Lecture 29: Chemical digestion

Question 1

What is chemical digestion?

Why do we need chemical digestion?

Answer 1

Chemical digestion is the chemical hydrolysis of good by enzyme - breakdown of food into its basic molecules by enzymes so that they are of an appropriate size for absorption

As it reduces the nutrients to as size that allows them to cross the epithelial lining of GI tract

Question 2

Chemical digestion is a two step process, what are the two step?

Answer 2

1. Luminal digestion
   Initial digestion involves enzymes being secreted into the lumen
2. Contact digestion
   - brushborder villi = site of contact digestion
   - increases surface area therefore increases the amount of enzymes for contact digestion
   - digestion is completed by enzymes produced by enterocytes which are attached to the brush border vlli

Question 3

How are carbohydrates chemically digested?

Answer 3

• Salivary and pancreatic amylase digest polysaccharides down to disaccharides = luminal digestion
• Disaccharides are then broken down into monosaccharides by disaccharidases which are enzymes that break down disaccharides
• Suncrase, maltase and lactase are disaccharides which are bound to the brush borders of the small intestine = contact digestion

Question 4

How are proteins chemically digested?

Answer 4

Luminal digestion
- Pepsin in stomach
- Trypsin and chymotrypsin are secreted in by the pancreas and work in the small intestine
= converts proteins into polypeptides and proteases
Contact digestion
-Involves peptidases which are attached to the brush border os the small intestine, they convert polypeptides to individual amino acids

Question 5

How are lipids chemically digested?

What are the phases of chemical digestion?

Answer 5

There is NO contact digestion

1. Emulsification of lipids
2. Stabilisation
3. Hydrolysis
4. Formation of micelles

Question 6

Chemical digestion of lipids

- explain emulsification of lipids

Answer 6

• Motility breaks large fat droplets into smaller droplets
• Mechanical digestion increases surface area for digestion
• Occurs in stomach = retropuslion and small intestine = segmentation

Question 7

Chemical digestion of lipids

- explain stabilisation

Answer 7

Lecithin and bile salts

• are secreted in bile and function of stabilise the emulsion droplets
• allow formation of smaller droplets which increase surface area further

Question 8

Chemical digestion of lipids

- explain hydrolysis

Answer 8

• occurs in the lumen of the small intestine at the surface of emulsion droplets
• involves lipase and co-lipase = secreted by pancreas
  Co-lipase anchors lipase to the surface of emulsion droplets where lipase converts triglycerides to monoglycerides and free fatty acids

Question 9

Chemical digestion of lipids

- Formation of micelle

Answer 9

Products of at are insoluble in water so they are kept in solution through the formation of micelles
MICELLES are small fat droplets consisting of 20-30 molecules
- bile salts/lecithin = amphipathic compounds
- fatty acids
- monoglycerides

Question 10

What are the main features of carbohydrates?

Answer 10

• Average amount ingested = 250-800g per day
• important source of energy
• consist of chains of monosaccharides
  INGESTED CARBS
• starch and glycogen = storgage polysaccharides a-1-4 glycosidic bonds
• disaccharides = suncrose, lactose, maltose
• Cellulose = cannot be digested b-1-4 glycosidic bonds

Question 11

What are the main features of proteins ?

Answer 11

• Ingest 70-100 per day in our food
• Require 10 essential amino acids in diet
  INGESTED PROTEINS
• long chain of amino acids linked by peptide bonds

Question 12

What are the main features of lipids?

Answer 12

• Ingest 100-150g per day

- Consists mainly of triglycerides - glycerol backbone with 3 fatty acids attached by ester bonds