Lecture 5 - Nutrient Digestion I (Carbohydrates and Proteins)

Question 1

What is the basic principle of digestion?

Answer 1

Taking large complex molecules and breaking them down into monomeric units

Question 2

What is the problem with digestion?

Answer 2

You unlock all these tiny molecules which causes water to flood in

Question 3

How do you get past the problem with digestion?

Answer 3

Stomach releases small amount of content at once to prevent influx of water
Molecules are repackaged into larger molecules (e.g. liver stores glycogen)

Question 4

What is dumping syndrome?

Answer 4

A condition where the stomach moves its contents to the duodenum too quickly –> massive water influx

Question 5

What are the principle dietary constituents?

Answer 5

Carbs - main energy source
Protein - build + repair body 
Fat - most calorie rich 
Vitamins
Minerals
Water

Question 6

What are monosaccharides?

Answer 6

Sugars that cannot be hydrolysed to give a simpler sugar

Question 7

What are the 3 monosaccharides we absorb in the GIT?

Answer 7

3 Hexose sugars (6C): galactose, fructose, glucose

Question 8

Where are the monosaccharides absorbed?

Answer 8

Small intestine

Question 9

What bond links two monosaccharides to form a disaccharide?

Answer 9

Glycosidic

Question 10

Where and by what are disaccharides broken down?

Answer 10

Brush border enzymes break them down in the small intestine

Question 11

What are the 3 important disaccharides?

Answer 11

Lactose
Sucrose
Maltose

Question 12

What is lactose?

Answer 12

Glucose + galactose

Question 13

What is sucrose?

Answer 13

Glucose + fructose

Question 14

What is maltose?

Answer 14

Glucose + glucose

Question 15

What enzyme breaks down lactose?

Answer 15

Lactase

Question 16

What enzyme breaks down sucrose?

Answer 16

Sucrase

Question 17

What enzyme breaks down maltose?

Answer 17

Maltase

Question 18

Can the body absorb disaccharides?

Answer 18

No

Question 19

What causes diarrhoea in lactose intolerance?

Answer 19

Lack of expression of lactase –> inability to breakdown lactose –> brings in too much water from lumen of small intestine

Question 20

What are the three main polysaccharides?

Answer 20

Starch
Cellulose
Glycogen

Question 21

What is the starch?

Answer 21

Plant form of glucose

Question 22

What is the composition of starch?

Answer 22

Alpha-amylase (glucose linked in straight chains)

Amylopectin (glucose chains highly branched)

Question 23

What bonds link the glucose monomers in starch?

Answer 23

Alpha-1, 4 glycosidic bonds

Question 24

What enzymes can hydrolyse the alpha 1-4, glycosidic bonds?

Answer 24

Salivary and pancreatic amylases

Question 25

What is cellulose?

Answer 25

Constituent of plant cell walls

Question 26

What is the composition of cellulose?

Answer 26

Unbranched, linear chains of glucose monomers linked by beta-1, 4, glycosidic bonds

Question 27

How is cellulose broken down?

Answer 27

It is a dietary fibre, i.e. require bacterial cellulase to break it down

Question 28

What is glycogen?

Answer 28

Animal storage form of glucose

Question 29

What bonds link the glucose monomers in glycogen?

Answer 29

Alpha 1, 4 glycosidic bonds

Question 30

What kind of bonds can be broken down by alpha-amylase?

Answer 30

Alpha 1, 4 glycosidic bonds

Question 31

What are the function of the villi in the GIT?

Answer 31

Increase surface area for absorption

Question 32

What are found on the surface of the villi?

Answer 32

Microvilli

Question 33

What are the two membranes on the epithelial cells?

Answer 33

Basolateral and apical membrane (outward facing)

Question 34

What is present between the columnar cells in the small intestine?

Answer 34

Tight junctions

Question 35

How do the molecules travel into the small intestine?

Answer 35

Molecules have to get through apical membrane

Question 36

What are the two ways that molecules can get across the apical membrane?

Answer 36

Can go through the cells (transcellular pathway) | Can go between epithelial cells (paracellular pathway)

Question 37

What are the different transcellular pathways?

Answer 37

Lipid soluble - cross lipid membrane | Transport proteins for charged molecules

Question 38

What pump creates a driving force for sodium to enter cells on the intestinal epithelium?

Answer 38

Na-K-ATPase

Question 39

How is glucose absorbed in the small intestine?

Answer 39

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

Question 40

How is water absorbed in the small intestine?

Answer 40

Follows Na absorption paracellularly

Question 41

How is fructose absorbed from the gut?

Answer 41

GLUT5 receptor on apical membrane and GLUT2 out basolateral membrane

Question 42

What are proteins?

Answer 42

Polymers of amino acids linked together by peptide bonds

Question 43

What are amino acid chains of 3-10 known as?

Answer 43

Peptides

Question 44

What are the names of enzymes which hydrolyse peptide bonds?

Answer 44

Proteases - breakdown proteins | Peptidases - breakdown peptidases

Question 45

Where do endopeptidases work?

Answer 45

Centre of protein

Question 46

Where do exopeptidases work?

Answer 46

Terminal amino acid

Question 47

How are proteins absorbed in the small intestines?

Answer 47

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

Question 48

How are dipeptides absorbed across the small intestine?

Answer 48

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)