tracheids and vessel elements have no cytoplasm; inside of cells empty to move water and minerals (dead cells)
Sieve-tube elements have few organelles in their cytoplasm (alive)
Perforation plates connect vessel elements & porous sieve plates connect sieve-tube elements
Transpiration: bulk flow up, using xylem; evaporation of water from a plant’s surface
Water and mineral absorption occurs near root tips, where there are root hairs and the epidermis is permeable to water
Endodermis: innermost layer of cells in the root cortex (external part of root)
surrounds vascular cylinder & is last checkpoint for selective passage of minerals from cortex to vascular tissue
Casparian strip: barrier at checkpoint of the endodermal wall, blocking apoplastic transfer of minerals from the cortex to the vascular cylinder
Endodermal cells discharge water and minerals from their protoplasts into their own cell walls
Xylem Sap: fluid in xylem containing water and dissolved minerals
transported from roots to leaves by bulk flow
Water flows in from root cortex, generating root pressure (push of xylem sap)
Sometimes results in guttation
Guttation: exudation of water droplets on tips or edges of leaves
Positive root pressure is weak and is a minor mechanism of xylem bulk flow
Cohesion-tension hypothesis: transpiration and water cohesion pull water from shoots to roots
Cohesion: water sticks to water
Adhesion: water sticked to other surfaces
prevents water from falling back down
Surface tension: water won’t spill over & bugs don’t sink in water
Xylem sap is normally under negative pressure/tension
Transpirational pull: positive + negative pressure
Water vapor in the air spaces of leaf diffuse down water potential gradient and exits via stomata
Air-water interface retreats into mesophyll cell walls
Creates negative pressure potential
Negative pressure potential lowers water potential
Water molecules pulled from more hydrated areas of lead by negative pressure potential created by air-water interface
Positive pressure → pushes up
Negative pressure → pulls up
Drought stress or freezing can cause a break in chain of water molecules through cavitation
Cavitation: formation of water vapor pocket; break in hydrogen bond
How does bulk flow differ from diffusion?
Driven by differences in pressure potential rather than solute potential
Occurs in hollow dead cells rather than membranes of living ells
Moves the entire solution instead of just water or solutes
Much faster
Changes in turgor pressure open & close stomata
Turgid→ guard cells bow outward and pore opens (expand & leave gap)
Flaccid→ guard cells become less bowed and pore closes (deflate & cover surface)
Generally open during day and close at night to minimize water loss
Opening triggered by light, CO2 depletion, and internal clock in guard cells
Circadian rhythms: 24 hr cycles; genes that control hormone release are exact
ex/ most heart attacks if at risk occur at 10am
Hormone Abscisic Acid (ABA) produced in response to water deficiency; causes closure of stomata
Sunny, warm, dry, and windy conditions cause evaporation & increase transpiration
More water molecules lost to atmosphere
Water sometimes leaks through cuticle
If uptake & transport can’t replace lost water, plant wilts
Transpiration→ evaporative cooling (lower temp of leaf)
Trade-off with losing water
Water has a high specific heat
Adaptations that Reduce Evaporative Water Loss
Xerophytes: plants adapted to dry conditions and climates
Crassulacean acid metabolism (CAM): stomatal gas exchange occurs at night
Reverse reaction
Change CO2 into a 4-carbon molecule and store it
Translocation: opposite of transpiration; goes down using phloem
Phloem sap: aqueous solution high in sucrose
Travels from sugar source to sugar sink
Phloem made of sieve-tube elements
Moves through bulk flow driven by positive pressure called pressure flow
Sugar source: organ that is a net producer of sugar (ex/ mature leaves)
Pressure is high
Sugar sink: organ that is a net consumer or depository of sugar (ex/ roots, buds, fruits)
Sugar must be loaded into sieve-tube elements before being exported to sinks
Pressure is low
Companion cells: enhance solute movement between apoplast and symplast
Self-thinning: dropping of sugar sinks (ex/ flowers, seeds, fruits)
Occurs when there are more sugar sinks than sources can support
Usually don’t want to get rid of flowers because they attract pollinators
Plasmodesmata (plant can have multiple) open and close in response to turgor pressure, cytosolic Ca2+ levels or cytosilic pH
plant viruses can cause it to dilate, allowing viral RNA to pass between cells
plants try to stop spread of virus by closing plasmodesmata
Phloem is key for transport of macromolecules and viruses
Systemic communication through phloem allows the integration of plant functions
Electricity: movement of any charge (ion)
Triggers cells to act/behave differently
Adaptations aid in the acquisition of resources, including water, minerals, carbon dioxide, and light
Early nonvascular (no xylem & phloem) land plants lived in shallow water and had aerial shoots
Nothing could live on land until plants colonized it
Xylem: transports water and minerals from roots to shoots
Phloem: transports photosynthetic products from where they are made to where they are needed
Trade-off between growing tall & branching
More energy invested in branching→ less energy available for height growth
Water availability signals leaf growth
Phyllotaxy: arrangement of leaves on a stem; a species-specific trait important for light capture → more leaves
Angle between leaves is 137.5 degrees
Minimizes shading of lower leaves
If there are too many top leaves, will shade bottom leaves
Competition among plants
Self-pruning: shedding of lower shaded leaves
Occurs when they respire more than they photosynthesize
Costs more than it benefits
Community: multiple different species living in the same area & interacting
Leaf-area index: ratio of total upper leaf surface of a plant divided by the surface area of land on which it grows
7→ shading so self-pruning occurs
less leaves→ smaller leaf area index
Roots are less competitive with other roots from the same plant than with roots from different plants
Nitrates necessary for growth
Mycorrhizae: mutualistic associations formed between roots and hyphae of soil fungi (roots/extensions of soil fungi)
ex/ increase surface area for absorbing water and minerals, especially phosphate (necessary for making DNA & phospholipids)
2 major transport pathways
Apoplast: everything external to the plasma membrane
Includes cell walls, extracellular spaces, and interior of dead cells like vessel elements and tracheids (water-conducting cell in xylem)
Symplast: consist of cytosol of all living cells & plasmodesmata
everything inside plasma membrane
Apoplastic route: through cell walls and extracellular spaces
Symplastic route: where water and solutes cross a plasma membrane once and then travel through cytosol
Transmembrane route: water and solutes repeatedly cross membranes when they pass from cell to cell
Can also occur in cells with the same plasmodesmata (some nutrients may not want to travel through the cytosol)
Active transport: need ATP
Proton pumps establish membrane potential by pumping H+ and establishing a proton gradient (plants)
Membrane potential is established by pumping Na+ by sodium-potassium pumps (animals)
Plant cell membranes have ion channels that only allow certain ions to pass
Osmosis: the diffusion of water into our out of a cell that is affected by solute concentration and pressure
Water potential: quantity that includes effects of solute concentration and physical pressure
Determines direction of movement of water
Flows from regions of higher water potential to regions of lower water potential
Potential→ refers to water’s capacity to perform work
Unit of pressure is a megapascal
Diffusion: high to low concentration
Solute potential: osmotic potential; directly proportional to molarity
Pressure potential: physical pressure on a solution (can be positive or negative)
Turgor pressure: positive pressure exerted by plasma membrane against cell wall & cell wall against protoplast
Protoplast: living part of cell, includes plasma membrane
Turgor loss results in wilting, which can be reversed by watering the plant
Flaccid: limp/deflated cell
If flaccid cell placed in environment with higher solute concentration, cell will lose water and undergo plasmolysis
Plasmolysis: protoplast shrinks and pulls away from cell wall
If flaccid cell placed in solution with lower solute concentration, cell will gain water and become turgid (plump, lots of water)
Aquaporins: transport proteins in cell membrane that facilitate passage of water
Opening & closing of aquaporins affect rate of osmotic water movement across membrane
Hydrophilic head & hydrophobic tail
Bulk-flow: the movement of a fluid driven by a pressure gradient (long-distance transport)
Water and solutes move through tracheids and vessel elements (in xylem, tube-like components) of xylem & sieve-tube elements (tube-like element of phloem) of phloem
Enhanced by structural adaptations of xylem and phloem
Mature tracheids and vessel elements have no cytoplasm; inside of cells empty to move water and minerals (dead cells)
Sieve-tube elements have few organelles in their cytoplasm (alive)
Perforation plates connect vessel elements & porous sieve plates connect sieve-tube elements
Transpiration: bulk flow up, using xylem; evaporation of water from a plant’s surface
Water and mineral absorption occurs near root tips, where there are root hairs and the epidermis is permeable to water
Endodermis: innermost layer of cells in the root cortex (external part of root)
surrounds vascular cylinder & is last checkpoint for selective passage of minerals from cortex to vascular tissue
Casparian strip: barrier at checkpoint of the endodermal wall, blocking apoplastic transfer of minerals from the cortex to the vascular cylinder
Endodermal cells discharge water and minerals from their protoplasts into their own cell walls
Xylem Sap: fluid in xylem containing water and dissolved minerals
transported from roots to leaves by bulk flow
Water flows in from root cortex, generating root pressure (push of xylem sap)
Sometimes results in guttation
Guttation: exudation of water droplets on tips or edges of leaves
Positive root pressure is weak and is a minor mechanism of xylem bulk flow
Cohesion-tension hypothesis: transpiration and water cohesion pull water from shoots to roots
Cohesion: water sticks to water
Adhesion: water sticked to other surfaces
prevents water from falling back down
Surface tension: water won’t spill over & bugs don’t sink in water
Xylem sap is normally under negative pressure/tension
Transpirational pull: positive + negative pressure
Water vapor in the air spaces of leaf diffuse down water potential gradient and exits via stomata
Air-water interface retreats into mesophyll cell walls
Creates negative pressure potential
Negative pressure potential lowers water potential
Water molecules pulled from more hydrated areas of lead by negative pressure potential created by air-water interface
Positive pressure → pushes up
Negative pressure → pulls up
Drought stress or freezing can cause a break in chain of water molecules through cavitation
Cavitation: formation of water vapor pocket; break in hydrogen bond
How does bulk flow differ from diffusion?
Driven by differences in pressure potential rather than solute potential
Occurs in hollow dead cells rather than membranes of living ells
Moves the entire solution instead of just water or solutes
Much faster
Changes in turgor pressure open & close stomata
Turgid→ guard cells bow outward and pore opens (expand & leave gap)
Flaccid→ guard cells become less bowed and pore closes (deflate & cover surface)
Generally open during day and close at night to minimize water loss
Opening triggered by light, CO2 depletion, and internal clock in guard cells
Circadian rhythms: 24 hr cycles; genes that control hormone release are exact
ex/ most heart attacks if at risk occur at 10am
Hormone Abscisic Acid (ABA) produced in response to water deficiency; causes closure of stomata
Sunny, warm, dry, and windy conditions cause evaporation & increase transpiration
More water molecules lost to atmosphere
Water sometimes leaks through cuticle
If uptake & transport can’t replace lost water, plant wilts
Transpiration→ evaporative cooling (lower temp of leaf)
Trade-off with losing water
Water has a high specific heat
Adaptations that Reduce Evaporative Water Loss
Xerophytes: plants adapted to dry conditions and climates
Crassulacean acid metabolism (CAM): stomatal gas exchange occurs at night
Reverse reaction
Change CO2 into a 4-carbon molecule and store it
Translocation: opposite of transpiration; goes down using phloem
Phloem sap: aqueous solution high in sucrose
Travels from sugar source to sugar sink
Phloem made of sieve-tube elements
Moves through bulk flow driven by positive pressure called pressure flow
Sugar source: organ that is a net producer of sugar (ex/ mature leaves)
Pressure is high
Sugar sink: organ that is a net consumer or depository of sugar (ex/ roots, buds, fruits)
Sugar must be loaded into sieve-tube elements before being exported to sinks
Pressure is low
Companion cells: enhance solute movement between apoplast and symplast
Self-thinning: dropping of sugar sinks (ex/ flowers, seeds, fruits)
Occurs when there are more sugar sinks than sources can support
Usually don’t want to get rid of flowers because they attract pollinators
Plasmodesmata (plant can have multiple) open and close in response to turgor pressure, cytosolic Ca2+ levels or cytosilic pH
plant viruses can cause it to dilate, allowing viral RNA to pass between cells
plants try to stop spread of virus by closing plasmodesmata
Phloem is key for transport of macromolecules and viruses
Systemic communication through phloem allows the integration of plant functions
Electricity: movement of any charge (ion)
Triggers cells to act/behave differently