6. Why Doesn't Everything Just Mix Together?
Key Question
If molecules are always moving, why doesn’t everything simply mix together?
What You’ll Learn
- What a concentration gradient is
- How random molecular motion produces diffusion
- Why net movement tends to occur from high concentration to low concentration
- What dynamic equilibrium means
- Why equilibrium does not mean that particles stop moving
- Why living systems need to preserve important concentration differences
Concentration Gradients
A concentration gradient is simply a difference in concentration between two regions.
If one area contains many particles and another contains fewer, random molecular motion tends to produce a net movement from the more concentrated region toward the less concentrated region.
This process is called diffusion.
Dynamic Equilibrium
Diffusion does not mean that particles eventually stop moving.
At equilibrium, particles continue moving randomly in both directions.
The difference is that there is no longer any net movement in one direction.
This is called dynamic equilibrium.
Why Biology Is Different
Living cells often need to maintain unequal concentrations of substances.
Neurons, for example, depend on different concentrations of ions inside and outside the cell.
If every ion simply became evenly distributed, neurons would lose the gradients they need to generate electrical signals.
Life therefore depends not just on movement, but on preserving the differences that matter.
Key Takeaway
Diffusion tends to reduce concentration differences.
But living systems often maintain those differences because they are essential for biological function.
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