Transport213-page3 - + plays a vital role in opening and...

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Transport in Plants - 3 Review of Membrane Transport Mechanisms The plasma membrane is selectively permeable. Materials that enter and leave a cell must pass through the membrane. The direction of water movement is dependent on the gradient between a cell’s internal and external environment. Although membranes are permeable to water, small membrane transport proteins, called aquaporins, also facilitate the diffusion of water, particularly across the central plant vacuole tonoplast. Most solutes, even if permeable, do not diffuse readily, and rely on transport proteins or membrane protein transport channels for facilitated diffusion. Recall that transport proteins have a selective binding site for the solute to attach on one side of the membrane and then carry the solute through the membrane releasing it on the other side. Channels allow free passage of specific substances, and some channels are gated to control solute passage. K + channels are common in the membranes of plant cells, and K
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Unformatted text preview: + plays a vital role in opening and closing of stomata (see later) . Plant cells also rely on active transport. The H + proton pump is a common use of active transport. The cell creates a proton gradient by pumping H + out of the cell, which is a source of potential energy (for the passive diffusion of the H+ back into the cell through the membrane), and creates the membrane potential (the differential charge between the interior, more negative charge, and external more positive charge), a second source of potential energy. Proton Gradient and Membrane Potential Membrane Potential favors diffusion of + ions The membrane potential facilitates the movement of positively charged ions into cells. The ion movement is “passive” but is a consequence of the active movement of H + through the proton pump that maintains the membrane potential. The proton pump moves many minerals into cells in quantities greater than what is found in the environment....
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This note was uploaded on 01/08/2012 for the course BIO 213 taught by Professor Makina during the Fall '09 term at SUNY Stony Brook.

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