State that protein carriers move molecules or ions across a membrane during active transport.
3.3 Active Transport
Objective: State that protein carriers move molecules or ions across a membrane during active transport.
When asked about active transport, remember to mention that it requires energy (usually ATP) and involves protein carriers that change shape to move substances against their concentration gradient.
What is Active Transport?
Active transport is the movement of molecules or ions across a cell membrane against their concentration gradient. This means the substance moves from an area of lower concentration to an area of higher concentration.
How Protein Carriers Work
- ATP binds to the carrier protein, providing the energy needed.
- The carrier protein changes shape, exposing the substance to the other side of the membrane.
- The substance is released into the destination side.
- ATP is hydrolysed to ADP + Pi, and the carrier returns to its original shape, ready for another cycle.
Analogy: Delivery Truck 🚚
Think of a protein carrier as a delivery truck that needs fuel (ATP) to cross a border (the membrane). Even if the destination side is already full (high concentration), the truck can still deliver goods because it has the fuel to push through.
Examples in the Body
- Na⁺/K⁺‑ATPase pump: moves 3 Na⁺ out and 2 K⁺ in each cycle.
- Glucose transporter (GLUT4) in muscle cells, which transports glucose into cells during exercise.
- H⁺‑ATPase in stomach lining, pumping acid into the stomach.
Key Points
- Active transport requires energy (ATP).
- It moves substances against their concentration gradient.
- Protein carriers change shape to transport molecules.
- It is essential for maintaining ion balances and nutrient uptake.
Comparison: Passive vs Active Transport
| Feature | Passive Transport | Active Transport |
|---|---|---|
| Energy Requirement | None | ATP |
| Direction Relative to Gradient | Along gradient (downhill) | Against gradient (uphill) |
| Typical Carriers | Channels, carriers | Pumps, active carriers |
Why does the Na⁺/K⁺‑ATPase pump need ATP?
Answer: Because it moves Na⁺ and K⁺ against their concentration gradients, which requires energy.
Revision
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