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Cells need transport of materials across cell membrane simply because they cannot produce everything they need for growth and sustenance and they also don't need everything they produce. In simpler terms, cells requires substrate (food) and nutrients and need to get rid of excess materials (beyond their utility in the cell). Thus transport of materials across the membrane is necessary for cell's growth and survival (a buildup of excess/waste material will be detrimental to cell). The cell membrane helps in this process. This membrane is selectively permeable, that is, it only allows entry of certain molecules in the cell, while preventing access to all others. The transport of material can be passive (without expenditure of energy) or could be active (requires expenditure of energy by cell).
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