More Exchange and Transport Systems - Digestion and Absorption

Question 1

During hydrolysis, what are carbohydrates broken down into?

Answer 1

Disaccharides and then monosaccharides

Question 2

During hydrolysis, what are fats broken down into?

Answer 2

Fatty acids and then monoglycerides

Question 3

Durng hydrolysis, what are proteins broken down into?

Answer 3

Amino acids

Question 4

What catalyses the breakdown of starch?

Into what?

Answer 4

Amylase and membrane-bound disaccharidases catalyse the conversion of starch (a polysaccharide) into the smaller sugar maltose. This involves the hydrolysis of the glycosidic bonds in starch.

Question 5

Where is amylase produced?

Answer 5

The salivary glands (releases into mouth) and the pancreas (releases into small intestine)

Question 6

What are membrane-bound disaccharidases?

Answer 6

Enzymes attached to the cell membranes of epithelial cells lining the ileum (the final part of the small intestine). They help to break down disaccharides (e.g. maltose, sucrose and lactose) into monosaccharides (e.g. gluctose, fructose and galactose).
This involves the hydrolysis of glycosidic bonds.
Monosaccharides can be transported across the membranes of the ileum epithelial cells via specific transporter proteins.

Question 7

How are lipids broken down?

Answer 7

By lipase (with the help of bile salts)

1) Lipase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids. This involves the hydrolysis of the ester bonds in lipids.
2) Lipases are made in the pancreas. They work in the small intestine.
3) Bile salts are produced by the liver and emulsify lipids – this means they can cause lipids to form small droplets.
4) Bile salts are really important in the process of lipid digestion. Several small lipid droplets have a bigger surface area than a single large droplet (for the same volume of lipid). So the formation of small droplets greatly increases the surface area of the lipid that’s available for lipases to work on.
5) Once the lipid has been broken down, the monoglycerides and fatty acids stick with the bile salts to form tiny structures called micelles.

Question 8

What adaptations does the small intestine have that make it efficient for absorption? (3)

Answer 8

1) There is a rich blood supply,
which maintains a concentration gradient.

2) There are villi and microvilli,
which largely increase the surface area.

3) Its cells have many mitochondria,
which means there’s lots of ATP providing energy for active transport.

Question 9

What is co-transport?
What’s required?
Is energy needed?
What is an example of a co-transporter for monosaccharides?

Answer 9

To move one substance against a concentration gradient, while another moves with a concentration gradient.

A carrier protein is required.

It is passive – no energy/ATP is required.

An example of a co-transporter is the Na+ and glucose co-transporter Glut1

Question 10

In short, what are proteins broken down by?

Answer 10

A combination of different proteases (or peptidases).

These are enzymes
that catalyse the conversion of proteins into amino acids
by hydrolysing the peptide bonds between amino acids.

Question 11

What are the two types of peptidases?

Answer 11

ENDOPEPTIDASES

• Hydrolyse peptide bonds WITHIN a protein

(Trypsin and chymotrypsin are examples
which are synthesised by the pancreas and secreted into the small intestine)

• Pepsin is another endopeptidase
  which is released into the stomach by cells in the stomach lining,
  and only works in acidic conditions – provided by HCl in the stomach

EXOPEPTIDASES

• Hydrolyse peptide bonds at the ENDS of protein molecules.
  They remove single amino acds from proteins.
• Dipeptidases are exopeptidases that work specifically on dipeptides.
  They act to separate the two amino acids that make up a dipeptide by hydrolysing the peptide bond between them.
• Dipeptidases are often located in the cell-surface membrane of epithelial cells in the small intestine.

Question 12

What are dipeptidases?

Answer 12

Dipeptidases are exopeptidases that work specifically on dipeptides.

They act to separate the two amino acids that make up a dipeptide by hydrolysing the peptide bond between them.

Question 13

Where are dipeptidases located?

Answer 13

Dipeptidases are often located in the cell-surface membrane of epithelial cells in the small intestine.

Question 14

What are the products of digestion absorbed across and into?

Answer 14

They’re absorbed across the ileum epithelium and into the bloodstream.

Question 15

What are the products of carbohydrate digestion?

How are these products absorbed?

Answer 15

Monosaccharides

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

Question 16

What are the products of lipid digestion?

How are these products absorbed?

Answer 16

Monoglycerides and fatty acids

• Micelles help to move monoglycerides and fatty acids towards the epithelium.

Because micelles constantly break up and reform
they can ‘release’ monoglycerides and fatty acids,
allowing them to be absorbed –
whole micelles are not taken up across the epithelium.

Monoglycerides and fatty acids are lipid-soluble, so can diffuse directly across the epithelial cell membrane.

Question 17

What are the products of protein digestion?

How are these products absorbed?

Answer 17

Amino acids

These are absorbed via co-transport, in a similar way to glucose and galactose.

Sodium ions are actively transported out of the ileum epithelial cells into the blood.

This creates a sodium ion concentration gradent. Sodium ions can then diffuse from the lumen of the ileum into the epithelial cells through sodium-dependent transporter proteins,
carrying amino acids with them.