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Hormones
• chemical substances produced in small quantities in one part of an organism and then transported to another part of an organism where they bring about a physiological response
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Target Cells
• have receptors that allow them to recognize the hormone and respond
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Hormone Action
• Depends on :– Which hormone – the effect on the target cells
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Figure 45.4 One chemical signal, different effects
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Site of Hormone Production
• In animals– usually produced in specialized tissue that
serves only to produce hormones
• In plants– produced by tissue that is also used for some
other function (in the apical meristem, in seeds, in fruits etc)
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Auxin
• -increases the plasticity of cell walls and promotes elongation of stems
• -involved in the response of plants to light (phototropisms)
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Figure 39.4 Early experiments of phototropism
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Figure 39.5 The Went experiments
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Plant HormonesFive major types:
• Typically work together to control various aspects of plant growth, reproduction and development
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Plant HormonesFive major types:
• 1. Auxins – promotes growth, stem elongation, cell division– Promotes lateral bud dormancy
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Plant HormonesFive major types:
• 2. Cytokinins– promotes cell division (with auxin)– Promotes bud formation (lateral buds when
auxin not present)
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Figure 39.8 Apical dominance: with apical bud (left), apical bud removed (right)
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Plant HormonesFive major types:
• 3. Gibberellins– promotes stem elongation– Speed seed germination
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Figure 39.10 Treating pea dwarfism with a growth hormone
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Plant HormonesFive major types:
• 4. Ethylene– controls leaf, flower and fruit abscission and
fruit ripening• 5. Abscisic acid
– promotes leaf senescence, controls stomatal closure and seed dormancy
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Hormones in Animals
• secreted by – specialized nerve cells called neurosecretory
cells• neurons that receive signals from other neurons and
respond by releasing hormones
– specialized cells called endocrine cells• usually organized into an endocrine gland
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Glands
• Secretory organs• Endocrine glands
– Produce hormones and secrete them into body fluids
– Are ductless
• Exocrine glands– Produce variety of substance– Convey them directly to the target via ducts
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Figure 45.5 Human endocrine glands surveyed in this chapter
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Chemical messengers in Animals
• Hormones– Secreted by endocrine glands and carried by
circulatory system
• Neurotransmitters– Secreted by neurons and affect next cell
• Paracrine Regulators– Secreted by cells and act within the same organ
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Hormones• lipophilic
– Fat soluble
– Easily enter cell and bind to receptors in the cytoplasm or nucleus
– Steroid hormones
• Lipophobic (polar)– Cannot cross cell membrane
– Bind to membrane receptors
– Need secondary messengers
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Vertebrate Hormone Example
• Islets of Langerhans in the Pancreas
• Control of blood glucose levels…– Alpha cells produce glucagon (raises blood
glucose level)– Beta cells produce insulin (takes glucose out of
the blood)
• Insulin and glucagon are antagonistic hormones
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Figure 45.10 Glucose homeostasis maintained by insulin and glucagon
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Anterior Pituitary Gland
• “master gland”
• Many tropic hormones– That stimulate growth in their target organs
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Figure 45.6b Hormones of the hypothalamus and pituitary glands
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Invertebrate Hormone Example
• Molting in Insects…
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Figure 45.2 Hormonal regulation of insect development (Layer 1)
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Figure 45.2 Hormonal regulation of insect development (Layer 2)
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Figure 45.2 Hormonal regulation of insect development (Layer 3)
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Interaction of Nervous System and Endocrine System
• often cooperate and interact to maintain homeostasis of the individual
• some endocrine glands are controlled by the nervous system
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Nervous System Endocrine System
• More structurally complex
• Network of neurons branching throughout the body
• Neurons conduct electrical signals directly to the target
• Very fast conduction of signal
• less complex
• organized into glands
• hormones released into the blood and travel throughout the body but only affect target
• may take minutes to hours to days for response to occur