The most commonly found carbohydrate in the human diet is starch. This is broken down into glucose in two stages:

amylase maltase

starch ? maltose ? glucose

• In the mouth, salivary amylase begins to digest the starch. Although not much digestion takes place it helps to rid the mouth of starch which helps prevent bacterial infection.
• The rest of the starch is digested by pancreatic amylase in the duodenum to create maltose, a disaccharide. Glycogen is also digested.
• Disaccharides are digested in monosaccharides by disaccharidases. This includes, maltose, sucrose and lactose using three different enzymes:

maltase

maltose ? glucose

sucrase

sucrose ? glucose + fructose

lactase

lactose ? glucose + galactose

Disaccharidases are generally found within the ileum epithelial cell membrane so glucose can be made in the region that it needs to be absorbed.

During digestion, carbohydrates are absorbed by epithelial cells in the small intestine using different methods:

• Glucose is taken up by active transport via a protein carrier. The protein carrier works by secondary active transport and both glucose and Na+ are transported across the membrane. Glucose then passes on into capillaries via facilitated diffusion
• Fructose relies entirely on facilitated diffusion.
• Amino acids are transported by co-transport carrier proteins using secondary active transport. The proteins absorb both the amino acids and Na+. The amino acids then move from the epithelial cells into the capillaries by facilitated diffusion.