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Every cell is surrounded by a plasma membrane. The cell membrane is selectively permeable- only certain substances can cross it freely. The structure of the membrane allows materials to move across it by 2 main means: active and passive transport. Password transport involves two processes: Diffusion and Osmosis. Passive transport allows some substances such gases and water to move into and out of the cell without using energy. Active transport includes processes such as "ion pumps", "…

…concentrated) solution to a stronger (more concentrated) solution. It is the way water enters and leaves a cell. Osmosis is a special case of the diffusion of water. Osmosis occurs when water diffuses across a semi permeable membrane. Semi-permeable membranes will let small molecules pass through it easily, but will hold back larger molecules like sucrose's. It acts as if it has tiny holes in it, too small for the larger molecules to squeeze through.