GI digestion and absorption Flashcards
What is digestion?
The breakdown of nutrients into absorbable molecules
What is absorption?
Movement of nutrients, water and electrolytes from the gut lumen into the internal enviroment
What is the surface of the small intestine arranged in?
Circular folds of Keckring
What is the apical surface of epithelial cells covered in?
Microvilli - creates brush border
What are enterocytes?
Simple columnar epithelial cells which line the inner surface of the small and large intestines
What do lacteals transport?
Most fats to the lymph
What is meant by protein which comes from ‘endogenous sources’?
Protein which is not from diet, old gut cells (35-200g)
What can be absorbed in the mouth?
almost none (certain drugs can)
What is absorbed in the colon?
Almost nothing (apart from water), some “indigestible” substances used as fuel by gut flora
What kind of carbohydrates can be absorbed?
Only monosaccharides
How is glucose absorbed in the SI?
- Na+ dependant cotransport
- SGT 1 - sodium dependant glucose transporter 1 - located on the apical membrane
- Secondary active transport
How is galactose absorbed in the SI?
- Na+ dependant cotransport
- SGT 1 - sodium dependant glucose transporter 1 - located on the apical membrane
- Secondary active transport
How is fructose absorbed in the SI
- Facilitated diffusion
- GLUT 5 (glucose transporter 5) transports fructose across apical membrane
What are starch (from plants and glycogen (from animals) joined by?
alpha 1-4 glycosidic bonds
What are alpha 1-4 glycosidic bonds (present in starch and glycogen) digested by?
Amylase
What is cellulose (from plants) made up from?
Linear chains linked by B 1-4 glycosidic bonds. No enzymes in humans to digest this
What does pancreatic amylase produce when long chain carbohydrates are broken down?
- Maltotriose
- Maltose
- Alpha-limit dextrins
What are products of amylase and oligosaccharidases hydrolysed by?
Disaccharidases which are attached to the brush border membrane
What are some examples of disaccharidases?
- Maltase - produces glucose
- Sucrase - produces glucose and fructose
- Lactase - produces galactose and glucose
Describe the structure of maltose?
- 2 glucose molecules bound together by a alpha 1-4 glycosidic bond
Describe the structure of sucrose`
- GLUCOSE-FRUCTOSE
- bound together by an alpha 1-2 glycosidic bond
Describe the structure of lactose
- GALACTOSE-GLUCOSE
- Bound together by a B 1-4 glycosidic bond
What are the final products of carbohydrate digestion?
- Fructose
- Glucose
- Galactose
What does protein digestion begin with?
Stomach with pepsin
Where is protein digestion completed?
SI with pancreatic and brush-border proteases
How can the enzymes which digest proteins be classified?
- Endopeptidases - hydrolyse the interior peptide bonds of proteins
- Exopeptidases hydrolyse one amino acid at a time
Give 4 examples of endopeptidases
- Pepsin
- Trypsin
- Chymotrypsin
- Elastase
What are the products of stomach and pancreatic luminal enzymes?
- Oligopeptides
- Amino acids
What are the products of the action of brush border enzymes?
- Amino acids
- Dipeptides
- Tripeptides
What are protein-derived structures further broken down into in enterocytes
Amino acids
How are amino acids transported into SI?
Through Na+ - dependant cotransport
How are dipeptides transported into SI?
H+ - dipeptide cotransport
How are tripeptides transported into SI?
H+-tripeptide cotransport
How are lipids digested and absorbed?
- Dietary lipids are hydrophobic (insoluble in water) so causes problems with di/absorption
- Must be solubilised b
- Digestion begins in stomach with the action of lingual and gastric lipases
- Completed in the SI with the action of the pancreatic enzymes
What are the 4 main types of lipids?
- Fats/oils
- Phospolipids
- Cholesterol and cholesterol esters
- Fatty acids
What are the 3 main types of enzymes which digest lipids?
- Lipases
- Phospholipases
- Cholesterol esterases
Describe the process of lipid digestion in the mouth and stomach
- Salivary / gastric lipases
- Heat and movements in stomach mix food with lipases
- Hydrolysis initially slow due to largely seperate aqueous/lipid interface
- As hydrolysis procedes, rate increases due to fatty acids produced acting as surfactants breaking down lipid globules aiding emulsification
- Emulsified fats ejectd from stomach and duodenum
Describe lipid digestion in the duodenum
- Bile salts, lysolecithin and products of lipid digestion emulsify dietary lipids
- Emulsification produces small droplets of lipid dispersed in aqueous solution creating a large surface area for pancreatic enzyme digestion
- Pancreatic enzymes (pancreatic lipase, cholesterol ester hydrolase and phosphlipase A2) and the protein, colipase are secreted to complete digestion
What’s the difference between chylomicrons and micelles?
- Micelles - gut lumen
- Chylomicrons - in enterocytes and exocytosed into the lymphatic system
Describe the structure of chylomicrons?
- 100 nm in diameter
- Core of triglycerides and cholesterol ester
- Phospholipids and apoproteins on the outside (80%/20%)
How do chylomicrons enter vascular capillaries?
- Too big to enter themselves
- Enter lacteals by moving between endothelial cells that line lacteals
How do chylomicrons enter the bloodstream?
- Enter lacteals
- Lymphatic circulation carries chylomicrons to the thoracic duct which empties them into the bloodstream
How does the body deal with not having enough bile salts for the average meal?
- 3-4g in body but 3-15g required per meal
- Most reabsorbed from terminal ileum into the liver via the enterohepatic circulation (via hepatic portal veins)
What percentage of bile salts are lost in feces?
5%
How is water absorbed in the colon?
- Na+ actively absorbed (in exchange for K+)
- K+ reabsorbed in exchange for H+
- Cl- absorbed (in exchange for HCO3-)
- H2O follows due to osmosis
What is water absorption controlled by?
Enteric nerve plexi and hormonal control
- Aldosterone (increased water absorption)
What are formed in the colon
- B vitamins (B12, thiamin, riboflavin)
- Formation of vit K (B and K absorbed passively, probably)
How much bacteria is present in the feces?
10^11 bacteria / g of faeces
What can be absorbed in digestion?
- Certain bile constituents
- Other ‘indigestable’ matter
How much fluid enters the GI tract each day?
~ 9L (approx 2.3L from ingestion, the rest from secretion)
How much fluid is reabsorbed in the small intestine?
~8L
What percentage of the last litre of fluid is reabsorbed by the colon?
~90%
Through osmosis through cell walls into vascular capillaries inside villi
