Cells are "the unit of life" or "the building blocks" of which living things are made. Though all cells nearly have the same basic structure, the plant cell and the animal cell vary in some characteristic features. For example, plant cells are surrounded by a rigid layer of cellulose, which gives a great deal of support to the plant. On the other hand, almost all types of animal cell are naked, surrounded by only semi-permeable membrane. Plant cells also have plastids, which allow plants to capture the energy of the Sun in energy-rich molecules.