GED Science Practice Test: Proteins

Proteins are large organic molecules that have a broad array of roles within cells.  They are a part of cell membranes, act as enzymes in cellular reactions, transport materials, generate movement, and are an important part of our immune system, which protects us from disease and infection.  Animals not only take in proteins as nutrition from animal sources and some plant sources, but they make certain proteins required by cells for functioning.

Chemically, proteins are very complex molecules that are made up of strings of amino acids, like beads on a long necklace. A dipeptide has two amino acids linked together, a polypeptide, more than two. (Notice the similarity in naming to carbohydrates, which include disaccharides, polysaccharides, etc.). Although there are hundreds of different amino acids that exist, most organisms use only the 20 amino acids in the chart below to build proteins. Plants can make all 20 of the required amino acids, while humans can only make 10 of the require amino acids.  The 10 amino acids that humans cannot make are called essential amino acids.

Biochemists describe proteins in terms of their structure.  A protein’s primary structure is its sequence of amino acids. A protein’s secondary structure refers to how amino acids in the chain interact with other amino acids to form a repeated structure or shape.  The tertiary structure of the protein refers to how a protein’s shape folds on itself to form a 3-D structure.  The quaternary structure of the protein refers to how different chains of proteins interact with each other to form complexes.  The following diagram shows the different levels of structure for proteins:


The following diagram shows the quaternary structure of hemoglobin, which is a complex protein which carries oxygen in our blood.  Notice that hemoglobin is made up of four protein chains.  Unless that hemoglobin protein has a mistake in it (like in people with sickle cell anemia), the hemoglobin protein will have the same primary, secondary, tertiary, and quaternary structure in every hemoglobin molecule:


Enzymes are a particularly important type of protein that function as catalysts, meaning that they significantly accelerate the rate of certain metabolic reactions, such as the digestion of food or the synthesis of DNA. However, they do not directly participate in the reaction that they are catalyzing, and can be used repeatedly to catalyze different reactions.  Enzymes are very specific about the reactions they catalyze – most can only catalyze one kind of reaction – and each kind of enzyme has a unique tertiary or quaternary structure that allows it to fit with the substrates, or reactants, in the reaction it is catalyzing. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. The following diagram shows how an enzyme breaks down the disaccharide, sucrose, into the monosaccharides, glucose and fructose:



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