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.
Active transport can also be known as active processes and uses energy. This must always happen across a cell membrane.
It uses membrane proteins (proteins embedded in the membrane) to move molecules from a low to a high concentration, in other words against a concentration gradient.
ATP is used to provide energy for active transport. Cells where active transport takes place typically have many mitochondria. They release energy, which is needed for active transport. Aerobic respiration occurs in mitochondria, releasing energy.
Active transport needs energy because the molecules are moving against their concentration gradient.
A co-transporter protein can transport two different molecules at a time by active transport. An example of one is the sodium-potassium co-transporter protein which is important in the membrane of nerve cells called neurones.
Finally, it does not needed to be known in depth, but bulk transport involves the movement of large amounts of molecules across a membrane. For example, cells that produce hormones in large quantities transport them out in bulk.
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.