Chapter 36 Transport in Plants
Levels of Plant Transport 1. Cellular 2. Short Distance 3. Long Distance
The transport of solutes and water across cell membranes. Types of transport: 1. Passive Transport 2. Active Transport 3. Water Transport Cellular Transport
1. Passive Transport • Diffusion and Osmosis. • Requires no cellular energy. • Materials diffuse down concentration gradients.
Problems • Usually very slow. • How can diffusion be assisted? • Transport Proteins • Ex. K+ channel
Potassium Channel • Found in most plant cell membranes. • Allow K+ but not Na+ to pass. • Often “gated” to respond to environmental stimuli (see cell signaling)
2. Active Transport • Requires cell energy. • Moves solutes against a concentration gradient. • Ex: Proton Pumps
Proton Pump • Uses ATP to move H+ out of cells. • H+ creates a membrane potential. • H+ allows cotransport.
Membrane Potentials • Allow cations to moved into the cell. • Ex: Ca+2, Mg+2 • Allow anions to move by co-transport. • Ex: NO3
3. Water Transport • Osmosis - water moves from high concentration to low concentration.
Water Potential • The potential energy of water to move from one location to another. • Abbreviated as y
Problem • Cell wall creates a pressure in the cells. • Water potential must account for this pressure. • Pressure counteracts the tendency for water to move intoplant cells.
Water Potential • Has two components: • Pressure potential: yr • Solute potential: yp y = yr + yp
Comment • See the Ts lab handout for more on water potential.
Bulk Flow • The movement of water between two locations due to pressure or tension.
Bulk Flow • Much faster than osmosis. • Tension (negative pressure) pulls water from place to place. • May cause bulk flow against the diffusion gradient.
Plant Vacuoles • Create Turgor Pressure against the cell wall. • Affect water potential by controlling water concentrations inside cells.
Tonoplast • Name for the vacuole membrane. • Has proton pumps. • Comment – genetic modification of these pumps gives plants salt tolerance.
Proton Pumps • Drives solutes inside the vacuole. • Lowers water potential (yp)inside the vacuole.
Result • Water moves into the vacuole. • Vacuole swells. • Turgor pressure increases.
Turgor Pressure • Important for non-woody plant support. • Wilting: • Loss of turgor pressure. • Loss of water from cells.
Aquaporins • Water specific facilitated diffusion transport channels. • Help water move more rapidly through lipid bilayers.
Short Distance Transport 1. Transmembrane route 2. Symplast route 3. Apoplast route
Materials cross from cell to cell by crossing each cell's membranes and cell walls. 1. Transmembrane
The continuum of cytoplasm by plasmodesmata bridges between cells. 2. Symplast
Long Distance Transport • Problem: diffusion is too slow for long distances. • Answer: tension and bulk flow methods.
Main site of absorption of water and minerals. Comment - older roots have cork and are not very permeable to water. Root Hairs
Very spongy. Apoplast route very common. Root Cortex
Problem • Can't control uptake of materials if the apoplast route is used.
Endodermis with its Casparian Strip. Solution
Casparian Strip • Waxy layer of suberin. • Creates a barrier between the cortex and the stele. • Forces materials from apoplast into endodermis symplast.
Casparian Strip Endodermis
Result • Plant can now control movement of materials into the stele.
Xylem Sap • Solution of water and minerals loaded into the xylem by the endodermis. • Endodermis - also prevents back flow of water and minerals out of the stele.
Xylem Sap Transport Methods 1. Root Pressure 2. Transpiration (Ts)
Root Pressure • Root cells load minerals into xylem. • Water potential (yp) is lowered. • Water flows into xylem.
Result • Volume of water in xylem increases • Xylem sap is pushed up the xylem tissues creating root pressure.
Root Pressure: limited way to move xylem sap. Most apparent at night. Excess water may leave plant through Guttation. Comments
Transpiration (Ts) • Evaporation of water from aerial plant parts. • Major force to pull xylem sap up tall trees.
TCTM Theory • Transpiration • Cohesion • Tension • Mechanism
How does TCTM work? • Water evaporates from leaves, especially from the cell walls of the spongy mesophyll. • Reason: water potential of the air is usually much less than that of the cells.