The calcium in a signal often comes from the ER
- The idea
- Predict
- See it
- Why
- Check
The idea
“When a signal raises calcium in a cell, the calcium has to come in from outside the cell.”
Calcium channels sit in the plasma membrane, and the fluid outside is full of calcium. The cell keeps a closer supply in its ER.
The short version
The mistake: When a signal raises calcium in a cell, the calcium has to come in from outside the cell.
What's actually true: In many pathways the signal makes the ER open its calcium channels, and the calcium stored there floods the cytosol.
- Cells keep cytosolic Ca2+ low. Pumps spend ATP to move it out of the cell and into the ER, so the ER holds far more than the cytosol.
- Many signals lead the cell to make a second messenger called IP3. It opens calcium channels in the ER, and calcium rushes out down its gradient.
- Some cells also let calcium in from outside. The ER release works without it.
On the exam: When a signal raises Ca2+, think of the ER first. IP3 opens that store.