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Group 1: Hormone Overview and Discovery – Hormones are signaling molecules in multicellular organisms that regulate physiology and behavior. – Various molecules like eicosanoids, steroids, […]

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Group 1: Hormone Overview and Discovery

– Hormones are signaling molecules in multicellular organisms that regulate physiology and behavior.
– Various molecules like eicosanoids, steroids, amino acid derivatives, proteins, and gases can act as hormones.
– Hormones communicate between organs and tissues.
– Arnold Adolph Berthold discovered testosterone’s role in roosters in 1849, while Charles and Francis Darwin laid the foundation for plant hormone discovery in the 1870s.
– George Oliver and Edward Albert Schäfer discovered adrenaline as the first hormone in 1894.
– William Bayliss and Ernest Starling discovered secretin as a hormone in 1902.

Group 2: Hormone Signal Transduction and Endocrine Glands

– Hormonal signaling involves biosynthesis, secretion, and transport to target cells, initiating signal transduction pathways.
– Endocrine glands secrete hormones directly into the bloodstream, regulated by specific signals and negative feedback mechanisms.
– Water-soluble hormones travel through the circulatory system, while lipid-soluble hormones bind to carrier proteins for transport.
– Hormones can be released fully active or as prohormones that require activation.

Group 3: Types and Effects of Hormones

– Hormones can be endocrine, paracrine, autocrine, or intracrine, with effects on nearby or distant cells.
– Hormones regulate growth, circadian rhythms, mood, metabolism, immune responses, hunger, reproduction, and homeostasis in humans.
– Hormones prepare the body for different activities and life phases, inducing apoptosis when necessary.

Group 4: Hormone Regulation and Therapeutic Use

– Hormone biosynthesis and secretion are regulated by negative feedback mechanisms essential for maintaining hormone balance and homeostasis.
– Hormones like estrogens, progestogens, thyroxine, and steroids are used therapeutically for various conditions.
– Insulin is commonly prescribed for diabetic patients, showcasing the therapeutic use of hormones in healthcare.

Group 5: Plant Hormones, Comparison, and Evolution

– Plant hormones modulate developmental stages from germination to senescence, with spatial distribution changing based on age and environmental factors.
– Hormones act over larger spatial and temporal scales than neurotransmitters, influencing behaviors related to survival, reproduction, and social interactions.
– Hormones have played a significant role in the evolution of organisms, adapting signaling pathways to changing environments and behaviors.

Hormone (Wikipedia)

A hormone (from the Greek participle ὁρμῶν, "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs or tissues by complex biological processes to regulate physiology and behavior. Hormones are required for the correct development of animals, plants and fungi. Due to the broad definition of a hormone (as a signaling molecule that exerts its effects far from its site of production), numerous kinds of molecules can be classified as hormones. Among the substances that can be considered hormones, are eicosanoids (e.g. prostaglandins and thromboxanes), steroids (e.g. oestrogen and brassinosteroid), amino acid derivatives (e.g. epinephrine and auxin), protein or peptides (e.g. insulin and CLE peptides), and gases (e.g. ethylene and nitric oxide).

Left: A hormone feedback loop in a female adult. (1) follicle-stimulating hormone, (2) luteinizing hormone, (3) progesterone, (4) estradiol. Right: auxin transport from leaves to roots in Arabidopsis thaliana

Hormones are used to communicate between organs and tissues. In vertebrates, hormones are responsible for regulating a wide range of processes including both physiological processes and behavioral activities such as digestion, metabolism, respiration, sensory perception, sleep, excretion, lactation, stress induction, growth and development, movement, reproduction, and mood manipulation. In plants, hormones modulate almost all aspects of development, from germination to senescence.

Hormones affect distant cells by binding to specific receptor proteins in the target cell, resulting in a change in cell function. When a hormone binds to the receptor, it results in the activation of a signal transduction pathway that typically activates gene transcription, resulting in increased expression of target proteins. Hormones can also act in non-genomic pathways that synergize with genomic effects. Water-soluble hormones (such as peptides and amines) generally act on the surface of target cells via second messengers. Lipid soluble hormones, (such as steroids) generally pass through the plasma membranes of target cells (both cytoplasmic and nuclear) to act within their nuclei. Brassinosteroids, a type of polyhydroxysteroids, are a sixth class of plant hormones and may be useful as an anticancer drug for endocrine-responsive tumors to cause apoptosis and limit plant growth. Despite being lipid soluble, they nevertheless attach to their receptor at the cell surface.

In vertebrates, endocrine glands are specialized organs that secrete hormones into the endocrine signaling system. Hormone secretion occurs in response to specific biochemical signals and is often subject to negative feedback regulation. For instance, high blood sugar (serum glucose concentration) promotes insulin synthesis. Insulin then acts to reduce glucose levels and maintain homeostasis, leading to reduced insulin levels. Upon secretion, water-soluble hormones are readily transported through the circulatory system. Lipid-soluble hormones must bond to carrier plasma glycoproteins (e.g., thyroxine-binding globulin (TBG)) to form ligand-protein complexes. Some hormones, such as insulin and growth hormones, can be released into the bloodstream already fully active. Other hormones, called prohormones, must be activated in certain cells through a series of steps that are usually tightly controlled. The endocrine system secretes hormones directly into the bloodstream, typically via fenestrated capillaries, whereas the exocrine system secretes its hormones indirectly using ducts. Hormones with paracrine function diffuse through the interstitial spaces to nearby target tissue.

Plants lack specialized organs for the secretion of hormones, although there is spatial distribution of hormone production. For example, the hormone auxin is produced mainly at the tips of young leaves and in the shoot apical meristem. The lack of specialised glands means that the main site of hormone production can change throughout the life of a plant, and the site of production is dependent on the plant's age and environment.

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