Ch36WordLectureOutline

Ch36WordLectureOutline - CHAPTER 36 TRANSPORT IN PLANTS...

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CHAPTER 36 TRANSPORT IN PLANTS Introduction The algal ancestors of plants were completely immersed in water and dissolved minerals. The evolutionary journey onto land involved the differentiation of the plant body into roots, which absorb water and minerals from the soil, and shoots, which are exposed to light and atmospheric CO 2 . This morphological solution created a new problem: the need to transport materials between roots and shoots. Roots and shoots are bridged by vascular tissues that transport sap throughout the plant body. A. An Overview of Transport Mechanisms in Plants Transport in plants occurs on three levels: (1) The uptake and loss of water and solutes by individual cells. (2) Short-distance transport of substances from cell to cell at the level of tissues or organs. (3) Long-distance transport of sap within xylem and phloem at the level of the whole plant. 1. Transport at the cellular level depends on the selective permeability of membranes T he selective permeability of a plant cell’s plasma membrane controls the movement of solutes between the cell and the extracellular solution. Molecules tend to move down their concentration gradient, and when this occurs across a membrane it is called passive transport and occurs without the direct expenditure of metabolic energy by the cell.
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Transport proteins embedded in the membrane can speed movement across the membrane. Some transport proteins bind selectively to a solute on one side of the membrane and release it on the opposite side. Others act as selective channels , providing a selective passageway across the membrane. For example, the membranes of most plant cells have potassium channels that allow potassium ions (K + ) to pass, but not similar ions, such as sodium (Na + ). Some channels are gated, opening or closing in response to certain environmental or biochemical stimuli. In active transport, solutes are pumped across membranes against their electrochemical gradients. The cell must expend metabolic energy, usually in the form of ATP, to transport solutes “uphill”—counter to the direction in which the solute diffuses. Transport proteins that simply facilitate diffusion cannot perform active transport. Active transporters are a special class of membrane proteins, each responsible for pumping specific solutes. 2. Proton pumps play a central role in transport across plant membranes The most important active transporter in the plasma membrane of plant cells is the proton pump . It hydrolyzes ATP and uses the released energy to pump hydrogen ions (H + ) out of the cell. This creates a proton gradient because the H + concentration is higher outside the cell than inside. It also creates a membrane
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Ch36WordLectureOutline - CHAPTER 36 TRANSPORT IN PLANTS...

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