Digestion TOPIC H.2

Assessment Statements H2.1 State that digestive juices are secreted into the alimentary canal by glands, including salivary glands, gastric glands in the stomach wall, the pancreas and wall of the small intestine. H2.2 Explain the structural features of exocrine glands. H2.3 Compare the composition of saliva, gastric juice, and pancreatic juice. H2.4 Outline the control of digestive juice secretion by nerves and hormones using the example of gastric juice secretion. H 2.5 Outline the role of membrane bound enzymes on the surface of epithelial cells in the small intestine in digestion. H 2.6 Outline the reasons for cellulose not being digested in the alimentary canal. H2.7 Explain why pepsin and trypsin are initially synthesized as inactive precursors and how they are subsequently activated. H2.8 Discuss the roles of gastric acid and Helicobacter pylori in the development of stomach ulcers and stomach cancers. H 2.9 Explain the problem of lipid digestion in a hydrophilic medium and the role of bile in overcoming this.

Digestion- An Overview  Food moves from the mouth to the anus via the alimentary canal.  Along the way, digestive juices are added.  Secreted by exocrine glands (salivary, pancreas, and gland cells in stomach and small intestine).

Exocrine Glands  Secrete into ducts.  Made of secretory cells forming acinus around duct.

Digestive Juices FluidSourceComposition SalivaSalivary GlandsWater, electrolytes, salivary amylase, mucus, lysozyme Gastric JuicesStomachWater, mucus, enzymes including: pepsin, rennin, HCl Pancreatic JuicePancreasWater, bicarbonate, enzymes including: amylase, lipase, carboxypeptidase, & trypsinogen

Controlling the Juices  Nerves and hormones play a role.  Good example is gastric juices.

Gastric Juices 1. Sight or smell of food causes brain to send nerve impulses to gland cells in the stomach, causing secretion of compounds of gastric juices. 2. If chemoreceptors detect peptides in stomach or if stretch receptors detect distention, impulses are sent to the brain. 3. Brain sends impulses via vagus nerve to duodenum and part of the stomach closest to duodenum stimulating secretion of gastrin.

4. Hormone of gastrin stimulates secretion of acid and pepsinogen by two types of exocrine glands. 5. If pH falls too low, hormones of secretin and somatostatin inhibit gastrin secretion.

Ulcers and Cancer  Ulcers are sores on the stomach wall caused by digestion of the wall by pepsin and HCl.  Cancer is the growth of tumors in the stomach wall.

 Used to be thought that both were caused by stress and excessive gastric juice.  Now it is known most ulcers are caused by Helicobacter pyloris.  Perhaps linked with gastric cancer as well.

Zymogens  Inactive enzyme precursors that are altered after secretion to become active.  Done to prevent self digestion of cells that produce the enzyme.  Includes pepsinogen and trypsinogen.  Pepsinogen becomes pepsin under acidic conditions of stomach.  Trypsinogen becomes trypsin by enteropeptidase of small intestine.

Cellulose  Broken down by enzyme cellulase.  Humans lack this enzyme, preventing us from breaking it down.  Some animals have symbiotic relationship with bacteria allowing for energy absorption from cellulose.

Lipids  Lipids are water insoluble due to their hydrophobic nature.  Need to be accessible by enzyme lipase to break down into a fatty acid and monoglycerides.

Bile  Secreted by liver through canaliculi (narrow tubes) to the gallbladder for storage.  Bile acts like a detergent, allowing lipids to form emulsions.  Allows lipase to do its job.