Summary
Passive movement of small nonpolar molecules down their concentration gradient directly across the membrane.
In depth
Molecules move randomly and, on average, from high to low concentration until equal. No energy from the cell is needed. The rate depends on the concentration gradient, temperature, distance, surface area and the size and lipid solubility of the molecule.
Fick's law summarises this: rate is proportional to surface area times concentration difference divided by thickness. This is why gas exchange surfaces are large, thin and well supplied with blood.
Gases move this way: oxygen enters cells and carbon dioxide leaves, and the speed is proportional to the gradient. Steroid hormones and anaesthetics cross because they dissolve in lipids. The partition coefficient (how soluble a molecule is in oil versus water) predicts how quickly it passes.
Diffusion is fast over short distances but slow over long ones, which is why large organisms need circulatory systems. It also never runs against a gradient, so cells that must concentrate a substance use active transport.
Example: in the lung, oxygen diffuses across a barrier of roughly 0.5 micrometres between alveolar air and blood in a fraction of a second.
Frequently asked questions
What is Simple Diffusion?
Passive movement of small nonpolar molecules down their concentration gradient directly across the membrane.
Which area of biology covers Simple Diffusion, and how advanced is it?
Simple Diffusion is one of the 52 topics in Cell Biology. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include Cell Size and Surface Area, Facilitated Diffusion, Osmosis.
Can you give an example of Simple Diffusion?
Oxygen diffuses from the alveoli into blood, and carbon dioxide diffuses the other way.
What should I learn before Simple Diffusion?
Work through the Beginner topics in Cell Biology first, such as Cell Theory, Prokaryotic Cell, Eukaryotic Cell, then return to Simple Diffusion. Intermediate topics assume the Beginner vocabulary.
What topics are related to Simple Diffusion?
The closest topics to Simple Diffusion are Cell Size and Surface Area, Facilitated Diffusion, Osmosis, Eukaryotic Cell. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Simple Diffusion?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Simple Diffusion. To practise the whole of Cell Biology, use the Cell Biology quiz.
How is Simple Diffusion different from Cell Size and Surface Area?
Simple Diffusion: Passive movement of small nonpolar molecules down their concentration gradient directly across the membrane. Cell Size and Surface Area: Cells stay small because a high surface-area-to-volume ratio allows efficient exchange of materials with the environment. Read both side by side and note what each one explains.
How should I study Simple Diffusion step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Cell Size and Surface Area and explain the difference aloud. Finally, take the Cell Biology quiz and come back to this page for any question you miss.