Transport in Plants 

Transport Mechanisms

How does water move up to the top of a 10-story high tree?

• Water first enters the $roots$.
• Then moves to the $xylem$.
• Water rises through the xylem because of a combination of ($hydrogen bonding$ and $water potential$).
• Most of that water exits through the stomata in the leaves ($transpiration$).

Long-Distance Movement

-Local changes result in long-distance movement of materials.

-Water and dissolved minerals travel great distances in xylem.

• Most of the force is “pulling” caused by $transpiration$.
  • Evaporation from thin films of water in the stomata.
  • Occurs due to $cohesion$ (water molecules stick to each other) and $adhesion$ (stick to walls).

Movement of Water at Cellular Level

Water can diffuse through plasma membranes by osmosis

• $Osmosis$ is enhanced by aquaporins.

Other substances depend on protein transporters

• $Facilitated diffusion$ or $active transport$.
• $ATP-dependent hydrogen ion pumps$ often fuel active transport.
• Unequal concentrations of solutes drive osmosis.

Water Potential

• Potentials are a way to represent free energy
• Water potential (Ψw) is used to predict which way water will move
  • Water moves from higher to lower Ψw.
• Measured in units of pressure called megapascals (MPa)

Aquaporins

Aquaporins allow for the rapid osmosis that is needed for plants.

• Water- selective pores in plasma membrane increase the rate of osmosis by facilitating the diffusion of water.

Osmosis and Cellular Changes

Osmosis

• If a single plant cell is placed into water.
  • Water moves into cell by osmosis.
  • Cell expands and becomes $turgid$.
• If cell placed in high concentration of sucrose.
  • Water leaves cell.
  • Cell shrinks - $plasmolysis$.

Water Absorption

• Most of the water absorbed by the plant comes in through the region of the root with $root hairs$.
  • Surface area further increased by $mycorrhizal fungi$.
• Once absorbed through root hairs, water and minerals must move across cell layers until they reach the vascular tissues.
• Water and dissolved ions then enter the xylem and move throughout the plant.

Transport Routes

3 transport routes exist through cells

• $Apoplast route$ - movement through the cell walls and the space between cells
  • Avoids membrane transport
• $Symplast route$ - cytoplasm continuum between cells connected by plasmodesmata
• $Transmembrane route$ - membrane transport between cells and across the membranes of vacuoles within cells

Inward Movement of Water

• Eventually on their journey inward, molecules reach the $endodermis$
• Any further passage through the cell walls is blocked by the $Casparian strips$
  • Apoplast route is blocked by waterproof material suberin.
• Molecules must pass through the cell membranes and protoplasts of the endodermal cells to reach the xylem.

Movement of Ions

• $Mineral ion$ concentration in the soil water is usually much lower than it is in the plant
• $Active transport$ is required for increased solute concentration in the $stele$.
• Plasma membranes of endodermal cells contain a variety of protein transport channels that allow for active transport.

Xylem Transport

• The aqueous solution that passes through the endodermal cells moves into the tracheids and vessel elements of the xylem
• As ions are actively pumped into root, their presence decreases the water potential
• Water then moves into the plant via osmosis, causing an increase in turgor pressure
• Lowest water potential is usually in leaves due to transpiration

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

Guard Cells

• Only epidermal cells containing chloroplasts
• Have thicker cell walls on the inside and thinner cell walls elsewhere
  • Bulge and bow outward when they become turgid.
  • This causes the stoma between two guard cells to open.
• Turgor in guard cells results from the active uptake of solute.
• Addition of solute causes water potential to drop.
• Water enters osmotically and cells become turgid.

Changing Transpiration Rates

-Transpiration rates increase with temperature and wind velocity because water molecules evaporate more quickly

-Several pathways regulate stomatal opening and closing

• $Abscisic acid$ (ABA, a hormone) initiates a signaling pathway to close stomata in drought.
  • Opens K+, Cl−, and malate channels.
  • Water loss follows, making guard cells flaccid.

Also Affecting Stomatal Opening

-Close when CO2 concentrations are high (sufficient for Calvin Cycle)

-Open when blue wavelengths of light promote uptake of ions by the guard cells (ready for Light Reactions)

-Close when temperature exceeds 34°C and water relations unfavorable (Too hot or drought conditions)

• Alternative photosynthetic pathways, such as Crassulacean acid metabolism (CAM), reduce transpiration.

Water Stress Responses

Many morphological adaptations allow plants to limit water loss in drought conditions

• Dormancy.
• Loss of leaves – deciduous plants.
• Covering leaves with cuticle and wooly trichomes.
• Reducing the number of stomata.

Phloem Transport

-Most carbohydrates produced in leaves are distributed through phloem to rest of plant

-This process, called $translocation$, provides building blocks for actively growing regions of the plant

Phloem also transports:

• hormones.
• mRNA.
• amino acids.
• proteins.
• ions.

*Aphids feed on the nutrient-rich content of the phloem, which they extract through their piercing mouthparts called stylets. When an aphid is separated from its stylet and the cut stylet is left in the plant, the phloem fluid oozes out of it and can then be collected and analyzed.

Pressure-Flow Hypothesis

Most widely accepted model describing the movement of carbohydrates in phloem

• Dissolved carbohydrates flow from a $source$ (where the material is) to a $sink$ (where the material needs to go).
• Sources include photosynthetic tissues.
• Sinks include growing root and stem tips as well as developing fruits.
• Food-storage tissue can be sources or sinks.

Phloem-Loading

$Phloem-loading$ occurs at the source

• $Solute$ (usually sucrose) enter the sieve tubes in the smallest veins at the source.
• $Sieve cells$ must be alive to use $active transport$ to load sucrose.
• Water flows into sieve tubes by osmosis.
• At sink, solute is actively removed and water follows by osmosis.