How does solute concentration affect rate of diffusion?

Assuming you are talking about simple diffusion, then:

The rate of diffusion increases as the concentration gradient increases.  If the solutes are more evenly distributed between both sides of the membrane, then rate of diffusion is slower. 

Osmosis: diffusion causes to move across a membrane but NOT solutes
- Simple diffusion & osmosis = does NOT require energy expenditure
•   Passive process & distinct from active transport
- Facilitated Diffusion: movement of solute across a membrane when membrane itself (PURE LIPID BILAYER) is impermeable to that solute
•   Without energy expenditure
- Facilitated Diffusion: Channels – channel proteins in plasma membrane allow material that can’t pass through membrane by simple diffusion to flow through plasma membrane
•   Gated – if channel is open in response to specific stimuli
•   Voltage gated – response to change in electrical potential across membrane
•   Ligand gated – opens in response to binding of specific molecule like neurotransmitter
- Facilitated Diffusion: Carriers – don’t form tunnel through membranes like ion channels
•   Bind molecule to be transported at 1 side of membrane & undergo conformational change to move molecule to other side of membrane
- Active Transport: molecule movement through plasma membrane against gradient THEREFORE requires energy input & ALWAYS involves protein!
•   Primary active – molecule transport couple to ATP hydrolysis
•   Secondary active – not DIRECTLY to ATP hydrolysis