chapter 6: Cell Communication and Signaling Mechanisms

Cell signaling

Mechanism for communication between cells and organisms

involves signaling molecules, receptor on a target cell, signal transduction, and a target cell response

Signaling molecule

Hormone or other molecule - neurotransmitter

Receptor on target cell

Binds signaling molecule

Signal transduction

After receptor binding, conversion of an extracellular to intercellular signal

Target cell responds

the cellular process altered

Electrical signals

Neurotransmission within the nervous system

Cell-cell contact

Immune system T cell and B cell interactions

Chemical signals

Chemicals, including hormones, released into body fluid - blood or interstitial fluid

Cell response

Ligand pinching triggers a response stimulated by that receptor; different cell types may respond differently to the same ligand

Hydrophilic molecules

Circulate freely in plasma move out of plasma into interstitial fluid

Receptor location (hydrophilic)

Receptors on surface of target cells

Hydrophobic molecules

Move through plasma on transport molecules

cross membrane - lipid soluble

bind intracellular receptors - cytoplasmic or nuclear on membrane

Receptor location (hydrophobic)

Cytosolic: binds ligand then moves to nucleus

example: aldosterone

inter-nuclear: travel to nucleus and binds receptor

example: hydrophobic (NO gas), steroid or thyroid hormones

result from hydrophobic molecule receptions

gene activation

Agonist action

Hormone binding stimulates a receptor response

Antagonist action

Hormone binding fails to imitate a response

Additive effect

Effect of having two hormones is sum of the effect of each one individually

Synergistic effect

Effect of one hormone increased by presence of another but not fully additive

Non-chemical signals

Examples include rhodopsin in vertebrate eyes - light

phytochromes in plants - red light

hearing organ - sound waves

pressure - pacinian corpuscles in skin

Up-regulation

Increases number of cell receptors

Down-regulation

Decrease number of receptors; often involves transporting receptors to lysosome

receptor level: affinity

How easily, tightly hormone binds

receptor level: specificity

Similarity in hormone structure and receptor binding site

regulating reception and response: extracellular level

concentration of signal molecule - synthesis, amount bound, breakdown

regulating reception and response: intracellular level

response determined by intercellular molecules activated

altered membrane permeability

altered metabolism

altered gene activity

Ligand-gated (ion channel) receptors

Ligand binding leads to signaling molecule for activation

change in conformation opens or closes channel, allowing ions to move in or out of cell

example: acetylcholine

G protein-linked (G-protein-coupled) receptors

Ligand binding activates intracellular inactive g protein, which can open ion channels or alter enzyme activity

two major amplifier enzymes

adenylyl cyclase and phospholipase C

Enzyme-linked receptors

Two main types are tyrosine kinase and guanylyl cyclase; common ligands are insulin and growth factors

Signal amplification

Activating one receptor activates multiple second messenger molecules, each affecting cellular processes