Resource Acquisition and Transport in Vascular Plants

Plant Adaptations

• 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

Shoot Architecture and Light Capture

• 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

Root Architecture and Acquisition of Water & Minerals

• 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)

Transport

• 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

Absorption of Water & Minerals by Root Cells

• ==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

Transport of Water and Minerals into the Xylem

• ==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

Pushing Xylem Sap: Root Pressure

• 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

Pulling Xylem Sap: The Cohesion-Tension Hypothesis

• ==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

Xylem Sap Ascent by Bulk Flow

• 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

Mechanisms of Stomatal Opening and Closing

• 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

Movement from Sugar Sources to Sugar Sinks

• ==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

Misc Info

• 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