|Human Brain Cells
All cells share four characteristics. First, every cell is enclosed in a thin cell membrane, which provides shape and acts as a barrier between the cell and its environment. The membrane is semipermeable, composed largely of lipids (fats, oils, and fatty substances such as cholesterol), with embedded proteins that regulate the transport of molecules into and out of the cell.
Second, all cells are filled with cytoplasm. Cytosol, the fluid portion of cytoplasm, contains nutrients, enzymes, and other dissolved materials vital for cell metabolism. Also in the cytoplasm are specialized structures called organelles, held in place by a network of protein filaments.