Membrane proteins are associated with or integrated in membranes of cells and organelles, where they fulfil many functions vital for cellular physiology. They transport solutes across membranes to provide building blocks for maintenance, to achieve homeostasis, to generate metabolic energy or to link biochemical pathways on either side of the membrane. They are involved in signalling required for communication within cells, and between cells and their environment. They also have important roles in division, reshaping, recognition and interconnection of cells.