Proteins rule life. Essentially all chemical reactions that sustain life are catalyzed by enzymes, which are proteins. Other proteins play vital roles in transport processes, ranging from electron transfer to carrying oxygen, hormones and other substances. Membrane proteins control the passage of ions and metabolites across membranes that separate biological cells and cell organelles from their surroundings. Immunoproteins, the antibodies, defend the organism against foreign invaders. In muscles, proteins take care of converting chemical energy into mechanical work and the elasticity and structure of connective tissues and bones are provided by proteins.