Biology - Digestion
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- What is intracellular digestion?
- It is digestion that occurs within the cell
- What is extracellular digestion?
- It refers to the digestive process that occurs outside of the cell, within a lumen or tract
- Do Mammals have a two or one way digestive tract?
- It is one way
- What is it known as?
- The alimentary canal
- Are mammalian digestive tracts complex or simple?
- They are complex
- What are they organized into regions specialized for?
- They have regions for digestion and absorption of specific nutrients
- Where does the digestive tract begin?
- With the oral cavity
- What is the path of the rest of the tract and where does it end?
- It goes through the pharynx, the esophagus, the stomach, the small intestine, and the large intestine, ending in the anus.
- What are the accessory organs involved with digestion?
- Salivary glands, the pancreas, the liver, and the gall bladder, all have essential roles
- What are epithelial cells?
- They are the linings of most body surfaces
- Are they loosely or tighly connected together?
- They are tightly connected
- What is the purpose of this?
- To act as a barrier against mechanical injury, invading organisms, and fluid loss
- Can the free surface of epithelium be exposed to air or liquid?
- Yes
- Is it ciliated?
- It can be
- What is the inner surface attached to underlying connective tissue by?
- The basement membrane
- How is epithelium classified?
- According to the number of layers and the shape of its cells
- What is simple epithelium?
- It is a single layer of cells
- What is stratified epithelium?
- It has multiple layers of cells
- What is pseudostratified epithelium?
- It is single layered but appears stratified because its cells vary in height
- What are the three types of cell shape?
- Cuboidal, columnar, and squamous (scale-like)
- Do epithelial cells absorb or secrete solutions?
- Yes, some are specialized
- What is an example of this?
- Simple cuboidal cells in the small intestine absorb nutrients
- What is a mucous membrane or mucosa?
- It is a specialized epithelium that lines most body cavities
- Where does mechanical digestion take place?
- It takes place in the oral cavity
- Does mechanical digestion lead to changes in the molecular composition of food?
- No
- What does it do?
- It increases the surface area of the food, allowing for faster and more efficient enzymatic action
- What is chemical digestion?
- It is the enzymatic breakdown of macromolecules into smaller molecules
- Where does it begin?
- It actually begins in the mouth when salivary glands secrete saliva
- What is the purpose of saliva?
- It has several purposes such as facilitating swallowing through lubrication, and it provides a solvent for food particles
- Saliva is secreted why?
- In response to a nervous reflex triggered by the presence of food in the oral cavity
- What enzyme does saliva contain that hydrolyzes starch into simple sugars?
- Salivary amylase (ptyalin)
- During swallowing, what does the epiglottis do?
- It covers the opening of the trachea, thereby preventing food particles from going down the wrong passageway
- What is peristalsis?
- The rhythmic waves of involuntary muscular contractions
- What is the ring of muscle in the lower esophagus that opens during peristalsis?
- It is called the lower esophageal sphincter or cardiac sphincter
- Do sphincters normally remain open or closed?
- Closed
- What are the walls of the stomach lined by?
- Thick gastric mucosa, which contains the gastric glands and pyloric glands
- How does the brain stimulate the gastric glands?
- It responds to the sight, taste, and or smell of food, stimulating nervous impulses
- What are the three types of gastric glands?
- They are mucous cells, chief cells, and parietal cells
- What do mucous cells do?
- They secrete mucous, which protects the stomach lining from the low pH (2) present in the stomach
- What is gastric juice comprised of?
- It is composed of the secretions of the chief cells and the parietal cells
- What do chief cells secrete?
- Pepsinogen
- What is pepsinogen?
- It is the zymogen of the protein-hydrolyzing enzyme pepsin
- What do parietal cells secrete?
- They secrete HCl
- What does HCl do in the stomach?
- It kills bacteria, dissolves the glue holding food tissues together, and facilitates the conversion of pepsinogen to pepsin
- What does pepsin do?
- It hydrolyzes specific peptide bonds to yield polypeptide fragments
- What do the pyloric glands secrete?
- They secrete the hormone gastrin
- Why do they secrete gastrin?
- In response to the presence of certain substances in food
- What does gastrin do?
- It stimulates the gastric glands to secrete more HCl, and also stimulates muscular contractions of the stomach
- What does this churning produce?
- Chyme
- What is at the junction of the stomach and small intestine?
- The pyloric sphincter
- What is its purpose?
- It contains enzymes that neutralize the acidity of chyme and contains enzymes that digest carbohydrates, proteins, and lipids
- What can be absorbed into the bloodstream through capillaries in the stomach wall?
- Alcohol and certain drugs such as aspirin
- Where is chemical digestion completed?
- In the small intestine
- What are the three sections of the small intestine?
- The duodenum, the jejunum, and the ileum
- Why is the small intestine so long?
- In order to maximize the surface area available for digestion and absorption
- Why are villi in the stomach?
- They extend out of the intestinal submucosa even further
- Do they have projections off of them too?
- Yes, microvilli project from the surface of individual cells lining the villi
- Where does most digestion occur in the small intestine?
- Within the duodenum
- Where do the secretions of the intestinal glands, pancreas, liver, and gall bladder mix together with the acidic chyme?
- In the duodenum
- What does the presence of chyme in the duodenum trigger?
- Hormonal release, which in turn stimulates and regulates the secretions of the small intestine and its accessory organs
- What are some of the intestinal mucosa enzyme secretions that hydrolyze carbohydrates into monosaccharides?
- Maltase, lactase, sucrase, and peptidases
- What does the hormone secretin do and where does it come from?
- It is released by the duodenum in response to the acidity of chyme, stimulating the pancreas to secrete pancreatic juice
- Do the enzymes of the small intestine function better at an acidic or basic pH?
- A basic pH
- Is pancreatic juice acidic or basic?
- Basic
- What is trypsinogen?
- It is a proteolytic zymogen secreted by the pancreas
- What is its active form, and what converts it to the active form?
- The active form is trypsin, which is converted by an enzyme called enterokinase
- What does trypsin then convert?
- Another pancreatic zymogen, chymotrypsinogen into chymotrypsin
- What does each of these enzymes do?
- They cleave specific peptide bonds within proteins, producing polypeptide fragments
- What else does the pancreas secrete?
- Carboxypeptidase, which is also secreted as a zymogen and converted by trypsin
- It along with what digests polypeptides into amino acids?
- It along with aminopeptidase digests polypeptides into amino acids
- What is the hormone CCK (cholecystokinin)?
- It stimulates the secretion of pancreatic enzymes and the release of bile
- What secretes CCK?
- The duodenum
- Why?
- It slows down the release of chyme into the small intestine
- What is bile?
- An alkaline fluid synthesized and secreted by the liver
- Where is it stored?
- It is stored in the gall bladder
- Where is it released?
- Into the duodenum
- What is bile composed of?
- Bile salts, bile pigments, and cholesterol
- What are bile salts?
- Molecules with a water-soluble region on one and a fat-soluble region on the other
- What does this structure allow?
- It allows bile salts to emulsify fat globules as well as surround and maintain these particles in finely dispersed complexes called micelles
- What is this process known as?
- The emulsification of fat
- Why is it done?
- To increase the surface area to the actions of lipases, which hydrolyze molecules of fat into glycerol and fatty acids
- How much bile is released?
- The amount released is proportional to the amount of fat ingested
- If the chyme is fatty, what does the duodenum release?
- A hormone called enterogastrone
- What does it inhibit?
- Stomach peristalsis
- In addition to hormonal regulation, what else are digestive processes stimulated and inhibited by?
- They are stimulated by the parasympathetic nervous system and inhibited by the sympathetic nervous system
- Where does the majority of nutrient absorption occur?
- Across the walls of the jejunum and ileum
- Does any absorption occur in the duodenum?
- Yes, a very small amount
- How are monosaccharides absorbed?
- They are absorbed via active transport and facilitated diffusion into the epithelial cells lining the villi
- How are amino acids absorbed?
- They are absorbed into the epithelium via active transport
- How do monosaccharides, amino acids, and small fatty acids diffuse directly into the intestinal capillaries and enter portal circulation?
- Via the hepatic portal vein
- Where do larger fatty acids, glycerol, and cholesterol diffuse?
- Into the mucosal cells
- What do fatty acids and glycerol recombine then?
- They combine to form triglycerides
- Triglycerides, which, along with phosphoglycerides and cholesterol, are packaged into protein-coated droplets called what?
- Chylomicrons
- Where are chylomicrons secreted?
- Into tiny lymph vessels within the villi called lacteals
- What do lacteals do?
- They lead into the lymphatic system
- Where does the lymphatic system converge with venous blood at the thoracic duct?
- In the neck
- Where are chylomicrons processed?
- In the bloodstream
- Where are they then delivered?
- To the liver
- Once there, repackaging allows their release into the bloodstream as what?
- LDLs, VLDLs, and HDLs
- Where are vitamins and minerals absorbed?
- In the small intestine
- How are the fat soluble vitamins such as A, D, E, and K absorbed?
- They are absorbed along with fats
- Most water soluble vitamins such as the vitamin B complexes and vitamin C are absorbed how?
- Via simple diffusion into the circulatory system
- How many liters of fluid enter the small intestine every day?
- About seven
- Where does most of it go?
- Most is absorbed through the walls of the small intestine
- How long is the large intestine, and what does it consist of?
- It is about 1.5m long, and it consists of the cecum, the colon, and the rectum
- What is the cecum?
- It is the blind out pocketing at the junction of the small and large intestines
- What is at the tip of the cecum?
- A small finger-like projection called the appendix
- What is the appendix?
- A vestigial structure
- What does it contain?
- Lymphoid tissue that is often surgically removed if it becomes infected
- How does the colon function?
- It works in the absorption of salts and the absorption of any water not already absorbed by the small intestine
- If digested matter moves through the colon too quickly, too litter water is absorbed, causing what?
- Diarrhea and dehydration
- What causes constipation?
- When movement through the bowels is too slow, causing too much water to be absorbed
- What does the rectum do?
- It stores feces
- What are feces?
- Bacteria, particularly E. coli, water, undigested food, and unabsorbed digestive secretions like enzymes and bile
- What is the anus?
- It is the opening through which wastes are eliminated and is separated from the rectum by two sphincters that regulated elimination