Diffusion: random motion can give directional net transfer
| English | 中文 | Pinyin |
|---|---|---|
| diffusion/dɪˈfjuːʒn/ | 扩散 | kuò sàn |
| concentration gradient/ˌkɒnsənˈtreɪʃn ˈɡreɪdɪənt/ | 浓度梯度 | nóng dù tī dù |
What would explain this observation?
- Oxygen enters a respiring cell while carbon dioxide can leave it. The direction of net diffusion 扩散 depends on the concentration difference for each substance.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Diffusion is the spreading of particles in a solution or gas, giving net movement from higher concentration to lower concentration. Oxygen and carbon dioxide diffuse during gas exchange. Urea can diffuse from cells into blood plasma before excretion. Particles move in both directions, while the concentration difference determines the net result.
- diffusion: Net spreading of particles from higher to lower concentration; concentration gradient 浓度梯度: A difference in concentration between two regions.
Which change increases net diffusion rate when other conditions match?
A larger concentration difference produces a greater net diffusion rate under comparable conditions. Higher temperature increases particle motion, and a larger membrane surface area allows more transfer at once. A thinner exchange surface provides a shorter route, as developed in the exchange-surface case.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- A larger concentration difference produces a greater net diffusion rate under comparable conditions. Higher temperature increases particle motion, and a larger membrane surface area allows more transfer at once. A thinner exchange surface provides a shorter route, as developed in the exchange-surface case.
- Use a teacher-approved model or provided concentration data. State which substance is being tracked, identify its high and low concentration sides, and keep temperature, exposed area and time consistent when comparing one variable. Do not treat a coloured model substance as a direct measurement of every gas or solute.
Which two habits make the investigation or model in this case more defensible?
Use a teacher-approved model or provided concentration data. State which substance is being tracked, identify its high and low concentration sides, and keep temperature, exposed area and time consistent when comparing one variable. Do not treat a coloured model substance as a direct measurement of every gas or solute.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: in a model interval, 70 particles cross from A to B and 25 from B to A. Net transfer A→B =70−25=45 particles. If both directions later show 40 crossings, net transfer is zero, although 80 crossings occurred. This illustrates why zero net movement does not mean molecules stop moving.
In one interval 62 particles cross A→B and 22 cross B→A. Calculate net transfer A→B. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
In one interval 62 particles cross A→B and 22 cross B→A. Calculate net transfer A→B.
The result is 40 particles. Known: in a model interval, 70 particles cross from A to B and 25 from B to A. Net transfer A→B =70−25=45 particles. If both directions later show 40 crossings, net transfer is zero, although 80 crossings occurred. This illustrates why zero net movement does not mean molecules stop moving.
Check the conclusion and its limits
- Diffusion does not require respiration to supply energy for the particles’ net movement. A concentration gradient is specific to a substance; oxygen and carbon dioxide can have opposite gradients across the same surface.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
At zero net diffusion, every particle has stopped moving. This claim is false: Diffusion does not require respiration to supply energy for the particles’ net movement. A concentration gradient is specific to a substance; oxygen and carbon dioxide can have opposite gradients across the same surface.
Diffusion: random motion can give directional net transfer: A larger concentration difference produces a greater net diffusion rate under comparable conditions. Higher temperature increases particle motion, and a larger membrane surface area allows more transfer at once. A thinner exchange surface provides a shorter route, as developed in the exchange-surface case.
At zero net diffusion, every particle has stopped moving.
Diffusion does not require respiration to supply energy for the particles’ net movement. A concentration gradient is specific to a substance; oxygen and carbon dioxide can have opposite gradients across the same surface.
Net spreading of particles from higher to lower concentration: write the technical term.
diffusion means Net spreading of particles from higher to lower concentration.