Digestion And Absorption

Front 1

Briefly describe how is carbohydrate digested in the body.

Back 1

0. starch is composed of amylose and amylopectin

1. Thedigestion of starch begins in the mouth by the secretion of α-amylase in the saliva. This continues in thestomach but is eventually inhibited by the acidic conditions in the stomach.

2. The pancreas also secretesα-amylase into the duodenum where starch digestion is completed.

3. α-Amylase attacks theα-1,4-linkages in amylose to yield maltose (disaccharide) and glucose (monosaccharide), and inamylopectin and glycogen to give various poly- and oligosaccharides.

4. Isomaltase in the intestine thenattacks the α-1,6-linkages in the poly- and oligosaccharides to yield maltose and glucose.

5. Maltose isthem split into two glucose molecules by maltase.

6. Our diet also contains two other disaccharides,sucrose and lactose. These are broken down in the intestine by sucrase and lactase, respectively. Theproducts of sucrose digestion are glucose and fructose, and the products of lactose digestion are glucoseand galactose.

Each of the monosaccharides are then absorbed by transporters situated in themembranes of absorptive cells in the villi of the intestine.

Front 2

Briefly describe how is protein digested in the body

Back 2

1. Proteins are comprised of amino acids and are broken down by peptidases which cleave the peptidebonds linking the amino acids together.

2. Protein digestion begins in the stomach through the action ofpepsin, but this accounts for only a small part of the protein in the diet.

3. The remainder is digested in theduodenum by peptidases (trypsin, chymotrypsin, elastase and carboxypeptidases A and B) secreted bythe pancreas. These attack proteins at various points to yield either single amino acids or di- andtri-peptides which are then taken up by carriers in the villi membrane.

4. Intracellular di- andtri-pepetidases then cleave the small peptides into single amino acids.

Front 3

briefly describe how is fat digested in the body

Back 3

1. The bulk of ingested fat (lipids) is in the form of triacylglycerol, although some phospholipids andcholesterol esters are also present in the diet.

2. These are digested by enzymes (lipases, phospholipasesand cholesterol esterases) secreted by the pancreas.

3. A major problem with fat digestion is that theenzymes are water soluble but the lipids are not. This is overcome by the process of emulsificationwhere bile acids released from the gall bladder coat the lipid droplets and prevent them from coalescing.A complex of lipase and colipase is then able to hydrolyse triacylglycerol to monoacylglycerol and fattyacids. These too are insoluble in water so are aggregated together (approximately 20 molecules) in 4micelles before absorption by diffusion can take place. This process also involves bile acids. Lipidsoluble vitamins (A, D, E and K) are also absorbed by this route.

Front 4

what is enteric nervous system?

Back 4

The gastrointestinal tract is innervated by both the sympathetic and parasympathetic of theautonomic nervous system.

Autonomic nerves form synapses directly on smooth muscle and secretorycells, however, the gastrointestinal system possesses a nervous system of its own (the enteric nervoussystem) which also receives autonomic innervation

Front 5

what are the types of enteric neurons exist?

Back 5

Motor neunones

Inter neurones

Sensory neurones

Front 6

how is the function of motor and inter neurones controlled ?

Back 6

Motor and interneurones can be either excitatory orinhibitory, and act to regulate muscle contraction/relaxation and secretions.

Front 7

How is the function of sensory neurones controlled?

Back 7

Sensory neurones areactivated by mechanoreceptors (which detect stretching of the muscle) and chemoreceptors (whichdetect irritant substances in the gut lumen).

Front 8

what are the neurotransmitters used in the enteric nervous system?

Back 8

Although some of these neurones use acetylcholine as atransmitter, many do not. Such neurones are termed non-adrenergic non-cholinergic (NANC) and usevariety of transmitters, such as ATP, 5-hydroxytryptamine (5-HT), substance P, vasoactive intestinalpolypeptide (VIP) and nitric oxide (NO)

Front 9

what is the name of the cell that secrete HCl in the stomach?

Back 9

parietal cell together with intrinsic factor

Front 10

Briefly describe the mechanism of HCl secretion

Back 10

Acid secretion involves a number ofevents. Carbonic anhydrase in the parietal cell cytoplasm catalyses the conversion of CO2 and H2O toH2CO3 which dissociates into H+and HCO3-. The H+ions are then actively pumped into the lumen ofthe stomach via H+/K+ ATPases (proton pumps) which also move K+ions in the opposite direction. TheHCO3-ions are transported into the bloodstream by antiporters in exchange for Clionswhich are thencarried by symporters into the lumen together with K+ions. The overall effect is the secretion of anisotonic solution (150 mM) of HCl at pH 1 into the lumen of the stomach.

Front 11

What is the role of gastrin?

Where is it released from?

Back 11

Gastrin is one of the chemical signals released from and/or acting on the other cell types in the gastric mucosa.

G cells release gastrin into the blood vessels perfusing the stomach wall.Gastrin then re-enters the mucosal cells to promote acid secretion by activating CCK type B receptorspresent on parietal cells. In addition, gastrin acts (through the same receptors) onenterochromaffin-like (ECL) cells present within the mucosa

Front 12

what is the role of ECL cell?

Back 12

These cells release histamine (a localmediator) causing acid secretion via the stimulation of H2 receptors on the parietal cells.

Front 13

Briefly explain how is the secretion of HCl controlled

Back 13

G cells,parietal cells and ECL cells also receive innervation from the vagus nerve. The vagus releasesacetylcholine which acts on muscarinic receptors on these cells thus promoting the secretion of gastrin,acid and histamine, respectively. G cells are also stimulated to release gastrin by the presence ofpeptides and amino acids (products of protein breakdown)in the lumen of the stomach, and by gastrinreleasing peptide which is released from local neurones in the mucosa that are activated bymechanoreceptors. These sense the stomach distension that occurs when food enters.

Front 14

briefly explain the negative feedback system of HCl secretion

Back 14

Acid secretion isalso under negative feedback control. D cells in the gastric mucosa are sensitive to the concentration ofH+ions in the lumen. Once activated, D cells release the peptide somatostatin which inhibits acidsecretion from parietal cells directly, or indirectly by inhibiting histamine and gastrin secretion fromECL and G cells, respectively. Endocrine cells in the duodenum also release hormones that inhibit acidsecretion. Acid and fats in the chyme promote the secretion of sectretin and CCK into the bloodstream. 5These inhibit the secretion of acid from parietal cells in the stomach.