An animal cell is a form of cell of the Eukaryota that makes up many tissues in animals. The animal cell is distinct from other eukaryotes, particularly plant cells, as they lack cell walls and chloroplasts, and they have smaller vacuoles. Due to the lack of a rigid cell wall, animal cells can adopt a variety of shapes, and a phagocytic cell can even engulf other structures. There are many different cell types. For instance, there are approximately 210 distinct cell types in the adult human body.
Mitochondria are the organelles that function as the cell "powerhouse", generating ATP, the universal form of energy used by all cells. It converts food nutrients such as glucose, to a fuel (ATP) that the cells of the body can use. Mitochondria are tiny sac-like structures found near the nucleus. Little shelves called cristae are formed from folds in the inner membrane. Cells that are metabolically active such as muscle, liver and kidney cells have high energy requirements and therefore have more mitochondria.
Mitochondria are unique in that they have their own mitochondrial DNA (separate from the DNA that is in the nucleus). The Theory of Endosymbiosis posits that eukaryotes evolved from one cell living inside another cell, because the resulting mitochondria share many traits with current free-living bacteria (similar chromosomes and ribosomes and measured by modern molecular sequencing.