Nutrient Digestion & Absorption 1

Question 1

What types of carbohydrates do we digest and absorb?

Answer 1

Polysaccharides:
- Starch (Alpha-amylose & Amylopectin)
- Cellulose
- Glycogen

Disaccharides:
- Lactose
- Sucrose
- Maltose

Monosaccharides:
- Glucose
- Galactose
- Fructose

Question 2

How are disaccharides formed?

Answer 2

Two monosaccharides bound by a glycosidic bond
Glucose + Galactose = Lactose
Glucose + Fructose = Sucrose
Glucose + Glucose = Maltose

Question 3

What enzymes break down disaccharides?

Answer 3

Lactose = Lactase
Maltose = Maltase
Sucrose = Sucrase

Question 4

Explain the structure of alpha-amylose vs amylopectin:

Answer 4

Both are chains of glucose monomers bound by alpha-1,4-glycosidic bonds:
- Alpha-Amylose is straight chain
- Amylopectin is highly branched.

Question 5

What enzyme breaks down starch?

Answer 5

Alpha-Amylase
Of which there are two types. One in saliva and one from the pancreas

Question 6

Explain the structure of cellulose and how we digest it

Answer 6

A plant wall constituent its made of straight glucose chains. the difference against alpha-amylose starch is that cellulose monomers are linked by beta-1,4-glycosidic bonds.

We dont digest it, its a dietary fibre. Only bacteria express cellulase not animals

Question 7

Explain the structure and use of glycogen:

Answer 7

Glucose linked by alpha-1,4,-glycosidic bonds.
Animals like us store glucose in the liver as glycogen.

Question 8

What are intestinal villi?

Answer 8

Fingerlike folds of columnar epithelium which increase the surface area in the gut for absorption/secretion. They are in turn covered in microvilli

Question 9

Explain the membranes of villi

Answer 9

The epithelial cells that make up villi have two cell membranes:
- Apical at the top
- Basolateral on the sides/base

They are connected by a tight junction which also complexes with those on adjacent villi to form a tight junctional complex

Question 10

Explain molecule transport across intestinal villi:

Answer 10

Transcellular:
- in the top of the cell and out the bottom
- Since it goes through 2 different membranes it requires at least two transport proteins. (because water soluble molecules cant diffuse through a cell membrane remember?!)

Paracellular:
- Transport out the lumen between the villi

Question 11

How does glucose move out the gut lumen into the blood?

Answer 11

1) Na+K+ATPase moves Na+ from the epithlium into the blood creating a Na+ conc. gradient

2) glucose & Na+ pass through SGLT1 (sodium glucose transporter 1) from the gut into the cell using the Na+ gradient as a driving force

3) Glucose pass out the cell into the blood through the Glut-2 transporter

This is known as secondary active transport, as glucose is moving agianst its conc. gradient by being coupled to Na+ going along its own

Question 12

How does fructose get from the gut lumen into the blood?

Answer 12

Unlike glucose, fructose in the blood is used up so theres no conc. gradient to work against.
Therefore fructose moves into the cell without Na+ and exits into the blood through the GLUT-5 transporter

Question 13

What are proteins and peptides?

Answer 13

Polymers of amino acids bound by peptide bonds.
Small ones (i.e. 3-10 monomers) are called peptides rather than proteins

Question 14

How are glycoproteins and lipoproteins etc produced?

Answer 14

Proteins undergo post-translational modification

Question 15

What do we call enzymes that break down proteins and peptides?

Answer 15

• Proteases if acting on proteins
• Peptidases act on peptides
• Endopeptidases break down internal peptide bonds creating two smaller peptides/proteins
• Exopeptidases break the terminal peptide bond reducing the peptide by one monomer

Exopeptidases are split into aminopeptidases and carboxypeptidases based on whether they act on the amino terminal end or the carboxy terminal end

Question 16

How are amino acids transported out the gut lumen into the blood?

Answer 16

Exactly the same as glucose just with different transporters:
SAAT1 instead of SGLT1 etc

Question 17

Can polypeptides be transported out the gut without being broken down to amino acids? If so how?

Answer 17

Some di and tripeptides can.

So:
1) the surface of the gut around the villi is kept in an acidic micro climate
2) this produces a H+ conc gradient into the epithelial cell
3) this drives the movement of peptides into the cell (just like Na+ & glucose)
4) The H+ is then pumped back into the gut lumen through a transporter swapping it with Na+, the sodium gradient provides the driving force for this which is in turn provided by a Na+K+ATPase transporter between the epithelium and blood
5) Another transporter pumps the peptides into the blood from the epithelial cell.

Question 18

All of this comes in diagrams in the lecture notes

Answer 18

Much much easier to learn from