Chapter 9

intracellular digestion

• a part of metabolism

• involves the oxidation of glucose and fatty acids for energy.

extracellular digestion

the process by which these nutrients are obtained from food occurs within the lumen of the alimentary canal

alimentary canal

runs from the mouth to the anus and is sectioned off by sphincters

sphincters

circular smooth muscles around the canal that can contract to allow compartmentalization of function

Digestion

the breakdown of food into its constituent organic molecules: starches and other carbohydrates into monosaccharides, lipids (fats) into free fatty acids and glycerol, and proteins into amino acids

Mechanical digestion

• physical breakdown of large food particles into smaller food particles

• involves mastication

Chemical digestion

the enzymatic cleavage of chemical bonds, such as the peptide bonds of proteins or the glycosidic bonds of starches\

relies on enzymes from saliva produced by the three pairs of salivary glands

Absorption

the transport of products of digestion from the digestive tract into the circulatory system for distribution to the body’s tissues and cells.

oral cavity → pharynx → esophagus → stomach → small intestine → large intestine → rectum

What is the pathway of the digestive tract

enteric nervous system

a collection of one hundred million neurons that govern the function of the gastrointestinal system.

peristalsis

rhythmic contractions of the gut tube, in order to move materials through the system

triggered by neurons in the digestive tract

oral cavity

plays a role in both mechanical and chemical digestion of food

mastication

the breaking up of large food particles into smaller particles

using the teeth, tongue, and lips.

Salivary amylase

capable of hydrolyzing starch into smaller sugars (maltose and dextrins),

lipase

catalyzes the hydrolysis of lipids

bolus

mass of chewed food formed by the tongue

pharynx

the cavity that leads from the mouth and posterior nasal cavity to the esophagus

The nasopharynx, the oropharynx, and the laryngopharynx

What are the three parts of the pharynx

nasopharynx

behind the nasal cavity

oropharynx

at the back of the mouth

laryngopharynx

above the vocal cords

epiglottis

a cartilaginous structure that folds down to cover the laryngeal inlet

esophagus

• muscular tube that connects the pharynx to the stomach.

• top third of the esophagus is composed of skeletal muscle, the bottom third is composed of smooth muscle, and the middle third is a mix of both.

skeletal muscle

somatic (voluntary) motor control

peristalsis

rhythmic contraction of smooth muscle that propels food toward the stomach

squeezes, pushes, and propels the bolus toward the stomach

emesis (vomiting)

reversal of peristalsis

upper esophageal sphincter

• muscles of the oropharynx

• responsible for swallowing

lower esophageal sphincter (cardiac sphincter)

muscular ring that relaxes and opens to allow the passage of food as the bolus approaches the stomach

weakess here can cause GERD, which causes heartburn

stomach

• in the upper left quadrant of the abdominal cavity, underneath the diaphragm

• uses hydrochloric acid and enzymes to digest food, creating a fairly harsh environment.

• thick mucosa to prevent autodigestion

Sections of the stomach

fundus and body - gastric glands

antrum and pylorus - pyloric glands

pyloric glands

contain G-cells that secrete gastrin, a peptide hormone

Gastrin

induces the parietal cells in the stomach to secrete more HCl and signals the stomach to contract, mixing its contents

chyme

acidic, semifluid mixture produced from digesting solid food in the stomach

gastric glands

respond to signals from the vagus nerve of the parasympathetic nervous system

vagus nerve

activated by the brain in response to the sight, taste, and smell of food

Gastric juice

combination of secretions from the other two cell types in the gastric glands: chief cells and parietal cells.

chief cells

secrete pepsinogen

pepsinogen

the inactive, zymogen form of pepsin, a proteolytic enzyme.

parietal cells

• secrete HCl

• H+ in the HCl cleave pepsinogen to pepsin

• secrete intrinsic factor

intrinsic factor

a glycoprotein involved

in the proper absorption of vitamin B12.

Pepsin

digests proteins by cleaving peptide bonds near aromatic amino acids, resulting in short peptide fragments

activated by the acidic environment, so most active in at a low pH

Helicobacter pylori

• stomach bacteria

• infection is usually asymptomatic but can cause inflammation, ulcers, and even certain gastric cancers).

What six products does the stomach secrete?

1. HCl

2. Pepsinogen

3. Mucus

4. Bicarbonate

5. water

6. Intrinsic factor

Why does the stomach secrete HCl?

kills microbes, denatures proteins, and converts pepsinogen into pepsin

Why does the stomach secrete mucus and bicarbonate?

it protects the mucus membrane (mucosa)

Why does the stomach secrete water

dissolves and dilutes ingested material

What are the three sections of the small intestine?

1. the duodenum

2. the jejenum

3. the ileum

duodenum

responsible for the majority of chemical digestion and has some minor involvement in absorption

secretes enteropeptidase

secretes hormones like secretin and cholecystokinin (CCK) into the bloodstream

Enteropeptidase (formerly called enterokinase)

an enzyme critical for the activation of trypsinogen, a pancreatic protease, to trypsin, which then initiates an activation cascade

can also activate procarboxypeptidases A and B to their active forms

jejunum and ileum

Where does most of the absorption in the small intestine take place

pyloric sphincter

Food leaves the stomach through the ___________ and enters the duodenum.

Brush-border enzymes

present on the luminal surface of cells lining the duodenum and break down dimers and trimers of biomolecules into absorbable monomers.

Secretin

• peptide hormone that causes pancreatic enzymes to be released into the duodenum

• regulates the pH of the digestive tract by reducing HCl secretion from parietal cells and increasing bicarbonate secretion from the pancreas

• enterogastrone, a hormone that slows motility through the digestive tract.

cholecystokinin (CCK)

secreted in response to the entry of chyme (specifically, amino acids and fat in the chyme) into the duodenum.

stimulates the

release of both bile and pancreatic juices and also acts in the brain, where it promotes satiety

Bile

complex fluid composed of bile salts, pigments (especially bilirubin from the breakdown of hemoglobin), and cholesterol

Bile salts

derived from cholesterol

emulsify fats and cholesterol into micelles.

without it, fats would spontaneously separate out of the aqueous mixture in the duodenum and would not be accessible to pancreatic lipase, which is water-soluble

gallbladder

• stores and concentrates bile

• just beneath the liver

• common site of cholesterol or bilirubin stone formation

pancreas

release insulin, glucagon, and somatostatin— peptide hormones necessary for the maintenance of proper blood sugar levels

made of exocrine cells called acinar cells that produce pancreatic juices

Pancreatic amylase

breaks down large polysaccharides into small disaccharides and is therefore responsible for carbohydrate digestion

pancreatic peptidases

• trypsinogen, chymotrypsinogen, and carboxypeptidases A and B

• released in their zymogen form, but once activated are responsible for protein digestion

pancreatic lipase

• secreted by the pancreas

• capable of breaking down fats into free fatty acids and glycerol

bile ducts

connect the liver with both the gallbladder and small intestine

hepatic portal vein

• in liver

• receives all blood draining from the abdominal portion of the digestive tract

liver

• takes up excess sugar to create glycogen, the storage form of glucose

• stores fats as triacylglycerols

• can also reverse these processes, producing glucose for the rest of the body through glycogenolysis and gluconeogenesis and mobilizing fats in lipoproteins

• detoxifies both endogenous compounds (those made in the body) and exogenous compounds (those brought in from the environment).

villi

• small, finger-like projections from the epithelial lining

• each one has a capillary bed for absorbing water-soluble nutrients and a lacteal (lymphatic channel that takes up fats for transport into the lymphatic system)

mnemonic for the parts of the small intestine

Dow Jones Industrial

(Duodenum

Jejunum

Ileum)

chylomicrons

packaged triglycerides and esterified cholesterol molecules

fat-soluble vitamins

• A, D, E, and K

• dissolve directly into chylomicrons to enter the lymphatic circulation

water-soluble vitamins

• B complex and C

• are taken up, along with water, amino acids, and carbohydrates, across the endothelial cells of the small intestine, passing directly into the plasma.

large intestine

primarily involved in water absorption

three sections: the cecum, the colon, and the rectum

cecum

an outpocketing that accepts fluid exiting the small intestine through the ileocecal valve and is the site of attachment of the appendix.

colon

divided into the ascending, transverse, descending, and sigmoid colons

function is to absorb water and salts (such as sodium chloride) from the undigested material left over from the small intestine.

anus

the opening through which wastes are eliminated and consists of two sphincters: the internal and external anal sphincters.

internal anal sphincters

involuntary control (autonomic)

external anal sphincters

under voluntary control (somatic)