Here is a compilation of term papers on ‘Hormones’. Find paragraphs, long and short term papers on ‘Hormones’ especially written for school and college students.
Term Paper on Hormones
Term Paper # 1. Definition of Hormones:
The word ‘hormone’ is derived from the Greek word ‘hormaein’ which means ‘to arouse’ or ‘to excite’. Classically, hormones are defined as chemical messengers secreted by endocrine or ductless glands into the blood in response to an appropriate signal and carried to other sites in the body, where they act on target cells some distance away from the site of release.
However, there are chemical messengers secreted by cells other than those of the endocrine glands into the surrounding interstitial fluid that exert their effects on target cells nearby. Therefore, the definition of a hormone has been extended to include the aforementioned categories.
Term Paper # 2. Classification of Hormones:
ADVERTISEMENTS:
1. The following classes of hormones are described, based on the cellular source, route of distribution and target cell:
i. Endocrine Hormones:
These are long-range chemical messengers secreted by classic endocrine cells into the blood that act on a distant target cell.
ii. Neurohormones:
ADVERTISEMENTS:
These chemicals are released by neurons into the bloodstream and carried to distant target cells, e.g., vasopressin. Thus, like endocrine cells, these neurons release blood-borne chemical messengers, whereas ordinary neurons secrete short-range neurotransmitters into a confined space.
iii. Paracrine Hormones:
These are chemical messengers secreted by cells of one type that diffuse through the interstitial fluid and act on neighboring cells of another type, e.g., somatostatin secreted by D cells of the islets of Langerhans acts on the A and B cells.
iv. Autocrine Hormones:
ADVERTISEMENTS:
This class includes chemical messengers that regulate neighboring cells of the same type as the source, e.g., prostaglandins.
Note:
The same chemical messenger may function as an endocrine, paracrine or autocrine hormone, depending on the route by which it is delivered, e.g., insulin, secreted by B cells of the pancreatic islets, may act as:
i. An endocrine hormone when released into the blood, influencing various metabolic pathways.
ADVERTISEMENTS:
ii. A paracrine hormone when it is secreted into the interstitial fluid and acts on neighboring A cells.
iii. An autocrine hormone when, on secretion into the interstitial fluid, it regulates the function of the B cells themselves (as B cells possess insulin receptors).
2. Depending on their chemistry, hormones are classified into:
i. Proteins:
ADVERTISEMENTS:
a. Short-Chain Peptides ― For example, ADH and oxytocin.
b. Long-Chain Polypeptides ― For example, insulin and parathyroid hormone.
ii. Steroid Hormones ― For example, glucocorticoids
iii. Amino Acid Derivatives ― For example, thyroid hormones.
ADVERTISEMENTS:
iv. Amines ― For example, catecholamine.
Term Paper # 3. General Characteristics of Hormones:
The chemical nature of a hormone determines:
i. How it is synthesized, stored and released?
ii. How it is transported in blood?
iii. It is biological half-life (the period of time needed for the concentration of the hormone to be reduced by half) and mode of clearance.
iv. It is cellular mechanism of action.
Term Paper # 4. Features of Hormones:
The salient features of each class of hormones with regard to the above aspects are presented:
I. Protein/Peptide Hormones:
i. Protein/peptide hormones are synthesized on polyribosomes as large preprohormones. The preprohormones are processed by removal of a signal peptide in the endoplasmic reticulum to produce the hormone or the prohormone, which requires further cleavage to form the mature hormone. This final cleavage occurs while the prohormone is in the Golgi apparatus or the secretory granule.
ii. They are stored in the gland in membrane-bound secretory granules and are released by regulated exocytosis in response to a stimulus.
iii. Being water-soluble, they circulate predominantly in the unbound form. Therefore, they tend to have short biological half-lives.
iv. Protein hormones are readily digested if administered orally. Hence, they are administered through parenteral routes.
v. As they do not cross cell membranes readily, they signal through membrane receptors.
II. Steroid Hormones:
i. Steroid hormones are synthesized from cholesterol and contain the cyclopentanoperhydrophenanthrene ring.
ii. They usually circulate bound to transport proteins as they are not readily soluble in blood.
iii. Being hydrophobic, steroid hormones pass through cell membranes easily and are not stored. Instead, the hormone precursors are stored as lipid droplets in steroidogenic cells.
iv. Steroid compounds are absorbed readily in the gastrointestinal tract and therefore, are administered orally.
v. The receptors of steroid hormones are intracellular and the hormones act by regulating gene expression.
III. Thyroid Hormones:
i. Thyroid hormones are derived from tyrosine.
ii. They are sparingly soluble in blood and 99% of circulating thyroid hormone is transported bound to serum binding proteins.
iii. They cross cell membranes by both diffusion and transport systems.
iv. They are stored extracellularly in the thyroid gland as an integral part of a glycoprotein molecule called thyroglobulin. Hormone secretion occurs when the amines are split from the thyroglobulin molecule, and the free hormones are then released into the bloodstream.
v. Thyroid hormones are similar to steroid hormones in that the thyroid hormone receptor is intracellular and acts as a transcription factor.
IV. Catecholamines:
i. Catecholamines are also derived from tyrosine.
ii. They are stored in membrane-bound granules.
Term Paper # 5. Transport of Hormones in Circulation:
i. There is equilibrium between the concentrations of protein-bound hormone and free hormone. If free hormone levels drop, hormone is released from the transport proteins. Thus, bound hormone represents a “reservoir” of hormone and serves to “buffer” acute changes in hormone secretion.
ii. The free form is the biologically active form for target organ action and feedback control. Therefore, when evaluating hormonal status, free hormone levels must sometimes be determined rather than total hormone levels alone.
Signal Transduction (Intracellular Signaling):
Hormones bring about cell responses by signal transduction. The term signal transduction refers to the process by which incoming signals are conveyed into the target cell where they are transformed into the dictated cellular response. Hormones bind to specific receptors on a target tissue. This binding induces conformational changes in the receptor. This is referred to as a signal.
The signal is transduced into the activation of one or more intracellular messengers. Messenger molecules then bind to effector proteins, which modify specific cellular functions. The signaling pathway comprises the hormone-receptor binding, activation of intracellular messengers and the regulation of one or more effector proteins. The final outcome is referred to as the cellular response.
Signaling from Membrane Receptors:
Binding of a hormone (also known as the first messenger) to its specific surface membrane receptor brings response by three general means:
1. By activating second messenger pathways via G protein-coupled receptors
2. By activating receptor enzymes
3. By opening or closing chemically gated receptor channels.
G Protein-Coupled Receptors:
The G protein-coupled receptors represent the largest family of hormone receptors. The intracellular portions of the receptor are coupled to G proteins. G proteins are molecular switches that are active when bound to GTP and inactive when bound to GDP. They have intrinsic GTPase activity. The G proteins that directly interact with the receptors are termed heterotrimeric G proteins as they are composed of an α subunit (Gα) and a β/g subunit dimer.
The Gα is bound to GDP. On hormone binding, GDP is exchanged for GTP, thereby activating Gα. The Gα subunit separates from the β/g subunit and brings about biologic effects. The intrinsic GTPase activity of the Gα subunit then converts GTP to GDP and this leads to re-association of the α subunit with the β/g subunit and termination of the effector activation.
There are many types of Gsα proteins:
i. Gsα stimulates the membrane enzyme, adenylyl cyclase, which generates cAMP from ATP. cAMP activates protein kinase A, which phosphorylates numerous proteins and thereby, alters cell function. cAMP is called a second messenger because it is not the hormone (the first messenger) itself that directly brings about the intracellular changes.
ii. Giα inhibits adenylyl cyclase.
iii. Gqα activates phospholipase C, which generates diacylglycerol (DAG) and inositol triphosphate (IP3) from the membrane lipid, phosphatidylinositol bisphosphate (PIP2).
Diacylglycerol activates protein kinase C, which then phosphorylates a large number of proteins, leading to cellular response.
IP3 binds to its receptor, which is a large complex including a Ca2+ channel, on the endoplasmic reticulum membrane and promotes Ca2+ efflux from the endoplasmic reticulum into the cytoplasm. Calcium ions have their own second messenger effects, such as smooth muscle contraction and changes in cell secretion.
Receptor Tyrosine Kinases:
This family of receptors has intrinsic tyrosine kinase activity. Binding of the hormone to its receptor induces this tyrosine kinase activity and tyrosine residues in the receptor are phosphorylated, generating phosphotyrosines. The phosphotyrosines function to recruit intracellular proteins that specifically recognize them.
Receptors Associated with Cytoplasmic Tyrosine Kinases:
These receptors exist as dimers and do not have intrinsic tyrosine kinase activity. Instead, their cytoplasmic domains are stably associated with tyrosine kinases of the Janus kinase (JAK) family. Hormone binding induces a conformational change, bringing the two JAKs associated with the dimerized receptor closer together and causing their transphos-phorylation and activation.
JAKs then phosphorylate tyrosine residues on the cytoplasmic domains of the receptor. The phosphotyrosine residues recruit latent transcription factors called signal transducers and activators of transcription (STAT) proteins. STATs are phosphorylated by JAKs, which cause them to dissociate from the receptor, dimerize and translocate to the nucleus, where they regulate gene expression.
Receptor Serine/Threonine Kinases:
These receptors exist as dissociated heterodimers in the unbound state. Hormone binding to the receptors induces dimerization and activation of the receptor by phosphorylation.
Receptors Regulating Ion Channels:
Hormone binding to these receptors opens ion channels, the most common of which are calcium channels.
Signaling from Intracellular Receptors:
Intracellular receptors act as transcription regulators. They may be located in the cytoplasm or the nucleus.
i. Cytoplasmic Receptors:
In the absence of hormone, the cytoplasmic receptors are held in an inactive state through interactions with chaperone proteins (also called ‘heat-shock proteins’ because their levels increase in response to elevated temperatures and other stresses). Hormone binding induces a conformational change in the receptor, causing the hormone-receptor complex to dissociate from heat-shock proteins.
This exposes the nuclear localization signal and the dimerization domains, so that the receptors dimerize and enter the nucleus. Once in the nucleus, these receptors bind to specific DNA sequences called hormone-response elements (HREs). Bound to their respective HREs, the receptors recruit other proteins called co-activators that activate gene transcription.
ii. Nuclear Receptors:
They are bound to co-repressors in the absence of hormone. The receptor-co-repressor complexes are bound to the specific hormone- response elements and keep the expression of neighboring genes repressed.
Regulation of Hormone Secretion:
Negative Feedback:
Secretion of most hormones is regulated by negative feedback. Negative feedback means that the hormone secreted acts directly or indirectly on the secretory cell in a negative way to inhibit further secretion, e.g., a rise in blood glucose detected by the B cells of the pancreas causes them to release insulin, which stimulates glucose uptake into tissues and thereby decreases blood glucose concentration. With restoration of blood glucose to the set-point level, the B cells are not stimulated any further and secretion of insulin is inhibited.
Positive Feedback:
Positive feedback means that the hormone secreted brings about stimulation of further secretion by its actions, e.g., effect of oxytocin on uterine muscle during childbirth. In this case, the stimulus for oxytocin secretion is dilation of the uterine cervix.
Sensory nerves transmit this information to the brain and the brain signals release oxytocin from nerve endings in the posterior pituitary gland. Enhanced uterine contraction in response to oxytocin results in greater dilation of the cervix, which strengthens the signal for oxytocin release and so on until the infant is expelled from the uterine cavity.
Term Paper # 6. Measurement of Hormone Concentrations in Blood:
As most hormones are present in extremely minute quantities in blood, they cannot be measured by the usual chemical means. An extremely sensitive method employed to measure hormones, their precursors and their metabolic end-products is called radioimmunoassay.
Radioimmunoassay:
Principle:
This method employs antibodies formed against the hormone whose concentration is to be determined and a radioactively labeled hormone prepared in the laboratory.
Method:
A small quantity of the antibody and an appropriate amount of the radioactive hormone are simultaneously mixed with the fluid sample containing the hormone to be measured. One specific condition to be met is that there must be too little antibody to bind completely both the radioactive hormone and the natural hormone in the fluid to be assayed.
Therefore, the natural hormone in the assay fluid and the radioactive hormone compete for the binding sites of the antibody. In the process of competing, the quantity of each of the two hormones, the natural and the radioactive, that is bound to the antibody is proportional to its concentration in the assay fluid.
After binding has reached equilibrium, the antibody-hormone complex is separated from the remainder of the solution and the quantity of radioactive hormone bound in this complex is measured by radioactive counting techniques. If a large amount of radioactive hormone is bound with the antibody, it is clear that there was only a small amount of natural hormone to compete with the radioactive hormone; conversely, if only a small amount of radioactive hormone has bound, it is clear that there was a large amount of natural hormone to compete for the binding sites.
A “standard curve” is plotted by determining the percentage of antibody bound to radioactive hormone in test samples with different concentrations of the natural hormone. By comparing the radioactive counts recorded from the unknown assay procedure with the standard curve, one can determine the concentration of the hormone in the unknown assayed fluid.
Enzyme-Linked Immunosorbent Assay (ELISA):
This highly sensitive method can be used to measure almost any protein. It is performed on plastic plates that each have 96 small wells. Each well is coated with and antibody (AB1) that is specific for the hormone being assayed. Samples or standards are added to each of the wells, followed by a second antibody (AB2) that is also specific for the hormone but binds to a different site of the hormone molecule. A third antibody (AB3) is added that recognizes AB2 and is coupled to an enzyme that converts a suitable substrate to a product that can be easily detected by colorimetric or fluorescent optical methods.
In contrast to competitive radioimmunoassay methods, ELISA methods use excess antibodies so that all hormone molecules are captured in antibody-hormone complexes. Therefore, the amount of hormone present in the sample or in the standard is proportional to the amount of product formed.
Term Paper # 7. Hormonal Rhythms:
Most living cells have rhythmic fluctuations in their function that are about 24 hours in length; these rhythms are called circadian (diurnal; circa “about” + dia “day”) rhythms. Rhythms with a periodicity of less than 24 hours are referred to as ultradian rhythms.
Biological rhythms are set by an internal clock or pacemaker and persist even in the absence of cues. The internal clock that drives a circadian rhythm can be synchronized to time cues in the environment, such as the light/dark cycle. This process of synchronization to an external stimulus is called entrainment.
A rhythm that runs at a frequency that is independent of external cues is called a free-running rhythm. If a free-running nocturnal animal is exposed to periodic light and dark, the onset of activity soon becomes synchronized to the beginning of the dark period. The shift of activity produced by a synchronizing stimulus is referred to as a phase shift, and the process of shifting the rhythm is called entrainment.
Circadian rhythms are entrained (synchronized to the day-night cycle in the environment) by the paired suprachiasmatic nuclei (SCN) above the optic chiasm. These nuclei receive information about the light-dark cycle via the retinohypothalamic fibers that pass from the optic chiasm to the SCN.
Efferents from the SCN initiate neural and humoral signals that entrain a wide variety of circadian rhythms. These include the sleep-wake cycle, the rhythms in the secretion of ACTH and other pituitary hormones and the secretion of the pineal hormone melatonin. The nocturnal peaks in melatonin secretion appear to be an important hormonal signal entraining other cells in the body.