Digestion and absorption of carbohydrates and proteins Flashcards
What are monosaccharides?
Dietary carbohydrates
Monomers
What are oligosaccharides?
Dietary carbohydrates
Short polymers
What are polysaccharides?
Dietary carbohydrates
Long polymers
Which dietary carbohydrates can be directly absorbed by the small intestine?
Monosaccharides
What are the two categories of polymers?
Digestible
Non digestible
What is dietary fibre?
Both soluble and insoluble forms of fibre
Found in fruits, vegetables and cereals
Describe insoluble fibre
Non water soluble
Not easily fermented
Cellulose and hemicellulose
Aids the digestive system and eases elimination
Describe soluble fibre
Water soluble
Pectins, gums, mucilage
Easily digested by bacteria in the colon
What forms 45-60% of dietary carbohydrate?
Starch
Describe starch
Starch is a carbohydrate primarily found in plants
Polysaccharide
Consists of amylose and amylopectin
Describe amylose
Straight chain glucose polymer
Connected by alpha-1,4 linkages
Describe amylopectin
Branched glucose polymer
Alpha-1,4 and alpha-1,6 linkages
What is glycogen?
The “animal starch”
Structure like amylopectin but even more highly branched
What makes up 30-40% of dietary carbohydrate?
Disaccharides
Sucrose
Lactose
What makes up 5-10% of dietary carbohydrate?
Monosaccharides
Glucose
Fructose
Where does sucrose come from?
Sugar cane
Sugar beets
Where does lactose come from?
Milk
Are disaccharides directly absorbed by the small intestine?
No
What are the two steps to digesting dietary carbohydrates?
Intraluminal hydrolysis
Membrane digestion
Which enzymes are involved in luminal digestion?
Salivary amylase and pancreatic amylase
What inactivates salivary amylase?
Gastric acid
What induces pancreatic amylase?
CCK
Describe amylase
An endoenzyme
Hydrolyses internal alpha-1,4 linkages
What are the products of starch hydrolysis by amylase?
Maltose
Maltotriose
Alpha-limit dextrins
How many oligosaccharidases are there in the human brush border?
3
Name the 3 oligosaccharidases
Lactase
Maltase
Sucrase-isomaltase
What is the substrate of lactase and what are the products?
Lactose
Glucose and galactose
What is the substrate of maltase and what are the products?
Alpha-1,4 linkages in straight chains
Produces glucose
What enzymes cleave alpha-1,4 linkages?
Maltase
Sucrase
Isomaltase
What is the substrate and products of sucrase?
Sucrose
Glucose and fructose
What is the substrate and products of isomaltase?
The only enzyme that can split the alpha-1,6 linkages of alpha limit dextrins
Where is the highest concentration of oligosaccharides?
Proximal jejunum
What are the two stages to absorption of glucose, galactose and fructose?
Uptake across the apical membrane
Exit across the basolateral membrane
What is responsible for the uptake of glucose and galactose?
Na/glucose transporter SGLT1
Active transport
Energy from Na+ gradient
What is responsible for the uptake of fructose?
GLUT5
Facilitated diffusion
What is responsible for the exit of monosaccharides across the basolateral membrane?
GLUT2
What are the symptoms of lactase deficiency?
Diarrhoea and cramps
High hydrogen breath levels
Lactose is metabolised by colonic bacteria
How are proteins absorbed?
Digested into constituent oligopeptides and amino acids
Where is protein sourced from?
50% diet
50% endogenous
