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.

Digestive Enzymes Enzymes are contained within the digestive juices that are secreted into the digestive tract by exocrine glands.  An exocrine gland is a gland that secretes substances through ducts or tubes onto a body surface or into a cavity.  This is in contrast to endocrine glands, which secrete substances directly into the blood stream.

Exocrine Glands The exocrine glands that play an important role in the digestive system are the:  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 inactive enzyme precursor pepsinogen  Some digestive enzymes are secreted as inactive precursors because, in their active state, they would be potentially very harmful to the exocrine gland cells that produce and secrete them.

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 inactive enzyme 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 The breakdown of lipids by pancreatic lipase poses special problems, because lipids are insoluble in the aqueous environment of the digestive tract.  As food travels through the digestive tract, the lipids within them melt and coalesce into large droplets.  Lipase is water-soluble, so is unable to enter the lipid droplets and would only be able to breakdown the lipids on the surface of the droplets.  Food does not remain in the digestive tract long enough for lipase to be able to completely digest the lipids in this manner.

Bile This problem is overcome by the action of bile. Bile molecules have 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.

Control of Digestive Juice Secretion The secretion of digestive enzymes is regulated by both nerves and hormones. Nerves carry signals to and from the brain and cause specific responses throughout the body. Hormones are chemical regulators that are secreted in one part of the body (by endocrine glands) and transported by the bloodstream to another part, where they cause a response.

The sight or smell of food causes the brain to send nerve impulses to the stomach to start secreting gastric juices. Much more gastric juice is secreted once food enters the stomach:  Receptors in the stomach lining send impulses to the brain, which then sends more impulses to the exocrine gland cells  Impulses are also send to endocrine glands in the stomach wall, which then release the hormone gastrin into the bloodstream.  Gastrin travels to the upper part of the stomach where it stimulates increased secretion of hydrochloric acid. Control of Digestive Juice Secretion