Mechanical Digestion

Overview of Digestion

Composition of Food

Mechanical Digestion

Movement of Food through the GI tract

Chemical Digestion

What is Digestion?

Digestion: the breakdown of food into smaller parts, which can be absorbed and utilized by the body

carbohydrates are digested into monosaccharides proteins are digested into amino acids fats are digested into fatty acids and glycerol

Composition of Food

Some components of food are not digested, but are absorbed directly (water, salts, monosaccharides, most vitamins)

Some components of food require digestion prior to absorption (carbohydrates, proteins, fats, nucleic acids)

Some components of food are not digested, and are never resorbed (cellulose, from plants)

Mechanical Digestion

The majority of mechanical digestion occurs in the oral cavity

Grinding and tearing of food decreases its size, increasing its surface area

Mastication is controlled by the mastication reflex, which is stimulated by the presence of a bolus of food

Deglutition (swallowing)

There are three phases to deglutition: voluntary phase: the tongue pushes a bolus of food toward

oropharynx

Deglutition (swallowing)

pharyngeal phase: bolus in the oropharynx stimulates a reflex response:- soft palate elevates- pharynx elevates

- pharyngeal constrictor muscles constrict- upper esophageal sphincter relaxes- vocal cords close- epiglottis closes laryngeal opening

Deglutition (swallowing)

esophageal phase: bolus moved toward stomach by peristaltic waves

- peristaltic waves cause relaxation of the lower esophageal sphincter

- peristaltic waves stimulated by presence of food in the esophagus

- food and liquid can be moved against the force of gravity by peristalsis in the esophagus

Movement of Food through the GI tract

In the stomach, food is mixed by tonic contractions (last phase of mechanical digestion)

- mixing waves (every 20 seconds)

- peristalic waves (more powerful) Chyme: mixture of food in the stomach Gastric emptying: pyloric contractions move food through

the pyloric sphincter

- rate depends on component of food

- influenced by the autonomic system

Movement of Food (cont.)

In the small intestine, mixing and propulsion of contents continues

Segmental contractions mix the intestinal contents Peristaltic contractions move the contents along the

digestive tract Contractions increase in response to filling of intestinal

lumen Also regulated by parasympathetic system, pH, tonicity of

contents Peristaltic waves continue to move material through colon

to rectum

Chemical Digestion

Digestion of food components takes place in specific locations of the digestive tract, by specific enzymes

Carbohydrate digestion begins in the oral cavity:- Salivary amylase: digests complex CHOs to dextrans, maltose, simple sugars- optimum pH: 6.9- starches are not digested in the oral cavity

In the stomach, the pH drops to < 3.0- salivary amylase stays active for about 20 minutes- acidic environment inactivates amylase after that

Carbohydrate Digestion (cont.)

In the small intestine, digestion of carbohydrates continues:

- pH rises

- pancreatic amylase released into the duodenum

- large CHOs further digested

- dissacharides cleaved by dissacharidases on microvilli

Digestion of Protein

No digestion of protein occurs in the oral cavity In the stomach, pepsin begins to break down peptide bonds

- pepsin formed from pepsinogen (zymogen)

- pH optimum for pepsin = 1 to 3

- pepsin is inactivated at higher pH

- only about 15% of protein digestion occurs in the stomach

Digestion of Proteins (cont.)

In the small intestine, protein digestion is completed- pH = 7 to 9- pancreas secretes trypsinogen, chymotrypsinogen, and carboxypetidase

Trypsinogen => trypsin (activated by enterokinase); digests at arginine or lysine residues

chymotrypsinogen => chymotrypsin; digests at carboxy linkages of hydrophobic amino acids

carboxypeptidase: digests from carboxy terminus aminopeptidase (from small intestine): digests from amino

terminus

Digestion of Lipids

The first step in lipid digestion is emulsification with bile

- bile salts secreted by liver, stored in the gall bladder

- digestive enzymes are water soluble, act at surface of lipid droplets

- bile is secreted into the duodenum (small intestine) Pancreatic lipase converts lipids to free fatty acids and

glycerol Cholesterol esterase converts cholesterol esters into

cholesterol and free fatty acids

NEXT LECTURE.....

Accessory Digestive Organs