There are two main types of cells: prokaryotic cells which lack nuclei and eukaryotic cells which contain nuclei. Eukaryotic cells include plant and animal cells. All cells are surrounded by a plasma membrane made of phospholipids, carbohydrates, proteins and cholesterol that contains organelles like the nucleus, mitochondria and chloroplasts. Transport across the cell membrane can occur through passive diffusion, osmosis and facilitated diffusion which don't require energy or through active transport which uses ATP-powered transport proteins to move molecules against their concentration gradient.

1. Review of Cells