Water flows through the cell wall and gaps between cells (does not cross plasma membrane) o This is because the cell wall is selectively permeable o The substances that travel through the apoplastic route do not have to go through security “cell membrane” until they hit the Casparian strip, and if they want to go on to the xylem/phloem, they have to pass through the membrane o This is so that toxic substances do not get into the xylem/phloem, because once they’re in, they have access to the whole plant and can kill the plant Symplast: consists of the entire mass of cytosol of all the living cells in a plant, as well as the plasmodesmata (the cytoplasmic channels that interconnect them) o Symplastic route: water and solutes move along the continuum of cytosol o This route requires substances to cross a plasma membrane once, when they first enter the plant o After entering one cell, substances can move from cell to cell via plasmodesmata o Substances also have complete access to the xylem and phloem and the rest of the plant Transmembrane route: water and solutes move out of one cell, across the cell wall, and into the neighboring cell, which may pass them to the next cell in the same way o Requires repeated crossings of plasma membranes as substances exit one cell and enter the next
M o s t substances may use more than one route Short-Distance Transport of Solutes Across Plasma Membranes The selective permeability of the plasma membrane controls the short-distance movement of substances into and out of cells H + ions (rather than Na + in animals) play the primary role in basic transport in plants o For example, in plant cells the membrane potential (voltage across the membrane) is established mainly through the pumping of hydrogen ions by proton pumps, rather than the pumping of sodium ions by sodium-potassium pumps H + is cotransported in plants o Na + is cotransported in animals o During cotransport, plant cells use the energy in the hydrogen ion gradient and membrane potential to drive the active transport of many different solutes o Cotransport with H + is responsible for absorption of neutral solutes, such as sugar sucrose, by phloem cells and other cells o H + /sucrose contransporter couple movement of sucrose against its concentration gradient with movement of H + down its electrochemical gradient Short-Distance Transport of Water Across Plasma Membranes Absorption or loss of water by a cell occurs by osmosis , the diffusion of free water, across a membrane
Water potential: predicts the direction in which water will flow; a quantity that includes the effects of solute concentration and physical pressure Free water moves from regions of high water potential low water potential o If a plant cell/seed is immersed in a solution that has a higher water potential, water move into the cell or seed, causing it to expand Ψ = water potential Ψ of pure water in a container open to the atmosphere under standard conditions is 0 mPa. How Solutes and Pressure Affect Water Potential Ψ = Ψ s + Ψ P Ψ = water potential Ψ s = solute potential (osmotic potential) – affects the direction of osmosis Ψ P
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