Digestion and Absorption

Question 1

Name the 2 functional periods the body undergoes in providing energy for cellular activities.

Answer 1

Absorptive

Postabsorptive

Question 2

What occurs during the absorptive state?

Answer 2

Ingested nutrients enter the blood from the GI tract. Some nutrients provide energy for the body and others are added to energy stores.

Question 3

What occurs during the postabsorptive state?

Answer 3

The GI tract is empty of nutrients and the body’s own stores must supply energy.

Question 4

Name 3 important fatty acids we absorb.

Answer 4

Palmitic, stearic, oleic acid

Question 5

Where does triglyceride digestion predominantly occur?

Answer 5

Small intestine

Question 6

Where is lipase synthesised?

Answer 6

Pancreas.

Question 7

Give the equation for the splitting of a triglyceride by lipase.

Answer 7

Triglyceride —> Monoglyceride + 2 Fatty acids

Question 8

Name the process which converts lipids in ingested food to very small droplets.

Answer 8

Emulsification

Question 9

Give 2 things that emulsification requires.

Answer 9

1. Mechanical disruption of large droplets into smaller droplets. Provided by motility of GI tract
2. Emulsifying agent: provided by phospholipids and bile salts. They prevent the aggregation of fat into larger droplets by repelling.

Question 10

The coating of lipid with emulsifiers impairs the accessibility of lipase. How is this overcome?

Answer 10

Pancreas secretes colipase which binds onto the droplet as well as binding to lipase, holding it in place.

Question 11

What are micelles?

Answer 11

Holding stations for lipids

Question 12

In the small intestine, the fatty acids and monoglycerides are re-synthesised into triglycerides. In which organelles does this take place?

Answer 12

Smooth ER. The fat droplets are then pinched off in vesicles to the Golgi apparatus where they are modified to chylomicrons. The chylomicrons fuse with the plasma membrane and enter the interstitial fluid via exocytosis.

Question 13

What do chylomicrons contain?

Answer 13

triglycerides, phospholipids, cholesterol and fat soluble vitamins

Question 14

Give the sources of fatty acids for triglyceride synthesis.

Answer 14

1. Glucose that enters adipose tissue is broken down to provide building blocks for the synthesis of fatty acids
2. Glucose that is used in the liver to form VLDL triglycerides
3. Ingested triglycerides, transported in blood in chylomicrons and taken up by adipocytes

Question 15

Where are vitamins A,D,E + K (fat soluble vitamins) absorbed?

Answer 15

Ileum

Question 16

Where are vitamins C+B (water soluble vitamins) absorbed?

Answer 16

Jejunum

Question 17

One exception of absorption is vitamin B-12. How is this absorbed?

Answer 17

Vitamin B-12 must first bind to intrinsic factor (secreted by parietal cells). Intrinsic factor bound to B-12 binds to epithelial cells in the lower ileum so it can be absorbed via endocytosis.

Question 18

What is vitamin B-12 used for? What can a deficiency lead to?

Answer 18

Vitamin B-12 is needed for erythrocyte formation. Deficiency can lead to pernicious anaemia - usually caused by lack of intrinsic factor.

Question 19

Where is protein mainly digested and absorbed?

Answer 19

Duodenum

Question 20

How many amino acids can we not manufacture?

Answer 20

8 - these are essential.

Question 21

Amino acids exist as optical isomers. Which form is found in the proteins we use?

Answer 21

L-form

Question 22

Give the peptide bond.

Answer 22

CONH (HN-C=O)

Question 23

Describe the digestion of protein.

Answer 23

1. Begins in stomach. Pepsin cleaves peptide linkage to form fragments
2. In small intestine, fragments are further broken down by endopeptidases e.g. trypsin or exopeptidases e.g. carboxyl dipeptidases

Question 24

Where is pepsinogen I found?

Answer 24

HCL secreting region (body) of stomach

Question 25

Where is pepsinogen II found?

Answer 25

Pyloric region of stomach

Question 26

What is the optimal PH for pepsin?

Answer 26

1.6-3.2, so its action is terminated on exit from stomach

Question 27

Where does digestion and absorption of carbohydrates occur?

Answer 27

In duodenum

Question 28

Name some important monosaccharides.

Answer 28

Glucose, fructose, galactose

Question 29

Name some important disaccharides.

Answer 29

Lactose - beta linkage (those who are lactose intolerant do not have sufficient beta enzymes to break down lactose)

Sucrose - alpha linkage

Question 30

Name some important polysaccharides.

Answer 30

Starch, glycogen

Question 31

Which isomer of sugar molecules are utilised in metabolism?

Answer 31

D-isomers

Question 32

Describe the linkage in glycogen.

Answer 32

Alpha 1,4 glycosidic bonds, alpha 1,6 glycosidic bonds

Question 33

Describe the linkage in starch.

Answer 33

Majority alpha 1,4 glycosidic bonds, less alpha 1,6 bonds than glycogen

Question 34

Describe how starch is digested.

Answer 34

1. Starch is degraded by ptyalin (alpha amylase) in the mouth. Ph=6.7
2. Small intestine is responsible for 95% of starch digestion. Here, pancreatic alpha amylase catalyses alpha1,4 bonds to form: maltose, maltotriose, glucose
3. These products are further digested by oligosaccharidases in the brush border of microvilli to form: maltase, lactase, sucrase
4. These products are further broken down to monosaccharides glucose, galactose and fructose.

Question 35

Describe the absorption of carbohydrates and the products they are converted to.

Answer 35

1. Hexoses and pentoses are rapidly absorbed across the intestinal mucosa and drain into the hepatic portal vein
2. The carbohydrates enter the liver and they are converted to glucose
3. Liver also converts glucose to glycogen for storage
4. Liver converts some glucose into alpha-glycerol phosphate and fatty acids which are later used to synthesise triglycerides (occurs in hepatocytes as well as adipocytes).

Question 36

What does VLDL stand for?

Answer 36

Very low density lipoproteins - contain more fat than protein an fat is less dense than protein

Question 37

Where are bile salts reabsorbed in the GI tract?

Answer 37

Terminal ileum