Biology Workbook For Dummies
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In addition to the nervous system’s electrical signals, animals also regulate their bodies with chemical messengers called hormones. Endocrine glands produce the hormones and then release them into the blood to travel through the body until they reach their target cells, the cells that respond to the hormone. For a cell to respond to a particular hormone, it must have receptors for that hormone.

Hormones in vertebrates can be divided into two groups:

  • Peptide hormones, such as insulin, are short chains of amino acids; think of them as very small proteins. Peptide hormones are hydrophilic (water-loving), so they don’t pass easily through cell membranes. Cells put the receptors for peptide hormones in their plasma membranes so the hormones can bind at the cell surface.

  • Steroid hormones, such as testosterone and estrogen, are lipids, so they’re hydrophobic (water-fearing) and can pass easily through the hydrophobic layer of the plasma membrane and enter cells. Thus, the receptors for steroid hormones are located inside the cell.

When a signal like a hormone reaches a cell and is relayed to molecules within the cell, scientists call it signal transduction.

Hormones trigger cellular responses in three basic steps:

  1. Reception: The hormone binds to its receptor.

    Hydrophilic hormones typically bind to receptors on the cell surface, whereas hydrophobic hormones can pass through the plasma membrane and bind to a receptor inside the cell.

  2. Signal transduction: When the hormone binds to its receptor, it causes a change in the receptor that is passed to molecules inside the cell.

    After the receptor changes, it causes changes in another molecule, which causes changes in another molecule, and so on. Scientists call the molecules in this signaling chain relay molecules or second messengers.

  3. Cellular response: A final member of the relay molecules causes a change in the cell’s behavior.

    The relay molecule may cause the cell to access a gene to build a new protein or to stop making a protein. Or the relay molecule may interact with an enzyme to increase or stop its activity.

The endocrine system is the system that handles hormone production and secretion within an organism. It keeps a check on cellular processes and the bloodstream’s components and can make adjustments as necessary.

For questions 1–6, use the following terms to label the events of transduction of a hormone signal shown in the figure.


a. Hormone (primary messenger)

b. Relay proteins (secondary messengers)

c. DNA

d. Cytoplasm

e. Nucleus

f. Receptor

For question 7, use different colored pencils or highlighters to mark the sections of the figure that show the major events that occur during the transduction of a hormone signal:

a. Reception of hormone signal

b. Signal transduction

c. Response by changes in cellular activity

The following is how the figure should be labeled:

  1. a. Hormone (primary messenger)

  2. f. Receptor

  3. b. Relay proteins (secondary messengers)

  4. c. DNA

  5. e. Nucleus

  6. d. Cytoplasm

  7. The following is how the figure should be colored:

    • a. Reception of hormone signal should highlight the hormone and the receptor

    • b. Signal transduction should highlight everything from the inside edge of the receptor through the relay proteins up to the DNA

    • c. Response by changes in cellular activity should highlight the arrow that moves off the side of the relay proteins

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