Proteins are large, complex macromolecules composed of smaller units called amino acids, which are linked together in long chains. These chains fold into unique three-dimensional structures, and this specific shape dictates the protein’s job within the body. Found in every cell, they are the primary components of living tissues and are fundamental to virtually all biological processes.
Building the Body’s Framework
Many proteins function as structural components, providing shape, strength, and support to cells and tissues. These fibrous proteins act as the internal scaffolding for cells and the connective material for the body at large. They are organized into long strands or sheets, which gives them the durability required for their architectural roles.
One example is collagen, the most abundant protein in mammals, making up a significant portion of bone, skin, tendons, and ligaments. Collagen forms strong, rope-like helices that give connective tissues their tensile strength, allowing them to withstand pulling forces. This protein provides the framework that keeps bones firm and ensures the integrity of the skin.
Another structural protein is keratin, the main component of hair, nails, and the outermost layer of the skin. Keratin is a tough, insoluble protein that forms protective layers against the external environment. Complementing these is elastin, a protein that allows many tissues to resume their shape after stretching or contracting. Found in the walls of arteries, lungs, and skin, elastin’s properties permit these tissues to expand and recoil as needed.
Catalyzing Chemical Reactions
A large group of proteins, known as enzymes, function as biological catalysts. They accelerate the rate of chemical reactions within cells, allowing metabolic processes to occur at a pace necessary to sustain life. Enzymes are not consumed in the reactions they facilitate, meaning a single enzyme molecule can be used repeatedly. Each enzyme is highly specific, acting on only one type of substance, or substrate.
This catalytic activity is observed in the digestive system, where enzymes break down large food molecules into smaller, absorbable units. For instance, amylase in saliva begins the breakdown of carbohydrates into simpler sugars. In the stomach and small intestine, proteases like trypsin dismantle proteins into amino acids, and lipases break down fats into fatty acids and glycerol.
Beyond digestion, enzymes drive nearly every reaction in metabolic pathways. Cellular respiration, the process by which cells convert nutrients like glucose into adenosine triphosphate (ATP), is managed by a complex series of enzymatic steps. These metabolic enzymes work in organized sequences, where the product of one reaction becomes the substrate for the next, ensuring the efficient production of energy.
Transport and Storage of Molecules
Proteins are responsible for the transport and storage of molecules and ions throughout the body. Transport proteins bind to specific molecules to move them across cell membranes or through the bloodstream. Storage proteins hold a reserve of important substances for later use.
A well-known transport protein is hemoglobin, which is found in red blood cells and carries oxygen. This protein binds to oxygen molecules in the lungs, where oxygen concentration is high. It then travels through the bloodstream to tissues, where it releases the oxygen for use in cellular respiration. Hemoglobin also transports a portion of carbon dioxide back to the lungs to be exhaled.
In addition to systemic transport, proteins facilitate movement across the membranes of every cell. Channel and pump proteins are embedded within the cell membrane, creating regulated passageways for ions and nutrients. For storage, the body uses proteins like ferritin, which safely stores iron. Ferritin sequesters iron in a stable, non-reactive form, releasing it only when needed for processes like making new hemoglobin.
Communication and Regulation
Proteins are central to the body’s communication networks, acting as signaling molecules and regulators. They transmit information between cells, tissues, and organs, ensuring different parts of the body work together. This regulatory role extends to controlling gene expression and maintaining the body’s internal balance.
Hormones are an example of protein-based signaling. Insulin, a small protein hormone produced by the pancreas, is released into the bloodstream in response to high blood glucose. Insulin travels throughout the body and binds to receptors on cells, signaling them to absorb glucose from the blood for energy or storage. This action effectively lowers blood sugar.
Proteins also regulate cellular activity at the genetic level. Transcription factors are proteins that bind to specific DNA sequences, controlling the rate at which genetic information is copied into messenger RNA. By activating or repressing gene expression, these proteins dictate which proteins a cell produces. Proteins like albumin in the blood also help regulate the body’s fluid balance by maintaining osmotic pressure.
Movement and Immunity
Proteins are involved in generating movement, from the contraction of muscles to the motility of individual cells. They are also a core part of the body’s defense system, identifying and neutralizing foreign invaders.
The ability to move is powered by contractile proteins. In muscle tissue, the most prominent of these are actin and myosin. These proteins are organized into long filaments that, upon receiving a nerve signal, slide past one another. This sliding action shortens the muscle fiber, generating the force that results in muscle contraction.
The immune system relies on a class of proteins called antibodies to protect the body from pathogens. Antibodies are produced by immune cells and circulate in the bloodstream, where they can identify and bind to specific antigens—molecules on the surface of foreign substances like bacteria and viruses. This binding action neutralizes the invader directly or marks it for destruction by other immune cells.