Slide 1
Random motion creates net spread
Diffusion spreads particles through their random thermal motion. When concentration differs across space, the net movement tends toward the less concentrated region.
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Individual particles still move in both directions. The imbalance arises because more particles initially leave the crowded side, so a directional net flow can emerge without every particle following a planned route.
Slide 2
Equilibrium does not stop motion
Once concentrations become uniform in a simple closed system, random motion continues. Opposite transfers balance, removing the net concentration-driven flow.
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A dye molecule does not recognize that equilibrium has been reached. The statistical pattern changes because equal regions now contain similar numbers of particles available to move, not because molecules switch off their motion.
Slide 3
A membrane can limit the route
Diffusion across a membrane requires a pathway the substance can use. Channels or carriers can enable passive transport without supplying energy directly.
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Facilitated diffusion remains passive even though a protein is involved. Its direction follows the relevant gradient, while the protein’s selectivity and abundance determine which substances move and how quickly.
Slide 4
Distance strongly affects time
Diffusion works efficiently over short distances but becomes slow over long ones. Typical diffusion time grows approximately with distance squared.
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Doubling a distance requires about four times as long in the same simple diffusion conditions. This helps explain why large organisms use bulk flow to complement diffusion rather than relying on diffusion across their entire bodies.
Slide 5
Pumps maintain what diffusion erodes
Active transport can create concentration differences that passive movement tends to reduce. Living cells combine both processes to maintain useful conditions.
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Water movement across a selectively permeable membrane is called osmosis. If solutes cannot cross readily, the resulting water redistribution can change cell volume, linking permeability to mechanical consequences.
Sources
- Principles of Membrane Transport
NCBI Bookshelf
- Cell Membrane Plasma Membrane
National Human Genome Research Institute