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LS. 3 The Cell In Its Environment The cell membrane is selectively permeable, which means that some substances can pass through it. Substances that can move into and out of a cell do so by one of two methods: Passive transport or active transport Passive Transport is the movement of materials through the cell membrane without using energy. •Diffusion – the process by which molecules tend to move from an area of high concentration to an area of lower concentration. (No energy required.) Osmosis – the diffusion of water molecules through a selectively permeable membrane. Osmosis is important because cells cannot function properly without water. (No energy required.) Facilitated Diffusion is a process in which proteins in the cell membrane form channels through which molecules such as sugar pass. Facilitated Diffusion uses no cellular energy and is another form of passive transport. •Osmosis and Diffusion are both types of passive transport. •Active transport – the movement of materials through a membrane using energy. (Low concentration to high concentration) The main difference between passive and active transport is that active requires the cell to use energy while passive does not. Two methods of active transport are: Endocytosis and Exocytosis