types of direct contact
gap junctions between animal cells
plasmodesmata between plant cells
A secreting cell acts on nearby target cells by discharging molecules of a local regulator into the extracellular fluid.
A nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell.
Specialized endocrine cells secrete hormones into body fluids, often blood. Hormones reach virtually all body cells, but are bound only by some cells.
This is when the target cell detects a signaling molecule coming from outside the cell. A chemical signal is "detected" when the ligand (signaling moleclue) binds to a receptor protein located at the cell's surface or inside the cell (for water-soluble signal molecules). A shape change in a receptor is often the inital transduction of the signal.
Three main types of receptors
G-protein coupled receptors
receptor tyrosine kinases (attach phosphates to tyrosine amino acids on receptors to activate them)
ligand-gated ion channels (common on nerve cells)
G protein coupled receptors
When GDP is bound to the G protein, G protein is inactive. When the appropiate signaling molecule binds to the extracellular side of the receptor, the receptor is activated and changes shape. Its cytoplasmic side then binds an inactive G protein, causing GTP to displace the GDP. This activates the G protein. The activated G protein dissociates from the receptor, diffuses along the membrane, and then binds to an enzyme, altering the enzyme's shape and activity. Once activated, the enzyme can trigger a cellular response along the signal transduction pathway. Then the G protein hydrolyzes its bound GTP to GDP. Now inactive again, G protein leaves the enzyme and returns to its original state.
receptor tyrosine kinases
More than one signal transduction pathway can be triggered at once. The ability of a single lingand-binding event to trigger many pathways is the difference between receptor tyrosine kinases and G protein-coupled receptors.
Before signaling molecule binds, the receptors exist as monomers, each with an extracellular ligand-binding site. The binding of a signaling molecule causes two receptor monomers to associate closely with each other forming a dimer. The dimerization activates the tyrosine kinase region of each monomer, adding a phosphate from an ATP molecule to a tyrosine on the tail. Now that the receptor is fully activated, it is recognized by specific relay proteins inside the cell. Each protein binds to a specific phosphorylated tyrosine, undergoing a resulting structural change that activates the bound protein. Each activated protein triggers a transduction pathway, leading to a cellular response.
ion channel receptors
The gate in the ligand-gated ion cannel receptor remains closed until a ligand binds to the receptor. When the ligand binds to the receptor and the gate opens, specific ions can flow through the channel and rapidly change the concentration of that particular ion inside the cell. This change may diretly affect the activity of the cell in some way. When the ligand dissociates from this receptor the gate closes and ions no longer enter the cell.
These are found in cytoplasm or nucleus of target cells.
They bind to hydrophobic molecules such as lipids or small molecules that diffuse through cell membrane
Often act as a transcription factor
Succession of kinase activation that causes cellular response driven by phosphorylation:
The binding of the signaling molecule changes the receptor protein in some way, initating the process of transduction and changing the shape of the protein. Many signal transduction pathways include phosphorylation cascades and protein phosphatases. The balance between the two regulates the activity of proteins. Second messengers are also often involved in transduction process.
a series of protein kinases each add a phosphate group to the next one in line, activating the protein. This often amplifies signal because it activates large number of proteins.
small molecules or ions that are a part of a signal pathway
Because second messengers are small and water-soluble, they can readily spread throughout the cell by diffusion.
promotes protein production by activating transciption factors or activating proteins in the cytoplasm
A signaling pathway with lots of steps between the initail signaling at the cell surface and the cell's response results in signal amplification.
The response is specific to cells & receptor molecules
scaffolding proteins increase signal transduction.
programmed cell death
signal pathways activate proteases and nucleases (breaks down nucleic acid) that cause cell death
a secreted chemical that are formed in body fluids
acts on specific target cells
changes the taget cell's functioning
signal transduction pathway
a series of steps linking a mechanical, chemical, or electrical stimulus to a specific cellular response