Plant cells differ from animal cells in their possession of chloroplasts, a cell wall, a large central vacuole and their relative lack of an internal cytoskeleton. Plant cells gain their form and rigidity from the pressure of their large, water-filled vacuoles against their tough cellulose cell walls. Animal cells must rely on protein-based skeletal structures within the cell to maintain their shape.
Animal cells vary widely in size and shape. Although the typical animal cell is roughly spherical, many are flattened or irregular in shape. Animal cells are surrounded with a pliable sheath called a plasma membrane.
A plant cell is essentially like a Mylar balloon filled with water. The cells resist deformation because of the pressure of the fluids against the tough container. If some of the water is released, the balloon droops and is much easier to deform. In the same way, plants that lack sufficient water wilt, becoming floppy and shrivel as their large vacuoles empty of water. In mature plant cells, these vacuoles can occupy up to 90 percent of the total volume of the cell. Plant cells have an internal cytoskeleton, but it is less extensive than that in an animal cell. It is used to maintain certain structures within the cell, rather than the shape of the cell as a whole.
Plants also possess chloroplasts, the organelles that allow them to create carbohydrates from water, carbon dioxide and sunlight.
The metabolisms of the cell types also differ. Plants are autotrophic and produce nutrients internally from sunlight. Animal cells require nutrients outside themselves and cannot synthesize proteins without an external food source.