... is selectively permeable, which means that only certain substances may pass through the membrane. Cell membranes consist mainly of phospholipids. These phospholipids have two parts; a polar phosphate "head" and two nonpolar fatty acid "tails." These molecules are arranged in a way that is known as a bilayer' (or double layer) so that the polar heads face ... means that the particles move further from the concentrated area, resulting in fewer within a particular space. Another form of diffusion is known as facilitated diffusion'. This process uses a carrier, generally in the form of a pro...

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• Grade Level: Undergraduate