Lecture 5 - Nutrient Digestion I (Carbohydrates and Proteins) Flashcards
What is the basic principle of digestion?
Taking large complex molecules and breaking them down into monomeric units
What is the problem with digestion?
You unlock all these tiny molecules which causes water to flood in
How do you get past the problem with digestion?
Stomach releases small amount of content at once to prevent influx of water
Molecules are repackaged into larger molecules (e.g. liver stores glycogen)
What is dumping syndrome?
A condition where the stomach moves its contents to the duodenum too quickly –> massive water influx
What are the principle dietary constituents?
Carbs - main energy source Protein - build + repair body Fat - most calorie rich Vitamins Minerals Water
What are monosaccharides?
Sugars that cannot be hydrolysed to give a simpler sugar
What are the 3 monosaccharides we absorb in the GIT?
3 Hexose sugars (6C): galactose, fructose, glucose
Where are the monosaccharides absorbed?
Small intestine
What bond links two monosaccharides to form a disaccharide?
Glycosidic
Where and by what are disaccharides broken down?
Brush border enzymes break them down in the small intestine
What are the 3 important disaccharides?
Lactose
Sucrose
Maltose
What is lactose?
Glucose + galactose
What is sucrose?
Glucose + fructose
What is maltose?
Glucose + glucose
What enzyme breaks down lactose?
Lactase
What enzyme breaks down sucrose?
Sucrase
What enzyme breaks down maltose?
Maltase
Can the body absorb disaccharides?
No
What causes diarrhoea in lactose intolerance?
Lack of expression of lactase –> inability to breakdown lactose –> brings in too much water from lumen of small intestine
What are the three main polysaccharides?
Starch
Cellulose
Glycogen
What is the starch?
Plant form of glucose
What is the composition of starch?
Alpha-amylase (glucose linked in straight chains)
Amylopectin (glucose chains highly branched)
What bonds link the glucose monomers in starch?
Alpha-1, 4 glycosidic bonds
What enzymes can hydrolyse the alpha 1-4, glycosidic bonds?
Salivary and pancreatic amylases
What is cellulose?
Constituent of plant cell walls
What is the composition of cellulose?
Unbranched, linear chains of glucose monomers linked by beta-1, 4, glycosidic bonds
How is cellulose broken down?
It is a dietary fibre, i.e. require bacterial cellulase to break it down
What is glycogen?
Animal storage form of glucose
What bonds link the glucose monomers in glycogen?
Alpha 1, 4 glycosidic bonds
What kind of bonds can be broken down by alpha-amylase?
Alpha 1, 4 glycosidic bonds
What are the function of the villi in the GIT?
Increase surface area for absorption
What are found on the surface of the villi?
Microvilli
What are the two membranes on the epithelial cells?
Basolateral and apical membrane (outward facing)
What is present between the columnar cells in the small intestine?
Tight junctions
How do the molecules travel into the small intestine?
Molecules have to get through apical membrane
What are the two ways that molecules can get across the apical membrane?
Can go through the cells (transcellular pathway)
Can go between epithelial cells (paracellular pathway)
What are the different transcellular pathways?
Lipid soluble - cross lipid membrane
Transport proteins for charged molecules
What pump creates a driving force for sodium to enter cells on the intestinal epithelium?
Na-K-ATPase
How is glucose absorbed in the small intestine?
Sodium-glucose linked transporter 1 co-transports glucose and sodium into the epithelial cell
Glucose moved across basolateral membrane via GLUT2 transporter
NaK-ATPase on basolateral membrane maintains a concentration gradient that favours Na moving in from the gut
How is water absorbed in the small intestine?
Follows Na absorption paracellularly
How is fructose absorbed from the gut?
GLUT5 receptor on apical membrane and GLUT2 out basolateral membrane
What are proteins?
Polymers of amino acids linked together by peptide bonds
What are amino acid chains of 3-10 known as?
Peptides
What are the names of enzymes which hydrolyse peptide bonds?
Proteases - breakdown proteins
Peptidases - breakdown peptidases
Where do endopeptidases work?
Centre of protein
Where do exopeptidases work?
Terminal amino acid
How are proteins absorbed in the small intestines?
Sodium coupled amino acid transporter - sodium electrochemical gradient driven by pump, sodium pulls in amino acid with it and specific transporter transports it across basolateral membrane
How are dipeptides absorbed across the small intestine?
PepT1 transporter absorbs di and tripeptides by coupling up with hydrogen ions (proton motive force)
Can be transported across basolateral membrane as intact dipeptide or broken down to AAs in cell
NHE3 transporter transports H out of cell again (this is important for creating acid microclimate)
