Concepts 7.3 and 7.4 • Facilitated diffusion is a type of passive transport, since the molecules are diffusing through the membrane and the cell expends no energy to move them. • Diffusion is the tendency for particles of any kind to spread out from where they are more concentrated to where they are less concentrated. • This process can also be described as molecules moving down their concentration gradient. • Diffusion across a biological membrane is called passive transport, since the cell expends no energy to move the molecules. • The plasma membrane is permeable to water molecules, and the movement of water into and out of cells is critical to life. • Diffusion of water molecules across a selectively permeable membrane is a special kind of passive transport called osmosis. • • Oxygen and carbon dioxide molecules move into and out of cells by passive transport.
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