Human Endocrine System

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

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.

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. Thus, options A and D are incorrect as the absence of receptors renders the cell unaffected by hormonal action. Option B is incorrect as hormone-receptor binding is an essential step in hormonal action. Thus, in absence of receptors, the target organ will no longer respond to the hormone. Hence, the correct answer is option C.

• A tropic hormone is a hormone that stimulates an endocrine gland to grow and secrete its hormones. (One hormone causes another hormone to do something) 
• In humans, tropic hormones are secreted by the adenohypophysis(anterior pituitary gland). Hence, A hormone that stimulates an endocrine gland to grow and secrete its hormone is known as Tropic hormones

• In both males and females have glandular tissue within the breasts however, in females the glandular tissue begins to develop after puberty in response to estrogen release. 
• Mammary glands only produce milk after childbirth. During pregnancy, the hormones progesterone and prolactin are released. Hence, Estrogen, Prolactin and Progesterone help in the formation of alveoli of the mammary glands.

Glucagon is a peptide hormone that binds to cell-surface receptors and utilizes cAMP as a second messenger. Estrogen, cortisol, and iodothyronines are lipid-soluble and act via intracellular receptors.

• Parathyroid hormone (PTH) is essential for the physiologic maintenance of mineral homeostasis. 
• PTH regulates the mineral transport in bone and kidney and through its secondary actions on mineral transport in the intestine. 
• Calcitonin, in many ways, acts as a physiologic antagonist to PTH. Hence,  Parathyroid hormone is antagonistic to calcitonin

Hormones are the chemical substance, released from endocrine glands. They are directly released into the blood and affect the targeted organ. They may be the peptide such as insulin or steroid such as cortisol. Thus, the correct answer is option B.

Hormonal actions are regulated by two mechanisms: feedback and antagonistic effects of two hormones. A feedback mechanism can work as a negative or positive mechanism. The negative feedback mechanism is activated by surplus and proceeds to correct the process that caused the surplus. Therefore, increased the blood level of a certain hormone or its effect on target organ act to prevent its further release. For example, increased blood glucose level stimulates insulin secretion. The positive feedback mechanism of hormone regulation includes stimulation of release of the same hormone caused by its increased blood level. For example, the release of oxytocin is stimulated by uterine contraction during labor pain. 

Gonadotropin is a hormone which stimulates production of sex hormones and development of secondary sexual characters.
Females : Ovaries : Oestrogen and Progesterone
Males : Testes (testicle) : Testosterone.