Digestion Flashcards
Amylase
- optimum pH: 7
- starch -> maltose
- produced in the salivary glands, released into the mouth
- produced in the pancreas, released into the duodenum
Pepsin
- optimum pH: 1.5
- protein -> polypeptide
- produced by the gastric glands, released into the stomach
Describe the pH of the stomach
- provided by HCl
* mucus lining provides protection
Maltase
- optimum pH: 7
- maltose -> glucose
- produced in the pancreases, released into the duodenum
Lipase
- optimum pH: 7
- lipid -> lipid components
- produced in the stomach and pancreas, released into duodenum
Trypsin
- optimum pH: 7
* polypeptides -> amino acids
Bile function
- neutralises pH
- sodium hydrogencarbonate (hydrogencarbonate ions -> alkaline)
- secreted by the pancreas and by Brunner’s glands in the duodenum
Brunner’s glands
• lead to crypts of Lieberkuhn
Crypts of Lieberkuhn
- tubular invaginations of glandular epithelia at the base of vili
- close to the base, there are actively dividing stem cells
- Paneth cells
Paneth cells
secrete microbial agents to protect stem cells in crypts of Lieberkuhn from infection
Bile
- produced by the liver
- stored in the gall bladder
- released via the bile duct
- neutralises HCl
- bile salts emulsify fats
Emulsification of fats
Break fat globules into tiny droplets, increasing SA
Ileum function
Absorption
Ileum adaptations
- many folds, convolutions and twists to increase SA
- thin walls
- high capillary density -> carries sugars, AAs, minerals, vitamins (A, C and E) and water-soluble substances
- villi
Villi adaptations
- épithélial cells with lots of mitochondria
* microvilli project themselves into the lumen of the small intestine -> forms a brush border
Absorption
Takes place via diffusion and active transport
Why is absorption of lipids hard?
- large and hydrophobic long-chain fatty acids are not easily suspended in the watery intensional chyme
- bile salts and lecithin enclose them in a micelle
Micelle
- tiny polar sphere with hydrophilic ends facing the watery environment and hydrophobic tails turned into the interior
- creates a receptive environment for long-chain fatty acids
- can easily squeeze between microvilli and get v. Close to the luminal cell surface
Why are micelles necessary?
lipids would sit on the surface of the chyme and never come into contact with the absorptive surfaces of the epithelial cells
Leaving the micelle
Having reached the cell surface, lipid substances exit the micelle and are absorbed by simple diffusion
The process of lipid exocytosis from epithelial cells
1) fatty acids and monoglycerides leave micelle and enter epithelial cells
2) fatty acids link to form triglycerides
3) fatty globules combine with proteins to form chylomicrons, inside the Golgi
4) chylomicrons are extruded from the epithelial cells and enter a lacteal
5) lymph in the lacteals transports chylomicrons away from intestine; exocytosis
Describe the stomach
- dotted with pits leading to tubular gastric glands
* gastric glands secrete gastric juices
Gastric juices
- mainly water
- mucus
- HCl
- pepsinogen
Mucus
- from goblet cells
* protection and lubrication
HCl
- from oxyntic cells
* aka pariétal cells
Pepsinogen
- from chief cells
- aka. peptic, zymogen cells
- a precursor; inactive form of an enzyme that is converted by HCl to pepsin (active enzyme)
Pepsin
- large polypeptides -> small polypeptides
* endopeptidase
Endopeptidases
- breaks the peptide links within a polypeptide chain; does not break the terminal peptide links
- produces almost no individual AAs
- creates more ends on which exopeptidases can work, with increased SA
- e.g. pepsin, trypsin
Trypsin
Derived from trypsinogen
Mechanical digestion
Muscles in the stomach wall churn contents, covering them in enzymes
Acid chyme
- uniform creamy paste
* creates by combining chemical and mechanical digestion
Exopeptidases
- break the terminal peptide bonds
- e.g. aminopeptidase
- e.g. carboxypeptidase
Aminopeptidase
Acts on terminal peptide bonds at the amino end of the polypeptide chain
Carboxypeptidase
Acts on terminal peptide bonds at the carboxyl end of the polypeptide chain
Dipeptidases
- secreted by enterocytes into the small intestines
- hydrolyse dipeptides
- membrane-bound
Enterocytes
Intestinal lining cell
Nucléase
Nucleic acids -> nucleotides
Chymotrypsin
Proteins -> small polypeptides
Entérokinase
Trypsinogen -> trypsin
Sucrase
Sucrose -> α-fructose + α-glucose
Lactase
Lactose -> α-glucose + β-galactose
Peptidases
Peptides -> amino acids
Proteases
Proteins -> polypeptide chains
Nucleotidases
Nucleotides -> bases, sugars and phosphates
Water in digestion
- essential for hydrolytic breakdown of food
* transport medium for secretions
