Steroid Hormones

Steroid hormones are soluble in the lipids that make up the bulk of cell membranes. For this reason, steroid hormones can easily diffuse into their target cells. Once inside a target cell, steroid hormones combine (usually within the nucleus) with specific protein receptors. The resulting hormone-receptor complex binds within the nucleus to a particular region of the DNA and activates specific genes. The activated genes, in turn, are transcribed into messenger RNA (mRNA), which enters the cytoplasm where it directs synthesis of specific proteins. These proteins bring about the cellular changes associated with the particular hormone (fig. 13.4, table 13.3, and Clinical Application 13.1). An example is the steroid hormone aldosterone (al'do-ster-on", al-dos'ter-on), from the adrenal gland, whose action is to stimulate sodium retention by the kidneys. In response to aldosterone, cells that form tubules within the kidney begin to synthesize more Na+/K+ pumps, the proteins that actively

Steroid Hormones Cells

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^Hormone-receptor t complex ure m

Figure

(a) A steroid hormone passes through a cell membrane and (b) combines with a protein receptor, usually in the nucleus. (c) The hormone-receptor complex activates messenger RNA synthesis. (d) The mRNA leaves the nucleus and (e) guides protein synthesis. Note: In the bloodstream, most lipid-soluble steroids are bound to proteins. Only the few that are not bound are free to enter cells, as shown here.

(a) A steroid hormone passes through a cell membrane and (b) combines with a protein receptor, usually in the nucleus. (c) The hormone-receptor complex activates messenger RNA synthesis. (d) The mRNA leaves the nucleus and (e) guides protein synthesis. Note: In the bloodstream, most lipid-soluble steroids are bound to proteins. Only the few that are not bound are free to enter cells, as shown here.

1. Endocrine gland secretes steroid hormone.

2. Steroid hormone diffuses through target cell membrane and enters nucleus.

1. Endocrine gland secretes steroid hormone.

2. Steroid hormone diffuses through target cell membrane and enters nucleus.

3. Hormone combines with a receptor molecule.

4. Steroid hormone-receptor complex binds to DNA and promotes transcription of messenger RNA.

5. Messenger RNA enters the cytoplasm and directs protein synthesis.

6. Newly synthesized proteins produce hormone's specific effects.

transport these ions across the cell membrane, retaining sodium.

Essentials of Human Physiology

Essentials of Human Physiology

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.

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