Digestion, Absorption & Transportation Flashcards
Provide the origin, target, response, enzymes, and action of the hormone gastrin.
Provide the origin, target, response, enzymes, and action of the hormone secretin.
Provide the origin, target, response, enzymes, and action of the hormone Cholecystokinin (CCK).
Provide the origin, target, response, enzymes, and action of the hormone Gastric Inhibitory Peptide (GIP).
How are carbohydrates absorbed and transported?
How are lipids absorbed and transported?
How are proteins absorbed and transported?
Define: digestion
- mechanical and chemical processes that transform foods into nutrients ready for absorption.
Define: absorption.
- uptake of nutrients into intestinal (absorptive) cells for transport into either the bloodstream or the lymph
- Macronutrients have been digested to basic building blocks
- Vitamins and minerals are freed from food components, absorbed intact
- Trillions of molecules of CHO, PRO, FAT, vitamins and minerals need to be absorbed following a meal – requires an efficient system
- Majority of absorption occurs in Duodenum and Jejunum
Define: transportation (w.r.t. digestion & absorption)
- movement of nutrients to body cells for metabolic use
What is the Gastrointestinal tract?
- long flexible, muscular tube from mouth to anus
What are the major organs in the G.I. tract?
-
Mouth
* muscles in mouth surface area of food -
Stomach
* ~ 1 litre capacity § pH ~1.7-1.5 -
Small intestine
* ~2.5 cm diameter, 300 cm length -
Large intestine (colon)
* ~4 cm diameter, 150 cm length
What are the accessory organs in the G.I. tract?
-
Liver
* produces bile – consists of bile salts, pigments, lecithin, cholesterol, water -
Gall bladder
* stores bile -
Pancreas
* secretes pancreatic juices into small intestine
What are the 5 types of muscle action in the G.I. tract?
-
Chewing
* breaks food apart, increases surface area of food -
Peristalsis
* longitudinal, wave-like muscular contractions that propel food along GI tract - Stomach churning
- strongest muscles of GI tract found in stomach
- mixing, churning stomach contents, breaking large pieces into smaller pieces
-
Segmentation
* cinching of muscles to break up food within a particular region of the GI tract -
Sphincter control
* valves that control movement of food along GI tract § found at junctions between major organs of GI tract
How many muscle layers does the G.I. tract have?
- 2 layers of muscle all along GI tract, except stomach has 3 layers
- longitudinal outer layer, circular inner layer
- diagonal innermost layer for stomach only
What are the three layers of muscle in the stomach?
Define: chyme
- particle of food suspended in liquid
What is peristalsis?
The small intestine has two muscle layers that work together in peristalsis and segmentation.
- Circular muscles are inside.
- Longitudinal muscles are outside.
What is segmentation?
Define: hormone
- substance produced at one site in body in response to a specific stimulus
- travels in blood to different site (target organ)
- elicits a response there
What are characteristics of enzymes?
- a protein catalyst that accelerates rates of reactions without being changed or consumed in the process
- synthesis, degradation, rearrangement and exchange reactions
- lock and key fit with substrate to produce end product
- ‘ase’ suffix, although not for all enzymes
- active and inactive (pro-enzyme) forms
What directs the chemical action in the G.I. tract?
In addition to muscle (mechanical) action on food in the GI tract, there is chemical action under the direction of hormones and performed by enzymes
What are the four main types of digestion hormones, where are they produced, and what tissues do they target?
Describe basic enzyme action.
What are the basic reactions for nutrient handling?
1. Hydrolysis (digestion)
- requires water
- catabolic, breakdown
- ex., sucrose digestion
2. Condensation (synthesis)
- releases water
- anabolic, synthetic
- ex., protein synthesis
3. Reduction/Oxidation
- gain (reduction) or loss (oxidation) of electron
- ex., iron absorption
What are the salivary glands?
- parotid glands, serous, watery secretion
- sublingual glands, mucus secretion
- submaxillary (mandibular) glands, mixed serous and mucus secretion
Describe digestion that occurs in the mouth.
A. Chewing
- bigger → smaller pieces
- increased surface area, increased efficiency of digestion
B. Fat melts
- warming up to body temperature
C. Mixing with saliva
- Salivary juices secreted by glands in mouth contain: water, digestive enzymes (salivary amylase), and R-factor (protein) necessary for vitamin B-12 absorption.
What are the functions of salivary juices?
- moistens, lubricates, and softens food
- stimulates taste buds
- initial dilution of food to make it isotonic with blood
- start of carbohydrate digestion through action of salivary amylase
- slows development of dental caries
- contains lysozymes which kill bacteria that enter mouth
Define: bolus
- bolus travels past the epiglottis and down the esophagus (peristalsis, gravity, lubrication) to enter stomach
- once a mouthful of food is swallowed, it is called a bolus
Describe how a person swallows.
What is choking?
How does the sphincter prevent backflow of acidic stomach content into near-neutral esophagus?
When the circular muscles of a sphincter contract, the passage closes; when they relax, the passage opens.
What are the chemical secretions made in the stomach?
- G cells: secrete the hormone gastrin from pyloric region
- Mucous-secreting cells: secrete an alkaline mucus; thick layer protects the stomach wall against shear stress and acid damage
- Parietal cells: secrete hydrochloric acid & intrinsic factor (IF)
- Chief cells: secrete pepsinogen pepsin, a proteolytic enzyme
Where does the bolus enter the stomach?
- bolus enters stomach through the CARDIAC or LOWER ESOPHAGEAL (LES) sphincter
How does the stomach respond to food?
In response to food present in stomach:
- neural system detects distention, muscles contract
- stomach G-cells in pyloric region produce hormone, GASTRIN
- Gastrin causes release of gastric juices
What do gastric juices contain?
- water
- hydrochloric acid (HCl) (parietal cells)
- PRO digestive enzyme, PEPSIN (acid-stable) (chief cells)
What are the functions of acidic gastric juices?
- denatures proteins – changes 3-D shape – including salivary amylase, no CHO digestion in stomach
- small degree of protein digestion (~10%) by PEPSIN
- destroys bacteria, hormones
- increases solubility of minerals (e.g., calcium, iron)
- secretion of Intrinsic Factor (protein) required for vitamin B-12 absorption
- stimulates mucous-secreting cells; the thick mucus protects stomach wall against acid erosion
How does the stomach control pH?
- pH feedback mechanism!
- once pH drops to ~1.5, gastric juices are turned off
- protective mechanism against acid damage
Why does the small intestine have a near neutral pH?
- allows digestive enzymes to function
- protective – small intestine has less of a mucus coating than stomach
How is chyme formed?
The stomach churns and mixes bolus with gastric juices to produce CHYME – particles of food suspended in liquid, no longer resembles food (looks like a puree)
How does chyme leave the stomach?
- chyme empties into small intestine from stomach through the PYLORIC sphincter
What are five factors that influence digestion that occurs in the small intestine?
- Hormones
- Pancreatic juices
- Pancreatic enzymes
- Intestinal juices
- Intestinal enzymes
What are the three segments of the small intestine?
Chyme travels into the small intestine, which has 3 segments:
- Duodenum
- Jejunum
- Ileum
Small intestinal cells have a short lifespan (2-3 days), discuss their life-cycle.
- ~5 mL of acidic chyme every 20 seconds – protects against acid erosion
- acidic chyme reaches intestinal side of sphincter, drop in pH signals sphincter to close tightly
- alkaline pancreatic juices neutralize acidic chyme
- pH rises, pyloric sphincter relaxes, more acidic chyme enters duodenum, cycle repeats
What function does the Duodenum/Jejunum (~300 cm) serve?
~90% of all nutrients digested and absorbed here!
- bile released into lumen to emulsify dietary fat
- pancreatic juices enter lumen to digest chyme
- goblet cells found on villi produce mucus
- crypt glands found between villi secrete intestinal juices into lumen of small intestine which contain: water & intestinal digestive enzymes
What 3 hormones are secreted in small intestine and why?
Secretin, CCK and GIP all help establish an optimal rate of movement of chyme through GI tract to:
- prevent acid damage to small intestine
- maximize contact with digestive enzymes
- optimize absorption
What stimulates the release of secretin, and what is its function?
- acidic chyme stimulates secretion of Secretin
- causes release of alkaline (& zinc-rich) pancreatic juices which contain: water, digestive enzymes, and bicarbonate (HCO3-)
Secretin signals the release of pancreatic juices. What are the digestive enzymes contained within them?
Carbohydrates - Pancreatic amylase
- Continues CHO digestion started in mouth
Lipids - Pancreatic lipase
- Starts lipid digestion
- (TG → MG + 2 free FA)
- (PL → lysoPL + 1 free FA)
Protein - Trypsin
- Continues protein digestion started in stomach
What is the role of the hormone CHOLECYSTOKININ (CCK) in small intestinal digestion?
- stimulates gall bladder to release bile into small intestine
- bile emulsifies dietary fat in lumen
- stimulates pancreas to release pancreatic juices
What is the role of GASTRIC INHIBITORY PEPTIDE (GIP) (hormone) in small intestinal digestion?
- inhibits secretion of gastric juices
- slows release of stomach content into small intestine
What are the enzymes contained within the intestinal juices secreted by Crypt glands?
CHO in chyme: disaccharidases digest disaccharides into monosaccharides:
- Maltase (maltose: glucose + glucose)
- Lactase (lactose: glucose + galactose)
- Sucrase (sucrose: glucose + fructose)
PRO in chyme: Di- and Tri-peptidases digest small peptides
** FAT in chyme: emulsified by bile, digested by pancreatic lipase – no intestinal enzymes involved!
What factors influence digestion time?
A meal can take up to 3-10 hours for digestion/absorption!
Influencing factors:
- fibre content
- fat, protein content
- binders in food
- health of GI tract
Chyme in small intestine usually digested/absorbed in about 30 minutes.
What is the function of the Ileum (~150 cm)?
- “reserve capacity” for digestion and absorption
- terminal ileum – only site for vitamin B-12 absorption and bile resorption/recycling
How do remnants of chyme leave the small intestine and enter the large intestine?
- remnants of chyme leave the small intestine and enter the large intestine through the ILEOCECAL sphincter
Describe digestion that occurs in the large intestine.
- very little digestion occurs in colon
- bacterial fermentation of dietary fibres, resistant starches, and/or undigested lactose produces short chain fatty acids (2C-4C) and gases (methane, CO2, hydrogen)
- bacterial synthesis of 2 vitamins – vitamin K and biotin
- waste products collect in rectum; rectal sphincter under involuntary control
- anal sphincter under voluntary control relaxes to allow excretion of waste products from body
Describe the anatomy & components of the small intestine.
- inner wall of small intestine (~2.5 cm diameter) has many FOLDS
- on these folds there are thousands of finger-like projections called VILLI
- each villus (singular) is composed of hundreds of INTESTINAL CELLS (a.k.a. absorptive cells, mucosal cells, enterocytes)
- on mucosal (lumen) side of each intestinal cell are MICROVILLI that are in constant motion
- nutrients get trapped in microvilli in preparation for absorption into intestinal cells
- total surface area of small intestine covers ~1/2 a badminton court
- ~2 kg of GI tract nourishes ~50-70 kg of body tissues
What are Nutrient Absorption Processes?
Diffusion
- no energy and no protein carrier required
- natural flow from area of high concentration to low concentration
- ex., water, small lipids, some minerals
Facilitated diffusion
- no energy required, but protein carrier required
- ex., fructose
Active transport
- energy and protein carrier required ex., glucose, galactose
Endocytosis
- energy required, used by infants to absorb immune bodies
- Intestinal cells may have 100’s of different protein carriers, pumps and enzymes which act on different nutrients.*
Describe microvilli.
How are carbohydrates absorbed?
- GLU and GAL – active transport into intestinal cells from lumen, facilitated diffusion out of intestinal cells into bloodstream
- FRU – facilitated diffusion into and out of intestinal cells
- monosaccharides are transported directly from intestinal cells in the bloodstream to the liver, entering through the PORTALVEIN
- once in the liver, most FRU and GAL is converted into GLU
How are proteins absorbed?
- amino acids – actively transported into and out of intestinal cells into bloodstream
- some small peptides: – 2 or 3 amino acids in length actively transported into intestinal cells – digested by di- and tri-peptidases inside intestinal cells – only single amino acids are actively transported out of intestinal cells into bloodstream
- amino acids are transported directly from intestinal cells in the bloodstream to the liver, entering the liver through the PORTAL VEIN
How are lipids absorbed?
Emulsification Prior to Digestion of Lipids
- bile emulsifies dietary fat (mixed TG)
Digestion of Lipids
- digestion of lipids happens mostly in the small intestine
- main hormones:
- Cholecystokinin (CCK) stimulates release of bile from gall bladder into the lumen of the small intestine to emulsify dietary lipids before enzymatic digestion
- Secretin – release of pancreatic juice into small intestine
- pancreatic enzyme, pancreatic lipase digests TG MG + 2 free FA
- lyso-phospholipase digests 1 FA from PL
- cholesterol remains intact, FA hydrolyzed from cholesteryl ester
What are small products of fat digestion?
Glycerol
Short Chain Fatty Acids
Medium Chain Fatty Acids
What are large products of fat digestion?
PL, lyso-PL = Phospholipids
F/S = Fat-soluble vitamins
LCFA = Long Chain Fatty Acids
MG = Monoglycerides
How are large products of fat digestion absorbed?
All products, except bile - diffuse into intestinal cell within a micelle.
- Micelles have a hydrophobic core and a hydrophili shell.
- Transformation inside intestinal cells: TG & PL re-made + proteins = Chylomicrons exit to lymph
What does acid production in the stomach require?
- phosphorus
- sodium
- potassium
- chloride
Chloride
How are small products of fat digestion absorbed?
- behave as water-soluble
- absorbed directly into the liver
Give a brief overview of the vascular system as it relates to transport of nutrients.
How are large products of fat digestion absorbed?
-via micelle
What is a chylomicron?
A type of lipoprotein.
Features of lipoproteins:
- Shell (hydrophilic): proteins, phospholipids
- Core (hydrophobic): fats (cholesterol, PL,TG)
Composition of a lipoprotein
- triglycerides
- phospholipids
- cholesterol
- protein
Discuss absorption that occurs in the large intestine (colon).
- ~95% of energy-producing nutrients were absorbed in the small intestine
- electrolytes absorbed into bloodstream
- some water reabsorbed into body
- Vitamins K and biotin absorbed into bloodstream
- semi-solid waste products (feces) collect and are expelled from body
- waste products consist of mostly undigested fibres, sloughed off intestinal cells, and connective tissue from animal foods
How are carbohydrates transported via the bloodstream from the small intestine?
- carbs are hydrophilic
- monosaccharides are transported directly from intestinal cells in the bloodstream to the liver, entering through the PORTAL VEIN
- once in the liver, most fructose and galactose are converted into glucose
What is the fate of monosaccharides in the liver?
Monosaccharides (MS) have several potential fates in the liver:
- glucose or other monosaccharides can be used as an immediate source of energy
- glucose can be released into the bloodstream as an energy source for other cells
- glucose can be used to synthesize GLYCOGEN (a carbohydrate storage form of energy) for later use of glucose
- Excess glucose can be converted into FAT for storage in adipose cells and later use
How are fats transported via the bloodstream from the small intestine?
- FAT (most = hydrophobic)
- most fat first travels in the lymph (as chylomicron) BEFORE it enters the bloodstream
How are proteins transported via the bloodstream from the small intestine?
- proteins are hydrophilic
- only single amino acids are transported out of the intestinal cell into the bloodstream
What is the fate of amino acides in the liver?
Amino acids have several potential fates in the liver:
- used to synthesize new proteins
- used as an immediate energy source by liver
- released into bloodstream for use by other cells
- converted to non-protein molecules (ex., GLU, FAT)
Discuss transport of nutrients from the small intestine via lymph.
FAT & FAT-SOLUBLE VITAMINS
- water-insoluble products of digestion (larger products of fat digestion – MG, LCFA, lysoPL) - re-formed into intact TG and PL
- chylomicrons formed inside the intestinal cell from TG, PL, dietary cholesterol, dietary fat-soluble vitamins and proteins synthesized inside the intestinal cell - diffuses from the intestinal cell into the lymph
- lipoproteins allow transport of water-insoluble nutrients in the aqueous environment of the human body
Describe transport of nutrients from the small intestine via lymph followed by the bloodstream.
FAT & FAT-SOLUBLE VITAMINS
- lymph system characteristics
- originates from blood plasma lost from circulation
- contains no red blood cells or platelets
- yellowish colour
- no pumping organ
- collects large products of fat digestion
- chylomicrons travel in the lymph vessels which converge as the THORACIC DUCT and drain the content into the bloodstream at the left SUBCLAVIAN VEIN behind the heart
What is the enzyme that acts on the chylomicron, where is it found, and what is its function?
LIPOPROTEIN LIPASE (LPL)
- LPL is found on the surface of cells lining the blood vessels & is on the outside of adipose and muscle cell
- Once circulating in the bloodstream to body tissues, fats (mostly triglycerides) in chylomicrons are removed by the cells for use (muscle) or for storage (adipose) by LPL
- LPL hydrolyzes triglycerides into fatty acids and glycerol
- fatty acides cross cell membrane into cell - stored in adipose, or used in muscle as fuel.
- glycerol travels in the blood to the liver to be converted to glucose
What is a chylomicron remnant?
- as fat is removed from chylomicrons (via action of LPL) and moved into cells, chylomicrons shrink in size to become a chylomicron remnant (CR)
- liver removes CR from circulation and uses what remains in the remnant (mostly cholesterol, protein) to make other compounds needed by the body (e.g., bile, glucose, vitamin D)
Identify the products of carbohydrate digestion, how they are transported, and to where.
f. d. = facilitated diffusion
a. t. = active transport
What is the key feature with the transport of dietary fats?
- the key feature with the transport of dietary fats is that body cells gain access to the fats before the liver does
- however, this is not an issue because the liver is able to synthesize all the fats the body needs (from CHO, PRO, other FAT, alcohol), therefore it isn’t essential that it have first opportunity at these nutrients
Identify the products of protein digestion, how they are transported, and to where.
f. d. = facilitated diffusion
a. t. = active transport
Identify the products of fat digestion, how they are transported, and to where.
f. d. = facilitated diffusion
a. t. = active transport
