Nutrient digestion I - carbohydrates and proteins

Question 1

What are the 6 primary dietary constituents?

Answer 1

• Carbohydrate
• Protein
• Fat
• Vitamins
• Minerals
• Water

Question 2

List the 3 main monosaccharides

Answer 2

Hexose sugars (6C) - glucose, galactose, fructose (ketohexose)

Question 3

Which of the 3 monosaccharides is a ketohexose?

Answer 3

Fructose

Question 4

List and explain disaccharides

Answer 4

Two monosaccharides linked together by glycosidic bond
Broken down to constituent monomers by brush border enzymes in small intestine to allow for human absorption
o Lactose = glucose + galactose (lactase)
o Sucrose = glucose + fructose (sucrase)
o Maltose = glucose + glucose (maltase) – most common

Question 5

What are the storage forms of glucose in plants?

Answer 5

Starch: amylose (straight) and amylopectin (branched)

Cellulose

Question 6

What breaks down starch?

Answer 6

Amylase

Question 7

Where is amylase produced?

Answer 7

Mouth and pancreas

Question 8

Which storage forms of glucose in plants can we digest?

Answer 8

Starch - amylose and amylopectin

Cant digest cellulose as we don’t have cellulase

Question 9

Whats the most highly branched storage form of glucose?

Answer 9

Glycogen

Question 10

At what blood glucose level does glycogen production start?

Answer 10

> 5mmol/L (until 18miles worth made, then for fat)

Question 11

Which glucose polymers act as dietary fibre?

Answer 11

cellulose

Question 12

Describe glucose transport from gut lumen to the blood stream

Answer 12

• SGLT1 (sodium coupled glucose transporter 1) is a co-transporter that allows the entry of Na and glucose into the epithelium from the lumen.
• Sodium is pumped out the cell by Na-K ATPase to maintain a low intracellular Na concentration
• In order to counteract the higher Na and glucose concentration, water moves via osmosis into the blood paracellularly to maintain the osmotic gradient
• Glucose is transported into the blood to be transported around the body and supply energy via GLUT-2 (glucose transporter)

Question 13

Describe fructose transport from gut lumen to the blood stream

Answer 13

• Fructose is transported via GLUT-5 (fructose transporter) into the epithelium cells
• Fructose, as well as all other monosaccharides, is then transported into the bloodstream via GLUT-2.

Question 14

Describe the two types of exopeptidases

Answer 14

o Aminopeptidases – snip off amino acid from end with amine group (-NH2)
o Carboxypeptidases – snip off an amino acid from carboxyl end (-COOH)

Question 15

Describe the transport of amino acids from the lumen into the bloodstream

Answer 15

• Similar to glucose, amino acids are transported along with sodium down its gradient into the membrane via SAAT1
• To maintain the sodium gradient, sodium is transported out the cell into the blood via the NA-K ATPase
• The amino acids are then transported into the blood via specific transporters to each amino acid.

Question 16

Describe the transport of short peptides from the lumen into the bloodstream

Answer 16

• PepT1 picks up dipeptides and tripeptides, and is coupled with hydrogen ions, to allow the transport of these short peptides into the membrane using the proton motive force.
• The hydrogen ions are then recycled via NHE3 (sodium-hydrogen exchanger) and swapped for sodium
• To maintain the low intracellular sodium concentration, again the Na-K ATPase swaps intracellular sodium for extracellular potassium.
• These intracellular dipeptides and tripeptides then are transported into the bloodstream via an unknown transporter