Absorption of carbohydrates and proteins Flashcards
How are sugars transported?
Sugars are highly water soluble and cannot simply diffuse across cell membranes hence require specific transporter proteins (proteins within the lipid bilayer) anchored in the membrane that form ‘pores’ in the membrane
There are two types of transport: active transport (against gradient, need energy) and facilitative transport (with gradient, no energy)
What are the glucose transporters across intestinal epithelia?
Glucose transport across the intestinal epithelia involves two glucose transporters, SGLT 1 (sodium glucose linked transporter, secondary (indirect) active transporter) and GLUT 2 (facilitate transporter)
How is glucose transported via SGLT 1 and GLUT 2 membrane transporters?
SGLT 1 transporter sense glucose and facilitates transport of glucose to epithelial cells - co-transports Na ions at the same time (glucose/Na symport)
Na concentration in intestinal tract = 120-140mmol/L, within epithelial cells =20-30mmol/L, when SGLT 1 brings Na in, Na concentration increases, if the Na concentration gets too high this inhibits SGLT 1 transporter
Na/K ATPase, hydrolyses ATP, releasing energy allowing transport of Na out of epithelial cell into blood circulation and K ions in
When glucose is imported into cell, concentration increases and GLUT 2 senses increased concentration, transports glucose out of epithelial cell into blood stream
How is Na+/K+ concentration balanced?
For every 3 sodium ions transported out of the cell, 2 potassium ions are transported into the cell
Hydrolysis of ATP and phosphorylation of the transporter leads to conformational change, enabling the transport of Na+ followed by K+, then dephosphorlyation of ATPase, conformational change of protein transporter which allows K+ to be transported back inside the cell
How are peptides absorbed?
Very little absorption of peptides longer than four amino acids (proteases need to hydrolyse and break down large amino acid chains)
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 in the blood circulation
How are amino acids absorbed in the GI tract?
Absorption from the lumen of small intestine by transepithelial transport
Semi-specific Na+ dependent transport system. Na+ dependent carriers transport both Na+ and an amino acid
Six different Na+ dependent carriers: neutral AA, proline and hydroxyproline, acidic AA, basic AA (Lys, Arg) and cystine
Na+ transported with AA, Na+ and AA concentration increases in epithelial cell, Na+ need to be coupled with ATPase pump (same as glucose transport system), so that AA can be transported out of epithelial cell into the blood stream via facilitated transporter
What is lactose intolerance?
Lactase enzyme deficiency - lactose cannot be hydrolysed causing bloating, flatulence and diarrhoea due to fermentation of lactose by intestinal bacteria
Avoid lactose in diet
Examples of diseases affecting digestive organs:
Pancreatitis - leads to inappropriate activation of zymogens (precursor enzymes/proenzymes e.g. proteases) resulting in ‘self digestion’
Stomach/peptic ulcers - due to breakdown of mucosa which normally protects against protease action
Cystic fibrosis - malabsorption (formation of a lot of material being generated that coats surfaces and can block pancreatic duct)
Coeliac disease - malabsorption
What is cystic fibrosis?
Causes thick mucous secretions which block pancreatic duct and secretion of pancreatic enzymes. Can be aided by taking supplements containing pancreatic enzymes i.e. pancreatin, a pancreas extract
What is coeliac disease?
Coeliac disease is a disease of the small intestine where the body reacts against gluten protein present in wheat. Antibodies react with transglutaminase causing villi to be flattened and as a result, nutrients not absorbed. Gastrointestinal symptoms
How are DNA and RNA digested?
DNA and RNA are subject to partial acid hydrolysis in the stomach
Intestinal endonuclease enzymes hydrolyse the phosphodiester bonds linking individual nucleotides. Exonuclease enzymes release individual nucleotides (nucleotide monophosphates) which are absorbed via nucleotide transporters
