Short distance transport (Lecture 11)
Plant Transport
*Water movement (transport) occurs at three levels:
Cellular
Lateral transport (short-distance)
Whole plant (long-distance)
Short-Distance Transport of Solutes Across Plasma Membranes
Transport begins with the absorption of resources by plant cells.
The movement of substances into and out of cells is regulated by selectively permeable membrane.
(Plasma membrane permeability controls short-distance movement of substances)
Both active and passive transport occur in plants.
Diffusion across a membrane is passive transport.
The pumping of solutes across a membrane is active transport and requires energy.
Most solutes pass through transport proteins embedded in the cell membrane.
The most important transport protein for active transport of ions/protons across the membrane is the proton pump.
Proton pumps in plant cells create a hydrogen ion gradient that is a form of potential energy that can be harnessed to do work.
They contribute to a voltage known as a membrane potential.
Membrane potentials determined by three factors:
1) the concentration of ions on the inside and outside of the cell;
2) the permeability of the cell membrane to those ions through
specific ion channels; and
3) by the activity of electrogenic pumps (are primary active transporters that hydrolyze ATP )
In animals, membrane potential is established through pumping Na+ by sodium-potassium pumps
Plant cells use energy stored in the proton gradient and membrane potential to drive the transport of many different solutes.
The “coat-tail” effect of cotransport is also responsible for the uptake of the sugar sucrose by plant cells.
Cotransport – a transport protein couples the diffusion of one solute to the active transport of another.
Short-Distance Transport of Water Across Plasma Membranes
To survive, plants must balance water uptake and loss
Osmosis determines the net uptake or water loss by a cell and is affected by solute concentration and pressure
Water potential is a measurement that combines the effects of solute concentration and pressure
Water potential determines the direction of movement of water
Water flows from regions of higher water potential to regions of lower water potential
Potential refers to water’s capacity to perform work
How Solutes and Pressure Affect Water Potential?
- The solute potential of a solution is proportional to the number of dissolved molecules (solutes).
- Solute potential is also called osmotic potential.
- Remember: More solute means less water.
- Pressure potential is the physical pressure on a solution.
- Turgor pressure is the pressure exerted by the plasma membrane against the cell wall, and the cell wall against the protoplast.
Measuring Water Potential
- Consider a U-shaped tube where the two arms are separated by a membrane permeable only to waterWater moves in the direction from higher water potential to lower water potential
Addition of Solutes reduces water potential.
Physical/positive pressure increases water potential.
Negative pressure decreases water potential.
Water Movement Across Plant Cell Membranes
Water potential affects uptake and loss of water by plant cells
If a flaccid cell is placed in an environment with a higher solute concentration, the cell will lose water and undergo plasmolysis
Plasmolysis occurs when the protoplast shrinks and pulls away from the cell wall
If a flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid
Turgor loss in plants causes wilting, which can be reversed when the plant is watered
Aquaporins: Facilitating Diffusion of Water
Aquaporins are transport proteins in the cell membrane that allow the passage of water
These affect the rate of water movement across the membrane
The rate of water movement is likely regulated by phosphorylation of the aquaporin proteins.
Long-Distance Transport -Bulk Flow in Vessels Xylem and Phloem
Efficient long distance transport of fluid requires bulk flow, the movement of a fluid driven by pressure
Water and solutes move together through tracheids and vessel elements of xylem, and sieve-tube elements of phloem
Efficient movement is possible because mature tracheids and vessel elements have no cytoplasm, and sieve-tube elements have few organelles in their cytoplasm
Transpiration drives the transport of water and minerals from roots to shoots via the xylem
*Plants can move a large volume of water from their roots to shoots
Absorption of Water and Minerals by Root Cells
Most water and mineral absorption occurs near root tips, where root hairs are located and the epidermis is permeable to water
Root hairs account for much of the surface area of roots
After soil solution enters the roots, the extensive surface area of cortical cell membranes enhances uptake of water and selected minerals
The concentration of essential minerals is greater in the roots than soil because of active transport
Transport of Water and Minerals into the Xylem
The endodermis is the innermost layer of cells in the root cortex
It surrounds the vascular cylinder and is the last checkpoint for selective passage of minerals from the cortex into the vascular tissue
Water can cross the cortex via the symplast or apoplast
The waxy Casparian strip of the endodermal wall blocks apoplastic transfer of minerals from the cortex to the vascular cylinder
Endodermis cells (stele) have suberin (waterproof)
Water and minerals in the apoplast must cross the plasma membrane of an endodermal cell to enter the vascular cylinder
The endodermis regulates and transports needed minerals from the soil into the xylem
Water and minerals move from the protoplasts of endodermal cells into their cell walls
Diffusion and active transport are involved in this movement from symplast to apoplast
Water and minerals now enter the tracheids and vessel elements
