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Medicine LibreTexts

1.03: Functions of Human Life

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  • Learning Objectives

    • Explain the importance of organization to the function of the human organism
    • Distinguish between metabolism, anabolism, and catabolism
    • Provide at least two examples of human responsiveness and human movement
    • Compare and contrast growth, differentiation, and reproduction

    The different organ systems each have different functions and therefore unique roles to perform in physiology. These many functions can be summarized in terms of a few that we might consider definitive of human life: organization, metabolism, responsiveness, movement, development, and reproduction.


    A human body consists of trillions of cells organized in a way that maintains distinct internal compartments. These compartments keep body cells separated from external environmental threats and keep the cells moist and nourished. They also separate internal body fluids from the countless microorganisms that grow on body surfaces, including the lining of certain tracts, or passageways. The intestinal tract, for example, is home to even more bacteria cells than the total of all human cells in the body, yet these bacteria are outside the body and cannot be allowed to circulate freely inside the body.

    Cells, for example, have a cell membrane (also referred to as the plasma membrane) that keeps the intracellular environment—the fluids and organelles—separate from the extracellular environment. Blood vessels keep blood inside a closed circulatory system, and nerves and muscles are wrapped in connective tissue sheaths that separate them from surrounding structures. In the chest and abdomen, a variety of internal membranes keep major organs such as the lungs, heart, and kidneys separate from others.

    The body’s largest organ system is the integumentary system, which includes the skin and its associated structures, such as hair and nails. The surface tissue of skin is a barrier that protects internal structures and fluids from potentially harmful microorganisms and other toxins.


    The first law of thermodynamics holds that energy can neither be created nor destroyed—it can only change form. Your basic function as an organism is to consume (ingest) energy and molecules in the foods you eat, convert some of it into fuel for movement, sustain your body functions, and build and maintain your body structures. There are two types of reactions that accomplish this:anabolism and catabolism.

    • Anabolism is the process whereby smaller, simpler molecules are combined into larger, more complex substances. Your body can assemble, by utilizing energy, the complex chemicals it needs by combining small molecules derived from the foods you eat
    • Catabolism is the process by which larger more complex substances are broken down into smaller simpler molecules. Catabolism releases energy. The complex molecules found in foods are broken down so the body can use their parts to assemble the structures and substances needed for life.

    Taken together, these two processes are called metabolism. Metabolism is the sum of all anabolic and catabolic reactions that take place in the body (Figure \(\PageIndex{1}\)). Both anabolism and catabolism occur simultaneously and continuously to keep you alive.

    Figure \(\PageIndex{1}\): Metabolism. Anabolic reactions are building reactions, and they consume energy. Catabolic reactions break materials down and release energy. Metabolism includes both anabolic and catabolic reactions.


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    Figure \(\PageIndex{2}\): Marathon Runners. Runners demonstrate two characteristics of living humans—responsiveness and movement. Anatomic structures and physiological processes allow runners to coordinate the action of muscle groups and sweat in response to rising internal body temperature. (credit: Phil Roeder/flickr)

    Development, growth and reproduction




    Chapter Review


    Interactive Link Questions


    Answer: Fatty acid catabolism.

    Review Questions

    Q. Metabolism can be defined as the ________.

    A. adjustment by an organism to external or internal changes

    B. process whereby all unspecialized cells become specialized to perform distinct functions

    C. process whereby new cells are formed to replace worn-out cells

    D. sum of all chemical reactions in an organism

    Answer: D

    Q. Adenosine triphosphate (ATP) is an important molecule because it ________.

    A. is the result of catabolism

    B. release energy in uncontrolled bursts

    C. stores energy for use by body cells

    D. All of the above

    Answer: C

    Q. Cancer cells can be characterized as “generic” cells that perform no specialized body function. Thus cancer cells lack ________.

    A. differentiation

    B. reproduction

    C. responsiveness

    D. both reproduction and responsiveness

    Answer: A

    Critical Thinking Questions

    Q. Explain why the smell of smoke when you are sitting at a campfire does not trigger alarm, but the smell of smoke in your residence hall does.

    A. When you are sitting at a campfire, your sense of smell adapts to the smell of smoke. Only if that smell were to suddenly and dramatically intensify would you be likely to notice and respond. In contrast, the smell of even a trace of smoke would be new and highly unusual in your residence hall, and would be perceived as danger.

    Q. Identify three different ways that growth can occur in the human body.

    A. Growth can occur by increasing the number of existing cells, increasing the size of existing cells, or increasing the amount of non-cellular material around cells.


    assembly of more complex molecules from simpler molecules
    breaking down of more complex molecules into simpler molecules
    changes an organism goes through during its life
    process by which unspecialized cells become specialized in structure and function
    process of increasing in size
    sum of all of the body’s chemical reactions
    process by which worn-out cells are replaced
    process by which new organisms are generated
    ability of an organisms or a system to adjust to changes in conditions