Question

• Distinguish between water-soluble (hydrophilic) and lipid-soluble (hydrophobic) hormones. List examples of each. Describe how each are secreted from cells, travel via the circulatory system, receptor location on target cells, and mechanism(s) for eliciting cellular responses.

• What are endocrine glands? List several examples. (Contrast these with exocrine glands. You learned about several of these when you studied digestion.) Note that many endocrine glands are also organs with other functions.

Answer 1

HYDROPHILIC vs HYDROPHOBIC HORMONES

Hydrophilic hormones (water-soluble) cannot diffuse through membranes. So they are secreted by exocytosis. They circulate dissolved in plasma. Examples of hydrophilic hormones include glycoproteins and peptide hormones e.g. thyroid-stimulating hormone, follicle-stimulating hormone, luteinizing hormone and insulin.

Hydrophilic hormones are not lipid-soluble and cannot diffuse through cell membranes. So, they interact with cell membrane receptors like G-protein cuopled receptors which are associated with a G protein, which becomes activated when the hormone binds to the receptor. This leads to activation of second-messenger systems resulting in a variety of intracellular consequences.

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Hydrophobic hormones (lipophilic) are able to diffuse through the membrane. So, they are secreted by diffusion into the blood. Being insoluble in plasma, they circulate bound to plasma proteins. Examples of hydrophobic hormones include steroid hormones like testosterone, estrogens, progesterone, aldosterone and cortisol.

Hydrophobic hormones can diffuse through the cell membranes and interact with an intracellular cytoplasmic or nuclear receptor leading to their activation.  

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ENDOCRINE vs EXOCRINE GLANDS

Endocrine glands are those glands that release chemical substances directly into the bloodstream or tissues. The chemical substances released by these glands are known as "hormones"

Eg:- Pituitary gland, Hypothalamus, Thyroid gland, Ovary, Testes, Parathyroid gland, Adrenal gland.

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Exocrine glands are those glands that release chemical substances by means of ducts. The chemical substances released by these glands are known as "enzymes"

Eg:- Sweat glands, Salivary glands, Sebaceous glands, Mucous glands.

NB: "Liver and pancreas are both exocrine and endocrine glands"