Hormones Essay, Research Paper

Hormones are organic substances that are secreted by plants and animals and that

function in the regulation of physiological activities and in maintaining

homeostasis. They carry out their functions by evoking responses from specific

organs or tissues that are adapted to react to minute quantities of them. The

classical view of hormones is that they are transmitted to their targets in the

bloodstream after discharge from the glands that secrete them. This mode of

discharge (directly into the bloodstream) is called endocrine secretion. The

meaning of the term hormone has been extended beyond the original definition of

a blood-borne secretion, however, to include similar regulatory substances that

are distributed by diffusion across cell membranes instead of by a blood system.

. Among animals, the hormones of the vertebrates–particularly those of humans

and other mammals–are the best known. Most vertebrate hormones originate in

specialized tissues, called endocrine tissues, and are carried to their targets

through the bloodstream. Endocrine glands. A major endocrine gland in

vertebrates is the pituitary, which consists of two distinct sections: the

anterior pituitary (or adenohypophysis) and the posterior pituitary (or

neurohypophysis). The anterior pituitary is sometimes called the "master

gland," because it secretes several hormones that affect the other

endocrine glands. For example, the anterior pituitary hormones thyrotropin and

adrenocorticotropic hormone (ACTH) regulate endocrine activity in the thyroid

and the outer region (cortex) of the adrenal glands, respectively. The anterior

pituitary also secretes hormones that affect the sex glands. One of these is

follicle-stimulating hormone (FSH), which stimulates egg production in the

ovaries and sperm production in the testes. Another is luteinizing hormone (LH).

In females, LH works in conjunction with FSH to regulate the female reproductive

cycle and the secretion of female sex hormones. In males, LH controls the

production of the male sex hormones. Other hormones produced in the anterior

pituitary include growth hormone, which is responsible for normal body growth,

and prolactin, which promotes milk production in female mammals. Its designation

as the master gland notwithstanding, the anterior pituitary itself is regulated

by substances called releasing hormones that are secreted by the hypothalamus,

the part of the brain located directly above the pituitary. These hypothalamic

hormones stimulate–or, in some cases, inhibit–the secretions of the anterior

pituitary. The posterior pituitary stores and releases two hormones: oxytocin,

which causes the uterus to contract during birth, and vasopressin, which acts on

the kidneys to restrict the output of urine. These two hormones are actually

produced by the hypothalamus, which is linked directly to the posterior

pituitary. Other endocrine glands in vertebrates include the thyroid,

parathyroids, adrenals, pancreas, and gonads (sex glands). The thyroid produces

hormones that control metabolic rate and oxygen consumption. Hormones from the

parathyroids are concerned with calcium concentration in the blood, and the

pancreas releases insulin and glucagon, hormones that, respectively, lower and

raise the blood-sugar level. Hormones from the adrenal cortex regulate glucose

and sodium metabolism. Those secreted by the central portion (medulla) of the

adrenals affect the heart and the circulatory and respiratory systems; these

hormones are important in helping an individual cope with stress. The heart

itself releases a hormone– atrial natriuretic peptide–that helps regulate

blood pressure, blood volume, and the salt and water balance within the blood.

(see also Index: thyroid hormone, parathormone) The female sex hormones–the

estrogens and progesterone–are produced by the ovaries. Together with FSH and

LH, these hormones control the cyclical changes in the female reproductive

system–the menstrual cycle in human females and the estrous cycle in other

female mammals. The estrogens also are responsible for female sexual

characteristics. Progesterone is concerned with the maintenance of pregnancy.

Male sex hormones–known as androgens–include testosterone, which is secreted

by the testes. Testosterone is responsible for the maintenance of male sexual

characteristics. Hormone chemistry. Structurally, vertebrate hormones fall into

two main classes. Those of the adrenal cortex and the sex organs are steroids, a

major class of lipid compounds. Virtually all other known vertebrate hormones

consist of amino acids. Most nonsteroidal hormones are composed of chains of

amino acids–either short chains (polypeptides) or long chains (proteins). The

hormones of the adrenal medulla, however, are composed of amino acid derivatives

called amines, those of the thyroid of a single amino acid combined with atoms

of iodine. It is believed that hormones achieve their effects on target tissues

and organs through either of two mechanisms. The steroid hormones and the

hormones of the thyroid can, as a result of their chemical structures, pass

through cell membranes. These hormones apparently enter a target cell and

combine with an intracellular receptor protein. The hormone-receptor complex

then enters the cell’s nucleus, where it apparently affects the activity of

specific genes. Genes carry the cell’s hereditary blueprint for protein

synthesis, and so the interaction of the hormone-receptor complex with the genes

influences the cell’s production of proteins. Because many proteins function as

enzymes within the cell, this influence on protein synthesis can have

far-reaching effects on the cell’s activities. The polypeptide, protein, and

amine hormones are believed to operate by a different mechanism. These hormones

do not enter the target cell; instead, they combine with a receptor protein on

the cell’s outer membrane. This hormone-receptor complex apparently triggers an

enzyme in the membrane, causing the synthesis of a so-called

"second-messenger" compound within the cell. This second messenger–in

many cases, cyclic adenosine monophosphate (cyclic AMP)–apparently activates

enzyme systems that bring about the desired action by the cell. It is

interesting to note that both proposed mechanisms of hormone action involve the

binding of the hormone to a specific receptor molecule. This feature accounts

for the specificity of hormones; a hormone can have an effect only on cells that

possess the appropriate receptor. Hormones probably exist in all invertebrates.

In insects, neurosecretory cells in the brain produce thoracotropic hormone.

This hormone stimulates glands in the thorax to secrete the hormone ecdysone,

which causes the periodic molting, or shedding, of the hard exoskeleton. Another

insect hormone, called juvenile hormone, maintains the larval state. A decrease

in juvenile hormone triggers the development of the adult characteristics.

Plants also have a hormonal system, which includes auxins, gibberellins, and

cytokinins, all of which promote growth. Plant hormones also include several

growth inhibitors, which regulate such activities as the fall of deciduous

leaves in autumn and the development of dormancy in buds and seeds.