Nutrition and Energy (Protein Absorption and Metabolism)

Question 1

What forms of protein digestion occur in the mouth?

Answer 1

Mechanical digestion: Teeth and jaws increase surface area for enzymatic action

Question 2

What forms of protein digestion occur in the stomach?

Answer 2

Mechanical digestion: Muscular movements of the stomach (peristalsis)
Chemical digestion: Pepsin (enzyme) hydrolyses peptide bonds to form large polypeptides

Question 3

What is the precursor of pepsin? How is it activated?

Answer 3

Pepsinogen is secreted into the stomach lumen and is activated by the HCl to form pepsin

Question 4

What forms of protein digestion occur in the small intestine?

Answer 4

Mechanical digestion: Muscular movements of the small intestine (peristalsis)
Chemical digestion: Enzymatic breakdown of polypeptides into individual amino acids

Question 5

What inactive enzymes does the pancreas secrete into the small intestine?

Answer 5

Trypsinogen, Chymotripsinogen and Procarboxypeptidase

Question 6

How are the inactive pancreatic protein-digesting enzymes activated?

Answer 6

Enterocyte cell membrane enzymes activate the inactive pancreatic enzymes
Trypsinogen → Trypsin by Enterokinase
Chymotrypsinogen → Chymotrypsin by Trypsin
Procarboxypeptidase → Carboxypeptidase by Trypsin

Question 7

What are the different enzymes that breakdown proteins in the small intestine?

Answer 7

Trypsin, Chymotrypsin (Break peptide bonds in intestinal lumen)
Procarboxypeptidase (Breaks peptide bonds from carboxyl end in intestinal lumen)
Aminopeptidase (Breaks peptide bonds from amino end on cell surface)
Dipeptidase (Breaks dipeptides on cell surface)
Peptidase (Breaks peptide chains into amino acids in cell lumen)

Question 8

Are amino acids stored?

Answer 8

No, they are sent to the liver where they are made into proteins for use around the body

Question 9

Between meals, what happens to proteins?

Answer 9

Proteins in the liver are broken down into amino acids, some of which can undergo gluconeogenesis to replenish blood glucose

Question 10

What happens to muscle after weeks of fasting?

Answer 10

Muscle proteins are broken down for amino acids for gluconeogenesis

Question 11

What is the byproduct of gluconeogenesis from amino acids? What is it converted into? How is it excreted?

Answer 11

The byproduct of amino acid gluconeogenesis is ammonia. This is converted into urea and excreted by the kidneys in urine