Digestive EnzymesDigestive 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 GlandsExocrine Glands

Enzymes production:

salivary glands

gastric glands in the stomach wall

pancreas

glands in the wall of the small intestine

SalivaSaliva

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 JuiceGastric Juice

Gastric juice is produced by the gastric glands in the stomach wall.

It contains:

mucus

hydrochloric acid

the precursor

PepsinPepsin

Pepsinogen is activated by the hydrochloric acid, which converts it into pepsin.

Pepsin converts proteins into peptides.

protein

pepsin

peptides

Pancreatic JuicePancreatic 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 AmylasePancreatic Amylase

Pancreatic amylase carries out the same reaction as salivary amylase:

starch

amylase

maltose

Pancreatic LipasePancreatic Lipase

Pancreatic lipase breaks down triglycerides into glycerol and fatty acids.

triglycerides

lipase

glycerol

fatty acids

Pancreatic LipasePancreatic Lipase

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

apart into smaller molecules.

BileBile

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.

http://www.biologyinmotion.com/bile/index.html

TrypsinTrypsin

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 peptidespeptides

MaltaseMaltase

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 EnzymesDigestive 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 EnzymesDigestive 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.