h2 option digestion

H2 option: DIGESTION

H.2.1 State that digestive juices are secreted into the alimentary canal by glands, including salivary glands, gastric glands in the stomach wall, the pancreas and the wall of the small intestine

From the mouth to the anus the food passes through a tubular structure

ALIMENTARY CANAL

Digestion requires a number of different enzymes Saliva: from salivary glands in the mouth Gastric juice: from gastric glands in the stomach wall Pancreatic juice: from the pancreas, released into duodenum Intestinal juice: from the wall of the small intestine

Gastric juice

Pancreatic juice

Intestinal juice

Specific enzymes from the pancreas and the duodenal wall have specific roles in digesting macromolecules.

Fig. 41.17Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

H.2.2 Explain the structural features of exocrine gland cells Exocrine glands have a duct to secrete products Examples: sweat glands; glands producing digestive enzymes(e.g. Chief cells) The secretory cells of an exocrine gland produce the product and secrete it into the acinus (singular). The acini (plural) collect the product and pass it into the duct that leads into a lumen or out of the body.

Exocrine versus endocrine glands Exocrine glands Having duct Secrete product into acini to the lumen or out of body Examples: sweat glands; digestive enzyme producing glands Endocrine glands Ductless Secrete product into the blood stream Examples: hormoneproducing cells

H.2.3 Compare the composition of saliva, gastric juice and pancreatic juice

Place releasedMouth Stomach (chief cells & parietal cells) Small intestine

pH7 2

GlandSalivary glands Gastric glands

SecretionSaliva Gastric juice HCl Pancreatic juice

EnzymeSalivary amylase Pepsin (pepsinogen) Pancreatic amylase Trypsin (trysinogen) Lipase

SubstrateStarch Protein

ProductMaltose Polypeptides

8

pancreas

Starch

Maltose

Polypeptides

Glycerol + fatty acids Phosphate, glycerol + fatty acids

Phospholipase

Bicarbonate ions

Not an enzyme

Neutralizes stomach acid

H.2.4 Outline the control of digestive juice secretion by nerves and hormones, using the examples of secretion of gastric juice

ANIMAL NUTRITION

The Mammalian Digestive System1. 2. 3. 4. 5. The oral cavity, pharynx, and esophagus initiate food processing The stomach stores food and performs preliminary digestion The small intestine is the major organ of digestion and absorption Hormones help regulate digestion Reclaiming water is major function of the large intestine

Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Hormones help regulate digestion Hormones released by the wall of the stomach and duodenum help ensure that digestive secretions are present only when needed. When we see, smell, or taste food, impulses from the brain initiate the secretion of gastric juice. During and after a meal, gastrin will be released by the stomach as to the presence of particular polypeptides and amino acids as well as physical stretching of the wall of the stomach.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Gastrin .

Is a hormone which travels from the stomach via the blood to the gastric glands and stimulates the release of gastric juice

H.2.5 Outline the role of membrane bound enzymes on the surface of epithelial cells in the small intestine in digestion

The process1. Some digestive enzymes, e.g. maltase, are immobilized in the plasma membrane of epithelial cells on the surface of intestinal villi. 2. They are positioned in the membrane, so that the active site is towards the lumen of the small intestine. 3. When the substrate (maltose) binds to the active site, it is broken down into product (glucose) . 4. The glucose is absorbed into the epithelial cell and passed on to the blood capillary.

H.2.6 Outline the reasons for cellulose not being digested in the alimentary canal

Alimentary canal is ..

Cellulose A polysaccharide Does not dissolve in water ( problem!!!) Contains -1,4 linkages (indigestible by amylase) .compare with starch Mammals do not possess cellulase to digest cellulose Some bacteria do have cellulase and they can live in a mutualistic relationship with herbivores

H.2.7 Explain why pepsin and trypsin are initially synthesized as inactive precursors and how they are subsequently activated

Why . Inactive precursors Pepsin and trypsin are proteases If they were produced in their active form, they would digest the cell which made them. So they are produced as inactive precursors (pepsinogen and trypsinogen) The precursors are activated in the presence of HCl and enterokinase Since HCl and enterokinase are produced separately from the precursors, The precursor and the activator do not meet until they are in the lumen of the digestive tract.

H.2.8 Discuss the roles of gastric acid and Helicobacter pylori in the development of stomach ulcers and stomach cancers..please refer to handout (page 358 of Minka Peeters Weem book, 3rd ed)

The stomach s second line of defense against self-digestion is a coating of mucus, secreted by epithelial cells, that protects the stomach lining. Still, the epithelium is continually eroded, and the epithelium is completely replaced by mitosis every three days. Gastric ulcers, lesions in the stomach lining, are caused by the acid-tolerant bacterium Helicobacter pylori. Ulcers are often treated with antibiotics.


Table 27.3, continued


H.2.9 Explain the problem of lipid digestion in a hydrophilic medium and the role of bile in overcoming this

Nearly all the fat in a meal reaches the small intestine undigested. Normally fat molecules are insoluble in water, but bile salts, secreted by the gallbladder into the duodenum, coat tiny fats droplets and keep them from coalescing, a process known as emulsification. The large surface area of these small droplets is exposed to lipase, an enzyme that hydrolyzes fat molecules into glycerol, fatty acids, and glycerides. Globule of fats drops droplets .emulsification


The end .

h2 option digestion

Documents