Glands & Types of Hormones

Need help preparing for the Biology section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Glands & Types of Hormones of the Endocrine System. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!

The endocrine system is made up of various glands and organs that help to regulate and control many of the body’s functions. In simple terms, glands are organs that synthesize substances for release. There are two different types of glands – endocrine glands and exocrine glands. The main distinction between the two types of glands is where they secrete their substances. The endocrine glands secrete their substances into the bloodstream, while the exocrine glands release substances onto epithelial surfaces.

Endocrine Glands

There are various endocrine glands throughout the body that secrete different types of hormones into the bloodstream. For example, the pancreas is an endocrine gland that secretes insulin and glucagon into the bloodstream to help regulate blood glucose concentrations. The thyroid gland releases thyroid hormone, which regulates basal metabolic rate. Similarly, the parathyroid glands secrete parathyroid hormone, which helps regulate blood calcium concentrations. The hypothalamus and pituitary gland secrete a variety of different hormones important for physiology, while the ovaries and testes secrete hormones important for reproductive processes.

Exocrine Glands

Exocrine glands secrete substances onto epithelial surfaces. One example of an epithelial surface is the skin. The sweat glands and sebaceous glands secrete sweat and oil onto the surface of the skin, respectively. Also, the mammary glands secrete milk, which is released onto the skin surface. Epithelial surfaces can also include surfaces within the body. One example is the gastrointestinal tract. Although the pancreas is an endocrine gland, it also has exocrine functions as well. It secretes digestive enzymes onto the surface of the gastrointestinal tract. The gastric glands secrete acid, pepsinogen, and mucus onto the stomach and other components of the gastrointestinal tract. Furthermore, the salivary glands secrete saliva into the mouth, which is also part of the gastrointestinal tract. Another gland, the prostate gland, secretes a component of semen that passes through the urinary tract in males.

Hormones

Hormones are signaling molecules released in the blood that regulate physiology and behavior. As stated earlier, hormones are released by endocrine glands. There are three types of hormones – peptide hormones, steroid hormones, and tyrosine derivatives. It is essential to understand the difference between all three types of hormones for the MCAT exam.

The two main types of hormones are peptide hormones and steroid hormones. Peptide hormones and steroid hormones differ. The main difference between these two types of hormones is the type of receptors they bind to and how they elicit responses in the cell. Peptide hormones are polar molecules, and in this way, they cannot pass through the phospholipid bilayer on cells. Instead, they bind to cell surface receptors, which activate a signal transduction pathway, leading to the phosphorylation or dephosphorylation of different proteins for different results. Steroid hormones, on the other hand, are nonpolar molecules and can pass directly through the cell membrane. In this way, they bind to intracellular receptors that will ultimately enter the nucleus and modify transcription. Like peptide hormones, steroid hormones will also lead to a change in cells to produce a response.

Aside from the receptor type they bind to, there are additional differences between peptide and steroid hormones. Peptide hormones, as their name implies, are made of amino acids. These amino acid sequences can be of varying sizes. Some sequences can consist of a few amino acids, while others may consist of many amino acids. In this way, peptide hormones can be different sizes relative to each other. Also, it is important to note that when peptide hormones bind to cell surface receptors, they often drive signaling cascades that involve second messengers. Signaling cascades allow for signal amplification, so the binding of a single peptide hormone can result in a larger response in a cell.

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