Membrane Permeability
Introduced| English | Meaning |
|---|---|
| diffusion/dɪˈfjuːʒn/ | The net movement of particles from higher to lower concentration due to their random motion. |
| concentration gradient/ˌkɒnsənˈtreɪʃn ˈɡreɪdɪənt/ | A change in a substance's concentration across a distance or between regions. |
| passive/ˈpæsɪv/ | passive |
Crossing without effort
- Some things enter and leave a cell all by themselves, with no energy spent.
- Oxygen drifts in; carbon dioxide drifts out — no pumping needed.
- This free movement is called diffusion.
- It is the simplest way a substance can cross the membrane.
Diffusion: high to low
- Diffusion 扩散 is the net movement of particles from where they are crowded to where they are sparse.
- Particles are always moving randomly, so they spread out on their own.
- The overall drift is from high concentration to low.
- Given time, the particles end up evenly spread.
In diffusion, particles move…
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Diffusion is the net movement of particles down their concentration gradient — high to low.
The concentration gradient
- The difference in concentration between two places is the concentration gradient 浓度梯度.
- A steeper gradient makes diffusion faster.
- As the two sides even out, the gradient shrinks and diffusion slows.
- When both sides are equal, net movement stops.
Diffusion across a membrane
Drag the concentrations. Particles diffuse down their gradient until both sides are equal.
The difference in concentration between two regions is the concentration ____.
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A steeper concentration gradient makes diffusion faster.
Passive: no energy needed
- Diffusion is passive 被动的 — the cell spends no ATP on it.
- The particles' own random motion does all the work.
- This is why simple gases cross the membrane for free.
- Warmth, a big surface, and a thin membrane all speed it up.
Diffusion is passive — it needs no energy from the cell.
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Diffusion is passive: particles move on their own random motion, so the cell spends no ATP.
Which change makes diffusion across a membrane faster?
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A steeper gradient (plus a thin membrane, big area, and warmth) speeds diffusion.
Select all true statements about diffusion.
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Diffusion goes high → low, never the reverse on its own. The other three are correct.
When both sides are equal, the particles do not stop moving. They keep darting about — but just as many cross each way, so there is no net change. Equal concentration means no net movement, not no movement at all.
Oxygen into your blood:
- Air in the lungs is rich in oxygen; blood arriving is low in it.
- That gradient drives oxygen to diffuse across the thin lung wall into the blood.
- No energy is spent — the gradient does the work, all by diffusion.
Diffusion is the net, passive movement of particles down a concentration gradient — from high to low — needing no ATP. A steeper gradient, a thinner membrane, and a larger area all speed it up. Net movement stops when both sides are equal, though particles keep moving.
Write from oxygen crossing freely
- Oxygen is small and nonpolar.
- A sodium ion is charged.
Which one crosses the bilayer by simple diffusion?
Small nonpolar molecules slip between the tails.
Which crosses the bilayer without a protein?
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Charge stops an ion from crossing the oily tails.