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Pineal Gland (location, composition, role)
1) Located at the base of the brain. 2) A single, small gland. 3) Responsible for the secretion of the hormone melatonin. It's secretion is triggered by light and dark cycles in the environment.
Melatonin
Hormone secreted by the pineal gland in response to light and dark cycles in the environment. This hormone plays a role in the regulation of the circadian rhythms - physiological cycles lasting 24 hours.
Chyme
Acidic, partially digested food coming from the stomach.
Circadian Rhythms
Physiological cycles lasting 24 hours. These are regulated partially by melatonin release of the pineal gland.
What are the basic mechanisms of hormones?
They come in either a steroid or peptide form, and trigger target cells via extracellular or intracellular receptors. 1) Peptide hormones act by binding to specific receptors on the surface of the target cell and trigger an enzymatic reaction within the cell. The peptide hormone is the first messenger and cyclic adenosine monophosphate (ATP conversion carried out within the cell when the peptide binds) or cAMP is often the second messenger which relays further messages, resulting in a cascade effect. 2) Steroid hormones are lipid based, so they enter the target cell directly and bind to a receptor within the cytoplasm. The receptor-hormone complex then enters the nucleus and directly activates the expression of genes via receptors on the chromatin.
Auxins
An important class of plant hormones that are responsible for growth patterns such as phototropism by indole-acetic acid and geotropism. The stimulate the production of new xylem cells.
Phototropism
A plant's response to grow towards the light source. This response is stimulated by the auxin hormone, indole-acetic acid. The side of the plant struck by light has a reduced supply of auxin and grows more slowly than the shaded side. The result is that the shaded side grows more quickly, and the asymmetrical growth causes the plant to bend.
Geotropism
A plant's response to grow away from, or toward gravity. Negative geotropism
Gibberellins
Plant hormones that stimulate the growth of phloem cells. They inhibit the formation of new root cells and stimulate the rapid elongation of stem cells. They also play a role in the termination of seed dormancy and plant flowering.
Kinins
Or cytokinin. Plant hormones that are important for cell division and, therefore, plant growth. They work in conjunction with auxins.
Ethylene
A gas produced by plant cells that stimulates fruit ripening and induces aging and senescence.
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