Lecture 25: ABSORPTION OF CARBOHYDRATES AND PROTEINS Flashcards
What are sugars?
Highly water soluble and cannot simply diffuse across cell membranes (lipid bilayer)
What does sugar transport require?
Specific transporter proteins anchored in the membrane that form pores in the membrane
What are the types of transport?
Active and facilitative
What is active transport?
Against a concentration gradient that needs energy from ATP
What is facilitative transport?
Passage down a concentration gradient
What does glucose transport across the intestinal epithelia involve?
Two glucose transporters (SGLT1, a secondary active transporter, and GLUT2, a facilitative transporter)
What happens to glucose when it is in the circulation?
It is taken up by tissues such as liver, muscle and brain via other glucose transporters - such as GLUT4 (adipose and muscle) and GLUT3 (brain)
What are the glucose transporters?
GLUT1, GLUT2, GLUT3, GLUT4, SGLT1
Where is the main site of GLUt1?
Ubiquitous
Where is the main site of GLUT2?
Liver, pancreas, kidneys, intestinal epithelia
Where is the main site of GLUT3?
Brain
Where is the main site of GLUT4?
Muscle, adipose tissue
Where is the main site of SGLT1?
Intestinal epithelia
What does glucose transport by SGLT1 involve?
Simultaneous transport (symport) of sodium ions (from intestinal lumen into cell)
What is extracellular sodium concentration?
120-140 mmol/L
What is intracellular sodium concentration?
20-30 mmol/L
What does GLUT2 do?
Transport glucose from inside the intestinal cells to the peripheral tissues/blood
What does Na+/K+ ATPase do?
Actively transports Na+ to maintain the low sodium concentration in epithelial cells and maintain the gradient for SGLT1
How does Na+/K+ ATPase work?
It takes up 3 sodium ions, hydrolysis of ATP causes a phosphate to bind and a conformational change to release the sodium ions and pick up 2 potassium ions. Hydrolysis of the phosphate bond causes release of the potassium ions
What is there very little absorption of?
Peptides longer than four amino acids
How are di- and tri-peptides absorbed in the small intestine?
By co-transport with hydrogen ions via membrane transporter PepT1
what happens with absorbed di- and tri-peptides?
They are further digested into individual amino acids by cytoplasmic peptidases and exported from the epithelial cells into the blood circulation
What is the transport system for absorption of amino acids?
Semi specific sodium dependent
What do sodium dependent carriers transport?
Sodium ions and an amino acid
How many different sodium dependent carriers are there?
At least 6
What are some of the sodium dependent carriers?
Neutral amino acids, proline and hydroxyproline, acidic amino acids, basic amino acids (Lys, Arg) and cysteine
When does uptake of intact proteins occur?
Only in a few circumstances, for example newborn animals. Uptake of immunoglobulins in colostral milk for acquisition of passive immunity
What diseases affect digestive organs?
Pancreatitis, stomach (or peptic) ulcers, cystic fibrosis and coeliac disease
What does pancreatitis lead to?
Inappropriate activation of zymogens (proenzymes) resulting in self digestion
What are stomach (or peptic) ulcers due to?
The breakdown of the mucosa which normally protects against protease action
What do cystic fibrosis and coeliac disease cause?
Malabsorption
What does cystic fibrosis cause?
Thick mucous secretions which block the pancreatic duct and affect the secretion of pancreatic enzymes
What can cystic fibrosis be aided by?
Takin supplements containing pancreatic enzymes - ‘pancreatin’, a pancreas extract
What is coeliac disease a disease of?
The small intestine (particularly the duodenum)
What does the body react to in coeliac disease?
Gluten protein present in wheat
What do antibodies react with in coeliac disease?
Transglutaminase
What happens to villi in coeliac disease?
They flatten and channels close causing nutrients to not be absorbed
What symptoms are observed in coeliac disease?
Gastrointestinal
What are DNA and RNA subject to?
Partial acid hydrolysis in the stomach
What do intestinal endonuclease enzymes do?
Hydrolyse the phosphodiester bonds linking individual nucleotides
What do exonuclease enzymes in the intestine do?
Release individual nucleotides (nucleoside monophosphate)
How are individual nucleotides absorbed?
Via nucleotide transporters
