MCAT Biology - The Digestive System

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Digestive System

take complex foods composed of polysaccharides, fats, and proteins and large macromolecules into smaller, simpler monosaccharides, fatty acids, and amino acids and then absorb them into the bloodstream; requires a complex system of mechanical and chemical agents; specialized sections with different functional roles

intracellular digestion

involves the oxidation of glucose and fatty acids for energy

metabolism

set of life-sustaining chemical reactions in organisms

1. the conversion of the energy in food to energy available to run cellular processes

2. the conversion of food to building blocks of proteins, lipids, nucleic acids, and some carbohydrates

3. the elimination of metabolic wastes

alimentary canal

runs from the mouth to the anus and is sectioned off by sphincters; where extracellular digestion occurs

extracellular digestion

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

sphincters

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

Digestion

involves 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

the physical breakdown of large food particles into smaller food particles

Chemical digestion

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

Absorption

involves 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 / mouth

First portion of the alimentary canal that receives food; plays a role in both mechanical and chemical digestion of food

pharynx

a shared pathway for both food entering the digestive system and air entering the respiratory system

esophagus

receives food from the pharynx and transports it to the stomach; top third is composed of skeletal muscle (voluntary), the bottom third is composed of smooth muscle (involuntary), and the middle third is a mix of both

stomach

highly muscular organ with a capacity of approximately two liters; upper left quadrant of the abdominal cavity, underneath the diaphragm; Chemical breakdown of food takes place by means of secreted digestive enzymes and gastric hydrochloric acid; thick mucosa to avoid autodigestion

small intestine

follows stomach; seven meters; three segments: the duodenum, the jejunum, and the ileum

large intestine

rectum

feces are stored until an appropriate time of release

salivary glands

pancreas

endocrine function: the release of insulin, glucagon, and somatostatin

exocrine function: pancreatic juices

liver

located in the upper right quadrant of the abdomen; makes bile and processes nutrient-rich blood; detoxifies both endogenous and exogenous compounds

gallbladder

located just beneath the liver; stores and concentrates bile; contracts and pushes bile out into the biliary tree when activated by CCK

enteric nervous system

collection of one hundred million neurons in the walls of the digestive tract that govern the function of the gastrointestinal system and trigger peristalsis; function independently of the brain and spinal cord, although it is heavily regulated by the autonomic nervous system

peristalsis

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

food coma

feeling sleepy and lethargic after eating a big meal due to parasympathetic activity

ingest

Consumption of a substance by an organism; accomplished by taking in a substance through the mouth into the gastrointestinal tract, such as through eating or drinking

thirst

need for water; encouraging the behavior of fluid consumption

triggered by ADH and aldosterone

glucagon

secreted by the pancreas; stimulate feelings of hunger

ghrelin

secreted by the stomach and pancreas; stimulate feelings of hunger

Leptin

made by adipocytes; stimulating feelings of satiety

mastication / chewing

breaking up of large food particles into smaller particles using the teeth, tongue, and lips; increase the surface area-to-volume ratio of the food, creating more surface area for enzymatic digestion as it passes through the gut tube; moderates the size of food particles entering the lumen of the alimentary canal

saliva

liquid with enzymes produced in the salivary galns; aids mechanical digestion by moistening and lubricating food

Salivary amylase

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

lipase

catalyzes the hydrolysis of lipids

bolus

ball of food formed by the tongue to be forced back to the pharynx and swallowed

emesis / vomiting

reversal of peristalsis due to exposure to chemicals, infectious agents, physical stimulation in the posterior pharynx, and even cognitive stimulation

upper esophageal sphincter

muscles of the oropharynx that initiated swallowing

lower esophageal (cardiac) sphincter

relaxes and opens to allow the passage of food into the stomach at the end of the esophagus

gastroesophageal reflux disease (GERD)

classic heartburn symptoms after eating; weakness in the lower esophageal sphincter; acid reflux irritates the less-protected mucosa, stimulating general burning pain

fundus

upper curved part; contain mostly gastric glands

body of stomach

main, central region of the stomach; contain mostly gastric glands

antrum

lowest part of stomach; contain mostly pyloric glands.

pylorus

part of the stomach leading into small intestine; contain mostly pyloric glands

lesser curvature

internal curvature of the stomach

greater curvature

external curvature of the stomach

rugae

lining of the stomach is thrown into folds

gastric glands

respond to signals from the vagus nerve

vagus nerve

parasympathetic nervous system; activated by the brain in response to the sight, taste, and smell of food

Mucous cells

produce the bicarbonate-rich mucus that protects the muscular wall from the harshly acidic (pH = 2) and proteolytic environment of the stomach

Gastric juice

combination of secretions from chief cells and parietal cells.

chief cells

secrete pepsinogen

zymogen

inactive form of an enzyme

pepsinogen

zymogen form of pepsin; cleaved into pepsin by hydrogen ions

Pepsin

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

activated by acidic environment

parietal cells

secrete hydrochloric acid

Helicobacter pylori

bacteria that can survive acidic stomach environment; infection is usually asymptomatic but can cause inflammation, ulcers, and even certain gastric cancers

intrinsic factor

glycoprotein involved in the proper absorption of vitamin B12; secreted by parietal cells

pyloric glands

contain G-cells

G-cells

secrete gastrin

Gastrin

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

chyme

acidic, semifluid mixture from the digestion of solid food in the stomach; significant increase in the surface area

few substances that are absorbed directly from the stomach

alcohol and aspirin

Zollinger–Ellison syndrome

rare disease resulting from a gastrin-secreting tumor (gastrinoma) causing excessive HCl production; sign: excessive HCl production

duodenum

responsible for the majority of chemical digestion; minor involvement in absorption; secretes brush border enzymes, enteropeptidase, secretin, cholecystokinin (CCK)

pyloric sphincter

where food leaves the stomach 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

ex. disaccharidases

enteropeptidase

involved in the activation of other digestive enzymes from the accessory organs of digestion

disaccharidases

digest disaccharides

Maltase

digests maltose

isomaltase

digests isomaltose

lactase

digests lactose

sucrase

digests sucrose

Peptidases

break down proteins or peptides

Aminopeptidase

removes the N-terminal amino acid from a peptide

Dipeptidases

cleave the peptide bonds of dipeptides to release free amino acids

Celiac disease

immune reaction against gluten, a protein found in grains, especially wheat; immune system develops antibodies against certain components of gluten; cross-react with elements of the small intestine, causing damage to the mucosa; not true allergies

symptoms: diarrhea, discomfort, malabsorptive syndromes

Enteropeptidase / enterokinase

enzyme critical for the activation of trypsinogen to trypsin and procarboxypeptidases A and B

trypsinogen

zymogen pancreatic protease

trypsin

pancreatic protease; initiates an activation cascade

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

hormone that slows motility through the digestive tract to allow digestive enzymes to act on chyme, especially fats

cholecystokinin (CCK)

synthesized and secreted by enteroendocrine cells in the duodenum in response to the entry of amino acids and fat in chyme into the duodenum; stimulates the release of both bile and pancreatic juices; acts in the brain, where it promotes satiety.

Bile

complex fluid composed of bile salts, pigments, and cholesterol

Bile salts

derived from cholesterol; important role in the mechanical digestion of fats; have hydrophobic and hydrophilic regions, allowing them to serve as a bridge between aqueous and lipid environments

emulsify

mix two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation

micelles

an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single-tail regions in the centre

Pancreatic juices

complex mixture of several enzymes in a bicarbonate-rich alkaline solution; helps to neutralize acidic chyme, as well as provide an ideal working environment for the digestive enzymes; pH ~8.5

accessory organs of digestion

outgrowths of endoderm from the gut tube during development that aid in the release of enzymes

acinar cells

exocrine cells that produce pancreatic juices

Pancreatic amylase

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

chymotrypsinogen

zymogen; responsible for protein digestion

carboxypeptidases A and B

zymogen pancreatic proteases

pancreatic lipase

capable of breaking down fats into free fatty acids and glycerol

Pancreatitis

inflammation of the pancreas from premature activation of pancreatic enzymes and autodigestion of the pancreatic tissue; may result in a long hospital stay and long-term consequences such as diabetes and the reduced digestion of proteins and fats

usually caused by gallstones or excessive consumption of alcohol

major / minor duodenal papillae

where pancreatic ducts empty into the duodenum

bile ducts

connect the liver with both the gallbladder for storage and small intestine to emulsify chyme

hepatic portal vein

receives all blood draining from the abdominal portion of the digestive tract for processing before draining into the inferior vena cava on its way to the right side of the heart

glycogen

storage form of glucose

triacylglycerols

storage form of fat

glycogenolysis

produces glucose from glycogen

gluconeogenesis

produces glucose from other biomolecules