9.2 GI_Protein & Fat digestion-gastric acid secretion regulation

Digestion Enzymes

1. Digestion Enzymes
Complex: Protein digestion begins with pepsin (activated from pepsinogen by HCl in the stomach), followed by trypsin (from trypsinogen via enteropeptidase in duodenum), then peptidases in the small intestine to produce amino acids. Fats are emulsified by bile salts, hydrolyzed by lipase + co-lipase into monoglycerides and fatty acids, and reassembled into chylomicrons for lymphatic absorption. Carbohydrates are broken down by amylases into disaccharides, then by disaccharidases into monosaccharides for absorption. Cellulose is indigestible by human enzymes and fermented by colonic bacteria.

Simple: Proteins → pepsin → trypsin → amino acids. Fats → bile salts + lipase → monoglycerides → chylomicrons → lymph. Carbs → amylase → disaccharides → monosaccharides → absorbed. Humans can’t digest cellulose.

Carbohydrate Absorption

• Glucose/galactose: absorbed with Na⁺ via SGLT, exits via GLUT2.

• Fructose: enters via GLUT5, exits via GLUT2.

• Only monosaccharides can be absorbed.

Fat Digestion and Absorption

• Emulsified by bile salts, broken down by lipase + co-lipase into monoglycerides & fatty acids.

• Form micelles → diffuse into enterocytes → recombine into chylomicrons → enter lymph (lacteals).

• Short-chain fatty acids (<10 carbons) can enter capillaries directly.

Vitamin Absorption

• Fat-soluble vitamins: A, D, E, K → absorbed with fats. Absorption efficiency: A, D, K = 50–80%; E = 20–30%.

• Water-soluble vitamins: B, C → absorbed with water.

• Vitamin B12: requires intrinsic factor (from parietal cells) for absorption in ileum; initially binds haptocorrin, then binds IF after pancreatic protease degradation.

Regulation of Gastric Acid Secretion

Complex: Cephalic phase: sight, smell, taste, thought, swallowing → vagus nerve → stimulate parietal cells. Gastric phase: stomach distension + partially digested proteins → G cells release gastrin → parietal cells secrete acid; ECL cells release histamine. Negative feedback: low pH → D cells release somatostatin → inhibits gastrin → reduces acid. Gastric phase accounts for 50–60% of total acid.

Simple: Brain sees/tastes/smells food → acid. Stomach stretch + protein → gastrin → more acid. Too much acid → somatostatin stops it.

Water-soluble vitamins

B & C.

B12

binds haptocorrin → intrinsic factor → absorbed in ileum.

GI hormones from stomach

gastrin & somatostatin.

Cellulose

fermented by bacteria in large intestine.

Amylase

starch → disaccharides → monosaccharides.

Fat leaving stomach

emulsion → bile salts → micelles → chylomicrons.

Enterogastric reflex

slows chyme emptying.

LES

contraction strengthened by gastrin.

Gastrin

stimulated by partially digested proteins.

Cephalic phase triggers

smell, sight, taste, thought, swallowing.

carry the signals that stimulate the salivary glands to secrete saliva?

Facial and glossopharyngeal nerves

Gastrin is released from the G cells of the gastric glands when:

Partially digested proteins in chyme

Contraction of the lower esophageal sphincter is strengthened when:

Gastrin enters the bloodstream

Enterogastric reflex:

Inhibits the emptying of the chyme through the pylorus

Fat within the chyme leaving the stomach:

Is in the form of emulsion of large fat droplets

Lipid droplets in the duodenum are coated with:

Bile salts

The structure formed within the smooth endoplasmic reticulum by combining triglycerides together with cholesterol and proteins is known as:

Chylomicrons

The enzyme amylase is used to digest:

Carbohydrate into disaccharides

Carbohydrate absorbed in our intestine are in the form of:

Monosaccharides

Digestion of cellulose in human:

Occurs in the large intestine by bacteria

Which of the following GI hormones are secreted by the stomach?

Gastrin and somatostatin

Which of the following vitamins are water soluble?

B and C

Vitamin B12 in the stomach is found binding with which of the following after being released from the food:

Haptocorrin