41.3 The structure and function of animal digestive tracts

incomplete digestive tracts

have a single opening, the mouth, through which the animal both ingests food and eliminates waste

gastrovascular cavity

a chamber that the mouth opens to, where digestion takes place in an incomplete digestive tract

complete digestive tract

have 2 openings, the mouth and anus

advantages of the complete digestive tract

different chemical and physical processes can be confined to different compartments within the tract so that they can occur independently of each other and in a prescribed sequence

material can be ingested and digested without interruption, instead of alternating with waste removal

accessory structures to the complete digestive tract

salivary glands, liver, gallbladder, pancreas mo

mouth

mechanical and chemical digestion of the food

salivary glands

secrete salivary amylase into the mouth and produce a slimy substance called mucus

digestive enzymes in the mouth

salivary amylase, responsible for carbohydrate digestion, secreted by salivary glands

lingual lipase, begins the digestion of lipids by breaking triglycerides into diglycerides and fatty acids, secreted by tongue cells

esophagus

connects the mouth and stomach

peristalsis

the contraction and relaxation of the smooth muscle in the esophagus in response to nerve signals to propel food down the esophagus

the bird crop

a modified esophagus in bird species where food can be stored and sometimes processed

stomach

chemical and some mechanical digestion

sphincters

muscular valves that close off the inferior and superior passageways of the stomach

digestive enzyme in the stomach

pepsin, responsible for digesting proteins by cleaving the peptide bonds next to certain amino acids, reducing long polypeptides to relatively small chains of amino acids

chief cells

secretes pepsinogen in the stomach

parietal cells

secretes HCl in the stomach to lower the pH and activate pepsinogen

mucous cells

secretes additional mucus that is found in gastric juice in the stomach

function of mucus in the stomach

to line the gastric epithelium and protect the stomach from damage of HCl

carbonic anhydrase

enzyme in parietal cells that catalyzes the formation of a proton and bicarbonate ion (HCO3-) from CO2 and H2O

the proton is used to make HCl

how does chlorine enter the parietal cell

chloride ions from the blood enter parietal cells in exchange for bicarbonate ions via a cotransporter

gastrin

a hormone that is produced by stomach cells in response to the arrival of food, and stimulates parietal cells to secrete HCl

small intestine

completes digestion and absorbs most nutrients and water

villi

fingerlike projection in the epithelial tissue of the small intestines

microvilli

the cells that line the surface of the villi that have tiny projections on their apical surfaces and project into the lumen of the digestive tract

function of the villi and microvilli

increase the surface area of the small intestine, which increases the rate of nutrient absorption

lacteal

small lymphatic vessel extending into the center of a villus in the small intestine where nutrients can pass quickly from epithelial cells into the body’s transport systems

macromolecules digested in the small intestine

carbohydrates, fats, and proteins

secretin

a hormone produced by the small intestine in response to the arrival of food from the stomach and stimulates the secretion of bicarbonate ions by the pancreas

cholecystokinin

stimulates the secretion of digestive enzymes from the pancreas and the secretion of molecules from the gallbladder that aid in the processing of lipids

emulsification

the process that breaks up the big globules of fat that emerge from the stomach

bile salts

mechanically digest fats by breaking up large fat globules into smaller ones

where are bile salts synthesized and stored

they are synthesized in the liver and secreted in a solution called bile, which is stored in the gallbladder

carboxypeptidase, chymotrypsin, and elastase (proteases)

synthesized in the pancreas

regulated by being released in the inactive form in response to cholecystokinin from the small intestine and activated by trypsin

breaks peptide bonds in polypeptides, releasing amino acids in the small intestine

gastric lipase

synthesized in the stomach

regulated by being released in response to gastrin from the stomach

breaks bonds in fats, releasing fatty acids and diglycerides in the stomach

lingual lipase

synthesized in the salivary glands

regulated by being released in response to taste and smell stimuli

breaks bonds in fats, releasing fatty acids and diglycerides in the mouth and stomach

nucleases

synthesized in the pancreas

regulated by being released in response to cholecystokinin from the small intestine

breaks apart nucleic acids, releasing nucleotides in the small intestine

pancreatic amylase

synthesized in the pancreas

regulated by being released in response to cholecystokinin from small intestine

breaks apart carbohydrates, releasing smaller sugars in the small intestine

pancreatic lipase

synthesized by the pancreas

regulated by being released in response to cholecystokinin from small intestine

breaks bonds in fats, releasing fatty acids and monoglycerides in the small intestine

pepsin

synthesized in the stomach

regulated by being released in inactive form and being activated by low pH in the stomach lumen

breaks peptide bonds between certain amino acids in proteins, releasing polypeptides in the stomach

salivary amylase

synthesized in the salivary glands

regulated by being released in response to taste and smell stimuli

breaks apart carbohydrates, releasing dextrins and disaccharides in the mouth

trypsin

synthesized in the pancreas

regulated by being released in the inactive form in response to cholecystokinin from the small intestine and activated by enteropeptidase from the small intestine

breaks specific peptide bonds in polypeptides, releasing amino acids in the small intestine

primary function of the large intestine

to form feces by absorbing additional water and compacting the waste that remain

fermentation

the process by which bacteria breaks down the cellulose to produce several important nutrients like vitamin K