Unit 3 - Digestion

Question 1

What do the salivary glands do?

Answer 1

Secrete saliva containing amylase - the enzyme which hydrolyses starch to maltose.

Question 2

What does the pancreas do?

Answer 2

Secretes pancreatic juice containing amylase (and other carbohydrates), endopeptidases and exopeptidases and lipases.

Question 3

What does the stomach do?

Answer 3

Food is mixed with gastric juice. This is acidic and so kills microorganisms. It also contains endopeptidases which hydrolysis proteins to amino acids.

Question 4

What does the small intestine do?

Answer 4

It’s adapted to provide a large surface area for the absorption of the products of digestion. Disaccharidase and dipeptidase enzymes are embedded within the epithelial cell membrane of the small intestine.

Question 5

What is digestion?

Answer 5

Large, insoluble molecules are hydrolysed to smaller, soluble molecules that can be absorbed across cell membranes.

Question 6

Salivary amylase?

Answer 6

Produced in the salivary glands and secreted into the mouth to carry out its function.

Question 7

Endopeptidases?

Answer 7

Produced in the stomach and carries out its function in the stomach.

Question 8

Pancreatic amylase, lipase, endopeptidase and exopeptidase?

Answer 8

Produced in the pancreas and secreted into the lumen of the ileum (small intestine).

Question 9

Dipeptidase and disaccharides?

Answer 9

Produced in the epithelial cells of the small intestine and are embedded into the membrane of the epithelial cells of the small intestine.

Question 10

What happens when food enters the mouth?

Answer 10

It is broken up by teeth, then mixed with saliva.

Question 11

What does salivary amylase then do?

Answer 11

It catalyses the hydrolysis of glycosidic bonds in starch to produce maltose (a disaccharide).

Question 12

What happens to salivary amylase in the stomach?

Answer 12

It gets denatured.

Question 13

What does pancreatic amylase then do?

Answer 13

It continues to hydrolyse the starch to maltose.

Question 14

What do maltase (disaccharide) enzymes then do?

Answer 14

They hydrolyse maltose to glucose. The glucose can then be absorbed by the blood.

Question 15

What are the 3 membrane-bound disaccharides?

Answer 15

1) Sucrase
Sucrose=glucose + fructose

Question 16

How are lipids (triglycerides) digested?

Answer 16

Triglycerides are hydrolysed into a monoglyceride (glycerol and 1 fatty acid) and 2 fatty acids. This reaction is catalysed by lipase enzymes. This involves the hydrolysis of ester bonds in lipids.

Question 17

Where are bile salts produced and what do they do?

Answer 17

They’re produced by the liver and emulsify lipids - this means that they cause lipids to form small droplets to increase surface area.

Question 18

How does the formation of small droplets by bile aid digestion?

Answer 18

Many small droplets have a larger S.A.:V than a single larger lipid droplet. It greatly increases the surface area of lipid that’s available for lipase to work on and so leads to a faster rate of hydrolysis.

Question 19

How are micelles formed and what do they do?

Answer 19

Once a lipid has been hydrolysed by lipase, the monoglyceride and 2 fatty acids stick with the bile salts to form tiny structures called micelles. They help the products of lipid digestion be absorbed. They also make fatty acids and monoglyceride more soluble in water.

Question 20

How are proteins digested?

Answer 20

They’re digested by a combination of different peptidases (proteases). These are enzymes that catalyse the conversion of proteins to amino acids by hydrolysing the peptide bond between amino acids.

Question 21

What are the 3 different types of peptidases?

Answer 21

1) Endopeptidases
2) Exopeptidases
3) Dipeptidases

Question 22

What do endopeptidases do?

Answer 22

They hydrolyse peptide bonds in the MIDDLE of a polypeptide to produce shorter polypeptides. They increase the number of ends for exopeptidases to work.

Question 23

What do exopeptidases do?

Answer 23

They hydrolyse peptide bonds at the ENDS of the polypeptide molecule to release single amino acids.

Question 24

What do dipeptidases do?

Answer 24

They work specifically on dipeptides. They act on the 2 amino acids that make up a dipeptide by hydrolysing the peptide bond between them.

Question 25

Exam Q: ‘The combined action of endopeptidases and exopeptidases is more efficient than exopeptidases on their own. Explain why?’

Answer 25

Endopeptidases increases the number of ends the polypeptide has for the exopeptidases to act on leading to faster hydrolysis.

Question 26

What are the products of digestion absorbed across?

Answer 26

The ileum epithelium then into the bloodstream.

Question 27

How are monosaccharides absorbed?

Answer 27

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

Question 28

Describe the process by which monosaccharides are absorbed?

Answer 28

1) Sodium ions are actively transported out of the epithelial cell, into the blood, using energy from the hydrolysis of ATP and a specific carrier protein.
2) This creates a sodium ion concentration gradient from the lumen of the small intestine, into the epithelial cell.
3) So glucose and sodium ions are co-transported across the membrane, down the sodium ion concentration gradient via facilitated diffusion. This involves a specific carrier protein that will only carry glucose and sodium molecules at the same time.
4) The glucose is now transported from the epithelial cell into the blood via facilitated diffusion down a concentration gradient, using a specific carrier protein.

Question 29

How do micelles play a role in the absorption of monoglycerides and fatty acids?

Answer 29

-Micelles carry monoglycerides and fatty acids to the epithelium of the small intestine.
-Micelles constantly break up and reform they can ‘release’ monoglycerides and fatty acids, allowing them to be absorbed - while micelles are NOT taken up across the epithelium.
-Monoglycerides and fatty acids are lipid-soluble, so can diffuse directly across the phospholipid bilayer of the epithelial membrane.

Question 30

How are monoglycerides and fatty acids absorbed once there in the epithelial cells?

Answer 30

-Monoglycerides and fatty acids are recombined into triglycerides in the smooth endoplasmic reticulum. They are then encased in a vesicle, which enters the Golgi.
-The Golgi apparatus modifies the triglycerides and combines them with proteins, forming a structure called a chylomicron. Chylomicrons are packaged into vesicles for secretion.
-The vesicle moves towards the membrane, and fuses with the membrane, releasing the chylomicrons through a process called exocytosis.
-The triglycerides are then absorbed by the lymphatic vessels.

Question 31

How are amino acids absorbed?

Answer 31

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

Question 32

Describe the process of how amino acids are co-transported?

Answer 32

1) Sodium ions are actively transported out of the epithelial cell, into the blood, using energy from the hydrolysis of ATP and a specific carrier protein.
2) This creates a sodium ion concentration gradient from the lumen of the small intestine, into the epithelial cell.
3) So amino acids and sodium ions are co-transported across the membrane, down the sodium ion concentration gradient via facilitated diffusion. This involves a specific carrier protein that will only carry amino acids and sodium molecules at the same time.
4) The amino acids is now transported from the epithelial cell into the blood via facilitated diffusion down a concentration gradient, using a specific carrier protein.

Question 33

What happens to the absorption of glucose if ATP production is inhibited?

Answer 33

Active transport can’t occur as there is no ATP. If the sodium ions are not actively transported into the blood from the cell then the concentration of sodium ions will rise in the cell so the concentration gradient between the lumen and the cell isn’t maintained. If this occurs then facilitated diffusion (co-transport) from the lumen will stop. As glucose cannot enter the cell without sodium ions this would stop glucose absorption.

Question 34

Structure of villi and their adaptations?

Question 35

Structure of the digestive system?