Topic 3---B: More Exchange and Transport Systems- 1. Digestion and Absorption

Question 1

Why do large biological molecules need to be broken down?

Answer 1

• Large biological molecules are too big to cross cell membranes.
• So they can’t be absorbed from the gut into the blood.
• So during digestion, the large molecules are broken down into smaller molecules which can move across cell membranes
• So they can easily be absorbed from the gut into the blood.

Question 2

What are used to break down biological molecules?

Answer 2

Digestive enzymes
- Since enzymes only work with a specific substrate different enzymes are needed to catalyse the breakdown of different food molecules

Question 3

The digestion of carbohydrates

Answer 3

Amylase- It works by:
- Hydrolysing the glycosidic bonds in starch to produce maltose
Membrane-bound disaccharides- It works by:
- Hydrolyses disaccharides into monosaccharides

Question 4

The digestion of lipids

Answer 4

1. Bile salts emulsify lipid droplets into smaller droplets- this provides a larger surface area for lipase to hydrolyse ester bonds.
2. Lipase hydrolyses ester bonds releasing monoglycerides and fatty acids.
3. Fatty acids, monoglycerides, bile salts form micelles and then fatty acids and monoglycerides are absorbed into the epithelium cells via micelles by simple diffusion as they are lipsoluble.
4. In the epithelium cells, fatty acids and monoglycerides are reformed into triglycerides in the smooth endoplasmic reticulum
5. Proteins/ cholesterol/ phospholipids are added to the triglycerides in the golgi apparatus called chylomicrons
6. They are packaged into a vesicle by the golgi which released chylomicrons into the intercellular space
7. They enter the lymphatic system which transports chylomicrons away from the intestine

Question 5

The digestion of proteins

Answer 5

• There broken down by a combination of different peptidases.
  (enzymes that catalyse the conversion of proteins into amino acids) by hydrolysing the peptide bond between amino acids

Question 6

Endopeptidases (protein digestion)

Answer 6

They hydrolyse peptide bonds within a polypeptide chain to produce smaller chains (more ends= faster hydrolysis)

Question 7

Exopeptidases (protein digetion)

Answer 7

They hydrolyse terminal peptide bonds

Question 8

Membrane bound dipeptidases

Answer 8

• They are exopeptidases that work specifically on dipeptidases.
• They separate the 2 amino acids that make up a dipeptide by hydrolysing the peptide bond between them.