Transport in Plants

• the uptake and release of water and solutes by individual cells
  • absorption of water and minerals from he soil by root cells
• short-distance transport of substances from cell to cell
  • loading of sucrose from photosynthetic cells into the sieve tube cells of the phloem
• long-distance transport of sap within the xylem and phloem
  • this is a whole plant phenomena - transport of photosynthate from leaf to root

Cellular-level Transport

Survival of the plant depends on balancing water uptake and water loss.
In an animal cell, water flows from hypotonic to hypertonic solutions, but in a plant cell, there is the added presence of the pressure created by the cell wall
The combination of solute concentration differences and physical pressure are incorporated into water potential, abbreviated with the Greek letter psi ()

• Water will flow through a membrane from a solution of high water potential to a solution of low water potential
• Water potential is measured in units of megapascals (MPa)
• Pure water has a water potential of 0 MPa ( = 0 MPa)
  • The addition of solutes lowers water potential ( = -0.2 MPa for instance)
  • An increase in pressure (by lowering a piston for example) will raise water potential
• These two forces combine to form the following equation:
  • = p + s
  • = total water potential
  • p = water potential due to pressure
    • May be positive or negative
  • s = water potential due solute concentration (also known as Osmotic Potential)
    • Always negative or zero

Symplastic Movement

• Movement of water and solutes through the continuous connection of cytoplasm (though plasmodesmata)
• No crossing of the plasma membrane (once it is in the symplast - however, if the solute was initially external to the cell, then it must have crossed one plasma membrane to enter the symplast)

• Movement of water and solutes through the cell walls and the intercellular spaces
• No crossing of the plasma membrane
• More rapid - less resistance to the flow of water

Absorption of Water and Minerals by Roots

• Root hairs - extensions of the root epidermal cells to increase surface area
• Mycorrhizae - fungal associations with roots - greatly increase surface area
  • as much as three meters of fungal hyphae can extend from each centimeter of root
  • this is an ancient association - some of the oldest terrestrial plant fossils have fungal associations
  • click here to download a pdf of a paper on this if you're interested (its on page 6)
• As water is drawn into the root, dissolved minerals are also brought into the root
• Water flows through the apoplast and the symplast on its way to the xylem
  • The majority of the water, however, travels through the apoplast

Water flowing through the apoplast contains many minerals that the plant needs - it may also contains toxins and substances that the plant may not want.  However, since the water is flowing through the apoplast, there is no way to prevent the passive transport of these toxins, until the water hits the endodermis.

Endodermis

Cells of the endodermis possess cell walls that are ringed by the Casparian Strip, a waxy layer (composed of suberin).

• The Casparian Strip is a wax and therefore prevents the apoplastic flow of water
• Water must pass through the plasma membrane and enter the symplast
• The plasma membrane of the endodermal cells contain many transport proteins to actively transport some molecules in and others to pump other molecules out
• Once water passes under the Casparian Strip in the endodermal cells, it is free to enter the apoplast again on its way to the xylem.

Transport of Xylem Sap

Transpiration-Cohesion-Tension: A Mechanism to Pull Xylem Sap up the Plant

Stomata open up during the day to let CO₂ in and inadvertently let H₂O escape

• There is a gradient in water potential, high water potential in the soil and very low water potential in the air
• Water vapor leaves the air spaces of the plant via the stomates
• This water is replaced by evaporation of the thin layer of water that clings to the mesophyll cells
• Remember, water has strong adhesive and cohesive properties - as the water leaves, it is replaced by water clinging to the inside of the cell walls
  • This creates a tension (pulling) on the water in the xylem and gently pulls the water toward the direction of water loss
  • The cohesion of water is strong enough to transmit this pulling force all the way down to the roots
  • Adhesion of water to the cell wall also aids in resisting gravity
• As we said before, the water column in the tallest trees can be 100m - the tension created by evaporation of water coupled with the cohesive and adhesive forces is enough to support this column against the forces of gravity

At night, transpiration is almost nil.  However, the root cells continue to actively transport minerals into the stele (the root stele is basically everything surrounded by the endodermis - primarily the xylem and the phloem).

• This active transport lowers the water potential within the stele
• Water passively flows into the roots, pushing the water up against gravity
• Water that reaches the leaves is often forced out, causing a beading of water upon the leaf tips known as guttation
• In most plants, however, root pressure is not the primary mechanism for transporting the xylem
  • Tall trees generate almost no root pressure (the weight of the water pushing down on the xylem more than counteracts any generated root pressure)

The Control of Transpiration

• Guard cells control the size of the stomatal openings and thus regulate gas and water exchange
• Water loss by a plant through stomatal openings is known as transpiration
• The efficiency of a plant can be measured by its transpiration-to-photosyntesis ratio
  • The amount of water lost per gram of CO₂ assimilated into organic material created by photosynthesis
  • A typical ratio for a C₃ plant is 600:1 - for a typical C₄ plant it is more like 300:1
• As long as plants can pull water from the soil as fast as it leaves from the leaves, there is no problem
• When water loss exceeds water uptake, the plants will wilt as the leaves lose turgor pressure
  • The conditions that favor wilting are hot, sunny, and windy days

Each stoma is flanked by a pair of guard cells that are capable of changing shape, thereby widening or narrowing the gap between the two cells

• When dicot guard cells take in water by osmosis, they become turgid and swell
  • Guard cells are not uniformly thick - this, along with a series of radically oriented cellulose microfibrils in the cell wall, cause the guard cell to buckle outwards.
• As they swell, the gap between the guard cells widens
• If the plant loses water, the guard cells become flaccid and the gap closes

• Stomata open when guard cells accumulate K+ from neighboring epidermal cells
  • How does this change the water potential () in the guard cells?
• Stomata close when K+ leaves the guard cells into the neighboring epidermal cells
  • The transport of K+ is probably coupled to the transport of H+ in an antiport system (see Fig 36.2)
• Stomatal opening is triggered by light
  • Blue light receptors are present on the membranes of guard cells
  • Stimulation of the blue-light receptors stimulates an ATP-poweed proton pump on the plasma membrane
  • The pumping of H+ out of the cell creates and electrical potential which drives in cations like K+
• Plants also observe a 24 hour cycle (a circadian rhythm)
  • If placed in total darkness, the plant will still open its stomates when it normally would if there was light

• Thick cuticles - prevent water loss from epidermal cells
• Succulent (thick) leaves - store water
• Loss of leaves/reduction of leaves to form spines - light is not limiting, so photosynthesis can be carried out by the shoot
  • What type of plant am I describing?
• White leaves/spines - light colors reflect light and heat, thereby cooling the plant
• Trichomes (hairs) - create a more humid microenvironment to reduce evaporative water loss
• Sunken stomates - like trichomes, a more humid microenvironment is created
• CAM photosynthesis - stomates open during the night (when it is cooler) and fix CO₂  into four-carbon acids
  • The light reaction occurs during the day, generating NADPH and ATP

The Translocation of Phloem

• In angiosperms, the specialized cells that transport food in the plant are called sieve-tube members, arranged end to end to form large sieve tubes
• Phloem sap is very different from xylem sap
  • sugar (sucrose) can be concentrated up to 30% by weight
• Phloem transport is bidirectional
  • Phloem moves from a sugar source (a place where sugar is produce by photosynthesis or by the breakdown of sugars) to a sugar sink (an organ which consumes or stores sugar)
  • What are some organs which would be sugar sinks?

• Sucrose manufactured in the mesophyll cells can travel via the symplast to sieve-tube members
• In some species, sugar can leave the symplast and enter the apoplast, where is it pumped back into the sieve-tube members and the companion cells
  • Some companion cells have cell wall ingrowths that facilitate apoplatic transport of sucrose into the symplast
• Sucrose is loaded into the phloem via a chemiosmotic ATPase mechanism coupled with a H+/sucrose symport (taken from Lecture 8 notes)
• Other Active and Transport Mechanisms - The H+ / Sucrose Pump
  • H+ is actively pumped out by hydrolyzing ATP
  • H+ accumulated outside the membrane, generating a concentration and electrochemical gradient
  • The H+ cannot cross the membrane, but there is a carrier protein
  • H+ binds to carrier protein, but sucrose must also bind. When both are bound, the configuration changes and the protein opens to the membrane interior.
  • Downstream, sucrose must be unloaded, again utilizing an H+ / Sucrose pump

• Phloem loading results in a high solute concentration at the source end of the
  • This creates hypnotic conditions in the phloem, causing water to flow into the phloem
  • Hydrostatic pressure builds in the sieve tube, but it is greatest in the source
• At the sink, osmosis occurs with the unloading of sugar - water flows out of the phloem
• The buildup of pressure at the source and the reduction of that pressure at the sink causes water to flow from source to sink, carrying the sugar along with it.
  • Water is recycled via transport in the xylem
• This explanation is very simplified - scientists are just now discovering the subtle details of phloem movement in plants