11: Physiology of digestion and absorption - carbohydrates, protein Flashcards
What are the three main constituents of food?
Carbohydrates
Lipids
Proteins
Which carbohydrate cannot be digested in the human GI tract?
Cellulose
Lipids can be oxidised to generate energy. What else are they used for?
Cell membrane structure
Signal transduction
What are proteins digested to obtain?
Essential amino acids
If dietary protein is an exogenous source of protein, what are some examples of endogenous protein sources?
Digestive enzymes, mucin, dead cells being digested in the GI tract
What is digestion?
Breakdown of larger molecules into smaller, absorbable molecules by enzymes
Where do most digestive processes occur?
Small intestine
Salivary amylase and gastric pepsin don’t do all that much
What name is given to the combined processes of digestion and absorption?
Assimilation
All dietary carbohydrates need to be converted to ___ for absorption in the GI tract.
monosaccharides (e.g glucose, fructose)
Which enzyme starts to digest carbohydrates for the first 30 minutes after ingestion?
Alpha amylase
Where is alpha amylase produced?
Salivary glands (mainly parotids)
Pancreas
What name is given to enzymes which break down long chain carbohydrates into monosaccharides?
Oligosaccharidases
e.g Lactase, Maltase
Oligosaccharidases are (integral / peripheral) proteins found on the (lumenal / cytoplasmic) surface of the cell membrane.
integral , lumenal
Which oligosaccharidase breaks down lactose into glucose and galactose?
Lactase
Why may a person be unable to digest lactose?
Lactase insufficiency
What is the cause of lactose intolerance?
Lactase insufficiency
What causes primary lactase deficiency?
Genetic mutation in the gene which expresses lactase
What is the cause of secondary lactase deficiency?
Damage / infection in the small intestine
What is congenital lactase deficiency?
Inability to produce lactase (so inability to digest lactose) from birth
usually autosomal recessive
What symptoms will be experienced by a person with lactose intolerance if they consume lactose?
Bloating
Abdominal pain
Flatulence
Diarrhoea (undigested lactose)
Which transporter, found on the apical membrane of enterocytes, absorbs glucose and galactose with the aid of sodium?
SGLT1
SGLT1 is a secondary active transporter - which ion facilitates the transport of glucose / galactose across the cell membrane of enterocytes?
Na+
Which transporter carries fructose across the apical membrane of enterocytes?
GLUT5
Which transporter allows the exit of all monosaccharides from enterocytes?
GLUT2
GLUT2 allows all monosaccharides to exit enterocytes by (passive / facilitated) diffusion.
facilitated
Which transporter facilitates the action of GLUT2?
Sodium potassium pump
as it keeps extracellular Na+ levels high
How many sodium ions bind to SGLT1 to facilitate entry of 1 glucose / galactose molecule?
2
Where in the small intestine does carbohydrate absorption occur?
Duodenum
Jejunum
In which three areas can peptides be digested into amino acids?
Lumen (by lumenal enzymes)
Brush border (by brush border enzymes)
Intracellularly
Which enzyme starts to digest proteins in the stomach but is not essential for protein digestion?
Pepsin
What are exopeptidases?
Enzymes which break bonds towards the ends of peptides to give individual amino acids
What are endopeptidases?
Enzymes which break bonds in the centre of peptides to give small peptides
Where are peptidases produced before they are secreted into the duodenum?
Pancreas
Which ion is required to transport peptides into an enterocyte by facilitated transport?
Na+
Other than Na+ dependent peptide transporters, what other ion can drive transport of peptides into enterocytes?
H+ (protons)
Exit of amino acids from an enterocyte is facilitated by the action of which transporter?
Sodium potassium pump
What are the two main types of digestion?
Lumenal digestion
Membrane digestion
What is the brush border of an enterocyte?
Cell membrane
What type of digestion occurs at the brush borders of enterocytes?
Membrane digestion
What are some endopeptidases found in the duodenum?
Trypsin
Chymotrypsin
Elastase
Is pepsin an endopeptidase or an exopeptidase?
Where is it found?
Exopeptidase
Stomach
