L40. Nutrient Digestion and Absorption Flashcards
(20 cards)
What is the overview of digestion and absorption?
- Dissolving action by saliva
- Mechanical digestion by teeth and tongue
- Beginning of chemical carbohydrate and lipid digestion
- Continued mechanical and chemical breakdown by gastric motility, stomach acid and enzymes
- By the duodenum and accessory organs
- Acidic chyme buffered
- Enzymes secreted by pancreas catalyze carbohydrate, lipid, protein, and nucleic acid digestion
- In the small intestine chemical digestion and nutrient absorption occurs
- In the large intestine, water and salt are reabsorbed
- Wastes excreted as feces
What is the overview of chemical digestion?
Stage 1 = Luminal digestion
- Initial digestion involving enzymes secreted into lumen
* Salivary glands –> salivary amylase
* Stomach –> pepsin
* Small intestine -> pancreatic enzymes; pancreatic amylase, trypsin, chymotrypsin, carboxypeptidase, lipase
Stage 2 = Contact digestion
- In small intestine
- Completes digestion before absorption
- Involves enzyme produced by enterocytes and attached to brush border of enterocytes
What does the digestion of carbohydrates involve?
Dietary carbohydrates 250-300g per day and ~50% of energy intake
Polysaccharides - long chains
- 50-70% of ingested carbohydrates
- Starch and glycogen
- Cellulose = fibre cannot be digested
Disaccharides - two units
- 30-40% of ingested carbohydrates
- Sucrose and lactose (minimal maltose)
Monosaccharides - one unit
- 5-10% of ingested carbohydrates
- Glucose and fructose (minimal galactose)
Only monosaccharides can be absorbed by the intestines
What is the digestion of polysaccharides?
Digestion of polysaccharides occurs in the lumen via a-amlyase
- Minimal (~5%) oral digestion - salivary a-amylase
- Mostly intestinal lumen - pancreatic a-amylase
Hydrolyses a1-4 bonds such as those found in maltose, a-limit dextrins and oligosaccharides
Polysaccharide bonds are named for the carbons (1 to 6) that the bonds are formed at
- a and B refers to the orientation of the hydrogen at the bond
- a bond can be digested
- B cannot be digested
What is the digestion of dissaccharides?
Disaccharides are digested by contact digestion by disaccharidases (enzymes)
- Disaccharidases are synthesized by GI tract epithelial cells
- Transported to the apical membrane of the cells
- Function to break disaccharides into monosaccharides (e.g. fructose into glucose and galactose) at the apical membrane
- Monosaccharides are then in close proximity to epithelial transporters
Types of dissacharidases: maltase, sucrase, isomaltase, lactase, glucoamylase
- Enzyme specific to the disaccharide and named for the disaccharide they digest e.g. lactase breaks down lactose into glucose and galactose
What are the two mechanisms for absorption of monosaccharides?
- Na+ dependent
e.g. glucose, galactose
- Apical SGLT1 - secondary active transport
- Basolateral GLUT2 transporters - passive - Na+ independent - absorbed via facilitated diffusion
e.g. fructose
- Apical GLUT5 - specific for fructose
- Basolateral GLUT2
What is lactose intolerance?
- Most humans produce lactase in infancy - required for human milk digestion (only food source in infancy)
- Lactase persistence is the ability to produce lactase beyond early childhood
- Genetic variability in lactase persistence
- If milk from other species is consumed by people with poor lactase persistence, undigested lactose stays in intestine
- Decreased lactose absorption = decreased water absorption and diarrhoea
- Lactose used as substrate by colonic bacteria
- Result: gas, bloating and inflammation
- Treatment; avoid milk products in diet, use lactase supplements, lactose free milk, fermented milk
What does the digestion of proteins involve?
Proteins supply amino acids (not normally a major energy source)
~50% from diet (40-90g per day)
~25% from discarded cells
~25% from secreted proteins
Proteins
- Large biomolecules
- Made of one or more chains of amino acids
- Dietary sources include meat, eggs, fish, dairy products, nuts and legumes
Polypeptides and oligopeptides
- Unbranched chains or amino acids
- Polypeptides = long, unbranched chains
- Oligopeptides = smaller chains (~2-20aa)
Amino acids (single units)
- Organic compounds containing amine (-NH2) and carboxyl (-COOH) groups
- 9 essential amino acids (body cannot synthesise)
Only amino acids and very small (di, tri, and tetra) peptides are absorbed by the intestines
What does gastrin protein digestion involve?
- In the stomach mechanical and chemical digestion breaks complex protein structures into simpler ones so that enzymes can work in the small intestine
Acid environment:
- Proteins can be hydrolysed to long peptides in an acidic pH
Pepsinogen (produced by Chief cells):
- Converted to active pepsin in acidic pH
- Cleaves polypeptides at neutral amino acids
What does the digestion of protein in the intestinal lumen involve?
- Most peptide digestion happens in the small intestine and results in the release of free amino acids, short (di, tri, and tetra) peptides
- Pancreatic proteases (digest protein) must be converted into their active form in the intestines
- Trypsinogen is activated (to Trypsin) by enterokinase (one type of brush border enzyme)
- The others are activated by trypsin
Enzymes that cleave peptides at internal peptide bonds:
- Trypsin, chymotrypsin, elastase (endopeptidases)
Enzyme that cleaves peptides at C-terminus:
- Carboxypeptidase (ectopeptidase)
What occurs with contact digestion of small peptides?
- Large number of brush border peptidases
- Release free amino acids
- Peptidases synthesised by GI tract epithelial cells
- Transported to the apical membrane
- Break short polypeptides into amino acids at the apical membrane
- Proximity to epithelial transporters
How are small peptides and amino acids absorbed?
Amino acids; Na+ dependent
- Na+/K+-ATPase (and basolateral K+ channel) maintain driving force for Na+ entry
- Apical Na+/amino acid cotransporters (multiple types specific to each amino acid) uses Na+ as a driving force
- Basolateral amino acid transporters allow amino acids to exit across the basolateral membrane (multiple types, amino acid specific)
Di, tri, or tetra peptides; H+ dependent
- Na+/K+-ATPase (and basolateral K+ channel) maintain electrical driving force
- Apical PEPT1 cotransports peptides with H+ (driving force = electrical)
- Cytosolic peptidases cleaver to amino acids
- Basolateral amino acid transporters allow amino acids to exit across the basolateral membrane (multiple types, amino acid specific)
What does the digestion of fats involve?
We consume ~100g of dietary fat per day
- High energy content and a source of fat soluble vitamins
- Contain volatile molecules that contribute to flavour/aroma
- Insoluble in water
Consumed fats
- 90% triglycerides
- 10% other fats: sterols, phospholipids, cholesterol
Digestion products
- Monoglycerides and fatty acids
Free fatty acids and monoglycerides are absorbed by simple diffusion
What are the initial steps of fat digestion?
- Emulsification - stomach and duodenum
- Reduction in size of fat droplets
Mechanism
- Motility processes - mixing
- Retropulsion - crude emulsion, segmentation - stabilised emulsion - Stabilisation - duodenum
- Allows fat to interact with water phase where the enzymes are
- Increases surface area
- Net effect: increases rate of triglyceride digestion
Mechanism
- Amphipathic bile salts act as emulsifying agents to stabilise small emulsion droplets
- Segmentation in presence of bile salts - stabilised emulsion
How is fat digested in the intestinal lumen?
- Enzyme digestion - intestinal lumen
- Digests triglycerides to monoglycerides and fatty acids
Mechanism
- Colipase binds to emulsion droplets
- Colipase is amphipathic (hydrophilic and hydrophobic parts)
- Colipase interacts with lipase
- Lipase hydrolyes triglycerides at surface and releases monoglyceride and 2 fatty acids - Micelle formation
- Formation of smaller diameter droplets that can interact with the brush border for absorption
Mechanism
- As smaller products of digestion are formed they are stabilised in smaller droplets called micelles - 4-7nm in diameter by bile salts
How is fat absorbed?
Lumen and apical membrane
- Free fatty acids and monoglycerides move by simple diffusion from micelle into cell
- Micelle not absorbed
- Bile salts absorbed in ileum
Intracellular
- Transported to the endoplasmic reticulum and resynthesized to triglycerides to maintain gradient for apical diffusion
- Secreted from golgi as chylomicrons
- Targeted to basolateral membrane
Basolateral membrane
- Exocytosed as chylomicrons
- Enter lymph via lacteals of villi
Why don’t lipids need a transporter for absorption?
The cell membrane is made up of a phospholipid bilayer with a lipid core
Fatty acids and monoglycerides are lipid soluble and can move across the lipid core
What is involved with vitamin absorption?
Vitamins are essential for the body but most cannot be synthesized
- Exception is vitamin D, synthesized in the skin from cholesterol in a light dependent reaction
- Vitamins do not require digestion, may require release from food
Fat soluble
- A, D, E, K
Water soluble
- B complex and C
How are lipid soluble vitamins absorbed?
Fat soluble vitamins
- Released from food during gastric digestion and transported in micelles
- Absorbed by simple diffusion
- Same process as monoglyceride and fatty acid absorption
- Dependent on micelle formation
- Fat soluble vitamins are stored in body fat reserves
GI tract related deficiencies:
- Defects in bile salt secretion in fat soluble vitamin deficiencies
- Vitamin A = night blindness
- Vitamin D = rickets, osteoporosis
How are water soluble vitamins absorbed?
Vitamin C
- Ionised by the intestinal lumen pH
- Sodium coupled co-transporters
- SVCT1 and 2 in the apical membrane
Vitamin B12 (example of a B vitamin)
- Binds to intrinsic factor (IF) in the stomach
- Absorbed in the ileum - endocytosis
- Vitamin B12 binds to transcobalamin II for transport through the cell
- Exocytosed across basolateral membrane
In general - water soluble vitamins are absorbed by mediated transport
