Transport in Plants Chapter 36

To get onto land, plants evolved way to keep from drying out, to stand upright. Transport nutrients and water both over long distance and short distances.

At cellular level - plasma membrane allows for transport into cell (transport proteins). Some transport proteins act as selective channels - determine what can go into/out of cell. Plant cell - proton pumps function in pumping H+ ions out of cell.

Proton pump can aid in cotransport - H+ is pumped out of cell aiding in pumping in/out (against concentration gradient) of another substance (glucose)

Plants rely on osmosis to survive. Direction of water movement depends on solute concentration and physical pressure. (water potential) Water moves from high water potential to low water potential. Water potential measured in MPa - abbreviated psi.

Applying pressure to water can reverse movement of water. Using syringe (negative pressure) can force water to move upwards. Combined effects of pressure and solute concentrations on water potential are incorporated into equation: psi = psi P ( pressure potential + psi s ( solute potential)

Flaccid cell, psi p = 0. Placed in solution with lower psi, water will leave cell. Cell will plasmolyze, shrinking and pulling away from wall. As cell swells, it will push against wall, producing turgor pressure.

Placed in pure water - cell will have lower water potential due to solutes and water will enter cell. Walled cell with greater solute concentration than its surroundings will be turgid or firm.

Aquaporins are specific transport proteins - aid in passive movement of water only. Cell wall gives plants shape, but not passing of materials.

Membrane that bounds vacuole (tonoplast) regulates molecular traffic between cytosol and contents of vacuole (cell sap) Plasmodesmata (connections between cells) connect symplast (cytoplasm stream) Cell walls of adjacent plant cells - apoplast.

Because of distance water and nutrients need to travel between roots and leaves, simple diffusion not efficient enough. Water and solutes move through xylem vessels and sieve tubes by bulk flow, movement of fluid driven by pressure.

Tension allows for transport of materials. Transpiration forces water to move up plant in stream (negative pressure) - allows materials to move in bulk. Larger diameter of stem, faster material can move.

Absorption of water by roots Water, mineral salts from soil enter plant through epidermis of roots, cross root cortex, pass into stele, then flow up xylem vessels to shoot system.

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Much of absorption of water and minerals occurs near root tips - epidermis is permeable to water and where root hairs are located. Root hairs allow for maximum uptake. Most plants form partnerships with symbiotic fungi for absorbing water and minerals from soil.

Water, minerals in root cortex cannot be transported to rest of plant until they enter xylem of stele. Endodermis, innermost layers of root cortex, surrounds stele, is last checkpoint for absorption into vascular tissue.

Transport of Xylem Xylem sap flows into veins of leaf providing them with water. Plants lose water through transpiration; water replaced through water transport. Xylem sap rises against gravity through pumping system. Accumulation of minerals in stele lowers water potential; generates positive pressure (root pressure) forces fluid up xylem.

Root pressure causes guttation - exudation of water droplets (seen in morning on tips of grass blades) Roots accumulate water during night, transpiration is low, water enters leaf at faster rate.

Xylem sap pulled through plant creating stream of water - cannot be broken. Cavitation (formation of water vapor pockets in xylem vessel) breaks chain. Occurs when xylem sap freezes in water. Cannot be fixed in trees, but stream can form around it.

Control of transpiration Guard cells control amount of water lost during day (through stomata). Transpiration also cools plant down.

When transpiration exceeds delivery of water by xylem, (soil begins to dry out) leaves begin to wilt as cells lose turgor pressure. Guard cells control diameter of stoma by changing shape, widening or narrowing gap between 2 cells.

Potassium helps in regulation of guard cells. Stomata open during day, closed at night to minimize water loss when too dark for photosynthesis. Regulated in 3 ways. 1st - blue-red wavelengths signal plant to start photosynthesizing.

2nd - depletion of CO2. 3rd - internal clock in plant cues plant to start photosynthesizing - started at dawn. Opening and closing cycle of stomata is an example of circadian rhythm, cycles that have intervals of approximately 24 hours.

Plants adapted to arid climates (xerophytes) - leaf modifications that reduce rate of transpiration. Some -smaller, thicker leaves. Some - shed leaves during extremely dry months. Some - stomata concentrated on lower (shady) leaf surface.

Phloem sap Phloem transports organic products of photosynthesis throughout plant via translocation. Phloem sap - aqueous solution - sugar (mostly disaccharide sucrose) most abundant solute.

Xylem - unidirectional movement; phloem movement - variable. Sieve tubes carry food from sugar source to sugar sink. Sugar source - plant organ (especially mature leaves) where sugar is being produced by either photosynthesis or the breakdown of starch.

Sugar sink - organ (growing roots, shoots, or fruit) - net consumer or storer of sugar. Storage organ (like a tuber) can be sink in summer (storing for winter) but source during beginning of spring.