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What does it mean if a molecule is moved against the concentration gradient?
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"Against the concentration gradient" means moving a substance from a region of lower concentration to a region of higher concentration (i.e., uphill). That movement is non‑spontaneous and therefore requires input of energy.
In cells this is called active transport. Energy sources include:
- ATP hydrolysis (primary active transport), e.g. the Na+/K+ pump that moves Na+ out and K+ in against their concentration gradients.
- Coupling to another spontaneous flux (secondary active transport), e.g. the sodium–glucose symporter uses the Na+ gradient to bring glucose into the cell against its concentration gradient.
- Other sources such as light-driven pumps or redox-driven transport.
For ions you must consider the electrochemical gradient (concentration plus electrical potential); the free‑energy change can be written for neutral solutes as ΔG = RT ln([inside]/[outside]) and for ions includes an electrical term (zFΔψ). If ΔG > 0 the transport is uphill and requires energy input.
In cells this is called active transport. Energy sources include:
- ATP hydrolysis (primary active transport), e.g. the Na+/K+ pump that moves Na+ out and K+ in against their concentration gradients.
- Coupling to another spontaneous flux (secondary active transport), e.g. the sodium–glucose symporter uses the Na+ gradient to bring glucose into the cell against its concentration gradient.
- Other sources such as light-driven pumps or redox-driven transport.
For ions you must consider the electrochemical gradient (concentration plus electrical potential); the free‑energy change can be written for neutral solutes as ΔG = RT ln([inside]/[outside]) and for ions includes an electrical term (zFΔψ). If ΔG > 0 the transport is uphill and requires energy input.
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