Down Is Free. Up Costs.

You'll learnwhy moving down a gradient is free but moving up costs the cell ATP.

The membrane is a gatekeeper, not a wall, and there is only ever one question at the gate: which way does the gradient point? Down the gradient the trip is free; up it, the cell pays in ATP. Whether a protein is involved decides nothing at all. Twelve short steps; press Next for each one.

12 STEPS · 6 QUICK CHECKS · PASSIVE = DOWN THE GRADIENT, NO ATP · ACTIVE = UP THE GRADIENT, ATP · A PROTEIN IS NOT A BATTERY · v5

down the gradient is free · up it, the cell pays

Before you start

What you're looking at

One membrane with two sides: solute dots crowded outside, scarce inside, and an arrow marking the gradient from high to low.

The question

For every crossing: which way does the gradient point, and does the cell pay or cross for free?

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