3.3 - Digestion & Absorption

Question 1

Define digestion.

Answer 1

The hydrolysis of large, insoluble molecules into smaller molecules that can be absorbed across cell membranes.

Question 2

Which enzymes are involved in carbohydrate digestion?

Answer 2

Amylase, maltase, sucrase & lactase.

Question 3

What is amylase?

Answer 3

Amylase is a digestive enzyme that catalyses the breakdown of starch.

Question 4

Where is amylase produced & found?

Answer 4

Amylase is produced by the salivary glands, which then releases it into the mouth.
It is also produced by the pancreas, which releases amylase into the small intestine.

Question 5

How does amylase work?

Answer 5

Amylase works by catalysing hydrolysis reactions that break the glycosidic bonds in starch (a mixture of 2 polysaccharides) to produce maltose (a disaccharide).

Question 6

What are membrane-bound disaccharidases?

Answer 6

Enzymes that are attached to the cell membranes of epithelial cells lining the ileum (final part of small intestine).
They help break down disaccharides into monosaccharides, through the hydrolysis of glycosidic bonds.

Question 7

Name the 3 membrane-bound disaccharidases involved in the digestion of carbohydrates.

Answer 7

Maltase, sucrase & lactase

Question 8

How does sucrase work?

Answer 8

Sucrase catalyses the breakdown of the disaccharide sucrose into the monosaccharides glucose & fructose, by the hydrolysis of the glycosidic bond between them.

Question 9

How does maltase work?

Answer 9

Maltase catalyses the breakdown of the disaccharide maltose into two glucose monosaccharides, via the hydrolysis of a glycosidic bond.

Question 10

How does lactase work?

Answer 10

Lactase catalyses the breakdown of the disaccharide lactose into the monosaccharides glucose & galactose via hydrolysis of the glycosidic bond between them.

Question 11

What happens to the monosaccharides produced in the digestion of carbohydrates?

Answer 11

The monosaccharides can be transported across the epithelial cell membranes in the ileum via specific transporter proteins.

Question 12

Where are lipids digested?

Answer 12

The small intestine.

Question 13

What needs to happen before lipids can be digested?

Answer 13

They must be emulsified by bile salts produced by the liver. This breaks down large fat molecules into smaller, soluble molecules called micelles, increasing surface area.

Question 14

What are lipase enzymes?

Answer 14

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

Question 15

Where are lipases produced & found?

Answer 15

Lipases are mainly made in the pancreas.
They’re then secreted into the small intestine where they act.

Question 16

Where are bile salts produced?

Answer 16

In the liver

Question 17

What do bile salts do?

Answer 17

Emulsify lipids

Question 18

What is mean by ‘bile salts emulsify lipids’?

Answer 18

They cause lipids to form small droplets.
Several small lipid droplets have a bigger surface area to volume ratio than a single large droplet. This increases the surface area of lipid available for lipases to work on.

Question 19

What happens once the lipid has been broken down by lipase?

Answer 19

The monoglycerides and fatty acids produced stick with bile salts to form micelles.

Question 20

What are micelles?

Answer 20

Tiny structured of bile salts, monoglycerides & fatty acids, that help the products of lipid digestion to be absorbed?

Question 21

How are lipids digested?

Answer 21

Lipase hydrolyses the ester bond between the monoglycerides and fatty acids.

Question 22

Name the 3 peptidases (enzymes) involved in protein digestion?

Answer 22

Endopeptidases
Exopeptidases
Dipeptidases

Question 23

What is the role of endopeptidases?

Answer 23

Endopeptidases catalyse the hydrolysis of peptide bonds WITHIN a protein.

Question 24

What is the role of exopeptidases?

Answer 24

Exopeptidases catalyse the hydrolysis of peptide bonds at the END of protein molecules.

Question 25

What is the role of dipeptidases?

Answer 25

Dipeptidases are a type of exopeptidases that work specifically on dipeptides.
They act to separate the 2 amino acids that make up a dipeptide, by catalysing the hydrolysis of the peptide bond between them.

Question 26

How are certain molecules absorbed into the ileum despite a negative concentration gradient?

Answer 26

Through co-transport

Question 27

Which molecules require co-transport?

Answer 27

Amino acids & monosaccharides.

Question 28

Explain how sodium ions are involved in co-transport.

Answer 28

Sodium ions (Na +) are actively transported out of the cell into the lumen, creating a diffusion gradient. Nutrients are then taken up into cells along with Na+ ions.

Question 29

Why do fatty acids and monoglycerides not require co-transport?

Answer 29

The molecules are non-polar, meaning they can easily diffuse across the membrane of the epithelial cells.

Question 30

Where are the products of digestion absorbed?

Answer 30

Across the ileum epithelium into the bloodstream.

Question 31

Explain the absorption of monosaccharides.

Answer 31

Glucose & galactose are absorbed by active transport with sodium ions via a co-transporter protein.
Fructose is absorbed via facilitated diffusion through a different transporter protein.

Question 32

Explain the absorption of monoglycerides & fatty acids.

Answer 32

Micelles help moves monoglycerides & fatty acids towards the epithelium.
Micelles constantly break up & reform, releasing monoglycerides & fatty acids, allowing them to be absorbed.

Question 33

Explain the absorption of amino acids.

Answer 33

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