Because of their tremendous structural diversity, proteins can serve a wider variety of functions than any other type of molecule in the body. Many proteins, for example, contribute significantly to the structure of different tissues and in this way play a passive role in the functions of these tissues. Examples of such structural proteins include collagen (fig. 2.28) and keratin. Collagen is a fibrous protein that provides tensile strength to connective tissues, such as tendons and ligaments. Keratin is found in the outer layer of dead cells in the epidermis, where it prevents water loss through the skin.
■ Figure 2.28 A photomicrograph of collagenous fibers within connective tissue. Collagen proteins strengthen the connective tissues.
Many proteins play a more active role in the body, where specificity of structure and function is required. Enzymes and antibodies, for example, are proteins—no other type of molecule could provide the vast array of different structures needed for their tremendously varied functions. As another example, proteins in cell membranes may serve as receptors for specific regulator molecules (such as hormones) and as carriers for transport of specific molecules across the membrane. Proteins provide the diversity of shape and chemical properties required by these functions.
Test Yourself Before You Continue
1. Write the general formula for an amino acid and describe how amino acids differ from one another.
2. Describe and account for the different levels of protein structure.
3. Describe the different categories of protein function in the body and explain why proteins can serve functions that are so diverse.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.