Cell Biology

Message converted from one form to another

Send a general communication throughout the body and are produced by endocrine cells

• Signals diffuse locally through the extracellular fluid, and stay within the region from which they're secreted
• E.g. Inflammation on wounds

• Can travel long distance, but signal is picked up by individual cells through their own lines
• Delivered quickly and specifically to individual target cells through private lines

Junctions where electrical signals are converted to neurotransmitters which diffuse across the gap

• Very short range and done through physical contact of signal molecules and receptors in plasma membrane
• In embryonic development, contact-dependent signals inhibit neighboring cells from becoming specialied in same way as cell signaling

It can alter the cell's shape, movement, metabolism, and gene expression

Most animal cells are programmed to kill themselves

• Each target protein is activated by only one type of signal
• This receptor receives an external signal and generates a new intracellular signal in response
• This internal signal is passed on until a response is generated

• Transform, or transduce, the signal into a molecular form suitable for passing the signal along or stimulating a response
• They relay the signal from the point in the cell at which it is received to the point at which the response is produced
• Amplify the signal received, making it stronger
• Can distribute the signal to influence several processes in parallel

• If acting on molecule already present in the cell, it will be quick
• If extracellular signals need changes in gene expression to make new proteins, it will take longer

• Large hydrophilic hormones cannot
• Small hydrophobic ones can (steroids)
• Binding of these small hydrophobic hormones to receptors in the cytosol or nucleus cause a large conformational change in their target proteins, that then either promote or inhibit gene transcription

• NO is a common gas that can diffuse (acts locally to vasodilate smooth muscle - vasodilate)
• Gases can alter conditions in seconds

• Binds and activates the enzyme guanylyl cyclase
• Stimulates the formation of cyclic GMP from the nucleotide GTP
• Cyclic GMP is the next signaling molecule in the chain, which leads to the cell response
• Local blood-vessel dilation occurs

Blocks the enzyme that degrades cyclic GMP, so the NO signal is maintained

• Relay signal onward, spreading it
• Amplify the signal, so few have a great affect
• Receive other signals from other pathways
• Distribute signal to other pathways

• Signal switches from active to inactive state
• If a signaling pathway is to recover after transmitting a signal and make itself ready to transmit another, every molecular switch must be reset to its original, unstimulated state
• For every activation mechanism there hase to be an inactivation mechanism

• Activated or deactivated by phosphorylation
• Protein kinase (adds phosphate group)
• Protein phosphatase (removes phosphate)

• Switch between active or inactive form depending on if GTP (active) or GDP (inactive) is bound
• They can hydrolyze the GTP on their own because of their GTPase activity

Opens ion channels and produces an electrical current

• Acive GTP-binding proteins
• Which then either activates an ezyme or ion channel

• Act as enzymes or work with enzymes inside the cell
• Switching on this enzymatic activity then generates a hose of additional signals, including small molecules that are released into the cytosol

• The natural ligand that is bound to the receptor can be mimicked by other molecules
• This ligand will either bind to the ligand-binding site or to another area on the receptor
• This will result in over-stimulating or blocking the receptor's activity

• Acts on the neurotransmitter receptor in the central nervous system and muscle cells which are also acted upon by acetylcholine
• Enhancing effects of epi and norepi

• Largest family of cell surface receptors
• Target for drug development (more than half of drugs work on GPCRs)
• Made of single protein chain
• Weaves back and forth between the plasma membrane 7 times

• Made of 3 subunits
• Attached by lipids to plasma membrane
• Unstimulated, GDP is bound to alpha subunit
• Stimulated, GTP is bound to alpha subunit

• Binds to alpha subunit, and prevents the GTPase activity, therefore it stays activated much longer than it should
• Causes prolonged release of Cl- and H2O
• Can be deathly diarrhea and dehydration

• Heartbeat regulated by 2 sets of nerves (1 speeds and 1 slows)
• Nerves that signal slowing releases acetylcholine
• By complex will o pen the K+ channels, and K+ flows out and inhibits the heart cells excitability

• Adenylyl cyclase (involved in making cyclic AMP)
• Phospholipase C (inositol triphosphate and diacylglycerol)

• Activated adenylyl cyclase catalyzes the synthesis of cyclic AMP from ATP
• Cyclic AMP activates cyclic-AMP-dependent protein kinase (PKA)
• Activated PKA catalyzes phosphorylation of serines or proteins
• Adrenaline signal ultimately causes the breakdown of glycogen in skeletal muscles

• Triggers increase in Ca++
• Activated phospholipase C removes the sugar-phosphate head it creates 2 signaling molecules: inositl 1,4,5-triphosphate (IP3) and diacylglycerol (DAG)
• IP3 remains in cytosol
• DAG stays in plasma membrane
• IP3 opens Ca++ channels of ER and Ca++ increases in cytosol

IP3 released into the cytosol will eventually encounter endoplasmic reticulum; there it binds to and opens Ca++ channels that are embedded in the endoplasmic reticulum membrane