Tag: endocrine system and chemical co-ordination

Peptide hormones are hydrophilic due to which they are unable to cross the lipid plasma membrane. They bind to the receptors on the plasma membrane and affect the activity of proteins inside the cell called secondary messenger. These secondary messengers then perform the action required. The steroid hormones are hydrophobic, they can cross the plasma membrane directly and are able to affect the synthesis of proteins directly in the cells.

Prolactin release inhibiting hormone is an amino acid derivative hormone produced in the hypothalamus.

Cyclic adenosine monophosphate is a secondary messenger important in many biological processes. It is a derivative of ATP. When a G-protein–coupled receptor on the surface of a cell binds to its particular ligand, the receptor may stimulate the intracellular production of cAMP by the adenylyl cyclase-associated with that receptor. cAMP can then diffuse through the cytoplasm, where it interacts with various targets. Thus, cAMP serves as a second messenger within a cell.

Messenger proteins (or signaling proteins) carry messages or signals. They provide a means by which cells in one part of the body communicate with cells in another location. In some cases, these proteins can carry a signal from one cellular location to another. The signals can trigger/modulate the immune response, promote cell development/differentiation, alter cell metabolism, or stimulate cell growth. Carrier proteins help specific molecules or ions pass through the plasma membrane. They do not extend through the membrane. They bond and drag the molecules through the bilipid layer and release them on the opposite side. 

Steroid hormones, being hydrophobic molecules, diffuse freely into all cells and act within the cell. Steroid hormones secreted by adrenal cortex, ovaries, and testes do not utilize the second messenger to exert their influence. Steroid hormones enter the cytoplasm of a target cell and bind with specific receptor proteins (mobile). Hormone-receptor complex than diffuses into the nucleus and activates specific genes to form a new protein. This protein carries out the specific response for a particular steroid hormone.

Lipid insoluble hormones such as epinephrine acts or functions on a particular site by binding to it. These hormones have their receptors on the plasma membrane of the target cells. The binding of hormone to the receptor initiates a series of events. These series of events are called as signal transduction. In this, the hormone first binds to the surface receptor. It leads to the activation of the enzyme adenylate cyclase. This enzyme converts the ATP into cyclic AMP or cAMP inside the cell. This cAMP acts as second messenger. This cAMP then activates cAMP-dependent protein kinase called protein kinase A. This proteni kinase A triggers a series of molecular interactions that leads to glycogenolysis. This results in the synthesize of glucose from glycogen.

Steroid hormone receptors are found on the plasma membrane, in the cytosol and also in the nucleus of target cells. They are generally intracellular receptors (cytoplasmic) and initiate signal transduction for steroid hormones, which lead to changes in gene expression over a time period of hours to days. 

Though hormones are released into the blood stream and hence can reach all cells, yet they affect only those cells which have hormone specific receptors. Thus, binding of hormone to receptor is crucial irrespective of presence of receptors inside the cell or on its surface. This makes options A and D incorrect as absence of receptors renders the cell unaffected by hormonal action. Option B is incorrect as hormone-receptor binding is essential step in hormonal action. Thus, in absence of receptors, the target organ will no longer respond to hormone. Option C is correct. 

Steroid hormones are a group of hormones, that belong to the class of chemical compounds known as steroids; they are secreted by three steroid glands, the adrenal cortex, testes and ovaries, during pregnancy by the placenta. All steroid hormones are derived from cholesterol. They are transported through the bloodstream to the cells of various target organs, where they carry out the regulation of a wide range of physiological functions.