Amino acids are small molecules, and they are freely filtered in the glomerulus, but more than 99% of them are reabsorbed in the proximal tubule by a sodium-dependent transport system. This process is an example of active transport since it requires energy in the form of ATP to move molecules against the concentration gradient. The reabsorption of amino acids is essential for the body because they are the building blocks of proteins, and their loss in urine can lead to protein malnutrition.