Lecture 23 - Digestion of food molecules

Question 1

What are the main macronutrients that contribute to energy production in the body?

Answer 1

Carbohydrates protein and fats

These are all polymers and so they must be broken down into smaller building block structures in order to be utilised

Question 2

Carbohydrate broken down into…

Answer 2

Monosaccharides

Question 3

Protein broken down into…

Answer 3

Amino acids

Question 4

Nucelic acids broken down into…

Answer 4

Nucleotides

Question 5

Fat broken down into…

Answer 5

Monoacylglyceride, free fatty acids and cholesterol

Question 6

How do we get macronutrients?

Answer 6

Macronutrients are consumed in the diet and pass through the gastrointestinal tract

Question 7

Salivary glands

Answer 7

Saliva (neutral pH) contains mucus and amylase which starts the digestion of carbohydrates

Question 8

Stomach

Answer 8

Storage and mixing of food with gastric juices, slowly releases chyme into the intestine

Secretes acid (0.1 M HCl) = denaturing

Secreted pepsinogen which is converted to the active form of pepsin = protein digestion

Secretes mucus layer (protective)

Question 9

Pancreas

Answer 9

Slightly alkaline pH 7

Secretes most of the digestive enzymes including amylase, lipase and several other proteases

Question 10

Liver

Answer 10

Synthesis of bile salts/acids (stored in the gallbladder) important for fat digestion

Question 11

Small intestine

Answer 11

Final phase of digestion and absorption

Question 12

Two main phases of digestion

Answer 12

1- Hydrolysis of bonds connecting monomer units in food macromolecules
Carbohydrate: Glycosidic bonds leads starch to turn into disaccharides
Proteins: peptide bonds
Fat: Triacylglycerol ester bonds

2- Absorption of products from the gastrointestinal tract into the body
Breaking of bonds enables the absorption of them which allows for utilisation

Question 13

Digestion of dietary carbohydrates

Answer 13

Provides 40-50% of energy intake

Starch i.e.alpha amylose, amylopectin

Simple sugars i.e. sucrose, lactose, fructose, glucose

Fibre - such as cellulose (undigestible by most mammals (because the glycosidic bonding cannot be processed as it is beta rather than alpha))

Amylopectin is the main component of plant starch - polymers of up to 1 million glucose units
Maltose is present in honey

Question 14

Cellobiose and lactose

Answer 14

Cellulose and lactose are stereoisomers of one another

The functional groups in monosaccharides can be in either of two orientations. There is a convention of numbering the carbons (alpha and beta)

Cellubiose is a repeating disaccharide unit in cellulose
Mammals do not have an enzyme that can hydrolyse the beta 1,4 glycosidic bonds in cellulose

Lactose is present in milk (different orientation of hydroxyl functional groups)
Some people do not have the lactase enzyme and are unable to hydrolyse lactose

Question 15

Sucrose hydrolysed to

Answer 15

Glucose and fructose

Question 16

Salivary amylase

Answer 16

Source is the salivary glands
Acts on starch
Sit of action is in the mouth

Question 17

Pancreatic amylase

Answer 17

Made in the pancreas
Acts on starch
Site of action is in the small intestine - more active on starch in the small intestine than in the mouth, it is another round of starch digestion

Question 18

Maltase

Answer 18

Made in the small intestine
Acts on maltose as the substrate
Site of action is in the small intestine

Question 19

Lactase

Answer 19

Made in the small intestine
Acts on lactose
Site of action is in the small intestine

Question 20

Sucrase

Answer 20

Made in the small intestine
Acts on sucrose
Site of action is in the small intestine

Question 21

Isomaltase

Answer 21

Made in the small intestine
Acts on isomaltose
Site of action is in the small intestine

Question 22

Hydrolysis of starch and glycogen

Answer 22

Starch from plants consists mainly of: Amylose (a linear polymer of alpha 1,4 linked glucose units), amylopectin (a branched polymer of alpha 1,4 and alpha 1,6 linked glucose units)

Glycogen has a similar branched structure to amylopectin

Glycogen can be present in consumed foods such as liver and muscle

Glycogen is synthesised in animals from glucose and stored in liver and muscle, and then broken down to glucose when required by the body

The breakdown of glycogen stored in liver and muscle cells to glucose requires a ‘debranching enzyme’

Question 23

Hydrolysis of starch

Answer 23

Starch
Amylose (plants)
Amylopectin (plants_

Starch digestion: the enzyme amylase hydrolyses alpha 1,4 glycosidic linkages
Repeated internal attack yielding smaller and smaller oligosaccharides producing maltose/isomaltose disaccharides as end products

Question 24

Final digestion of carbohydrates at ‘brush border’ after amylase digestion

Answer 24

Intestinal epithelial cells secrete (examples):
Maltase/isomaltase turns maltose/isomaltase into 2 glucose molecules
Sucrase turns sucrose into fructose and glucose
Lactase turns lactose into galactose and glucose

The monosaccharides are absorbed into the body

Question 25

Lactose intolerance

Answer 25

Lactase enzyme deficiency (genetic basis)
Causes bloating, flatulence and diarrhoea due to fermentation of lactose by intestinal bacteria
Need to avoid lactose in their diet because they cannot process it

No lactase - accumulation of lactose which is fermented to acids, H+ and CO2 - bloating and flatulence
Acids and lactose drive H2O into the intestine which results in diarrhoea

Question 26

Digestion of dietary protein

Answer 26

Supplies amino acids to make body proteins
Supplies essential amino acids
Source of nitrogen for purines, pyrimidines, ham
Carbon skeletons can be used as fuel (N converted to urea and excreted in urine)

Question 27

Essential amino acids

Answer 27

Leucine 
Lysine 
Threonine 
Tryptophan 
Isoleucine 
Methionine 
Phenylalanine 
Valine 

Humans cannot synthesise all the amino acids and these are the ones that we cannot make and must instead receive them from an outside source

Question 28

Kwashiorkor

Answer 28

Result of a deficiency of dietary protein which causes an osmotic imbalance in the GI system, causing the abdomen to swell (oedema) due to retention of water
In addition, the level of albumin in the blood is low affecting colloidal osmotic (oncotic) pressure and also transport of molecule e.g. hormones and drugs

Question 29

Protein digestion

Answer 29

Involves hydrolyses of specific peptide bonds, performed by several different proteases

All proteases secreted as inactive forms (zymogens or proenzymes) - evolved because it is important to keep proteases under control in the body so that they do not damage it
All proteases are activated by the cleavage of peptides from their structure - most are activated by the cleavage of part of the protease protein polymer from the structure which allows refolding of the protease structure into its active and functional form

Question 30

Protease

Answer 30

Protease specificity is determined by adjacent amino acid side chains (determines specificity of protease binding protein substrate)
Pepsin = aromatic i.e. Phe, Tyr
Trypsin = positively charged i.e. Lys, Arg
Chymotrypsin = aromatic i.e. Phe, Tyr

Question 31

Two stages of protein digestion

Answer 31

Endopeptodases attack peptide bonds within the protein (peptide) polymer
Exopeptidases attack peptide bonds at the end of protein (peptide) polymer

Question 32

Endopeptidases

Answer 32

Endopeptodases attack peptide bonds within the protein (peptide) polymer
Examples - pepsin, trypsin and chymotrypsin

Question 33

Exopeptidases

Answer 33

Exopeptidases attack peptide bonds at the end of protein (peptide) polymer
Examples -amino peptidases, carboxypeptidases

Question 34

Aminopeptidases

Answer 34

Hydrolyse peptides from the N terminal end

Source is the small intestine
Substrate is polypeptides
Site of action is in the small intestine

Question 35

Carboxypeptidases

Answer 35

Hydrolyse peptides from the C terminal end

Question 36

Pepsin

Answer 36

Stomach mucosa is the source
Substrate is proteins, pepsinogen
Site of action is the stomach

Question 37

Trypsin

Answer 37

Source is the pancreases
Substrate is polypeptides and chymotrypsinogen
Site of action is in the small intestine

Question 38

Chymotrypsin

Answer 38

Source is the pancreas
Substrate is polypeptides
Site of action is in the small intestine

Question 39

Carboxypeptidase

Answer 39

Source is the pancreas
Substrate is polypeptides
Site of action is in the small intestine

Question 40

Dipeptidase

Answer 40

Source is the small intestine
Substrate is dipeptides
Site of action is the small intestine

Question 41

Pepsinogen activation to pepsin

Answer 41

Pepsinogen is the inactive zymogen (proenzyme) is activated to pepsin following exposure of pepsinogen to HCl in the stomach

Once there is active pepsin in the environment, it can hydrolytic ally activate other pepsinogen proenzyme molecules

In the stomach acidic environment, part of the pepsinogen protein unfolds, which activates the pepsin protease and results in hydrolysis of part of the pepsinogen protein sequence to generate stably activate pepsin protease

Question 42

Proteases are initially produced as ….

Answer 42

Zymogen (proenzyme) inactive forms

Question 43

Sequential hydrolysis by proteases

Answer 43

Pepsin in the stomach - trypsin in the small intestine - chymotrypsin in the small intestine - carboxypeptidases in the small intestine - amino peptidases in the small intestine

Dietary protein hydrolysis gives the end products of amino acids, di- and tri- peptides