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| Hormones are chemical signalling molecules which are released from [[endocrine]] glands; other tissues, such as the [[Kidney]] and heart muscle, apart from glands can produce hormones, however the effects of these hormones are usually more local than those of hormones produced by [[endocrine]] glands. [[Endocrine]] tissues/glands which both males and females possess are the pineal gland, hypothalamus, pituitary gland, thyroid gland, parathyroid glands, thymus, adrenal glands and islets of Langerhans in the [[Pancreas]]; males also have [[testes]] which produce [[testosterone]], while females have [[ovaries]] which produce [[oestrogen]]. Each of the [[endocrine]] glands produces and secretes hormones; hormones are secreted from their glands into the blood stream and, generally, act on distant target cells which possess specific [[Receptor]] molecules to regulate cellular function. Receptors can be located either on the target cell [[Plasma_membrane]] or inside the cell depending on the type of hormone.
| | See [[Hormone|Hormone]] |
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| There are three different types of hormones:
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| *Steroid hormones -- derived from cholesterol, steroid hormones are small, [[Hydrophobic]], lipiphilic molecules which can diffuse across cell the [[Plasma_membrane]]. Steroid hormones are not released prior to release; following synthesis, steroid hormones are immediately released. The [[Receptor]]s for steroid hormones exist in the [[Cytoplasm]] or [[Nucleus]] of the target cell; there is no need for [[Plasma_membrane]] receptors as steroid hormones are able to diffuse across the [[Plasma_membrane]] to reach internal receptor molecules. An example of a steroid hormone is [[oestrogen]].
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| *Peptide hormones -- chains of [[Amino_acid]] residues encoded by [[Gene]]s; peptide hormones are long molecules which are synthesised then stored prior to release in a less active form (preprohormones). Peptide hormones are hydrophilic and lipiphobic therefore, cannot cross the [[Plasma_membrane]]; as they cannot cross the membrane, receptors for peptide hormones must be placed on the cell surface membrane. An example of a peptide hormone is [[Insulin]] which is synthesised in Beta cells of the Islets of Langerhans in the [[Pancreas]].
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| *Amino acid hormones -- mostly synthesised from tyrosine, amino acid hormones are stored for instant release upon reception of a signal. Depending on the molecule, amino acid hormones can act as either a steroid or peptide hormone; for example, thyroid hormone is an amino acid hormone which acts like a steroid, while epinephrine, which is also an amino acid hormone, acts as a peptide hormone.
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| Hormones can be released by various mechanisms:
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| *Continuous -- hormones are released continuously into the blood stream from their endocrine glands and its effects are controlled by presence/absence of its receptor on its target cells rather than up- or down-regulating hormone release. For example, Thyroid hormone under the control of TSH.
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| *Pulsatile -- hormones release is either on or off; release is up- or down-regulated according to the requirements of the individual. For example, Growth Hormone Releasing Hormone which regulates the release of Growth Hormone.
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| *Circadian -- the hormone is released as a step in a cycle. For example, the release of melatonin at night due to a decrease in light intensity creating the feeling of fatigue.
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| *Exocytosis on stimulus -- for example, the release of [[Insulin]] as a result of the detection of an increase in blood [[Glucose]] levels.
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| Hormone effects must be controlled in accordance with the individual's requirements; hormones can be controlled by these means:
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| *Modification -- adding or removing groups from the hormone molecule will up- or down-regulate its effects. Modification increases or decreases the hormone's activity according to requirements.
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| *Degradation -- the hormone is broken down and excreted, therefore removing its active effects from the system.
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| *[[Receptor]] down-regulation -- receptors on the target cells of the hormone can be internalised or brokwn down so that the hormone has nowhere to bind; if the hormone cannot bind to a receptor on the target cell, it cannot give rise to the effects it would normally cause.
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| *Termination of intracellular effects -- for exmaple, enzymes/proteins involved in the steps following the binding of the hormone to its receptor can be inactivated, therefore, eliminating the hormone's effects.
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| *Negative feedback -- action of the hormone shuts down further production of the hormone. For example, [[Insulin]] is produced when blood [[Glucose]] levels are deemed too high; [[Insulin]] acts to reduce blood glucose levels, therefore reducing the production of more [[Insulin]].
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| '''References'''
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| Alberts. B, Bray. D, Hopkin. K, Johnson. A, Lewis. J, Raff. M, Roberts. K, Walter. P (2004),''Essential Cell Biology, ''2nd Edition, New York: Garland Science p541
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