Cells come in different sizes and shapes. All cells have a cell membrane which controls what enters and exits the cell, and cytoplasm which is a gel-like material. Only plants, algae, fungi and bacteria have cell walls. The nucleus is the control center of the cell and contains genetic material. Organelles like mitochondria, ribosomes, the endoplasmic reticulum and golgi bodies carry out specific functions within the cell. Mitochondria convert energy from food into a form cells can use, while ribosomes produce proteins and the endoplasmic reticulum transports materials.