How can particles spread out if your cells don’t spend any energy?
Your perfume reaches the other side of a room even though nobody is pushing each scent particle there. That happens because particles are always moving in random directions, and when you start with more of them crowded in one place, the overall result is a bigger flow away from the crowded area than back into it.
When you see particles ending up more evenly spread, you call that diffusion.
When particles end up drifting overall from a region where they are more packed together to a region where they are less packed together, because of their own random motion, that process is called diffusion.
Your cells do not need ATP for this, so diffusion is a passive process.
Watch the wording: particles do not march in one neat direction. They still move all over the place. The key idea is net movement , if one side starts more crowded, more particles leave that side than return, so the overall shift is from high concentration to low concentration.
This keeps happening until you reach equilibrium, where concentration is the same throughout the space. At that point, particles still move, but there is no longer an overall change because the concentration gradient has disappeared.
Think of a chaotic group chat after exam results drop. If one chat has loads of people posting messages and another is almost silent, screenshots and replies spread out fast at first. Later, when both chats are equally active, messages still appear in both places, but there’s no obvious one-way flood anymore.
Don’t confuse passive with slow. Over tiny distances, diffusion is extremely quick. In your lungs, oxygen can move from the air spaces into your blood in under a second.
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