Animal cells are the basic building blocks of all animals. They are eukaryotic cells, meaning they have a nucleus that contains genetic material. Unlike plant cells, animal cells do not have a cell wall; instead, they have a flexible cell membrane that allows them to change shape. Inside, they contain various organelles, such as mitochondria for energy production and ribosomes for protein synthesis. These cells come in many shapes and sizes, depending on their function. For example, muscle cells are long and thin to help with movement, while nerve cells have long extensions to transmit signals. Together, animal cells work to form tissues and organs, enabling the body to function properly.