Lecture 26: CHEMICAL DIGESTION Flashcards
What are the ain nutrients that undergo chemical digestion?
Carbohydrates (sugars), proteins nd lipids (fats)
What are carbohydrates important for?
A source of energy
What are carbohydrates made of?
Large storage polysaccharides and complex chains of monosaccharides
What is the most common carbohydrate?
Starch and also glycogen
What are starch and glycogen?
Long chains of glucose joined by alpha 1,4 glycosidic linkage
What are some disaccharides?
Sucrose, lactose and maltose
What is sucrose made of?
Glucose and fructose
What is lactose made of?
Glucose and galactose
What is maltose made of?
2 glucose
What doe we ingest a limited amount of?
Monosaccharides (glucose)
Are proteins a source of energy?
No
What are proteins required for?
Amino acids
How many amino acids are there?
21 - 12 can be synthesised but other essentials such s His, Lys and Leu can’t co must come from the diet
What are sources of protein?
50% diet and 50% endogenous (enzymes and immunoglobulins which have been secreted into the lumen)
What is the structure of ingested proteins?
Long chains of amino acids linked by peptide bonds
Are lipids essential?
No
What are lipids an important source of?
Energy and fat soluble vitamins A, D, E and K
What effect do lipids have on gastric emptying?
Slow it
What are lipids mainly?
Triglycerides (glycerol backbone with 3 fatty acids attached)
What can fatty acids be?
Variable in chain length
How many carbons in short chain fatty acids?
Less than 6
How many carbons in medium chain fatty acids?
6-12
How many carbons in long chain fatty acids?
12-24
Why do we need chemical digestion?
We ingest nutrients in the form of large complex molecules but can only absorb nutrients as small molecules
What does chemical digestion do?
Decreases the size of nutrients to allow them to be absorbed
Where does chemical digestion occur?
At the surface of food particles
What does mechanical digestion do?
Breaks up food and increases surface area available for chemical digestion
What does chemical digestion utilise?
Digestive enzymes
What are digestive enzymes?
Extracellular organic catalysts (E+S<>ES complex<>E+P)
How are digestive enzymes described?
Very specific
What is needed with regards to digestive enzymes?
Many different enzymes for different substrates (amylase, protease, lipase)
What do enzymes have?
An optimal pH
What is the optimal pH for salivary enzymes?
Alkaline
What is the optimal pH for gastric enzymes?
Acidic
What is the optimal pH for small intestinal enzymes?
Alkaline
What is there large amounts of in diet?
Cellulose
What is cellulose?
A structural polysaccharide of plants. Long chains of beta 1,4 glycosidic bonds which cant be broken down but provide fibre
What are the stages of chemical digestion?
Luminal digestion and contact digestion
What is involved in luminal digestion?
Enzymes secreted into the lumen
What is contact digestion?
In the small intestine involving enzymes produced by enterocytes and attached to the brush border of enterocytes
What enzymes are involved in luminal digestion of carbohydrates?
salivary and pancreatic amylase
What happens in luminal digestion of carbohydrates?
Polysaccharides are converted to oligosaccharides and disaccharides
What happens in contact digestion of carbohydrates?
Disaccharides are converted to monosaccharides
What enzymes does contact digestion of carbohydrates involve?
Sucrase, lactase and maltase
Where are enzymes bound?
To the brush border and when shed become part of the protein intake
What enzymes are involved in luminal digestion of proteins?
Pepsin in the stomach and trypsin, chymotrypsin and carboxypeptidase in the small intestine
What happens in luminal digestion of proteins?
Proteins are converted into polypeptides
What enzymes are involved in contact digestion of proteins?
Many types of peptidases attached to the brush border
What happens in contact digestion of proteins?
Polypeptides are converted into individual amino acids
Where does chemical digestion of lipids occur?
In the small intestine
What nutrient doesn’t have contact digestion?
Lipids
What is the main and minor enzymes for chemical digestion of lipids?
Mainly pancreatic lipase but also lingual lipase and gastric lipase have minor roles
What is the problem with lipid digestion?
Digestive enzymes are dissolved in the luminal fluid. This is no problem for carbohydrates and proteins as they are water soluble but as lipids are insoluble in water a more complex process is required
What are the stages of chemical digestion of lipids?
Emulsification- motility, stabilisation - bile salts, digestion (hydrolysis)- enzymes, formation of micelles - bile salts
What happens in emulsification?
Motility breaks up lipid droplets into small droplets to form an emulsion
What is the size of emulsion droplets?
0.5-1.0 micrometres
What does emulsification do?
Increase the surface area for digestion
Where does emulsification occur?
Stomach (retropulsion) to form a simple emulsion and small intestine (segmentation) to form a more complex emulsion where bile salts stabilise (suspend in fluid) the droplets
Where does stabilisation occur?
In the small intestine
What do bile salts have?
A hydrophobic (water hating) and a negatively charged hydrophilic side (water loving) = amphipathic
What do bile salts do?
Stabilise the emulsion in the small intestine and also reduce the size of the emulsion droplets to further increase surface area for absorption
Where does hydrolysis occur?
In the small intestine at the surface of emulsion droplets
What does hydrolysis involve?
Lipase and cofactor lipase which are both secreted by the pancreas
What does colipase do?
Anchors lipase to the surface of droplets
What does lipase do?
Converts triglycerides to monoglycerides and free fatty acids
What are the products of fat digestion?
Insoluble in water (especially monoglycerides and long chain fatty acids)
How are products of fat digestion kept in solution?
Through formation of micelles
What are micelles?
Small droplets (4-6 nanometer diameter) which consist of 20-30 molecules (amphipathic bile salts, fatty acids and monoglycerides)
