Digestion of food molecules

Question 1

What are the main classes of macronutrients?

Answer 1

There are three major classes of molecules; carbohydrates, protein and fats, referred to as macronutrients which contribute to energy production in the body. Nucleic acid components also contribute and bring nitrogen into the body - important for cellular function

Question 2

Function of the salivary glands

Answer 2

Release salvia which helps to masticate the food, contains mucous and amylase which starts the initial hydrolysis of the complex carbohydrate molecules

Question 3

Function of the Stomach

Answer 3

Various gastric juices secreted including HCl to maintain acidic pH, secretion of first protease, pepsinogen which then becomes active to pepsin and starts the process of digestion of peptide bonds within protein polymer molecules. It also secrets a mucous layer to protect the inner surface

Question 4

Function of the Pancreas

Answer 4

Slightly alkaline environment, secretes most of digestion enzymes including amylase, lipase (hydrolysis of lipid molecules) and several proteases

Question 5

Function of the Liver and Small intestine

Answer 5

L: Synthesis of bile salts/acids (stored in gall bladder until required) important for fat digestion - solubilise the fat material
SI: Final phase of digestion and absorption

Question 6

What are the two main phases of digestion?

Answer 6

1. Hydrolysis of various covalent bonds connecting monomer units in food macromolecules
   E.g. carbohydrates -> disaccharides, proteins -> peptide bonds, fat-> triacylglycerol ester bonds
   Important as what happens subsequently is highly dependent on the efficiency of this initial hydrolysis process
2. Absorption of hydrolysis products from gastrointestinal tract into the body

Question 7

How are dietary carbohydrates digested?

Answer 7

Provides 40-50% of energy intake (varies)
From plants we have starch (amylopectin main component), we have various sugar molecules and other components such as fibre (from cellulose - undigestible by most mammals)

Question 8

Relationship between cellobiose and lactose

Answer 8

Cellobiose (repeating disaccharide unit in cellulose) and lactose are stereoisomers of one another - mammals do not have an enzyme that can hydrolyse the Beta(1->4) glycosidic bonds in cellulose and some people do not have the lactase enzyme hence cannot hydrolyse lactose

Question 9

What happens when sucrose is hydrolysed?

Answer 9

When sucrose is hydrolysed into glucose and fructose, the monosaccharides can be taken up into the body however fructose can cause ‘craving’ to consume more material. This is in relationship to an industrial trend to make products with ‘high fructose corn syrup’

Question 10

Hydrolysis of starch and glycogen

Answer 10

Starch from plants consists of mainly amylose and amylopectin. Glycogen has a similar branched structure to amylopectin, can be present in consumed foods such as liver and muscle, synthesised in animals from glucose and stored in liver and muscle, then broken down to glucose when required. The breakdown of glycogen to glucose requires a ‘debranching enzyme’

Question 11

Starch digestion

Answer 11

The enzyme amylase hydrolyses alpha (1->4) glycosidic bonds. Reappeared internal attack yielding smaller and smaller oligosaccharides producing maltose/isomaltose disaccharides as end products. The intestinal epithelial cells secrete enzymes that hydrolyse the maltase/isomaltase, sucrase and lactase hence the monosaccharides are able to absorbed into the body

Question 12

Dietary protein digestion in diet

Answer 12

Supplies amino acids to make body proteins, supplies essential amino acids, source of nitrogen for other nitrogen compounds (purines, pyrimidines, haem) and the carbon skeletons can be used as fuel (nitrogen converted to urea and excreted in urine)

Question 13

Protein digestion

Answer 13

Involves hydrolysis of specific peptide bonds. Performed by several different proteases. All proteases secreted as inactive forms (zygomens or proenzymes). All proteases activated by cleavage of peptides from their structure
Protease specificity is determined by adjacent amino acid side chains in protein substrate

Question 14

What are endopeptidases and exopeptidases?

Answer 14

Endopeptidases attack peptide bonds within the protein (peptide) polymer e.g. pepsin, trypsin, chymotrypsin
Exopeptidases attack peptide bonds at the end of protein (peptide) polymer e.g. aminopeptidase and carboxypeptidase

Question 15

What is pepsinogen?

Answer 15

Pepsinogen, the inactive zymogen is activated to pepsin following the exposure of pepsinogen to HCl in the stomach. In the stomach acidic environment, part of the pepsinogen protein unfolds, which actives the pepsin protease and results in hydrolysis of part of the pepsinogen protein sequence to generate stably activated pepsin protease