115

Apuntes

115 When we look at a cell through a microscope or analyze its electrical or biochemi cal activity, we are, in essence, observing proteins. Proteins constitute most of a cell’s dry mass. They are not only the cell’s building blocks; they also execute the majority of the cell’s functions. Proteins that are enzymes provide the intricate molecular surfaces inside a cell that catalyze its many chemical reactions. Pro teins embedded in the plasma membrane form channels and pumps that control the passage of small molecules into and out of the cell. Other proteins carry mes sages from one cell to another or act as signal integrators that relay sets of signals inward from the plasma membrane to the cell nucleus. Yet others serve as tiny molecular machines with moving parts: kinesin, for example, propels organelles through the cytoplasm; topoisomerase can untangle knotted DNA molecules. Other specialized proteins act as antibodies, toxins, hormones, antifreeze mole cules, elastic fibers, ropes, or sources of luminescence. Before we can hope to understand how genes work, how muscles contract, how nerves conduct elec tricity, how embryos develop, or how our bodies function, we must attain a d...