Chapter 7 - Digestion Flashcards
Alimentary canal
The tube that extends from the mouth to the anus and is the location of extracellular digestion in all mammals
Basic functions of the digestive tract
digestion and absorbtion
- Digestion: the breakdown of food into its constituent organic molecules: lipids into free fatty acids, carbohydrates into monosaccharides, and protiens into amino acids.
- Absorption: transport of the products of digestion from the digestive tract into the circulatory system for distribution to the cells.
Chemical digestion
The enzymatic breakdown of large molecules into smaller molecules.
Mechanical digestion
Breakdown if food particles into smaller particles through such activities as biting, chewing, and churning.
______ is the major reaction involved in digestion of macromolecules.
Hydrolysis
Anatomy of the digestive tract
(and order)
Mouth → esophagus → stomach → small intestine → large intestine → rectum → anus
α-amylase
Enzyme in saliva that begins breaking down straight chains of starch into polysaccharides (in the mouth)
Epiglottis
A flap of cartilage that covers the glottis when swallowing food in order to prevent food particles from entering the larynx.
Esophagus: what happens here?
Bolus is pushed down into the esophagus by swallowing, then moved down esophagus by peristaltic action (wave motion). No digestion occurs in the esophagus.
Peristalsis
Involuntary muscular contractions of the esophagus that push food down the digestive tract.
Define cardiac sphincter
A valve between the esophagus and the stomach that prevents the content of the stomach from going back up through the esophagus.
Stomach: what happens here?
Mixes and stores food, turning it into chyme. Pepsin (enzyme) begins protein digestion. Very acidic pH (2). No absorption occurs in the stomach.
Chyme
Combination of partially digested food and acid that forms in the stomach.
Major cell types in stomach
- Mucous cells
- Chief (peptic) cells
- Parietal (oxyntic) cells
- G cells
Mucous cells
Cells in the stomach that secrete mucous (glycoprotein + electrolytes) to both lubricate stomach so food can slide along w/o causing damage; and also protect cell wall from acidic environment of the stomach.
Chief cells
Aka “peptic” cells. Deep in exocrine glands of the stomach. Secrete pepsinogen, the zymogen precursor to the enzyme pepsin. Pepsin begins protein digestion.
Pepsinogen
Zymogen (inactive) form of the enzyme pepsin. Pepsinogen is activated to pepsin in the stomach by the stomach’s low pH. Upon activation, pepsin begins protein digestion.
Parietal cells
Aka “oxyntic” cells. In the stomach. Secrete HCl, which diffuses into lumen of stomach. CO₂ also involved. Purpose: lower pH of stomach (H⁺ ions expelled into lumen) and raise pH of blood (bicarbonate, HCO₃⁻, goes into interstitial fluid).
G cells
In stomach. They secrete gastrin (peptide hormone) into interstitium. Absorbed into blood and stimulate parietal cells to release HCl.
Gastric glands
Located in the stomach, the gastric glands secrete HCl and various enzymes (e.g., pepsin) when stimulates by gastrin.
Pyloric glands
Glands located in the walls of the stomach that secrete the hormone gastrin in response to certain substances in food.
Pyloric sphincter
A valve between the stomach and the small intestine that regulates the flow of chyme into the small intestine.
What are the three sections of the small intestine, and what are their general functions?villi
The small intestine can be subdivided into three sections: the duodenum, jejunum, and ileum. Most digestion takes place in the duodenum and most absorption takes place in the jejunum and the ileum.
Cholecystokinin (CCK)
CCK is a hormone that is secreted by the duodenum in response to the presence of chyme. CCK stimulates the release of bile and pancreatic enzymes into the small intestine.
What is bile
An alkaline fluid synthesized in the liver, stored in the gall bladder, and released into the duodenum. Bile aids in the emulsification, digestion, and absorption of fats.
Small intestine: what happens here?
Most of digestion (duodenum) and absorption (jejunum and ileum) occurs here.
Pancreas
Secretes pancreatic amylase, trypsin, chymotrypsin, and lipase into the small intestine.
Intestinal glands
Secretes maltase, sucrase, lactase, aminopeptidase, dipeptidase, and enterokinase into the small intestine.
Hepatic portal vein
Carries nutrients (monosaccharides, amino acids, and small fatty acids) absorbed in the small intestine to the liver, where they are modified to enter circulation.
Villi
Fingerlike projections that extend out of the small intestine in order to increase surface area for maximum absorption.
Lacteal
Lymph vessel located in each villus of the small intestine. Nutrients absorbed in small intestine pass into a capillary network and then into the lacteal.
Brush border
The name for the fuzzy looking coating (the microvilli) on the villi of the small intestine. Membrane bound digestive enzymes are found in the brush border.
Goblet cells
Epithelial cells of small intestine that secrete mucous to lubricate small intestine and protect brush border from damage.
Duodenum: environment?
pH of 6, because of bicarbonate ion secreted by pancreas.
Pancreatic enzymes
(serving as exocrine gland)
- Trypsin
- Chymotrypsin
- Pancreatic amylase
- Lipase
- Ribonuclease
- Deoxyribonuclease
NOTE: these are all used in the small intestine.
Typsin / chymotrypsin
Degrade proteins into small polypeptides in the small intestine.
Pancreatic amylase
Like salivary amylase, breaks down polysaccharides (in the small intestine), but much stronger (breaks them down into small glucose polymers).
Lipase
Degrades fat (triglycerides) in the small intestine.
Large intestine (main functions)
Section of the GI tract that consists of the cecum, the colon, and the rectum. The major function of the large intestine is to absorb electrolytes and water.
E. coli in the large intestine: function
Symbiotic relationship. Bacteria get our “leftovers.” They produce certain vitamins for us (K, B₁₂, thamin, riboflavin).
How is glucose absorbed into enterocytes?
Secondary active transport mechanism, down the concentration gradient of sodium ions. Sodium pumped out. Sodium reenters through a transport protein (after glucose attaches itself to sodium).
Glycogenesis: what is it, and where does it take place?
Formation of glycogen from glucose (when glucose is in high concentrations); takes place mainly in the liver.
Glucogenesis: what is it, and where does it take place?
Formation of glucose from glycogen (when blood glucose level is low); takes place in the liver.
How does glucose enter MOST body cells?
Down concentration gradient via facilitated diffusion (enterocytes and renal tubule are only exceptions).
Enterocyte
Cells of villus of the small intestine.
How are polypeptides absorbed and digested?
Polypeptides absorbed into enterocytes (down gradient of sodium, similar to glucose). Hydrolyzed to amino acids within enterocytes. Absorbed directly into the blood. (Transport may be facilitated/active, but NEVER passive, because amino acids are too big and polar.)
What is the byproduct from proteins of gluconeogenesis?
Ammonia; converted to urea by the liver and excreted into urine by the kidney.
How are fats digested and absorbed?
Broken down from triglycerides to monoglycerides and fatty acids, then taken to brush border by bile micells, then diffuse through enterocyte membrane. Then turned back into triglycerides at smooth ER. Major absorption of fat occurs in liver and adipose tissue.
Albumin
Protein that carries free fatty acids in the blood
What is the effect when liver mobilizes fat/protein for energy?
Blood acidity increases.
Liver functions (LOTS!)
- Blood storage
- Blood filtration
- Carbohydrate metabolism
- Fat metabolism
- Protein metablism
- Detoxification
- Erythrocyte destruction
- Vitamin storage
Blood storage: Where does this occur, and how?
Liver expands to act as blood reservoir for body.
Blood filtration: Where does this occur, and how?
Special liver cells phagocytize intestinal bacteria.
Carbohydrate metabolism: Where does this occur, and how?
Liver maintains normal blood glucose levels through gluconeogenesis, glycogenesis, and glycogen storage.
Fat metabolism: Where does this occur, and how?
Liver makes bile from cholesterol. Converts carbohydrates/proteins to fat. Oxidizes fatty acids (for energy). Forms most lipoproteins.
Protein metabolism: Where does this occur, and how?
Liver deaminates amino acids. Forms urea from ammonia in the blood. Synthesizes plasma proteins (clotting factors such as prothrombin and fibrinogen, also albumin, most globulins), synthesizes nonessential amino acids.
Detoxification: Where does this occur, and how?
Detoxified chemicals excreted by liver as part of bile or polarized so can be excreted by kidney.
Erythrocyte destruction: Where does this occur, and how?
Special liver cells destroy some irregular RBCs (although most of this occurs in spleen).
Vitamin storage: Where does this occur?
Liver stores A, D, and B₁₂ vitamins.
Kidney: functions
- Excrete waste products (i.e., urea, uric acid, ammonia, phosphate)
- Maintain homeostasis: fluid volume; solute composition
- Control plasma pH
Kidney: diagram
Nephron
(and components, in order)
Functional unit of kidney:
Renal corpuscle → proximal tubule → loop of Henle → distal tubule → collecting duct
Renal corpuscle
Region of nephron (kidney) where filtration occurs. Made up of glomerulus and Bowman’s capsule. Located in cortex.
Proximal tubule
Region of nephron (kidney) where reabsorption (water, glucose, proteins, etc.) and secretion (drugs, toxins, etc.) occurs. Located in cortex.
Loop of Henle
Region of nephron (kidney) where solutes are concentrated. Loop of Henle is located in the medulla. Descending loop: permeable to water. Ascending loop: impermeable to water; actively transports Na⁺ into kidney.
Distal tubule
Region of nephron (kidney) where osmolarity is lowered. In presence of ADH, water flows out of tubule, concentrating filtrate. Located in cortex.
Collecting duct
Region of nephron (kidney) that carries filtrate into highly osmotic medulla. In presence of ADH, concentrates urine. (Normally impremeable to water, but becomes permeable w/ADH.) Straddles cortex/medulla.
Juxaglomerular apparatus
Monitors filtrate pressure in the distal tubule.
How does plasma enter the glomerulus of the kidney for filtration?
Hydrostatic pressure forces plasma through sieve-like “fenestrations” (which keep out blood cells and large proteins).
Renin
Enzyme secreted by granular cells of juxtaglomerular apparatus. Stimulates the adrenal cortex to secrete aldosterone.
Aldosterone: effect on kidney?
Aldosterone acts on distal tubule, stimulating formation of membrane proteins that absorb Na⁺ and secrete K⁺. Raises blood pressure.
ADH: effect on kidney?
Antidiuretic hormone; aka vasopressin. Increases water retention. Acts at collecting tubule of distal tubule to increase permeability of cells to water, allowing water to flow OUT of tubule and concentrating urine. Also acts on collecting duct, making the normally impermeable cells permeable to water (water passively diffuses into medulle, concentrating urine again).
Epithelial cells: function and classification
First border and primary protection against the outside world. Prevent fluid loss and allow for selective absorption. In adults, epithelia are developed from all three germ layers.
Digestive Enzymes
What is the activation cascade for pancreatic peptidases
Enterokinase, produced by the small intestine, activates pancreatic trypsinogen to form trypsin. Trypsin subsequently activates other trypsinogen molecules, as well as chymotrypsinogen to chymotrypsin, and other zymogens.
Digestive Enzymes Targeting Carbohydrates
(Function and sites of production and function)
SP = site of production; SF = site of function; Fxn: Function
-
Salivary Amylase – Hydrolylyses starch to maltose.
- SP: Salivary glands, SF: Mouth
-
Pancreatic Amylase – Hydrolylyses starch to maltose
- SP: intestinal glands, SF: Small intestine
-
Maltase – Hydrolylyses maltose to two glucose
- SP: intestinal glands, SF: Small intestine
-
Sucrase – Hydrolylyses sucrose to glucose and fructose
- SP: intestinal glands, SF: Small intestine
- Lactase – Hydrolylyses lactose to glucose and galactose
- SP: intestinal glands, SF: Small intestine
Digestive Enzymes Targeting Proteins
(Function and sites of production and function)
Enzyme (Zymogen); SP = site of production; SF = site of function
-
Pepsin (pepsinogen) – Hydrolylyses specific peptide bonds.
- SP: Gastric glands, SF: Stomach
-
Trypsin (trypsinogen) – Hydrolylyses specific peptide bonds. Activates chymotrypsin.
- SP: pancreas, SF: Small intestine
-
Chymotrypsin (trypsinogen) – Hydrolylyses specific peptide bonds.
- SP: pancreas, SF: Small intestine
-
Carboxypeptidase – Hydrolylyses C-terminal peptide bonds.
- SP: pancreas, SF: Small intestine
- Aminopeptidase – Hydrolylyses N-terminal peptide bonds.
- SP: intestinal glands, SF: Small intestine
- Dipeptidase – Hydrolylyses amino acid pairs
- SP: intestinal glands, SF: Small intestine
- Enterokinase – Activates trypsin
- SP: intestinal glands, SF: Small intestine
Digestive Enzymes Targeting Lipids
(Function and sites of production and function)
Molecule; SP = site of production; SF = site of function
- **Bile **– Emulsifies fats (not an enzyme)
- SP: Liver; SF: small intestine
-
Lipase – Hydrolyzes lipids
- SP: Pancreas; SF: Small intestine
