Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 7.4 Active Transport Uses Energy to Move Solutes Against Their Gradients
Passive transport is great when molecules can move on their own—but sometimes, the cell needs to push substances against their gradients. That’s where active transport comes in, using energy (usually ATP) to move molecules in or out, even when it’s an uphill battle.
1. What Is Active Transport?
Definition: Active transport is the movement of a substance across a membrane against its concentration gradient, requiring energy input, typically from ATP.
Importance: It maintains concentration differences of ions and molecules critical for nerve signaling, nutrient uptake, and cell homeostasis.
2. The Sodium-Potassium Pump: A Classic Example
Mechanism:
Pumps 3 sodium ions (Na⁺) out of the cell.
Brings 2 potassium ions (K⁺) into the cell.
Uses ATP to drive this exchange.
Result: Creates an electrochemical gradient—more positive charge outside the cell and more negative inside.
Significance:
Helps maintain resting membrane potential.
Keeps Na⁺ levels low and K⁺ levels high inside cells.
3. Electrogenic Pumps and Membrane Potential
Electrogenic pumps are active transporters that generate voltage across membranes.
Example:
The proton pump (H⁺ pump) in plants and fungi transports H⁺ ions out, creating a charge imbalance (voltage) and proton gradient.
This gradient powers many cellular processes, including cotransport.
4. Cotransport: Powered by Gradients
In cotransport, the downhill diffusion of one substance powers the uphill transport of another.
Example:
In plants, a proton gradient established by a proton pump is used to drive sucrose uptake via a cotransporter.
Energy-efficient: The cell doesn't use ATP directly for the second molecule—it uses the energy stored in the gradient.
In a Nutshell
Active transport requires ATP to move substances against their gradients.
The sodium-potassium pump and proton pumps are classic examples of electrogenic transporters.
These pumps create electrochemical gradients essential for cellular function.
Cotransport takes advantage of these gradients to bring other substances into the cell without direct ATP use.