Despite this, water molecules can pass freely through the bilayer, as can oxygen and carbon dioxide. Ions such as sodium or chloride cannot pass through, however, and neither can larger molecules such as sugars or
amino acids. Instead, these materials must pass through the membrane via specialized proteins. This selective permeability allows the membrane to control the flow of materials in and out of the cell and its organelles.
Proteins and Membrane Transport
Proteins are long chains of amino acids. They have unique shapes and chemical properties that dictate their diverse functions. Proteins govern the range of materials that enter and leave the cell, relay signals from the environment to the interior, and participate in many metabolic reactions, harvesting or harnessing energy to transform raw materials into the molecules needed by the cell for growth, repair, or other functions. Cytoskeleton proteins give the cell its structure. Approximately half the weight of a membrane is due to the proteins embedded in it. Proteins give each organelle, and the cell as a whole, its unique character.
As noted, ions cannot pass freely through the cell's phospholipid membrane. Instead, most ions flow through special channels built from multiple protein subunits that together form a pore from one side of the membrane to the other.
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