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Cell Communication - Signal Reception Ch. 11 Section 11.2 Whiteboards please

How cells “hear” the call • The target cell has a receptor protein that recognizes the chemical signal  Recognition happens when the signal molecule binds to the receptor  They fit together like lock and key

Critical Vocab!! • Signal molecule = ligand

Signal Reception • • • •

Ligand binds to receptor Receptor protein changes shape This starts the transduction pathway Leads to cell response

Quick Think

• How do cells “hear” the message from other cells?

Chemical Signals • Most ligands (chemical signals) are too large to pass through the cell membrane • BUT… Too big

Intracellular receptor proteins • Some signal receptors are inside the target cell

 Dissolved in the cytosol  In the nucleus

• These chemical signals have to be hydrophobic or small enough to cross the membrane

• Examples are  steroid & thyroid (lipids) hormones in animals  Nitric oxide (NO) - small gas

Testosterone - a steroid signal molecule • Testosterone is a hormone, a • • •

steroid, secreted by the testis It travels through the blood stream It enters all kinds of cells But the only cells that can “hear” it are the target cells that have the right receptor molecule inside the cell Once inside, testosterone binds to the receptor and helps turn on genes that control male sex characteristics

How do genes get “turned on”? • Protein receptors that “turn on” genes are called transcription factors • Transcription factors control which genes are “turned on” or transcribed into mRNA

Quick Think

• Where are intracellular reception proteins?

• Why can their chemical signals pass into the cell?

Quick Think

• Nerve Growth Factor (NGF) is a water-soluble signal molecule. Would you expect the receptor for NGF to be intracellular or in the cell membrane? Why?

Most signal receptor proteins are in the cell membrane • There are 3 main types of cell membrane signal receptors  G-protein-linked receptors  Receptor tyrosine kinases  Ion-channel receptors

G-protein linked receptors • Made up of a receptor •

protein + a G protein They have 7 α helices

The G-protein part • The G-protein part is like an on/off switch for the receptor protein

• When GDP (cousin of ATP) is bound to it, the G protein is inactive

• Ligand attaches • G-protein binds to GTP and becomes active • G protein slides down cell membrane • G protein binds to an enzyme to start transduction

The G-protein • G-protein receptor systems

are really common and play a role in lots of different cell processes  Development of embryos  Vision & Smell

• Lots of diseases involve Gproteins

 Bacterial infection like botulism and cholera interrupt G-protein function  Most drugs (medicines) work by influencing a G-protein pathway

Quick Think

• How does a G-protein linked receptor function?

Tyrosine-kinase receptors • Best when the cell needs to trigger several cell responses at once

Tyrosine-kinase receptors • kinase - enzyme that catalyzes the transfer of phosphate groups

• So it is an enzyme

that functions to transfer a phosphate group from ATP to tyrosine

Tyrosine-kinase receptors • Has 3 parts:  Extracellular (outside the cell) signalbinding site  1 α helix  An intracellular (inside the cell) tail with a bunch of tyrosines on it

Tyrosine-kinase receptors • Once 2 signal molecules binds to 2 tyrosine-kinase receptors, they pair up to form a dimer

Tyrosine-kinase receptors • Once this happens, this causes the tyrosine-kinase part of the receptor to take phosphate from ATP and add it to the tyrosine tail

• This makes the receptor protein “fully activated”

Tyrosine-kinase receptors • Once the tyrosine-kinase

receptor is fully activated, it is recognized by other proteins in the cell

• Each tyrosine-kinase

receptor may activate many different proteins that may lead to many different cell responses

Quick Think

• Summarize how a tyrosine-kinase receptor functions.

Ligand-gated ion channel protein • Receptor protein that acts

as a gate for entry into the cell

• When a signal molecule

binds to the receptor protein, the gate opens to allow specific ions (like sodium or calcium) to flow through it

Ligand-ion-gated channel protein • When the ligand (the signal

molecule) binds, it changes the shape of the channel protein

• This causes the “gate” to open and allow ions through

• When the ligand

dissociates, the gate

Ligand-gated-ion channel protein • Important in the nervous system  Neurotransmitter binds as a ligand  Ion-channel proteins open  Ions flow into cell  Triggers an electrical signal that moves down the length of the cell

Quick Think

• Summarize how a Ligandgated ion channel functions.

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