Mechanisms of Absorption

Different nutrients are absorbed by different mechanisms:

• Simple diffusion: part of the glucose and amino acids, and electrolytes such as chloride ions, are absorbed simply by moving down their concentration gradient.
• Facilitated transport: some amino acids and substances such as fructose are absorbed with the help of carrier ions, such as Na+, that assist their movement across the membrane.
• Active transport: some ions, such as sodium, are absorbed against their concentration gradient, which requires energy. Water is absorbed passively, along the concentration gradient created by the absorption of solutes. …