Enzymes and digestion

Question 1

By which process do enzymes break down large molecules into smaller ones during digestion

Answer 1

Hydrolysis

Question 2

List the seven major parts of the digestive system

Answer 2

1) The oesophagus
2) The stomach
3) The ileum
4) The large intestine
5) The rectum
6) The salivary glands
7) The pancreas

Question 3

What is the function of the oesophagus

Answer 3

To carry food from the mouth to the stomach

Question 4

Describe the structure and function of the stomach

Answer 4

• The stomach is a muscular sac with an inner layer that produces enzymes.
• Its role is to store and digest food, especially proteins.
• It has glands that produce enzymes which digest protein.

Question 5

Describe the structure and function of the ileum

Answer 5

• The ileum is a long muscular tube.
• Food is further digested in the ileum by enzymes that are produced by its walls and by glands that pour their secretions into it.
• The inner walls of the ileum are folded into villi, which gives them a large surface area.
• The surface area of these villi is further increased by millions of tiny projections, called microvilli, on the epithelial cells of each villus.
• This adapts the ileum for its purpose of absorbing the products of digestion into the bloodstream.

Question 6

Describe the function of the large intestine

Answer 6

The large intestine absorbs water. Most of the water that is absorbed is water from the secretions of the many digestive glands.

Question 7

Describe the structure and function of the rectum

Answer 7

• The rectum is the final section of the intestines.
• The faeces are stored here before periodically being removed by the anus in a process called egestion.

Question 8

Describe the structure and function of the salivary glands

Answer 8

• The salivary glands are situated near the mouth.
• They pass their secretions via a duct in the mouth.
• These secretions contain the enzyme amylase, which hydrolyses starch into maltose.

Question 9

Describe the structure and function of the pancreas

Answer 9

• The pancreas is a large gland situated below the stomach.
• It produces a secretion called pancreatic juice.
• This secretion contains proteases to hydrolyse proteins, lipase to hydrolyse lipids and amylase to hydrolyse starch.

Question 10

Name the two stages of digestion in humans

Answer 10

1. Physical breakdown
2. Chemical digestion

Question 11

Describe the physical breakdown stage of digestion in humans

Answer 11

• Food is broken down into smaller pieces by structures such as the teeth.
• This makes it possible to ingest and means the food has a large surface area for chemical digestion.
• Food is churned by muscles in the stomach wall and this physically breaks it up.

Question 12

Describe how chemical digestion works

Answer 12

• Chemical digestion hydrolyses large, insoluble molecules into smaller, soluble ones.
• It is carried out by enzymes and all digestive enzymes function by hydrolysis.
• Enzymes are specific and more than one enzyme is needed to hydrolyse a large molecule.
• Usually one enzyme hydrolyses a large molecule into sections and these sections are then hydrolysed into smaller sections by additional enzymes.

Question 13

What are the three main types of digestive enzyme

Answer 13

1) Carbohydrases
2) Lipases
3) Proteases

Question 14

Summarise how starch digestion works

Answer 14

• The enzyme amylase is produced in the mouth and pancreas.
• Amylase hydrolyses the alternate glycosidic bonds of the starch molecule to produce the disaccharide maltose.
• The maltose is in turn hydrolysed into the alpha-glucose monosacharide by a second enzyme (a disaccharidase called maltase)
• Maltase is produced by the lining of the ileum.

Question 15

Describe in detail the process of the digestion of starch

Answer 15

1) Saliva enters the mouth from the salivary glands and is thoroughly mixed with the food during chewing.
2) Saliva contains salivary amylase. This starts hydrolysing any starch in the food to maltose. It also contains mineral salts that help to maintain the PH at around neutral. This is the optimum PH for salivary amylase to work.
3) The food is swallowed and enters the stomach, where the conditions are acidic. This acid denatures the amylase and prevents further hydrolysis of the starch.
4) After a time the food is passed into the small intestine, where it mixes with the secretion from the pancreas called pancreatic juice.
5) The pancreatic juice contains pancreatic amylase. This continues the hydrolysis of any remaining starch to maltose. Alkaline salts are produced by both the pancreas and the intestinal wall to maintain the PH at around neutral so that the amylase can function.
6) Muscles in the intestine wall push food along the ileum. Its epithelial lining produces the disaccharidase maltase. Maltase is not released into the lumen of the ileum but is part of the cell-surface membranes of the epithelial cells that line the ileum. It is therefore referred to as membrane bound disaccharidase.
7) The maltase hydrolyses the maltose from starch breakdown into alpha-glucose

Question 16

Aside from maltose, which are the two other key disaccharides in our diet that are hydrolysed during digestion

Answer 16

Sucrose and lactose

Question 17

What disaccharidase hydrolyses sucrose and what does it produce

Answer 17

• Sucrase hydrolyses the single glycosidic bond in the sucrose molecule.
• This hydrolysis produces the two monosaccharides glucose and fructose

Question 18

What disaccharidase hydrolyses lactose and what does the hydrolysis produce

Answer 18

• Lactase hydrolyses the single glycosidic bond in the lactose molecule.
• This hydrolysis produces the two monosaccharides glucose and galactose.

Question 19

Describe the process of lipid digestion

Answer 19

• Lipids are hydrolysed by enzymes called lipases
• Lipases are produced in the pancreas and they hydrolyse the ester bond found in triglycerides to form fatty acids and monoglycerides.
• Lipids are firstly split up into tiny droplets called micelles by bile salts, which are produced by the liver.
• This process is called emulsification and increases the surface area of the lipids so that the action of lipases is sped up.

Question 20

What is a monoglyceride

Answer 20

A glycerol with a single fatty acid molecule attached

Question 21

Describe how protein digestion works

Answer 21

Proteins are hydrolysed by a group of enzymes called peptidases. There are three key types of peptidase:
1) endopeptidases
2) exopeptidases
3) dipeptidases

Question 22

How do endopeptidases work

Answer 22

Endopeptidases hydrolyse the peptide bonds between amino acids in the central region of a protein molecule forming a series of peptide molecules.

Question 23

How do exopeptidases work

Answer 23

Exopeptidases hydrolyse the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases. In this way they progressively release dipeptides and single amino acids.

Question 24

How do dipeptidases work

Answer 24

Dipeptidases hydrolyse the bond between the two amino acids of a dipeptide. Dipeptidases are membrane-bound, being part of the cell-surface membrane of the epithelial cells lining the ileum.

Question 25

List the key products of the hydrolysis that takes place during digestion (the substances that the body must absorb)

Answer 25

Monosaccharides, amino acids, monoglycerides and fatty acids

Question 26

Which part of the body absorbs the products of digestion

Answer 26

The ileum

Question 27

Where are the villi found

Answer 27

Between the lumen (cavity) of the intestines and the blood and other tissues in the body.

Question 28

What is the main way in which the ileum is adapted for efficient absorption in digestion

Answer 28

The wall of the ileum is folded and contains finger-like projections called villi.

Question 29

Explain how the villi increase the efficiency of absorption

Answer 29

• They increase the surface area for diffusion.
• They are very thin walled, thus reducing the distance over which diffusion takes place.
• They contain muscle and so are able to move. This helps to maintain diffusion gradients because their movement mixes the contents of the ileum. This ensures that, as the products of digestion are absorbed from the food adjacent to the villi, new material rich in the products of digestion replaces it.
• They are well supplied with blood vessels so that blood can carry away absorbed molecules and hence maintain a diffusion gradient.
• The epithelial cells lining the villi possess microvilli. These are finger-like projections of the cell-surface membrane that further increase the surface area for absorption.

Question 30

How are amino acids and monosaccharides absorbed during digestion

Answer 30

By diffusion, facilitated diffusion and co-transport.

Question 31

Describe how triglycerides are absorbed into the bloodstream and body tissue during digestion

Answer 31

1) Once formed during digestion, monoglycerides and fatty acids remain in association with the bile salts that initially emulsified the lipid droplets.
2) The structures formed are called micelles.
3) These micelles come into contact with the epithelial cells lining the villi of the ileum.
4) The micelles break down, releasing monoglycerides and fatty acids. As these are non-polar molecules, they can easily diffuse across the cell-surface membrane and into the epithelial cells.
5) Once inside the epithelial cells, the monoglycerides and fatty acids are transported to the endoplasmic reticulum where they are recombined to form triglycerides.
6) Starting in the endoplasmic reticulum and continuing into the Golgi apparatus, the triglycerides associate with cholesterol and lipoproteins to form structures called chylomicrons.
7) Chylomicrons move out of the epithelial cells via exocytosis. They enter lymphatic capillaries called lacteals that are found at the centre of each villus.
8) From here, the chylomicrons pass, via lymphatic vessels, into the blood system.
9) The triglycerides in the chylomicrons are hydrolysed by an enzyme in the endothelial cells of blood capillaries, from where they diffuse into cells.

Question 32

What are chlyomicrons

Answer 32

Special particles adapted for the transport of lipids.