Substances need to move around the body of an organism – obviously. To do this they must cross biological membranes. There are a number of ways this can happen, including: passive transport, active processes and bulk transport.
Firstly, passive transport. This can be also known as diffusion. An example of a passive process is facilitated diffusion. It does not require any additional energy and relies upon the internal kinetic energy of the individual molecules to move across the membrane, either through a channel or carrier protein.
Diffusion allows substances to move down a concentration gradient.
Carrier proteins are found in cell membranes and transport molecules across the membrane that would not be able to cross otherwise.
Looking a little closer into membranes – they are arranged in what is called the fluid mosaic model. Oxygen is able to diffuse freely across cell membranes as it is small and uncharged so it can move through the phospholipid bilayer. Others either need a channel or carrier protein.
If you made a cell membrane thinner it would increase the rate of diffusion (take less time) simply because a molecule would not have as far to travel.
If you reduce the steepness of a concentration gradient the rate of diffusion will decrease, but remember that, although molecules move in both directions, it is the net movement that is important.