Gastrointestinal Physiology: Digestion Flashcards
What are the 3 main nutrients that undergo chemical digetion?
Carbohydrates, lipids and proteins
These are made up of long chains of glucose joined by alpha 1-4 glycosidic bonds.
Starch and glycogen
What are the 3 disaccharides?
Sucrose, lactose and maltose
Why are lipids not essential to our bodies?
Because we can produce them endogenously so no need for them to be in our diet
What are some of the properties of lipids in our body? (3)
- slow gastric emptying
- source of energy
- fat soluble vitamins A, D, E and K
What is the main component of lipids?
Triglycerides
This reduces the size of nutrients to allow them to be absorbed.
Chemical digestion
Where does chemical digestion occur? And what does it use to break down the food particles?
Chemical digestion occurs at the surface of food particles.
It uses digestive enzymes secreted by SI and pancreas to break the food down
This breaks up food and increases surface area available for chemical digestion. Usually occurs at the mouth.
Mechanical digestion
These are highly specific extracellular organic catalysts that are responsible for breaking down food particles during chemical digestion.
Digestive enzymes
What is the optimal pH for each of these enzymes:
a) salivary enzymes
b) gastric enzymes
c) small intestinal enzymes
a) alkaline
b) acidic
c) alkaline
What does it mean by digestive enzymes being highly specific?
Different enzymes bind to different substrates
- amylase => starch
- proteases => proteins
- lipase => lipids
What are the 2 stages of chemical digestion?
Luminal digestion and contact digestion
This is the initial digestion which involves enzymes secreted into the lumen.
Additional question: this can occur at 3 different organs in the GI tract, what are those 3 and what enzymes specifically carryout this type of digestion?
Luminal digestion
Luminal digestion occurs in the stomach, the small intestine and the mouth. The enzymes in the stomach are usually pepsin, small intestine: pancreatic enzymes (pancreatic amylase, trypsin, chymotrypsin etc., and in the mouth: salivary amylase
This is the type of digestion which occurs in the small intestine. It involves enzymes produced by enterocytes which are attached to the brush border of the enterocytes.
Contact digestion (via microvilli brush border enzymes)
Remember: these enzymes are produced by the epithelial cells (enterocytes) of the small intestine. Segmentation helps contact digestion occur.
How are carbohydrates chemically digested?
- what are they broken down into
- where do they get broken down into
- what enzymes are responsible for breaking them down
Carbohydrate digestion occurs in two stages.
Luminal digestion occurs in the mouth and the small intestine
- luminal digestion breaks polysaccharides into oligosaccharides and disaccharides using salivary amylase and pancreatic enzymes
Contact digestion occurs in the small intestine
- contact digestion breaks disaccharides and breaks them down into monosaccharides using brushborder disaccharidases (carbohydrate enzymes)
Where is the pancreatic amylase found?
in the small intestine; it is a pancreatic enzyme not a brushborder enzyme.
How are proteins chemically digested?
- what are they broken down into
- where do they get broken down into
- what enzymes are responsible for breaking them down
Luminal digestion occurs in the stomach and the small intestine
- luminal digestion breaks proteins down into polypeptides using pepsin in stomach, and trypsin, chymotrypsin, and carboxypeptidase in the small intestine
Contact digestion occurs in the small intestine
- contact digestion breaks proteins into indvidual amino acids using peptidases (which are attached to the brush border)
Where is carboxypeptidase made?
In the pancreas, secreted by the pancreas into SI
How are lipids chemically digested?
- what are they broken down into
- where do they get broken down into
- what enzymes are responsible for breaking them down
Lipids only undergo luminal digestion in the small intestine using pancreatic lipase as the main enzyme (lingual and gastric lipase also play minor roles)
Why do lipids have a more complex process of digestion?
Because the digestive enzymes are soluble in aqueous solutions, but lipids are not.
- so need to convert lipids into a water soluble form so it can be broken down by water soluble enzymes
What are the 4 stages of lipid digestion?
1) emulsification
2) stabilisation
3) digestion (hydrolysis)
4) formation of micelles
This is the process where lipids are converted into small droplets. These droplets increase surface area for digestion.
Emulsification
Where does emulsification of lipids occur? and what motility patterns are associated with it in these areas?
Stomach - retropulsion
Small intestine - segmentation
Where does stabilisation of lipids occur? what is responsible for stabilisation and where is it secreted from?
Stabilisation occurs in the small intestine
Bile salts stabilise lipid droplets
Bile is secreted from the liver and concentrated in the gallbladder
This is the process where the lipid droplets are reduced in size by bile salts in the small intestine. These also further increase surface area for digestion.
Stabilisation
Where does hydrolysis of lipids occur?
In the small intestine at surface of emulsion droplets.
What are the 3 main enzymes involved in hydrolysing lipids into monoglycerides and fatty acids?
Pancreatic lipase, lingual and gastric lipase and colipases as well (these anchor lipases to the triglyceride surface)
This is the process where products of fat/lipid digestion are packaged into small droplets of about 20-30 molecules which allow it to be kept in solution.
Formation of micelles/
What are micelles composed of?
Bile salts, fatty acids and monoglycerides
What are the 2 products of fat/lipid digestion?
Monoglycerides and fatty acids
