SBI 4U
January 9 th , 2012

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Hormones – chemical messengers
Puberty: timed and controlled by hormones
Secreted by cells, tissues & organs of endocrine system

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Regulates and coordinates the functions of organs throughout the body
Uses hormones to control organ and tissue functions
Endocrine glands  direct secretion into blood or extracellular fluid

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Most body cells constantly exposed to hormones  only target cells respond
Hormones cleared from body by enzymatic breakdown

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60 known hormones and local regulators in humans
Chemical structure
2 types: protein hormones and steroid hormones

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Water soluble
Amino acid chains
Diffuse well through blood and intracellular fluids
Ex: the growth factors

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Lipid soluble
Derived from cholesterol; not soluble in blood
Can easily pass through lipid bilayer
Can combine with hydrophilic carrier proteins  easily soluble in blood
Ex: testosterone and estradiol

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Most secreted as prohormones  converted by target cells or enzymes
Prohormones – usually protein hormones
Ex: angiotensinogen converted to angiotensin by angiotensin-converting enzyme (ACE)
Hormones secreted in small amounts  amplification magnifies their effects
Separate mechanisms: for water-soluble and lipid-soluble

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Water-soluble protein hormones, cannot easily cross membranes
Protein hormone binds to receptor molecules – receptor molecule changes shape, sending signal to internal environment  thereby triggering changes
Ex: glucagon. When glucagon binds to receptor on liver cells, triggers reactions  activation of enzyme that breaks down glycogen

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Binding to receptors inside cell  receptors in cytosol or nucleus
Hormone-receptor complex bind to control sequence on gene  activation or deactivation of protein that it synthesizes
Ex: aldosterone. Secreted from adrenal gland. Attaches to receptors in cells (kidneys, sweat glands, colon)  transcription for gene leading to synthesis of proteins that increase Na+ reabsorption