Bio307 - Ch. 21 - Digestion

Gastrointestinal Tract (GI)

moves nutrients, water, electrolytes from external environment to the internal environment

Chyme

soupy substance created as ingested food is broken down by mechanical and chemical digestion

Salivary Glands

add exocrine secretions containing enzymes and mucous to the lumen

ex: amylase, which helps digest carbohydrates

Digestion

chemical AND mechanical breakdown of foods into absorbable units

Absorption

The transfer of substances from the lumen of the GI tract to the ECF

Proenzyme

a biologically inactive precursor to an enzyme, which can be modified into an active enzyme when conditions are right

Motility

movement of food from mouth through the pharynx (throat), esophagus, stomach, small and large intestines and out of the body.

peristalsis

involuntary muscle contractions that create waves of movement, pushing the contents of the GI tract forward

segmentation

contractions of circular muscles, which involves mixing of the food to aid in digestion and absorption

lipase(s)

pancreatic enzyme that catalyzes the breakdown of fats to fatty acid + glycerol

amylase

enzyme that converts starch/glycogen to simple sugars

protease (endopeptidase)

endopeptidase: cleaves peptide bonds within the protein
exopeptidase: cleaves bonds at either terminal of a protein, resulting in cleavage of peptide bonds

gastroesophageal reflux (GERD)

when stomach acid repeatedly flows back into esophagus, and this backwash irritates the lining of the esophagus

gastric

relating to the stomach

diarrhea

feces is discharged frequently and in liquid form, a loose watery stool 3 or more times in a day

Recognize that the GI tract is a tube that is continuous with the external environment, is not subject to homeostatic regulation.

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Label Figure 21.1a and 21.1d

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List the 4 layers of the GI tract, inner to outer

Mucosa - epithelial
Submucosa - connective
Muscularis Propia (externa) - 2 layers of smooth muscle
Serosa - connective tissue

Provide the major functions of:

Saliva
Lower Esophageal Sphincter
Stomach
Gastric Acid
Pepsin
Small Intestine
Bicarbonate Secretion (into small intestine)
Brush Border enzymes
Bile Salts
Pancreas (digestive function)
The Liver (digestive functions)
gallbladder
large intestine
bacterial flora

Saliva: contains enzymes (amylase) which help initiate breakdown of food
Lower Esophageal Sphincter: region of high muscle tension that acts as a barrier between the esophagus and the stomach, where the tension relaxes allowing bolus to pass into the stomach:
Stomach: storage, digestion, defense
Gastric Acid: releases pepsin (enzyme which breaks down proteins), triggers somatostatin release from D cells, HCl denatures proteins by breaking bonds, helps kill bacteria, and inactivates salivary amylase, stopping breakdown of carbohydrates
Pepsin: breaks down proteins
Small Intestine: 95% of absorption of nutrients
Bicarbonate Secretion (into small intestine): acid buffer, maintain pH balance
Brush Border enzymes: anchored to luminal cell membrane, degrade of nutrients into absorbable units
Bile Salts: breakdown of fats
Pancreas (digestive function): enzymes for sugar, fat, starch digestion
The Liver (digestive functions): secrete bile, purify blood containing the newly absorbed nutrients

GI Tract: mouth to anus

1) Mouth
2) Esophagus
3) Stomach
4) Small Intestine
5) Large Intestine
6) Anus

Why digestive enzymes are synthesized and secreted as proenzymes

localized activation, can be transformed into the active state when needed

Contraction Mechanism:

Peristalsis: pushes the food forward
Segmentation: mixes and combines the food

How the small intestine provides a large amount of surface area on which nutrient absorption can occur

microvilli amplifies the surface area of diffusion