4. Digestion and Absorption

Study Guide

What digestive processes occur in the mouth

Food is mechanically broken down by chewing and mixed with saliva to form a bolus for swallowing 

What are the main functions of saliva in digestion?

lubricates food, begins chemical digestion, buffers acids, and protects the oral cavity from bacteria

What components of saliva aid digestion?

Salivary amylase begins carbohydrate digestion, and lingual lipase initiates fat digestion. 

Where are most nutrients absorbed in the GI tract?

In the small intestine, particularly in the duodenum and jejunum

Where are carbohydrates absorbed?

Mainly in the duodenum and jejunum as monosaccharides

Where are proteins absorbed

in the small intestine as amino acids, dipeptides, and tripeptides

Where are fats absorbed?

Primarily in the jejunum after emulsification and micelle formation

Where are water and electrolytes absorbed?

In the small intestine, with additional absorption in the large intestine

How are vitamins absorbed

With dietary fats

How are water soluble vitamins absorbed

By diffusion or active transport

How are carbohydrates digested

They are broken down by salivary and pancreatic amylase into disaccharides, then by brush border enzymes into monosaccharides

How are carbohydrates absorbed

Glucose and Galactose use the SGLT1 sodium dependent transporter, while fructose is absorbed by facilitated diffusion via GLUT5

How are proteins digested

By pepsin in the stomach and by pancreatic enzymes luke trypsin and chymotrypsin in the small intestine

How are proteins absorbed

Amino acids use Na+ dependent transporters, while dipeptides and tripeptides use H+ linked transporters in enterocytes

How are fats digested

They are emulsified by bile salts and digested by pancreatic lipase into monoglycerides and free fatty acids

What is emulsification

The process by which bile salts break large fat droplets into smaller ones to increase surface area for lipase action

What is a micelle and what is its role in fat absorption

A small complex of bile salts and lipids that transports fatty acids and monoglycerides to the intestinal brush border for absorption

How are absorbed lipids processed inside intestinal cells

They are re-esterified into triglycerides, packaged into chylomicrons, and transported into lymphatic lacteals

What causes lactose intolerance

Lactase deficiency, leading ot undigested lactose that is fermented by bacteria, causing gas and diarrhea

How is vitamin B12 deficiency related to the GI tract

It requires intrinsic factor from parietal cells for absorption; its deficiency can result from gastric atrophy or loss of intrinsic factor in pernicious anemia

What are the fat-soluble vitamins and how are they absorbed

A, D, E, and K; absorbed along with dietary fats in micelles

What are the water soluble vitamins and how are they absorbed

B-complex and C; absorbed by diffusion or carrier-mediated transport in the small intestine

How is vitamin B12 absorption unique

It binds to intrinsic factor in the stomach and is absorbed in the terminal ileum via receptor mediated endocytosis