Biology: Chapter 9 Flashcards
Intracellular digestion
involves the oxidation of glucose and fatty acids for energy
Extracellular digestion
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
Digestion
involves the breakdown of food into its constituent organic molecules; starches and other carbohydrates into monosaccharides, lipids into free fatty acids and glycerol, and proteins into amino acids
Mechanical digestion
-physical breakdown of large food particles into smaller food particles, but does not involve breaking chemical bonds
-mastication (chewing) breaks up large food particles into smaller particles using the teeth, tongue, and lips; tongue forms food into a bolus which is forced back to the pharynx and swallowed
Chemical digestion
-enzymatic cleavage of chemical bonds
-relies on enzymes from saliva produced by salivary glands; salivary amylase hydrolyzes starch into smaller sugars, lipase catalyzes the hydrolysis of lipids
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
Digestive tract
-oral cavity
-pharynx
-esophagus
-stomach
-small intestine
-large intestine
-rectum
-salivary glands, pancreas, liver, and gallbladder help to provide the enzymes and lubrication necessary to aid in the digestion of food
Enteric nervous system
-Collection of neurons that govern the function of the gastrointestinal system
-Trigger peristalsis, or rhythmic contractions of the gut tub, in order to move materials through the system
-Heavily regulated by autonomic nervous system but can function independently of the brain and spinal cord
-All of the glands of the body (except sweat glands) are innervated by the parasympathetic nervous system
ADH and aldosterone
trigger the sensation of thirst
Glucagon and ghrelin
glucagon secreted by the pancreas and ghrelin by the stomach and pancreas; stimulate feelings of hunger
Leptin and cholecystokinin
stimulate feelings of satiety
Oral cavity
Plays a role in both mechanical and chemical digestion
Pharynx
-Cavity that leads from the mouth and posterior nasal cavity to the esophagus
-Nasopharynx: behind the nasal cavity
-Oropharynx: at the back of the mouth
-Laryngopharynx: above the vocal cords
-Epiglottis: prevents food from entering the larynx during swallowing
Esophagus
-Muscular tube that connects the pharynx to the stomach
-Top third is composed of skeletal muscle (voluntary motor control), bottom third is composed of smooth muscle (involuntary nervous control), middle is a mix of both
-Upper esophageal sphincter: swallowing is initiated by the muscles of the oropharynx
-Lower esophageal sphincter: muscular ring just before the stomach
Stomach
-Fundus and body: contain mostly gastric glands
-Antrum and pylorus: contain mostly pyloric glands
-Lesser curvature of the stomach: inner curvature
-Greater curvature of the stomach: outer curvature
-Rugae: the lining of the stomach
-Mucosa of the stomach contains gastric glands and pyloric glands
Gastric glands
respond to signals from the vagus nerve of parasympathetic nervous system, which is activated by the brain in response to the sight, taste, and smell of food
Pyloric glands
contain G-cells that secrete gastrin which is a peptide hormone that induces parietal cells to secrete more HCl and signals the stomach to contract
Mucus cells (gastric glands)
produce the bicarbonate-rich mucus that protects the muscular wall from harshly acidic and proteolytic environment of the stomach
Chief cells (gastric glands)
secrete pepsinogen which is the inactive form of pepsin
Parietal cells (gastric glands)
secrete hydrogen ions in the form of hydrochloric acid that cleave pepsinogen into pepsin; secrete intrinsic factor which is a glycoprotein that absorbs vitamin B12
Pepsin (gastric glands)
digests proteins by cleaving peptide bonds
Duodenum
-First segment of the small intestine, responsible for a majority of chemical digestion and has some minor involvement in absorption
-Food leaves the stomach through the pyloric sphincter and enters duodenum
-Presence of chyme causes the release of brush-border enzymes which break down dimers and trimers of biomolecules into absorbable monomers
Peptidases (duodenum)
break down proteins
Enteropeptidase (duodenum)
enzyme critical for activation of trypsinogen to trypsin
Secretin (duodenum)
peptide hormone that causes the 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
Cholecystokinin (CCK) (duodenum)
secreted in response to the entry of chyme; stimulates the release of both bile and pancreatic juices
Bile (CCK)
complex fluid composed of bile salts, pigments, and cholesterol
Bile salts (CCK)
derived from cholesterol, serve an important role in mechanical digestion of fats and facilitate the chemical digestion of lipids; emulsify fats and cholesterol into micelles
Pancreatic juices (CCK)
neutralizes acidic chyme and provides ideal working environment for digestive enzymes; contain enzymes that can digest carbohydrates, fats, and proteins
Pancreas
-Endocrine functions: release of insulin, glucagon, and somatostatin which are peptide hormones necessary for the maintenance of proper blood sugar levels
-Acinar cells: produce pancreatic juices
Liver
-Bile ducts connect the liver with both the gallbladder and the small intestine
-Bile is produced in the liver and travels down bile ducts where it may be stored in gallbladder or secreted into duodenum
-Receives all blood draining from the abdominal portion of the digestive tract through the hepatic portal vein
-Takes up excess sugar to create glycogen
-Stores fats as triacylglycerols
-Detoxifies endogenous compounds (made in the body) and exogenous compounds (brought from the environment)
Gallbladder
-Located just beneath the liver and both stores and concentrates bile
-Upon release of CCK, the gallbladder contracts and pushes bile out into the biliary tree
-Common site of cholesterol or bilirubin stone formation which causes inflammation of the gallbladder
Jejunum
second part of the small intestine
Ileum
third part of the small intestine
Role of jejunum and ileum
-Involved in the absorption of nutrients
-Small intestine is lined with villi which each have many microvilli that increase surface area for absorption
-At the middle of each villus there is a capillary bed for the absorption of water-soluble nutrients and a lacteal, a lymphatic channel that takes up fats for transport into the lymphatic system
-Simple carbohydrates and amino acids diffuse from the epithelial cells into the capillaries and the absorbed molecules then go to the liver via the hepatic portal circulation
-Triglycerides and esterified cholesterol molecules are packaged into chylomicron
-Vitamins are also absorbed in the small intestine
-Water passes transcellularly (across the cell membrane) and paracellularly (squeezing between cells)
Large intestine
-Primarily involved in water absorption
-larger diameter but shorter length than small intestine
-3 major parts: cecum, colon, and rectum
Cecum
accepts fluid exiting the small intestine through the ileocecal valve and is the site of attachment of the appendix
Colon
divided into ascending, transverse, descending, and sigmoid colons. Main function to absorb water and slats from undigested material left over from the small intestine. Concentrates remaining material to form feces
Rectum
serves as a storage site for feces, which consists of indigestible material, water, bacteria, and certain digestive secretions that are not reabsorbed; anus is the opening through which wastes are eliminated and consists of two sphincters
Internal sphincter
under involuntary (autonomic) control
External sphincter
under voluntary (somatic) control
