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The structure that you are referring to is the cell membrane. It is a double membrane consisting of two layers of lipids with protein molecules in between. There are pore spaces and channels in the membrane for transport to occur between the environment outside the cell and the environment inside the cell. There are special protein molecules that have receptor sites and aid in transport across the membrane. This lipid bilayer is a selectively permeable membrane. It allows the passage of materials across the membrane if certain conditions are met. There must be a concentration gradient, or difference in concentration on the outside and inside of the membrane. This will allow materials to enter or leave the cell, as long as the size of the molecules are small enough. This type of transport is called diffusion and requires no additional energy. However, the cell membrane sometimes folds in during pinocytosis and transports larger molecules into the cell. Sometimes a membrane may surround and engulf a large particle the cell needs. Both of these methods are active transport and require the use of additional energy. The lipid bilayer that makes up the cell membrane is the boundary between the cell's surroundings and its interior.
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