Lecture 26: Chemical digestion in GIT Flashcards
What is the main reason for chemical digestion?
Digest food and break it up into small enough pieces to be absorbed
How many amino acids can be made by the human body?
▪ 12 amino acids can be made by the body in adults
▪ Essential amino acids cannot be made by the body:
- 8 in adults
Are lipids essential for humans?
▪ Most lipids are not essential
BUT
▪ Omega -3 and -6 fatty acids
▪ Fat soluble vitamins; A, D, E & K (KADE)
What are humans source(s) of protein?
▪ 50% diet: plant and animal protein
▪ 50% Endogenous (body) proteins
- Secreted into intestine
- Enzymes
- Immunoglobulins
What is the main role of amino acids in the body?
Source of amino acids to make body proteins (main job)
Not a major source of energy under normal conditions
What is the main structure of lipids that we eat?
Mostly triglycerides (TAGs):
▪ Glycerol backbone
▪ 3 fatty acids
What are the relative sizes of short, medium and long chain fatty acids?
Short chain fatty acids:
▪ < 6 carbons
Medium chain fatty acids:
▪ 6 - 12 carbons
Long chain fatty acids:
▪ 13 - 24 carbons
In what form can carbohydrates be absorbed into the GIT?
monosaccharides
In what form can proteins be absorbed into the GIT?
amino acids, small peptides (2 - 3 amino acids)
In what form can lipids be absorbed into the GIT?
free fatty acids, monoglycerides (glycerol + 1 fatty acid)
What are digestive enzymes?
Digestive enzymes are extracellular, organic catalysts
Can cellulose (fiber) be broken down and digested in humans?
Cellulose can’t be digested by the body:
▪ amylase can’t break β-bonds between glucose molecules
What is the optimal pH for salivary and pancreatic enzymes?
slightly alkaline pH
What is the optimal pH for gastric (stomach) enzymes?
acidic pH
Which of the following statements is CORRECT?
1. Polysaccharides are made up of many amino acids.
2. Proteins are an important source of energy in the diet.
3. Mechanical digestion increases the surface area of food
particles.
4. Fiber cannot be digested by humans because amylase is unable to break ⍺-bonds between glucose molecules.
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Chippy chips
What are the two stages of chemical digestion?
- Luminal digestion: Starts chemical digestion
- Chemical digestion by acid and enzymes which have been secreted into the lumen of the GIT and mixed through the food/chyme - Contact digestion: Finishes chemical digestion
- Chemical digestion by enzymes made by and attached to the brush border of enterocytes (small intestine epithelial cells)
What can be digested in the luminal digestion phase?
Carbohydrate, Protein and Lipids/Fat
What can be digested in the contact digestion phase?
Further break down Carbohydrate and Protein via Brush border disaccharidases and Brush border peptidases
What is the digestion process for carbohydrates?
- Luminal digestion
Starch (polysaccharide)
▪ Initiated in the mouth by salivary amylase
▪ Continued in the small intestine by pancreatic amylase - Contact digestion
▪ In the small intestine disaccharidases are bound to the brush border
▪ Different enzymes for different disaccharides:
▪ sucrase for sucrose, lactase for lactose, maltase for maltose
What is the digestion process for proteins?
- Luminal digestion
▪ Initiated in the stomach by HCl acid (hydrochloric acid) and pepsin (protease)
▪ Continued in the small intestine by pancreatic proteases:
- Trypsin
- Chymotrypsin
- Carboxypeptidase - Contact digestion
▪ In the small intestine peptidases are bound to the brush border
▪ Different enzymes for different peptide bonds
Short peptides are broken down into __________ as
result of contact digestion by ___________on the
brush border of the intestinal cells.
1. amino acids : peptidases
2. triglycerides : peptidases
3. amino acids : disaccharides
4. monosaccharides : lipases
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Are digestive enzymes water or fat soluble?
Digestive enzymes are water soluble and dissolved in
the fluid within the lumen of the GIT
Does fat (lipids) undergo contact digestion?
NO
Fats only undergo luminal digestion (no contact digestion), which occurs in the stomach and small intestine
What percentage of fats are digested in the stomach and what enzymes are responsible?
Lingual and gastric lipase digest ≈ 30% of TAGs in the stomach
Where does a majority of lipid digestion occur and what enzyme is responsible?
Pancreatic lipase is the main digestive enzyme for lipids and digests TAGs in the small intestine
What are the four stages of lipid digestion?
- Emulsification: Mixing
▪Motility - Emulsification: Stabilisation
▪Bile salts (& others) - Digestion (Hydrolysis)
▪Enzymes - Formation of micelles
▪Bile salts
Where are bile salts made and stored?
▪ Made and secreted by liver & concentrated in the gallbladder
▪ Released into small intestine with arrival of chyme
What is special about bile salts and how do they aid lipid digestion?
▪ Amphipathic: have a hydrophobic (water-hating) and a negatively charged hydrophilic side (water-loving)
▪ Stabilize the emulsion droplets in small intestine
▪ Also reduce the size of the emulsion droplets
▪ Which further increases the surface area for digestion
What is the process of emulsification?
Process by which large lipid droplets are broken down into smaller, stabilised droplets increasing the surface area for digestion
What motility patterns aid in emulsifacation?
▪ Stomach: retropulsion (simple emulsion)
▪ Small intestine: segmentation (more complex emulsion - includes bile salts)
What occurs during hydrolysis of lipids (stage 3)?
▪ Lipase converts triglycerides (TAGs) to:
▪ 2 x free fatty acids
▪ 1 x monoglyceride
What does hydrolysis of lipids require?
▪ Hydrolysis of TAGs requires BOTH lipase and its cofactor colipase, both secreted by the pancreas
▪ Colipase anchors lipase to surface of droplet
Where does hydrolysis of lipids occur?
▪ in the small intestine
▪ at surface of emulsion droplets
How are the products of fat digestion kept in solution?
Kept in solution through formation of micelles:
▪ Consist of 20-30 molecules
- Bile salts
- Fatty acids (insoluble especially long chain)
- Monoglycerides (insoluble)
Bile:
1. is a lipolytic enzyme.
2. solubilises the products of lipid digestion.
3. salts reduce the surface area of lipid droplets.
4. converts trypsinogen to trypsin.
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