3B Digestion and Absorption

Question 1

Why do food molecules need to be digested?

Answer 1

E.g. starch, proteins in the food are too big to cross cell membranes. Digestion breaks them into smaller molecules e.g. glucose, amino acids which can move across cell membranes. Then they can be absorbed from the gut into the blood.

Question 2

What is a hydrolysis reaction?

Answer 2

A reaction where a bond is broken by adding water.

Question 3

What happens to carbohydrates in hydrolysis?

Answer 3

They are broken down into disaccharides and then monosaccharides.

Question 4

What enzyme digests starch?

Answer 4

Amylase catalyses the hydrolysis reactions that break glycosidic bonds in starch. It produces maltose disaccharides.

Question 5

Where is amylase produced and released?

Answer 5

In the salivary glands where it is released into the mouth, and pancreas where it is released into the small intestine.

Question 6

Where are membrane bound disaccharidases found, and what is their function?

Answer 6

They are attached to the cell membranes of epithelial cells lining the ileum. They are enzymes that break down disaccharides into monosaccharides by catalysing hydrolysis reactions.

Question 7

What disaccharidase breaks down sucrose and into which monosaccharides?

Answer 7

Sucrose, into glucose and fructose.

Question 8

What disaccharidase breaks down maltose and into which monosaccharides?

Answer 8

Maltose, into glucose and glucose.

Question 9

What disaccharidase breaks down lactose and into which monosaccharides?

Answer 9

Lactase, into glucose and galactose.

Question 10

What happens to fats during hydrolysis?

Answer 10

Fats are broken down into fatty acids and monoglycerides.

Question 11

What enzyme digests lipids?

Answer 11

Lipase, by catalysing hydrolysis reactions that break ester bonds.

Question 12

Where are lipases produced and released?

Answer 12

They are mainly produced in the pancreas, and released into the small intestine.

Question 13

What role do bile salts play in lipid digestion and why is it useful?

Answer 13

They emulsify lipids, causing them to form small droplets. Several small droplets have a bigger surface area than a single large droplet so there is more surface area available for lipases to work on.

Question 14

Where are bile salts produced?

Answer 14

In the liver.

Question 15

How do micelles form?

Answer 15

When a lipid has been broken down, the fatty acids and monoglycerides stick with bile salts to form tiny structures called micelles.

Question 16

What happens to proteins during hydrolysis?

Answer 16

They are broken down into amino acids.

Question 17

What enzymes digest proteins?

Answer 17

A combination of different peptidases catalyse hydrolysis reactions that break peptide bonds between amino acids.

Question 18

What peptidase hydrolyses peptide bonds within a protein? Give some examples.

Answer 18

Endopeptidase.
Trypsin and chymotrypsin are endopeptidases. They are produced in the pancreas and released in the small intestine.
Pepsin is another endopeptidase. It is released into the stomach by stomach lining cells and only works in acidic conditions.

Question 19

What peptidase hydrolyses peptide bonds at the ends of protein molecules?

Answer 19

Exopeptidases. They remove single amino acids from proteins.

Question 20

What are dipeptidases? Where are they usually located?

Answer 20

Exopeptidases that work specifically on dipeptides. They catalyse the hydrolysis of a peptide bond between two amino acids.
They are often located in the cell surface membrane of epithelial cells in the small intestine.

Question 21

Describe how monosaccharides are absorbed across the ileum epithelium into the blood.

Answer 21

1. Glucose and Galactose are absorbed by active transport with sodium ions via a co-transporter protein.
2. Fructose is absorbed by facilitated diffusion through a different transporter protein.

Question 22

Describe how monoglycerides and fatty acids are absorbed across the ileum epithelium into the blood.

Answer 22

1. Micelles move the monoglycerides and fatty acids towards the epithelium.
2. Because micelles constantly break up and reform they can release monoglycerides and fatty acids, allowing them to be absorbed.
3. Whole micelles are not taken up across the epithelium.
4. Monoglycerides and fatty acids are lipid-soluble, so can diffuse directly across the epithelial cell membrane.

Question 23

Describe how amino acids are absorbed across the ileum epithelium into the blood.

Answer 23

1. Sodium ions are actively transported out of the epithelial cells into the ileum itself.
2. This creates an electrochemical gradient.
3. Then they diffuse back into the cells through sodium-dependent transporter proteins in the epithelial cell membranes, carrying the amino acids with them.