As energy flows within a cell the materials needed for growth, repair and replacement also flow. Single-cell organisms are surrounded by the water in which they live or in the fluids of their host's body. Here the cell uses energy to get molecules into or out of the cell against the gradient.
Active transport is a little bit like going the wrong way on a one way street.
Most materials move by simple diffusion through the semi permeable membrane surrounding the cell. A semi permeable membrane only allows certain types of molecules to enter or leave the cell. Other membrane pumps move glucose, amino acids and other raw materials into the cell. Cells require energy and raw materials so that they can perform physical tasks including movement, packaging and exporting materials, dividing and reproducing.