3.3.3 Digestion & Absorption

Question 1

(Digestion & Absorption) Define the term “digestion”.

Answer 1

The process in which large (insoluble) molecules (e.g. starch, proteins) are hydrolysed by enzymes into small, soluble molecules (e.g. glucose and amino acids), which can be absorbed and assimilated.

Question 2

(Digestion & Absorption) What do carbohydrases hydrolyse?

Answer 2

Carbohydrates to monosaccharides.

Question 3

(Digestion & Absorption) What do lipases hydrolyse?

Answer 3

Lipids (fats and oils) into glycerol and fatty acids.

Question 4

(Digestion & Absorption) What do proteases hydrolyse?

Answer 4

Proteins to amino acids.

Question 5

(Digestion & Absorption) Can you describe digestion of carbohydrates by amylases and membrane-bound disaccharidases in mammals? (5)

Answer 5

• Starch
• Amylase (salivary glands, duodenum)
• Disaccharide
• Maltase (duodenum and ileum - inside cells of gut mucosa)
• Monosaccharide (glucose)

Question 6

(Digestion & Absorption) What occurs in the mouth when carbohydrates are digested by amylases and membrane-bound disaccharidases in mammals? (3)

Answer 6

Mastication
Salivary amylase
Starch hydrolysed to maltose

Question 7

(Digestion & Absorption) What occurs in the stomach when carbohydrates are digested by amylases and membrane-bound disaccharidases in mammals? (1)

Answer 7

Acidic pH denatures amylase

Question 8

(Digestion & Absorption) What occurs in the small intestine (duodenum - ileum) when carbohydrates are digested by amylases and membrane-bound disaccharidases in mammals? (4)

Answer 8

Pancreatic amylase continues starch hydrolysis / neutral pH
Peristalsis
Maltase hydrolyses maltose to glucose
Maltase is a membrane-bound disaccharidase

Question 9

(Digestion & Absorption) Can you describe digestion of proteins by endopeptidases, exopeptidases, and membrane-bound dipeptidases in mammals?

Answer 9

Endopeptidases (stomach) produce multiple shorter polypeptides which allows more exopeptidases (pancreatic) to hydrolyse more terminal peptide bonds, releasing amino acids faster.

Dipeptidases (membrane bound - ileum) release further amino acids.

Question 10

(Digestion & Absorption) Can you describe the mechanisms for the absorption of the products of digestion by cells lining the ileum of mammals, including co-transport mechanisms for the absorption of amino acids and of monosaccharides?

Answer 10

Sodium ions are being actively pumped out of the cuboidal cells by active, ATP driven Na / K exchange pumps.

This sets up a sodium ion concentration gradient., with a higher concentration of sodium ions on the outside.

The co-transporter then facilitates the sodium ions diffusing in down their gradient to “pull in” glucose molecules into the cytoplasm against its gradient (maximum absorption).

The glucose can then passively diffuse out through other carrier proteins onto the other side / passing into the blood capillaries.

Question 11

(Digestion & Absorption) Can you describe digestion of lipids by lipase, including the action of bile salts in mammals? (5)

Answer 11

Bile salts secreted from the liver (also gallbladder) emulsify lipid (fat) globules. This increases the surface area for pancreatic lipase to hydrolyse the triglyceride molecules into glycerol and fatty acids.

Question 12

(Digestion & Absorption) Amylase is a digestive enzyme which catalyses the breakdown of what?

Answer 12

Starch.

Question 13

(Digestion & Absorption) How does amylase catalyse the breakdown of starch?

Answer 13

Catalysing hydrolysis reactions that break the glyosidic bonds in starch to produce maltose (a disaccharide).

Question 14

(Digestion & Absorption) What is amylose produced by and where is it released?

Answer 14

The salivary glands, which releases amylase into the mouth.
Also by the pancreas, which releases amylase into the small intestine.

Question 15

(Digestion & Absorption) What enzymes digest carbohydrates?

Answer 15

Amylase and membrane-bound disaccharidases.

Question 16

(Digestion & Absorption) What are membrane-bound disaccharidases, and what are they attached to?

Answer 16

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

Question 17

(Digestion & Absorption) What is the ileum?

Answer 17

The final part of the small intestine.

Question 18

(Digestion & Absorption) What do membrane-bound disaccharidases help? (What does it involve?)

Answer 18

To break down disaccharides into monosaccharides. This involves the hydrolysis of glyosidic bonds.

Question 19

(Digestion & Absorption) Explain what lipase enzymes do regarding the digestion of lipids.

Answer 19

The catalyse the breakdown of lipids into monoglycerides and fatty acids. This involves the hydrolysis of the ester bonds in lipids.

Question 20

(Digestion & Absorption) Where are lipases made and secreted?

Answer 20

They are mainly made in the pancreas and then secreted into the small intestine, where they act.

Question 21

(Digestion & Absorption) Where are bile salts produced?

Answer 21

By the liver.

Question 22

(Digestion & Absorption) What is the function of bile salts, and what does their function allow them to do?

Answer 22

Emulsify lipids, which means they cause the lipids to form small droplets.

Question 23

(Digestion & Absorption) Although bile salts are not enzymes they are really important in the process of lipid digestion. Why is this?

Answer 23

Several small lipid droplets have a bigger surface area than a single large droplet, so the formation of small droplets greatly increases the surface area of the lipid that is available for lipases to work on.

Question 24

(Digestion & Absorption) What are micelles and how do they occur?

Answer 24

Once the lipid has been broken down by lipases, the monoglyceride and fatty acid stick with the bile salts to form tiny structures (micelles).

Question 25

(Digestion & Absorption) What is the function of micelles?

Answer 25

To help the products of lipid digestion to be absorbed.

Question 26

(Digestion & Absorption) How are proteins broken down?

Answer 26

By a combination of different peptidases - enzymes that catalyse the conversion of proteins into amino acids by hydrolysing the peptide bonds between amino acids.

Question 27

(Digestion & Absorption) What is the function of endopeptidases?

Answer 27

To hydrolyse peptide bonds within a protein.

Question 28

(Digestion & Absorption) State 3 examples of endopeptidases.

Answer 28

Trypsin, chymotrypsin and pepsin.

Question 29

(Digestion & Absorption) Describe what happens to the endopeptidases, trypsin and chymotrypsin, regarding the digestion of proteins.

Answer 29

They’re synthesised in the pancreas and secreted into the small intestine.

Question 30

(Digestion & Absorption) Describe what happens to the endopeptidase, pepsin, regarding the digestion of proteins.

Answer 30

It’s released into the stomach by cells in the stomach lining. It only works in acidic conditions - provided by hydrochloric acid in the stomach.

Question 31

(Digestion & Absorption) What is the function of exopeptidases?

Answer 31

To hydrolyse peptide bonds at the ends of protein molecules. They remove single amino acids from proteins.

Question 32

(Digestion & Absorption) What are dipeptidases?

Answer 32

Exopeptidases that work specifically on dipeptides.

Question 33

(Digestion & Absorption) What is the function of dipeptidases?

Answer 33

To separate the two amino acids that make up a dipeptide by hydrolysing the peptide bond between them.

Question 34

(Digestion & Absorption) Where are dipeptidases often located?

Answer 34

In the cell-surface membrane of epithelial cells in the small intestine.

Question 35

(Digestion & Absorption) Where are the products of digestion absorbed across and into?

Answer 35

Across the ileum epithelium and into the bloodstream.

Question 36

(Digestion & Absorption) How is glucose (monosaccharide) absorbed regarding the absorption of the products of digestion?

Answer 36

By active transport with sodium ions via a co-transporter protein.

Question 37

(Digestion & Absorption) How is galactose (monosaccharide) absorbed regarding the absorption of the products of digestion?

Answer 37

By active transport with sodium ions via a co-transporter protein (same as glucose).

Question 38

(Digestion & Absorption) How is fructose (monosaccharide) absorbed regarding the absorption of the products of digestion?

Answer 38

Via facilitated diffusion through a different transport protein than glucose and galactose.

Question 39

(Digestion & Absorption) What is the function of micelles?

Answer 39

To move monoglycerides and fatty acids towards the epithelium.

Question 40

(Digestion & Absorption) What is the advantage of micelles constantly breaking up and reforming?

Answer 40

They can release monoglycerides and fatty acids, allowing them to be absorbed - whole micelles are not taken up across the epithelium.

Question 41

(Digestion & Absorption) Monoglycerides and fatty acids are lipid-soluble, what does this allow?

Answer 41

They can diffuse directly across the epithelial cell membrane.

Question 42

(Digestion & Absorption) How are amino acids absorbed?

Answer 42

Sodium ions are actively transported out of the epithelial cells into the ileum itself.
They then diffuse back into the cells through sodium-dependent transporter proteins in the epithelial cell membranes, carrying the amino acids.