Detail of lipids in cell membranes

The cell membranes of single-celled marine life are mostly made of fatty compounds called lipids. They have a distinctive structure with hydrophilic (water-loving) “heads” and hydrophobic (water-avoiding) “tails” and are aligned in a double layer, with the heads facing toward the watery interior and exterior of the cell. This creates a membrane that acts as a gateway for fluids and other molecules passing in either direction. Different kinds of microbes can have different types of lipids in their membranes. (Illustration by Amy Caracappa-Qubeck, © Woods Hole Oceanographic Institution)