Mineral uptake
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In plants, minerals follow the same path as water. Some mineral ions diffuse in-between the cells. In contrast to water, some minerals are actively taken up by plant cells. Mineral nutrient concentration in roots may be 10,000 times more than in surrounding soil. During transport throughout a plant, minerals can exit xylem and enter cells that require them. Mineral ions cross plasma membranes by a chemiosmotic mechanism. Plants absorb minerals in ionic form: nitrate (NO3-), phosphate (HPO4-) and potassium ions (K+); all have difficulty crossing a charged plasma membrane.
It has long been known plants expend energy to actively take up and concentrate mineral ions. Proton pump hydrolyzes ATP to transport H+ ions out of cell; this sets up an electrochemical gradient that causes positive ions to flow into cells. Negative ions are carried across the plasma membrane in conjunction with H+ ions as H+ ions diffuse down their concentration gradient.
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