One bound molecule outside can change thousands of molecules inside the cell.

Circle one:   True   False

A ligand docks on a GPCR. What happens next inside the membrane?

• The ligand crosses the membrane alone
• The receptor changes shape and the G protein swaps GDP for GTP
• The second messengers leave the cell

What is a second messenger?

• Small molecules flooding the cytosol to pass the message on
• Hormones travelling in the blood to the cell
• Genes turning on inside the nucleus

What turns a GPCR pathway off?

• The ligand is used up by binding
• The G protein hydrolyzes GTP to GDP and messengers are cleared
• The receptor dissolves into the membrane

Where does the amplification in a GPCR pathway come from?

• The ligand divides into many copies outside
• The receptor copies itself in the membrane
• One receptor feeds several G proteins and each enzyme makes many messengers

Epinephrine releases glucose from liver but quickens heart cells. Why?

• Same hormone, different inner wiring per cell type
• Different hormones arriving at each cell
• Stronger binding on the heart cells

A toxin blocks the G protein from hydrolyzing GTP. What happens?

• The signal weakens but still shuts off normally
• The G protein sticks on and the signal never shuts off
• The receptor stops binding ligand outside

Nora says the signal stops by itself once the ligand is used up. What is wrong?

• She is right; binding consumes the ligand
• Only the messengers need clearing, the rest resets itself
• Ligand may stay bound, so GTP hydrolysis and clearance must work