Hormones are Chemical messengers synthesized by endocrine glands. Hormones are responsible for growth, cellular activities, metabolism, and many physiological activities.
Hormones are released in response to a specific stimulus, and usually, their action reverses or negates the stimulus through a negative feedback mechanism.
Classification of Hormones
- Steroid Hormones
- Non-steroidal Hormones
Steroid Hormones
Steroid Hormones are synthesized from Cholesterol or its derivatives.
It is water-insoluble, but lipid-soluble in nature.
Steroid hormones easily cross the plasma membrane.
Examples: Testosterone, Progesterone, Glucocorticoids.
Non-steroidal Hormones
Non-steroidal Hormones are also called Protein hormones, and they are large or small peptides.
Non-steroidal Hormones are water-soluble and do not cross the plasma membrane because they are hydrophobic.
Examples: Insulin, Adrenalin, Glucagon.
Mechanism of Hormonal Action
Hormones do not act on the target cell directly. It combines with the receptor to form a hormone receptor complex.
This complex executes the hormonal action by one of the following mechanisms
- By altering the permeability of the cell membrane.
- By activating the intracellular components of the cell membrane.
- By acting on genes.
Hormone receptors
Hormone receptors are the large proteins present in the target cells. Each cells have thousands of receptors.
Each receptor is specific for a single hormone ( each receptor can combine with only one hormone).
Situation of Hormone Receptor
Hormone receptors are situated either in the cell membrane or cytoplasm, or nucleus of the target cell as follows
Cell membrane
Protein receptors and Adrenal medullary hormones ( catecholamines) are situated in the cell membrane.
Cytoplasm
Receptors of Steroid Hormones are present in the cytoplasm of target cells.
Nucleus
Receptors of Thyroid hormones are present in the nucleus of cells.
Examples of hormones
- Growth Hormone
- Insulin
- Estrogen
- Testostreone
- cortisol