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.
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 Facilitated Diffusion, Cell Size and Surface Area, Plasma Membrane.
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 Facilitated Diffusion, Cell Size and Surface Area, Plasma Membrane, Osmosis. 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 Facilitated Diffusion?
Simple Diffusion: Passive movement of small nonpolar molecules down their concentration gradient directly across the membrane. Facilitated Diffusion: Passive transport of ions and polar molecules down a gradient through channel or carrier proteins. 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 Facilitated Diffusion and explain the difference aloud. Finally, take the Cell Biology quiz and come back to this page for any question you miss.