How is diffusion involved in osmosis?
Osmosis is the diffusion of water.
But it always involves another molecule and that often is salt. A "law" of osmosis says that water always follows salt. Where salt is higher in concentration, water will follow.
If you have some wilted celery, put it into water. There is more salt in each cell of the celery than in the plain tap water you will use. Water will move into the cell because of the higher salt concentration and the celery will become its' original self.
Here is a video which shows the changes that take place in plant cells when they are placed into pure water and a salt water solution.
Osmosis always involves the cell membrane (selectively permeable). While diffusion doesn't necessarily involve one.
The smell of cooking does involve simple diffusion. Or a drop of food coloring in a container of water will also.
Here is a video of a lab conducted to test osmosis in eggs placed into different solutions.
Video from: Noel Pauller