Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 45.1 Hormones and Other Signaling Molecules Bind to Target Receptors, Triggering Specific Response Pathways
Hormones are chemical messengers that circulate through body fluids and act on specific target cells with matching receptors. Along with the nervous system, the endocrine system coordinates physiological functions, development, and behavior in animals.
1. Types of Chemical Signaling
- Endocrine signaling: hormones travel via bloodstream to distant target cells
- Paracrine signaling: local regulators affect nearby cells
- Autocrine signaling: cells respond to their own secreted signals
- Synaptic signaling: neurons release neurotransmitters across synapses
- Neuroendocrine signaling: neurohormones are secreted by neurons into the bloodstream
- Pheromones: secreted into the environment to communicate with other individuals (e.g., trail marking, mating)
2. Local Regulators
- Act over short distances via diffusion
- Include prostaglandins (inflammation, pain), cytokines (immune signaling), growth factors (development), and nitric oxide (NO)
- NO promotes vasodilation and sexual function (Viagra enhances NO signaling)
3. Hormone Classes
- Polypeptides (e.g., insulin): water-soluble
- Amines (e.g., epinephrine, thyroxine): may be water- or lipid-soluble
- Steroids (e.g., cortisol, estradiol): lipid-soluble, derived from cholesterol
4. Hormone Solubility and Receptors
- Water-soluble hormones: bind to cell surface receptors, trigger signal transduction cascades
- Lipid-soluble hormones: cross membranes, bind to intracellular receptors, often regulate gene expression
5. Signal Transduction Examples
- Epinephrine (adrenaline) binds to G protein-coupled receptor → cAMP → protein kinase A → glycogen breakdown
- Amplification allows massive response: one epinephrine molecule can release millions of glucose molecules
6. Lipid-Soluble Hormone Response
- Receptors often in cytoplasm or nucleus
- Hormone-receptor complex directly binds DNA to activate transcription
- Example: estradiol stimulates liver cells to produce vitellogenin, a yolk protein
7. Multiple Effects from One Hormone
- A single hormone can have different effects depending on:
- Receptor type
- Intracellular signaling components
- Epinephrine causes vasoconstriction in intestines but vasodilation in skeletal muscle
8. Endocrine Glands and Tissues
- Endocrine glands (e.g. thyroid, adrenal, gonads) secrete hormones into blood
- Exocrine glands (e.g. salivary glands) release products via ducts
- Some organs (e.g. pancreas) have both endocrine and exocrine functions
In a Nutshell
Hormones act as long-distance messengers, triggering specific cellular responses by binding to their receptors. Different signaling types—endocrine, paracrine, synaptic, and neuroendocrine—coordinate bodily functions. Depending on solubility, hormones use surface or intracellular receptors to regulate metabolism, development, and behavior with high specificity and broad impact.