Topic 3.2- Digestion and Absorption

Question 1

Describe the structure of the human digestive system.

Answer 1

The oesophagus leads into the stomach, which connects to the small intestine (made up of the duodenum, the jejunum and the ileum). These lead into the large intestine (the ascending, transverse and descending colon) which lead to the rectum and anus. Other organs such as the liver, gall bladder and pancreas secrete chemicals such as enzymes and hormones into the digestive tract.

Question 2

How is food physically broken down?

Answer 2

By teeth in the mouth and by muscular stomach

Question 3

How is food chemically broken down?

Answer 3

Hydrolysis of large insoluble molecules into small soluble molecules carried out by specific enzymes

Question 4

What are carbohydrates broken down into?

Answer 4

Disaccharides and then monosaccharides

Question 5

What are lipids broken down into?

Answer 5

Fatty acids and monoglycerides

Question 6

What are proteins broken down into?

Answer 6

Amino acids

Question 7

Name 2 enzymes that break down carbohydrates

Answer 7

• Amylase
• Membrane-bound disaccharidases

Question 8

What reaction does amylase catalyses?

Answer 8

Conversion of starch into maltose

Question 9

Where is amylase produced & where is it released to?

Answer 9

• Salivary glands - release amylase into mouth
• Pancreas - releases amylase into small intestine

Question 10

What are membrane-bound disaccharidases?

Answer 10

Enzymes that are attached to cell membranes of epithelial cells lining the ileum

Question 11

What reaction do membrane-bound disaccharidases catalyses?

Answer 11

Hydrolysis of disaccharides (e.g. maltose, sucrose, lactose) into monosaccharides (e.g. glucose, fructose and galactose)

Question 12

Fill in the blanks

Answer 12

Question 13

What reaction does lipase catalyse?

Answer 13

Hydrolysis of lipids into monoglycerides and fatty acids

Question 14

Where is lipase made and where does it work?

Answer 14

Pancreas - works in small intestine

Question 15

Where are bile salts produced?

Answer 15

Liver

Question 16

What do bile salts do?

Answer 16

Emulsify lipids which cause lipids to form small droplets

Question 17

What is the benefit of bile salts causing lipids to form small droplets?

Answer 17

Several small lipid droplets have bigger SA than single large droplet so lipase can digest lipids faster

Question 18

What happens after the lipid has broken down?

Answer 18

Monoglycerides and fatty acids stick with bile salts to form tiny structures called micelles

Question 19

Name the different proteases (or peptidases) that break down proteins

Answer 19

• Endopeptidases
• Dipeptidases
• Exopeptidases

Question 20

What do endopeptidases do?

Answer 20

Break polypeptides into smaller peptide chains

Question 21

Where are trypsin and chymotrypsin (endopeptidases) synthesised and secreted into?

Answer 21

Synthesised in pancreas and secreted into small intestine

Question 22

Where is pepsin (endopeptidases) synthesised and secreted into?

Answer 22

Released into stomach by cells in stomach lining

Question 23

What do exopeptidases do?

Answer 23

• Act to hydrolyse peptide bonds at ends of protein molecules
• Remove terminal amino acids

Question 24

What are dipeptidases?

Answer 24

Exopeptidases that work specifically on dipeptides

Question 25

What do dipeptidases do?

Answer 25

• Hydrolses dipeptides into amino acids
• Breaks peptide bond between them

Question 26

Where are dipeptidases located?

Answer 26

In cell-surface membrane of epithelial cells in small intestine

Question 27

How is glucose and galactose absorbed?

Answer 27

By active transport with sodium ions via co-transporter protein

Question 28

How is fructose is absorbed?

Answer 28

Via facilitated diffusion through different transporter protein

Question 29

Describe how glucose is absorbed from lumen (middle) of ileum by co-transport

Answer 29

1. Sodium ions actively transported out of ileum epithelial cells into blood by sodium-potassium pump. This creates concentration gradient.This creates a higher conc. of sodium ions in lumen than inside cell which causes sodium to diffuse into epithelial cell by facilitated diffusion (down their concentration gradient) via sodium-glucose co-transporter proteins. Glucose is absorbed with sodium ions against their concentration gradient which causes the concentration of glucose inside cell to increase. So glucose diffuses out of cell into blood, down its concentration gradient through protein channel by facilitated diffusion

Question 30

How are amino acids absorbed?

Answer 30

By co-transport (similar to glucose)

Question 31

Name two ways the concentration gradient between the inside of the ileum and blood is maintained

Answer 31

• Constant circulation of blood
• Ileum has muscles which uses peristalsis to keep food moving along the gut

Question 32

Describe how in detail how monoglycerides and fatty acids are absorbed and how they’re transported around the body

Answer 32

1. Micelles help to move monoglycerides and fatty acids towards epithelium. Micelles constantly break up and reform so they can ‘release’ monoglycerides and fatty acids which allows them to be absorbed. (This makes lipid-soluble so they can diffuse directly across epithelial cell membrane)
2. Monoglycerides and fatty acids transported to ER & combine to form triglycerides
3. Triglycerides associated with cholesterol and lipoproteins to form chylomicrons (lipid transport structures)
4. Chylomicrons enter lacteal (part of lymphatic system) by exocytosis
5. Only drain into bloodstream near heart

Question 33

The walls of the small intestine have lots of finger-like projections called ____

Answer 33

villi

Question 34

The epithelial cells on the surface of the villi have folds in their cell-surface membranes called ______

Answer 34

microvilli

Question 35

Explain the presence of bile in the colon (large intestine)

Answer 35

• Secreted by the liver
• Bile passes unchanged from small intestine to colon

Question 36

Suggest why the peptides released when gluten is digested cannot by absorbed by facilitated diffusion

Answer 36

• Too big/wrong shape
• Cannot pass through channel protein

Question 37

Describe the function of part X

Answer 37

Transports chylomicrons

Question 38

Suggest an advantage of having muscle cells in the villi (2)

Answer 38

• Enables villi to move
• Increased contact with food

Question 39

Suggest how chylomicrons leave the epithelial cells. Give an reason for your answer. (2)

Answer 39

• Exocytosis
• Too large to leave by other methods