The structure of a phospholipid bilayer makes membranes a very effective barrier, so it difficult for most substances to cross a membrane. There are several mechanisms that cells use to move chemicals across the cell membrane.
One type of transport is called facilitated diffusion. With diffusion, substances can only be moved from areas of high concentration to areas of low concentration, but if they are very water like, they will not be able to diffuse through the lipid inside of the membrane bilayer. One way cells have solved the problem is to have proteins sitting in the membrane that act as channels to guide the substances across the membrane. Because substances are moving from areas of high concentration to areas of low concentration, no energy is required for facilitated diffusion.
A few substances can move across membranes without any assistance. These chemicals always move from an area of high concentration to an area of low concentration. This is called passive diffusion and substances can move only with the concentration gradient.
A third type of transport is called active transport. Active transport allows substances to be moved against a concentration gradient, from areas of low concentration to areas of high concentration, but it requires energy to do this. A metal, sodium (Na+), plays a key role in this type of transport. Na+ is actively kept at low concentrations inside the cell and high concentrations outside the cell. The cell can use these different concentrations of Na+ inside and outside the cell to provide energy to move other substances against their concentration gradient.
For example, in the filtrate of the kidneys, amino acids are in a low concentration and need to be moved into the tubules where the concentration can be quite high already. There are special proteins that move both sodium and amino acids at the same time. The movement of sodium from high to low concentration provides the energy to move the amino acids from low to high concentration.
After the amino acids are moved into the tubules via active transport, the amino acids need to move out of the tubules into the blood. In the tubules, amino acids are in high concentration and in the blood they are at low concentrations. Movement from the tubules to the blood occurs via facilitated diffusion.