Proteins for Transport of Water and Mineral Nutrients across the Membranes of Plant Cells

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Abstract
The plasma membrane of the plant cell is a selectively permeable barrier that ensures the entry of essential ions and metabolites into the cell. Together with the vacuolar membrane (tonoplast), it permits the cytoplasm to maintain intracellular homeostasis. These membranes consist primarily of phospholipid bilayers with transmembrane proteins that permit the traversing of water, ions, and metabolites and the maintenance of a cytosolic pH that is one to three units higher than that of the cell exterior or the vacuole. Pure phospholipid bilayers are permeable to gases, such as O2 and CO2, but they are barely permeable to water and nearly impermeable to inorganic ions and other hydrophilic solutes, such as sucrose and amino acids. Proteins are required to transport protons, inorganic ions, and organic solutes across the plasma membrane and the tonoplast at rates sufficient to meet the needs of the cell.