Proteins are a ubiquitous part of the cell and to many are considered the basic building block of life. While a number of other molecules are necessary for life, including nucleic acids, carbohydrates and lipids, the sheer number of proteins and their huge variety of functions make them especially important when considering cellular biology. They are involved in almost every process in the cell, from forming structural components to catalyzing reactions to cell signaling and interactions.
Proteins are made of amino acids, which each have their own size, charge and hydrophobicity properties. These wide range of properties allow cells to build proteins with a wide range of functions, binding capabilities and reactivities. Because of this flexibility, proteins can be built that function in many ways.
So in what ways are proteins the building blocks of the cell?
Enzymes are proteins that are instrumental in controlling the rates of chemical reactions in the cell. Without this control, reactions would occur spontaneously and without limit. Enzymes are generally specific to just one or a few reactions in larger pathways and have very tight control. Enzymes are instrumental in some critical processes, including metabolism and DNA replication.
There are also a number of proteins that exist to function as cell signaling components. Many of these are embedded into the membrane of the cell to allow for cell-cell interactions. They also may function to bind ligands such as antibodies, which in turn causes a signaling cascade that allows the cell to respond to exterior stimuli.
Related to cell signaling is the role of proteins in the transport of molecules into the cell, out of the cell, and within the cell.. A whole class of proteins exist that move ions in and out of the cell through pores and ion transporters.
Many proteins are involved in cellular structure, including proteins like actin and tubulin which form the cytoskeleton. Some proteins are involved in cell motility, like myosin, which allows the cell to contract and stretch to move and expand. This is critical for cell replication, when the cell has to divide evenly so that daughter cells are fully functional. It is also important for muscle contractions in higher level organisms, where muscle cells must contract and expand to allow for whole organism movement.
Other proteins are used by cells to form extracellular connective or structural tissue, such as collagen, elastin and keratin which forms ligaments, cartilage, and other connective and filamentous structures.
As you can see from the functions listed, proteins are involved in almost every cellular process from replication, division, metabolism, structure, motility and signaling. With that in mind, it is easy to see why proteins can be easily nicknamed the "building blocks of life."