List the factors that affect diffusion...
1. Size of driving force
2. Membrane surface area
3. Membrane permeability

Describe the movement of molecules with respect to concentration gradient...
When molecules are moving from the region of higher concentration to the region of lower concentration, it is called going down their concentration gradient. On the other hand, when molecules are moving from the region of lower concentration to the region of higher concentration, it is called going up their concentration gradient which basically means going against its concentration gradient.

Define diffusion...
Diffusion is the spontaneous net movement of molecules moving from a region of higher concentration to a region of lower concentration.

What is concentration gradient?
I think concentration gradient is present when molecules are not evenly distributed and it kinds of forces the molecules in the region with higher concentration to go to the region with lower concentration.