Lecture 25 - absorption of carbohydrates and proteins Flashcards
Absorption of products from the gastrointestinal tract into the body
The small intestine has specialised structures that create a vast surface area for absorption which makes it more efficient
This includes villi, microvilli (Brush border)
Overview of carbohydrate processing in the gastrointestinal tract
Complex carbohydrate is eaten - starch, lactose, sucrose
Salivary alpha amylase in the mouth does some initial hydrolysis
In degrading starch and glycogen, the alpha amylase from the pancreases generates these two sugar units - linear chains generate the maltose and the branch points, the 1,6 linkages, are referred to as a disaccharide isomaltose
Further breakdowns by alpha amylase in the small intestine
In the small intestine still… Two enzymes - maltase can hydrolyse maltose into two glucose units and isomaltase can hydrolyse isomaltose to glucose then there are other enzyme that can turn things into their associated monosaccharide form
Fiber material that might be associated with the food we have eaten is not hydrolyses because it typically has beta 1,4 glycosidic linkages and we don’t have the enzymes to hydrolyse these and so it passes out of the body through the faeces
Sugar transport
Sugars are highly water soluble and cannot simply diffuse across cell membances
Require specific transporter proteins anchored in the membrane that form ‘pores’ in the membrane
Two types:
Active transport - against a concentration gradient that needs energy from ATP
Facilitative transport - passage down a concentration gradient
Glucose transporters
Example of sugar transport
Glucose transport across intestinal epithelia involves glucose transporters (SGLT 1, a secondary active transporter, and GLUT2, a facilitative transporter)
Once in the circulation, glucose is taken up by tissues such as the liver, muscle and brain via other glucose transporters such as GLUT4 (muscle, adipose) and GLUT3 (brain)
Tissue distribution (main sites) of SGLT1 and GLUT2
GLUT2 = liver, pancreas, kidney, intestinal epithelia
SGLT 1 = intestinal epithelia
The SGLT and GLUT2 membrane transporters
SGLT 1 transporter has evolved to simultaneously transport sodium ions into the epithelial cell. As a result of the sodium transfer, the inside of the epithelial cell is still maintained at a much lower level and this is achieved by the sodium potassium ATPase and it is a pumping system that runs all the time that pumps sodium out of the epithelial cell to maintain the lower level. This particular transporter has also evloved to transport potassium in the opposite direction so there is a balancing act in reducing sodium levels in the cell with the activity of the SGLT 1 transporter and at the same time the balancing of the potassium ions back into the cell and this pump needs energy and it gets this through the hydrolysis of ATP
Evolved a role on the opposite side of the epithelial cell to SGLT 1 to get glucose that has been taken up into the epithelial cell to be passed out of the epithelial cell and this glucose ultimately goes into bloodstream and it is circulated around the body and then there is insulin sensing the glucose levels in the body and signalising to other peripheral tissues to utilise the glucose
Glucose transport by SGLT1 involves …
Simultaneous transport of sodium ions
What actively transports sodium in epithelial cells and why?
Na+/K+ - ATPase actively transports sodium in order to maintain low sodium in the epithelial cells
Absorption of peptides
Very little absorption of peptides longer than four amino acids
Absorption of di- and tri- peptides in the small intestine by co-transport with H+ ions via membrane transporter PepT1
Absorbed di- and tri-peptides are further digested into individual amino acids by cytoplasmic peptidases and exported from the epithelial cells into the blood circulation
Absorption of amino acids from the gastrointestinal tract
Absorption from the lumen of the small intestine by transepithelial transport
Semispecific Na+ -dependent transport system
Na+-dependent carriers transport both Na+ and an amino acid
At least six different Na+-dependent carrier: neutral AA, proline and hydroxylproline, acidic AA, basic AA (lys, arg) and cistine
Uptake of intact proteins from the gastrointestinal tract
Occurs only in a few circumstance for example in new born animals
In the early development of the gastrointestinal tract of some animals, newborn and lactating animals are able to uptake some whole proteins particularly immunoglobulins from colostrum milk which is thought to provide the newborn with some passive immunity to protect it initially prior to the newborn developing its own immune system
Overview of protein processing in the gastrointestinal tract
Protein is ingested
Protein reacts the stomach…pepsin is released in the stomach and does some initial hydrolysis at low pH around 2 and then the protein is partially hydrolysed and transferring down the gastrointestinal tract
Then we have other protease released from the pancreas along with bicarbonate (in the small intestine) which neutralises the solution flow through the system which are released in their zymogen form initially like pepsin
Further hydrolysis which then reduces the protein material down to small peptides and amino acids and then we have uptake into the epithelial cells where there are Di and tri peptidases that reduce the small peptides into free amino acids which go into the bloodstream and circulate to the peripheral tissues and then uptaken into those tissues for various anabolic and in some cases catabolic mechanisms/reactions
Lactose intolerance
Lactase enzyme deficiency (genetic basis)
Causes bloating, flatulence and diarrhoea due to fermentation of lactose by intestinal bacteria
Need to avoid lactose in their diet because they cannot process it
No lactase - accumulation of lactose which is fermented to acids, H+ and CO2 - bloating and flatulence
Acids and lactose drive H2O into the intestine which results in diarrhoea
Lactose is a disaccharide
Examples of diseases affecting digestive organs
Pancreatitis leads to inappropriate activation of zymogens (proenzymes), resulting in self digestion
Stomach (or peptic) ulcers due to the breakdown of the mucosa which normally protects against protease action
Cystic fibrosis causes malabsorption
Coeliac disease also causes malabsorption
Pancreatitis
Pancreatitis leads to inappropriate activation of zymogens (proenzymes), resulting in self digestion
