Lecture 26: CHEMICAL DIGESTION

Question 1

What are the ain nutrients that undergo chemical digestion?

Answer 1

Carbohydrates (sugars), proteins nd lipids (fats)

Question 2

What are carbohydrates important for?

Answer 2

A source of energy

Question 3

What are carbohydrates made of?

Answer 3

Large storage polysaccharides and complex chains of monosaccharides

Question 4

What is the most common carbohydrate?

Answer 4

Starch and also glycogen

Question 5

What are starch and glycogen?

Answer 5

Long chains of glucose joined by alpha 1,4 glycosidic linkage

Question 6

What are some disaccharides?

Answer 6

Sucrose, lactose and maltose

Question 7

What is sucrose made of?

Answer 7

Glucose and fructose

Question 8

What is lactose made of?

Answer 8

Glucose and galactose

Question 9

What is maltose made of?

Answer 9

2 glucose

Question 10

What doe we ingest a limited amount of?

Answer 10

Monosaccharides (glucose)

Question 11

Are proteins a source of energy?

Answer 11

No

Question 12

What are proteins required for?

Answer 12

Amino acids

Question 13

How many amino acids are there?

Answer 13

20 - 12 can be synthesised but other essentials such as His, Lys and Leu can’t so must come from the diet

Question 14

What are sources of protein?

Answer 14

50% diet and 50% endogenous (enzymes and immunoglobulins which have been secreted into the lumen)

Question 15

What is the structure of ingested proteins?

Answer 15

Long chains of amino acids linked by peptide bonds

Question 16

Are lipids essential?

Answer 16

No

Question 17

What are lipids an important source of?

Answer 17

Energy and fat soluble vitamins A, D, E and K

Question 18

What effect do lipids have on gastric emptying?

Answer 18

Slow it

Question 19

What are lipids mainly?

Answer 19

Triglycerides (glycerol backbone with 3 fatty acids attached)

Question 20

What can fatty acids be?

Answer 20

Variable in chain length

Question 21

How many carbons in short chain fatty acids?

Answer 21

Less than 6

Question 22

How many carbons in medium chain fatty acids?

Answer 22

6-12

Question 23

How many carbons in long chain fatty acids?

Answer 23

12-24

Question 24

Why do we need chemical digestion?

Answer 24

We ingest nutrients in the form of large complex molecules but can only absorb nutrients as small molecules

Question 25

What does chemical digestion do?

Answer 25

Decreases the size of nutrients to allow them to be absorbed

Question 26

Where does chemical digestion occur?

Answer 26

At the surface of food particles

Question 27

What does mechanical digestion do?

Answer 27

Breaks up food and increases surface area available for chemical digestion

Question 28

What does chemical digestion utilise?

Answer 28

Digestive enzymes

Question 29

What are digestive enzymes?

Answer 29

Extracellular organic catalysts (E+S<>ES complex<>E+P)

Question 30

How are digestive enzymes described?

Answer 30

Very specific

Question 31

What is needed with regards to digestive enzymes?

Answer 31

Many different enzymes for different substrates (amylase, protease, lipase)

Question 32

What do enzymes have?

Answer 32

An optimal pH

Question 33

What is the optimal pH for salivary enzymes?

Answer 33

Alkaline

Question 34

What is the optimal pH for gastric enzymes?

Answer 34

Acidic

Question 35

What is the optimal pH for small intestinal enzymes?

Answer 35

Alkaline

Question 36

What is there large amounts of in diet?

Answer 36

Cellulose

Question 37

What is cellulose?

Answer 37

A structural polysaccharide of plants. Long chains of beta 1,4 glycosidic bonds which cant be broken down but provide fibre

Question 38

What are the stages of chemical digestion?

Answer 38

Luminal digestion and contact digestion

Question 39

What is involved in luminal digestion?

Answer 39

Enzymes secreted into the lumen

Question 40

What is contact digestion?

Answer 40

In the small intestine involving enzymes produced by enterocytes and attached to the brush border of enterocytes

Question 41

What enzymes are involved in luminal digestion of carbohydrates?

Answer 41

salivary and pancreatic amylase

Question 42

What happens in luminal digestion of carbohydrates?

Answer 42

Polysaccharides are converted to oligosaccharides and disaccharides

Question 43

What happens in contact digestion of carbohydrates?

Answer 43

Disaccharides are converted to monosaccharides

Question 44

What enzymes does contact digestion of carbohydrates involve?

Answer 44

Sucrase, lactase and maltase

Question 45

Where are enzymes bound?

Answer 45

To the brush border and when shed become part of the protein intake

Question 46

What enzymes are involved in luminal digestion of proteins?

Answer 46

Pepsin in the stomach and trypsin, chymotrypsin and carboxypeptidase in the small intestine

Question 47

What happens in luminal digestion of proteins?

Answer 47

Proteins are converted into polypeptides

Question 48

What enzymes are involved in contact digestion of proteins?

Answer 48

Many types of peptidases attached to the brush border

Question 49

What happens in contact digestion of proteins?

Answer 49

Polypeptides are converted into individual amino acids

Question 50

Where does chemical digestion of lipids occur?

Answer 50

In the small intestine

Question 51

What nutrient doesn't have contact digestion?

Answer 51

Lipids

Question 52

What is the main and minor enzymes for chemical digestion of lipids?

Answer 52

Mainly pancreatic lipase but also lingual lipase and gastric lipase have minor roles

Question 53

What is the problem with lipid digestion?

Answer 53

Digestive enzymes are dissolved in the luminal fluid. This is no problem for carbohydrates and proteins as they are water soluble but as lipids are insoluble in water a more complex process is required

Question 54

What are the stages of chemical digestion of lipids?

Answer 54

Emulsification- motility, stabilisation - bile salts, digestion (hydrolysis)- enzymes, formation of micelles - bile salts

Question 55

What happens in emulsification?

Answer 55

Motility breaks up lipid droplets into small droplets to form an emulsion

Question 56

What is the size of emulsion droplets?

Answer 56

0.5-1.0 micrometres

Question 57

What does emulsification do?

Answer 57

Increase the surface area for digestion

Question 58

Where does emulsification occur?

Answer 58

Stomach (retropulsion) to form a simple emulsion and small intestine (segmentation) to form a more complex emulsion where bile salts stabilise (suspend in fluid) the droplets

Question 59

Where does stabilisation occur?

Answer 59

In the small intestine

Question 60

What do bile salts have?

Answer 60

A hydrophobic (water hating) and a negatively charged hydrophilic side (water loving) = amphipathic

Question 61

What do bile salts do?

Answer 61

Stabilise the emulsion in the small intestine and also reduce the size of the emulsion droplets to further increase surface area for absorption

Question 62

Where does hydrolysis occur?

Answer 62

In the small intestine at the surface of emulsion droplets

Question 63

What does hydrolysis involve?

Answer 63

Lipase and cofactor lipase which are both secreted by the pancreas

Question 64

What does colipase do?

Answer 64

Anchors lipase to the surface of droplets

Question 65

What does lipase do?

Answer 65

Converts triglycerides to monoglycerides and free fatty acids

Question 66

What are the products of fat digestion?

Answer 66

Insoluble in water (especially monoglycerides and long chain fatty acids)

Question 67

How are products of fat digestion kept in solution?

Answer 67

Through formation of micelles

Question 68

What are micelles?

Answer 68

Small droplets (4-6 nanometer diameter) which consist of 20-30 molecules (amphipathic bile salts, fatty acids and monoglycerides)