Digestion
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| Where does extracellular digestion occur? | in the lumen of the alimentary canal, which is outside the cell borders, between the mouth and anus and sectioned off by sphincters
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| Describe the path of the digestive tract. | It begins with the oral cavity, then the pharynx, esophagus, stomach, small intestine, large intestine. There are salivary glands and accessory organs such as the pancreas, liver, and gall bladder.
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| What are epithelial cells? | sheets of cells that cover our exterior and interior surfaces, tightly joined, may be ciliated
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| What are mucous membranes? | epithelia for most of our body cavities, e.g., nasal cavities, inner eye lid, mouth, gastrointestinal tract
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| What is the basement membrane? | connective tissue layer that keeps the epithelium bound
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| Is the epithelium in the digestive tract replaced? | Yes, every few days, due to the harsh conditions
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| What are the types of epithelia based on layers? | simple (1 layer), stratified (multiple layers), and pseudostratified (looks like multiple layers due to differences in cell height, but is just 1)
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| What types of shapes can cells be in the epithelia? | cuboidal, columnar, and squamous (scalelike)
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| What is mastication? | a process of mechanical digestion in the mouth which involves breaking up large food particles into smaller particles by using the teeth, tongue, and lips. This increases the surface area of food for more efficient chemical digestion
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| What does saliva contain? | salivary amylase aka ptyalin, and lipase
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| What does ptyalin do? | it hydrolyzes starch into smaller sugars (maltose and dextrin)
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| What does lipase do? | it catalyzes the hydrolysis of lipids
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| What is a bolus? | that formation of food that is forced back to the pharynx by the tongue and swallowed
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| What is the pharynx? | the cavity that leads from the mouth and nose to the esophagus; it has a connection to the larynx, a part of the respiratory tract
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| How do we prevent food from getting into the respiratory tract? | by using our epiglottis, which folds down and covers the trachea during swallowing
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| What is the esophagus? | a muscular tube that starts out with striated muscle and transitions into smooth muscle in the thorax, serves as the connection from the mouth to stomach
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| What controls the esophagus? | Most of it is under involuntary control through the autonomic nervous system. The upper third, with striated skeletal muscle, is under voluntary motor control.
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| What is peristalsis? | involuntary muscle contraction that keeps food going through the digestive tract throughout the gastrointestinal tract
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| What is the lower esophageal sphincter? | aka the cardiac sphincter, it is a muscular ring that opens to allow the passage of food when the bolus approaches the stomach
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| Does digestion take place in the esophagus? | No mechanical or chemical digestion, only the continued enzymatic activity initiated in the mouth by the salivary enzymes
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| What is the capacity of the stomach? | about 2 liters
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| Where is the stomach located? | on the the right side of the upper abdomen under the diaphragm
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| What does the stomach do? | It uses acid and enzymes to digest food in a fairly harsh environment
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| What does the stomach mucosa contain? | gastric glands and pyloric glands
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| What do gastric glands do? | they respond to signals in the brain activated by the sight, taste, and smell of food
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| What are the cell types of gastric glands? | mucous cells, chief cells, and parietal cells
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| What do mucous cells do? | Produce mucus that protects the muscular wall from the acidic (pH 2) and proteolytic environment of the stomach (muscle is made of protein)
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| What is gastric juice? | the combination of secretions from chief cells and parietal cells in the gastric glands in stomach mucosa
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| What do chief cells do? | They secrete pepsinogen, the zymogen form of the proteolytic enzyme pepsin
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| What does pepsin do? | Digests proteins by cleaving peptide bonds near aromatic aas, resulting in short pp fragments.
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| What do parietal cells do? | Secrete HCl, which activates pepsin and kills most harmful bacteria and breaks down the intracellular glue that holds food together
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| What is Helicobacter pylori? | A bacteria that is not killed by HCl in the stomach. Infection with it is usually asymptomatic but can cause inflammation and ulcers.
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| What do pyloric glands do? | They secrete gastrin, a hormon
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| What does gastrin do? | It induces our stomach to secrete more HCl and to mix the contents of the stomach, producing an acidic, semifluid mix known as chyme
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| What is the stomach the site for? | Digestion, not absorption
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| What is the pyloric sphincter? | Where food leaves the stomach through, entering the duodenum of the small intestine
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| Where does most chemical digestion and absorption occur? | In the small intestine
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| What are the three sections of the small intestine? | duodenum, jejunum, and ileum
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| How long is the small intestine? | 6 meters
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| What are villi? | projections that cover the surface of the inner wall of the small intestine; villi are covered with microvilli; they increase the surface area to over 300 sq.m and the absorptive capabilities
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| What do bacteria do in the small intestine? | they reside throughout and assist with its digestive and absorptive functions; there are over 400 different species of bacteria in the gut!
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| Where in the small intestine does most digestion occur? | In the duodenum
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| What happens when chyme enters the duodenum? | It triggers the release of hormones that lead to secretions from the small intestine itself as well as from the accessory organs (liver, gall bladder, pancreas)
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| What is in pancreatic juice? | a complete mix of several enzymes in a bicarbonate (basic) solution; contains enzymes that can digest all 3 types of nutrients: carbs, fats, proteins
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| What does bicarbonate in pancreatic juice do? | It helps to neutralize acidic chyme and provides an ideal environment for the digestive enzymes (active around pH 8.5)
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| What pancreatic enzyme is responsible for carb digestion? | Pancreatic amylase, which breaks down large polysaccharides into small disaccharides
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| What pancreatic enzymes are responsible for protein digestion? | the peptidases - trypsinogen, chymotrypsinogen, elastinogen, carboxypeptidase - released in zymogen form
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| What is enterokinase? | the master switch; produced by the small intestine, it activates trypsinogen to trypsin, which activates the other zymogens
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| What pancreatic enzyme is responsible for lipid digestion? | lipase, which breaks down fats to free fatty acids
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| What is bile? | fluid made up of bile salts, bile pigments, and cholesterold; produced by the liver and stored in the gall bladder
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| What is cholecystokinin (CKK)? | A hormone that is released by the SI in response to the movement of chyme out of the stomach into the intestines, triggers the release of bile from the gall bladder into the duodenum by way of the bile duct
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| What is the pH of bile? | ~7.5-8.8, alkaline like pancreatic juice to help neutralize the acidity from the stomach
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| What do bile salts do? | They play an important role in mechanical digestion of fats and ultimately facilitate the chemical digestion of lipids.
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| How do bile salts assist in the digestion of fats? | They have a hydrophobic and hydrophilic region, so they allow fat to be emulsified in the SI; fats form micelles (mechanical dig.) which expose more surface to the actions of lipase (water-soluble) which hydrolyzes ester bonds holding lipids together (che
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| What does the SI do? | Chyme in the duodenum causes the SI to release disaccharidases (maltase, lactase, and sucrase), peptidases (including dipeptidases), enterokinase, secretin, and CCK
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| What is secretin? | a hormone that causes pancreatic juice to be released from the pancreas
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| What is CCK (again)? | a hormone that stimulates the release of bile and pancreatic juice
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| Bile release is tied to the level of ___ ingested. | fat
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| What is enterogastrone? | The hormone that is released by the duodenum to slow the movt of chyme and allow a greater time to digest fat when a lot of fat is ingested.
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| How does the autonomic nervous system exert control over the digestive system? | The parasympathetic division is involved in stimulation (rest and digest) and the sympathetic in inhibition (fight or flight) of digestive activities
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| Where does the absorptive process occur (how nutrients are taken up for use)? | the jejunum and ileum
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| How are simple sugars and amino acids absorbed? | By active transport and facilitated diffusion into the epithelial cells lining the gut; then they move across the cell into the intestinal capillaries by diffusion from a conc gradient. Then they go to the liver via the hepatic portal circulation.
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| How are small fatty acids absorbed? | They follow the same process as carbs and aas by diffusing directly into the intestinal capillaries. They don’t need transporters bc they’re nonpolar and can easily pass the cell membrane.
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| How are larger fats, glycerol, and cholesterol absorbed? | They move separately into the intestinal cells but then re-form into into triglycerides
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| What are chylomicrons? | Triglycerides and esterified cholesterol molecules are packaged into insoluble chylomicrons and enter the lymphatic circulation through lacteals, rather than entering the bloodstream
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| What are lacteals? | small vessels that form the beginning of the lymphatic system; they converge and enter the venous circulation through the lymphatic duct in the neck region (the thoracic duct)
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| What happens to chylomicrons? | They’re processed directly in the bloodstream into low-density lipoprotein (LDL - bad cholesterol)
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| What happens to LDL in the bloodstream? | If in excess, it can lead to atherosclerosis. LDL molecules are taken up by the liver, where they can be repackaged into high-density lipoprotein (HDL - good), very low-density lipoprotein (VLDL), or more LDL
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| How can vitamins be categorized for absorption? | fat- or water-soluble
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| There are only 4 fat-soluble vitamins. What are they? | A, D, E, K
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| How are fat-soluble vitamins absorbed? | alongside the fats; failure to digest fat properly would inhibit its proper absorption and may lead to a deficiency of the fat-soluble vitamins
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| How are water-soluble vitamins absorbed? | with water, aas, and carbs, across the endothelial cells and passed directly into the plasma of the blood
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| What is the final part of the gastrointestinal tract? | the large intestine
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| What is the large intestine mostly involved in? | Water absorption, although the overall water balance in the body is controlled by the kidneys
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| How large is the large intestine? | Larger than the SI in diameter, but only 1.5 m long
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| What are the 3 major sections of the LI? | the cecum, colon, and rectum
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| What is the cecum? | a pocket with no outlet that connects the small and large intestines and contains the appendix
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| What is the appendix? | a tiny structure once thought to be vestigial but recent evidence shows it may have a role in warding off certain bacterial infections
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| What is the colon responsible for? | Absorbing water and salts in the undigested material from the small intestines. It’s like a recycling system, sifting through the processed food and pulling last bits of nutrients out of the remaining waste products.
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| What is the rectum? | A storage site for feces - indigestible material, water, bacteria, and certain digestive secretions that aren’t reabsorbed (enzymes and bile)
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| What does the anus consist of? | 2 sphincters, the internal and external anal sphincters
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| The external and internal sphincters are under _____ control. | The external sphincter is under voluntary control (somatic), but the internal sphincter is under involuntary control (autonomic)
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| Which embryonic structure gives rise to the anus in humans? | The blastopore
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