Digestion

Question 1

What is amylase?

Answer 1

This enzyme is produced in the mouth (salivary glands) and pancreas.
Amylase hydrolyses the alternate glycosidic bonds of the starch molecule to produce the disaccharide maltose.

Question 2

What is maltase?

Answer 2

The dissacharidase maltase is produced in the lining of the ileum (small intestine).
It is not released into the lumen but is part of the cell-surface membranes of the epithelial cells, so is referred to as a membrane-bound disaccharide.
It hydrolyses maltose into the monosaccharide α-glucose.

Question 3

What are the three main important digestive enzymes?

Answer 3

Carbohydrases hydrolyse carbohydrates, ultimately to monosaccharides.
Lipases hydrolyse lipids into monoglycerides and fatty acids.
Proteases hydrolyse proteins, ultimately to amino acids.

Question 4

What is the process of carbohydrate digestion?

Answer 4

Saliva enters the mouth from the salivary glands and is thoroughly mixed with the food during chewing.
The salivary amylase starts hydrolysing any starch in the food to maltose. It also contains mineral salts that help to maintain the pH around neutral, the optimal pH for amylase.
The food is swallowed and enters the acidic stomach, which denatures the amylase and prevents further hydrolysis of the starch.

Question 5

What is the process of carbohydrate digestion - pancreas?

Answer 5

The food is passed into the small intestine, where it mixes with the secretion from the pancreas called the pancreatic juice.
This contains pancreatic amylase, which continues the hydrolysis of any remaining starch to maltose. Alkaline salts are produced by both the pancreas and the intestinal wall to maintain pH at neutral.
Muscles in the intestine wall push the food along the ileum, its epithelium lining produces maltase.
This hydrolyses the maltose from starch breakdown into α-glucose.

Question 6

What is sucrase?

Answer 6

Sucrose is found in many natural foods, especially fruits.
Sucrase, a membrane-bound disaccharide, hydrolyses the single glycosidic bond in the sucrose molecule.
This hydrolysis produces the two monosaccharides glucose and fructose.

Question 7

What is lactase?

Answer 7

Lactose is found in milk, and milk products.
Lactase, a membrane-bound disaccharide, hydrolyses the single glycosidic bond in the lactose molecule.
This hydrolysis produces the two monosaccharides glucose and galactose.

Question 8

What is lipase?

Answer 8

Lipids are hydrolysed by enzymes called lipases.
Lipases are produced in the pancreas and hydrolyse the ester bond found in triglycerides to form fatty acids and monoglycerides (a glycerol molecule with a single fatty acid molecule attached).

Question 9

How are lipids digested?

Answer 9

Lipids are firstly split up into tiny droplets called micelles by bile salts, produced by the liver.
This process is emulsification, and increases the surface area of the lipids so that the action of lipases is sped up.
Bile also neutralises stomach acids.

Question 10

What are peptidases?

Answer 10

Proteins are large, complex molecules that are hydrolysed by peptidases.
There are endopeptidases, exopeptidases and dipeptidases.

Question 11

What are endopeptidases?

Answer 11

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

Question 12

What are exopeptidases?

Answer 12

They hydrolyse the peptide bonds on the terminal (end) amino acids of the peptide molecules formed by endopeptidases.
They progressively release dipeptides and single amino acids.

Question 13

What are dipeptidases?

Answer 13

They 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 14

Why do endopeptidases work first?

Answer 14

This exposes more ends of the amino acid chains for the exopeptidases to work on.
It can create 4 ends, but if exopeptidases worked first, there would only be 2 ends to work on, which would be slower.

Question 15

What is physical digestion?

Answer 15

If the food is large, it is broken down into smaller pieces by structures such as the teeth.
This not only makes it possible to ingest the food but also provides a large surface area for chemical digestion.
Food is churned by the muscles in the stomach wall and this also physically breaks it up.

Question 16

What is chemical digestion?

Answer 16

This hydrolyses large, insoluble molecules into smaller, soluble ones, through enzymes.
All digestive enzymes function through hydrolysis, the splitting up of molecules by adding water to chemical bonds that hold them together.
Enzymes are specific so more than one is needed to hydrolyse a large molecule.
Usually one enzyme hydrolyses a large molecule into sections and these sections are then hydrolysed into smaller molecules.

Question 17

What is the human digestive system?

Answer 17

It is made up of a long muscular tube and its associated glands.
The glands produce enzymes that hydrolyse large molecules into small ones ready for absorption.
It is therefore an exchange surface through which food substances are absorbed.

Question 18

What is the oesophagus?

Answer 18

It carries food from the mouth to the stomach.

Question 19

What is the stomach?

Answer 19

A muscular sac with an inner layer that produces enzymes.
It stores and digests food, especially proteins.
It has glands that produce enzymes which digest proteins.

Question 20

What is the ileum?

Answer 20

A long muscular tube.
Food is further digested in the ileum by enzymes that are produced in 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.

Question 21

What is the large intestine?

Answer 21

It absorbs water.
Most of the water is from the secretions of the many digestive glands.

Question 22

What is the rectum?

Answer 22

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

Question 23

What are the salivary glands?

Answer 23

Situated near the mouth, they pass their secretions via a duct into the mouth.
These secretions contain the enzyme amylase.

Question 24

What is the pancreas?

Answer 24

A large gland situated below the stomach.
It produces a secretion called pancreatic juice.
This contains proteases, lipases and amylase.

Question 25

What is the structure of the ileum?

Answer 25

It is adapted for absorbing the products of digestion.
The wall is folded and possesses finger-like projections, about 1mm long, called the villi.
They have thin walls, lined with epithelial cells on the other side of which is a rich network of blood capillaries.
The villi considerably increase the surface area of the ileum and therefore accelerates the rates of absorption.

Question 26

What are the villi?

Answer 26

Villi are situated at the interface between the lumen of the intestines and the blood and other tissues inside the body.
They are part of a specialised exchange surface adapted for the absorption of the products of digestion.

Question 27

How do the properties of the villi increase absorption efficiency?

Answer 27

They increase the surface area for diffusion.
They are very thin walled, so reduce the diffusion distance.
The epithelial cells lining the villi possess microvilli that further increase the surface area for absorption.

Question 28

How do the villi properties increase efficiency - diffusion gradient.

Answer 28

They contain muscle and so can move. This helps to maintain the diffusion gradients because their movement mixes the contents of the ileum. This ensures that, as the products are absorbed from the food adjacent to the villi, new material rich in the products replaces it.
They are well supplied with blood vessels so that the blood can carry away absorbed molecules and hence maintain a diffusion gradient.

Question 29

How are amino acids and monosaccharides absorbed?

Answer 29

Amino acids and monosaccharides such as glucose, fructose and galactose are absorbed by the ileum by diffusion and co-transport.

Question 30

How are triglycerides absorbed?

Answer 30

Once formed during digestion, monoglycerides and fatty acids remain in association with the bile salts that initially emulsified the lipid droplets.
The micelles formed are tiny structures, 4-7nm in diameter.
Through the movement of material within the lumen of the ileum, the micelles come into contact with the epithelial cells lining the villi.
Here, the micelles break down, releasing the monoglycerides and fatty acids.
These are non-polar molecules, so easily diffuse across the cell-surface membrane into the epithelial cells.

Question 31

How are triglycerides absorbed inside the epithelial cells?

Answer 31

Once inside the epithelial cells, monoglycerides and fatty acids are transported to the endoplasmic reticulum where they are recombined to form triglycerides.
Starting in the endoplasmic reticulum and continuing to the Golgi apparatus, the triglycerides associate with cholesterol and lipoproteins to form chylomicrons.
These are special particles adapted for the transport of lipids.

Question 32

How are triglycerides absorbed - exocytosis?

Answer 32

Chylomicrons move out of the epithelial cells by exocytosis.
They enter lymphatic capillaries called lacteals that are found at the centre of each villus.