Lecture 25: ABSORPTION OF CARBOHYDRATES AND PROTEINS

Question 1

What are sugars?

Answer 1

Highly water soluble and cannot simply diffuse across cell membranes (lipid bilayer)

Question 2

What does sugar transport require?

Answer 2

Specific transporter proteins anchored in the membrane that form pores in the membrane

Question 3

What are the types of transport?

Answer 3

Active and facilitative

Question 4

What is active transport?

Answer 4

Against a concentration gradient that needs energy from ATP

Question 5

What is facilitative transport?

Answer 5

Passage down a concentration gradient

Question 6

What does glucose transport across the intestinal epithelia involve?

Answer 6

Two glucose transporters (SGLT1, a secondary active transporter, and GLUT2, a facilitative transporter)

Question 7

What happens to glucose when it is in the circulation?

Answer 7

It is taken up by tissues such as liver, muscle and brain via other glucose transporters - such as GLUT4 (adipose and muscle) and GLUT3 (brain)

Question 8

What are the glucose transporters?

Answer 8

GLUT1, GLUT2, GLUT3, GLUT4, SGLT1

Question 9

Where is the main site of GLUt1?

Answer 9

Ubiquitous

Question 10

Where is the main site of GLUT2?

Answer 10

Liver, pancreas, kidneys, intestinal epithelia

Question 11

Where is the main site of GLUT3?

Answer 11

Brain

Question 12

Where is the main site of GLUT4?

Answer 12

Muscle, adipose tissue

Question 13

Where is the main site of SGLT1?

Answer 13

Intestinal epithelia

Question 14

What does glucose transport by SGLT1 involve?

Answer 14

Simultaneous transport (symport) of sodium ions (from intestinal lumen into cell)

Question 15

What is extracellular sodium concentration?

Answer 15

120-140 mmol/L

Question 16

What is intracellular sodium concentration?

Answer 16

20-30 mmol/L

Question 17

What does GLUT2 do?

Answer 17

Transport glucose from inside the intestinal cells to the peripheral tissues/blood

Question 18

What does Na+/K+ ATPase do?

Answer 18

Actively transports Na+ to maintain the low sodium concentration in epithelial cells and maintain the gradient for SGLT1

Question 19

How does Na+/K+ ATPase work?

Answer 19

It takes up 3 sodium ions, hydrolysis of ATP causes a phosphate to bind and a conformational change to release the sodium ions and pick up 2 potassium ions. Hydrolysis of the phosphate bond causes release of the potassium ions

Question 20

What is there very little absorption of?

Answer 20

Peptides longer than four amino acids

Question 21

How are di- and tri-peptides absorbed in the small intestine?

Answer 21

By co-transport with hydrogen ions via membrane transporter PepT1

Question 22

what happens with absorbed di- and tri-peptides?

Answer 22

They are further digested into individual amino acids by cytoplasmic peptidases and exported from the epithelial cells into the blood circulation

Question 23

What is the transport system for absorption of amino acids?

Answer 23

Semi specific sodium dependent

Question 24

What do sodium dependent carriers transport?

Answer 24

Sodium ions and an amino acid

Question 25

How many different sodium dependent carriers are there?

Answer 25

At least 6

Question 26

What are some of the sodium dependent carriers?

Answer 26

Neutral amino acids, proline and hydroxyproline, acidic amino acids, basic amino acids (Lys, Arg) and cysteine

Question 27

When does uptake of intact proteins occur?

Answer 27

Only in a few circumstances, for example newborn animals. Uptake of immunoglobulins in colostral milk for acquisition of passive immunity

Question 28

What diseases affect digestive organs?

Answer 28

Pancreatitis, stomach (or peptic) ulcers, cystic fibrosis and coeliac disease

Question 29

What does pancreatitis lead to?

Answer 29

Inappropriate activation of zymogens (proenzymes) resulting in self digestion

Question 30

What are stomach (or peptic) ulcers due to?

Answer 30

The breakdown of the mucosa which normally protects against protease action

Question 31

What do cystic fibrosis and coeliac disease cause?

Answer 31

Malabsorption

Question 32

What does cystic fibrosis cause?

Answer 32

Thick mucous secretions which block the pancreatic duct and affect the secretion of pancreatic enzymes

Question 33

What can cystic fibrosis be aided by?

Answer 33

Takin supplements containing pancreatic enzymes - 'pancreatin', a pancreas extract

Question 34

What is coeliac disease a disease of?

Answer 34

The small intestine (particularly the duodenum)

Question 35

What does the body react to in coeliac disease?

Answer 35

Gluten protein present in wheat

Question 36

What do antibodies react with in coeliac disease?

Answer 36

Transglutaminase

Question 37

What happens to villi in coeliac disease?

Answer 37

They flatten and channels close causing nutrients to not be absorbed

Question 38

What symptoms are observed in coeliac disease?

Answer 38

Gastrointestinal

Question 39

What are DNA and RNA subject to?

Answer 39

Partial acid hydrolysis in the stomach

Question 40

What do intestinal endonuclease enzymes do?

Answer 40

Hydrolyse the phosphodiester bonds linking individual nucleotides

Question 41

What do exonuclease enzymes in the intestine do?

Answer 41

Release individual nucleotides (nucleoside monophosphate)

Question 42

How are individual nucleotides absorbed?

Answer 42

Via nucleotide transporters