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: Movement of substances against their concentration gradient using energy, typically from ATP.
- Importance: Maintains ion and molecule concentration differences crucial for nerve signals, nutrient uptake, and 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 the exchange
- Result: Creates an electrochemical gradient—positive outside, negative inside.
- Significance:
- Maintains resting membrane potential
- Keeps intracellular K⁺ high and Na⁺ low
3. Electrogenic Pumps and Membrane Potential
- Electrogenic pumps generate voltage across membranes by moving ions.
- Example: Proton pump in plants and fungi moves H⁺ out of the cell, creating voltage and a gradient.
- This gradient is used for cellular processes like cotransport.
4. Cotransport: Powered by Gradients
- In cotransport, the downhill movement of one substance powers the uphill transport of another.
- Example: In plants, H⁺ gradients help transport sucrose via a cotransporter.
- Energy-efficient: No direct ATP is used for the second molecule; the gradient's energy is harnessed.
In a Nutshell
- Active transport uses ATP to move substances against their gradients.
- Key pumps like the sodium-potassium pump and proton pump create vital electrochemical gradients.
- These gradients drive cellular work, including cotransport without direct ATP usage.