Primary vs Secondary Active Transport
In primary active transport, the energy comes directly from the ultimate source.
In secondary active transport, the energy comes from the ultimate source only, but indirectly.
Analogy of Mountain
We take a rock from base to the top of mountain
↓ The energy that we put in to take rock uphill, is stored in the rock in form of its high altitude
↓
Tie this rock to another rock at the base with a rope
↓
Let the top rock roll down the hill
↓
The rope pulls the second rock from bottom to top
Here the ultimate source of energy is us.
Movement of first rock is primary active because we are putting our energy directly on it.
The energy that we have put is stored in the rock as potential energy.
Movement of second rock is secondary active because the energy that we have put in the first rock is secondarily used to move that rock.
If we tie the second rock to a third rock and let the second rock roll down to move third rock up, the movement of third rock would be called tertiary active transport.
Example of Na-Ca Exchanger
Na-K ATPase pump moves Na from inside to outside of the cell using energy from ATP
↓
Energy from ATP is stored in Na outside the cell, in form of its high concentration
↓
Na-Ca exchanger is like a rope that couples Na and Ca movement
↓
It lets the Na diffuse down the concentration gradient
↓
Uses the energy from this downhill movement to push Ca uphill
Here the ultimate source of energy is ATP.
Movement of Na by Na-K ATPase pump is primary active because it uses ATP directly.
The energy that ATP has put, is stored in form of an electrochemical gradient of Na.
Movemnet of Ca by Na-Ca exchanger is secondary active because the energy that ATP has put on Na is secondary is used to transport Ca.
This stores the energy in Ca. Now if we use this energy from Ca to move a third substance uphill, it would be called tertiary active transport.