Transport in Plants

Transport Occurs on Three Levels Water loss and uptake from cells. Root cells absorb minerals form the soil. Transport of substances short distances. Transport of sugar through sieve tube members. Transport of substances long distances. Transport of water and sugar up the stem.

Transport on the Cellular Level Relies on selective permeability of the plasma membrane. Transport proteins embedded in the membrane Selective channels for potassium but not for sodium Set up electrochemical gradients. Gated channels require stimulus to open and close gates. Proton pumps Require ATP (active transport) Uses chemiosmosis to couple proton gradient to transport across the membrane. Sets up membrane potential. Facilitates potassium uptake by root cells. Cotransport of nitrates against their gradient when protons diffuse with their gradient into the root cell. This is how sucrose gets into cells.

Water Potential Drives water up the Stem. Water moves by osmosis and pressure potential from cells with a higher water potential to cells with a lower water potential. Pressure potential is pressure exerted by the plant cell wall. Solute potential causes water from a hypotonic environment to a hypertonic environment. The combination of pressure and solute potential determine water potential which is the tendency of water to leave one place in favor of another. Scientists measure water potential in megapascals.

Water Potential a Closer Look Pure water in open air has a water potential of zero. Solutes lower water potential. Water has a tendency to move into the cell. Physical pressure increases water potential. Water has a tendency to move out of the cell. Tension is negative pressure Pulling water up with a plunger Water will always move in the direction of lower or negative water potential

Aquaporins It is believed by scientist that water moves through gated channels called aquaporins. Affects the rate at which water travels across the membrane. Tonoplast is the membrane bound vacuole which may take up 90% of a plant cell. Plasmodermata are channels that connect the cytoplasm of two neighboring plant cells. Symplast is the continuous route through plant tissue due to plasmodermata. Apoplast is the continuous path between cells that resides outside the plasma membrane but inside the cell wall.

Three Routes of Lateral Transport From cell to cell through cell wall and plasma membrane. Must cross cell membrane twice. Symplast – Only have to cross cell membrane once and then can travel from cell to cell through plasmodermata. Apoplasts – never cross cell membrane or enter cell. Efficient for traveling longer distances.

Bulk Flow Necessary for Longer Distances Hydrostatic pressure. Transport of phloem sap Guttation Tension created by transpiration Decreasing the diameter of the xylem increases tension. Negative water potential of the cells surrounding the stomata pull water from cells with a higher water potential near the xylem and create tension. Older Xylem filled with resin decreases the pathways for water to move up the stem adds to adds to hydrostatic pressure.

Absorption of Water and Minerals By the Roots Route – soil – epidermis- cortex – xylem Most absorption occurs at the root tips. Symbiotic fungi called mycorrhizae facilitate the access of these minerals form the soil. Dilute soil solution moves through the cortex via apoplasts. Transport proteins in the plasma membrane accumulate K+.

From Cortex to Xylem Water moves through apoplasts until it reaches the endodermis. The endodermis is an inner membrane that encompasses the vascular bundle. It is selectively permeable and actively pumps minerals into the center of the root called the stele. Water moves into the root because it is hypertonic to the soil solution. The Casparian strip is a belt of waxy substance called suberin that forces water to cross the endodermis through symplasts. Allows the endodermis to be selective about the minerals that can enter the vascular system.

Pushy Xylem Sap Because the rot cortex has a higher water potential than the stele water moves into the stele creating positive pressure At night when the rate of transpiration is low guttation occurs because the endodermal cells continue to pump minerals into the stele. Water cannot go very far using root pressure. Most of the water is pulled by transpiration.

The Transpiration Pull Evaporation of water that coats cells within the leaf tissue keeps the relative humidity high in the airs spaces. As the water evaporates a film of water within these spaces forms a meniscus with an increasing curvature that creates tension. Tension is a negative pressure and water is pulled from areas where the hydrostatic pressure is greater. Water is being pulled by adhesive and cohesive forces. Water is replace by leaf xylem.

Ascent of the Sap Cohesion allows water molecules from below to be pulled from above. The small diameter of the tracheids and xylem vessels allows the much more of the hydrophillic walls of the xylem to be exposed. Adhesive forces facilitates for the climbing of the water column. Ascent of the sap is solar powered.

Transpiration Trade Off Allows nutrients to be distributed throughout all the tissues of the plant. Provides evaporative cooling.

How Stomata Open and Close Three triggers for opening: Blue light receptors in guard cells. promotes uptake of K= into guard cells Water follows and the buckle open due to uneven thickness of their cell walls. Light also stimulates photosynthesis and produces ATP necessary for the proton pumps. Reduced CO2 Internal clock Circadian rhythms Stomata to close when: temperatures increases Increases CO2 within the air spaces of the leaves.

Xerophytes Plants adapted to arid climates Small thick leaves Stomata located on shady part of the leaves or in depressions. Fleshy stems perform photosynthesis CAM and C=4+ plants.

Transport of Phloem Sap Translocation – transport of food(sugar) within the plant. Phloem sap is primarily sucrose. Source Where sugar is made by photosynthesis Where starch is broken down. Leaves mostly.

Transport of Sugar Sink Organ that consumes sugar. Organ that stores sugar. Growing roots, shoots, fruits and stems. Storage organ can be a sink or a source. Sink when storing nutrients to be consumed in the winter. The sinks use sources that are close by.

Phloem Loading and Unloading Phloem is transported and loaded into sieve tube members. Sucrose moves from mesophyll cells where it is made through symplasts. In some species sucrose moves through both symplasts and apoplasts. Companion cells help load sugar into sieve tube members. Sucrose is cotransported across the membrane with protons. Sucrose is accumulated in various parts of the plant and requires active transport. There is a sugar gradient that is created between the sink and source and water moves in by osmosis. This creates pressure between the sink and the source and sugar solution is squeezed by hydrostatic pressure to the sink.