Digestive Enzymes Throughout the digestive system, enzymes break down the food into useful substances.  Recall that enzymes are protein catalysts – they speed up chemical reactions.  The substance that an enzyme acts on is called its substrate.

Exocrine Glands Enzymes production:  salivary glands  gastric glands in the stomach wall  pancreas  glands in the wall of the small intestine

Saliva Saliva is produced by the salivary glands. It contains mucus and the enzyme salivary amylase.  breaks down starch into maltose (a disaccharide) starch amylase maltose

Gastric Juice Gastric juice is produced by the gastric glands in the stomach wall. It contains:  mucus  hydrochloric acid  the precursor

Pepsin Pepsinogen is activated by the hydrochloric acid, which converts it into pepsin. Pepsin converts proteins into peptides. protein pepsin peptides

Pancreatic Juice Pancreatic juice is produced by the exocrine glands in the pancreas It contains:  bicarbonate ions (alkaline)  many enzymes, including pancreatic amylase and pancreatic lipase  the precursor trypsinogen

Pancreatic Amylase Pancreatic amylase carries out the same reaction as salivary amylase: starch amylase maltose

Pancreatic Lipase Pancreatic lipase breaks down triglycerides into glycerol and fatty acids. triglycerides lipase glycerol fatty acids

Pancreatic Lipase Lipase reacts too slowly to break apart lipids. To overcome this problem, fat molecules are broken apart into smaller molecules.

Bile Bile a hydrophobic end and a hydrophillic end, so are able to interact with both the lipids and the water, causing the lipids to break up into smaller droplets. This process is called emulsification. This speeds up the digestion of the lipids in the small intestine.

Trypsin Trypsinogen is activated by an enzyme called enteropeptidase, which is secreted by the lining of the small intestine. Enteropeptidase converts trypsinogen to trypsin. Trypsin continues the breakdown of proteins. trypsin smaller peptides peptides

Maltase Glands in the wall of the small intestine produce enzymes such as maltase (a disaccharidase). Maltase breaks down maltose into two glucose molecules. maltose maltase glucose 2

Digestive Enzymes Enzymes produced by the wall of the small intestine are not secreted like the other enzymes of the digestive tract. Instead, they remain attached to the plasma membrane of the cells lining the intestine, with their active sites exposed to the food in the intestine. With this arrangement, the substrates can be digested and then the products of digestion can immediately be absorbed into the body.

Digestive Enzymes Some macromolecules cannot be digested by humans, for example, cellulose.  Humans cannot digest cellulose because they do not have the gene that produces the enzyme cellulase.