3.3.3 Digestion and Absorption

Question 1

What happens during digestion?

Answer 1

Large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell membranes

Question 2

Molecules that are digested in mammals

Answer 2

• Carbohydrates - by amylases + membrane-bound disaccharidases
• Lipids - by lipase, including the action of bile salts
• Proteins- by endopeptidases, exopeptidases, and membrane-bound peptidases

Question 3

Major parts of the digestive system

Answer 3

• Oesophagus
• Stomach
• Ileum
• Large intestine
• Rectum
• Salivary glands
• Pancreas

Question 4

Oesophagus

Answer 4

Carries food from mouth->stomach

Question 5

Stomach

Answer 5

• Muscular sac with an inner layer that produces enzymes
• Stores + digests food
• Has glands that produce enzymes that digest proteins

Question 6

Ileum

Answer 6

• Long muscular tube
• Food is further digested here by enzymes that are produced by its walls and glands
• Inner walls are folded into villi (which have microvilli) = large S.A.

Question 7

Why are villi and microvilli important?

Answer 7

Increases S.A. that adapts ileum for its purpose of absorbing the products of digestion into the bloodstream

Question 8

Large intestine

Answer 8

Absorbs water

Question 9

Rectum

Answer 9

• Final section of the intestines
• Stores faeces before being periodically removed via the anus

Question 10

Egestion

Answer 10

Removal of faeces from rectum

Question 11

Salivary glands

Answer 11

• In mouth
• Pass their secretion via a duct (contains amylase)

Question 12

Pancreas

Answer 12

• Large gland situated below the stomach
• Produces pancreatic juice

Question 13

What does pancreatic juice contain

Answer 13

Proteases
Lipase
Amylase

Question 14

By what process do enzymes break down a large molecule into its products?

Answer 14

• Hydrolysis

Question 15

Types of digestion

Answer 15

• Physical digestion
• Chemical digestion

Question 16

Physical digestion

Answer 16

Large food is broken down into smaller pieces mechanically (by teeth and churning in the stomach)

Question 17

Chemical digestion

Answer 17

Hydrolyses large, insoluble molecules into smaller, soluble ones

Question 18

How do enzymes work

Answer 18

Multiple enzymes are needed to break down a large insoluble molecule

• 1 enzyme will break a large molecule into smaller sections
• Another enzyme will break these sections into its monomers

Question 19

Types of digestive enzymes

Answer 19

• Carbohydrases
• Lipases
• Proteases

Question 20

Carbohydrases

Answer 20

Hydrolyses carbohydrates -> monosaccharides

Question 21

Lipases

Answer 21

Hydrolyse lipids -> fatty acids + monoglyceride

Question 22

Proteases

Answer 22

Hydrolyses proteins -> amino acids

Question 23

Carbohydrate digestion

Answer 23

• Amylase in mouth + pancreas: starch -> maltose (disaccharide). Does this by hydrolysing glycosidic bonds in starch
• Maltase in ileum: maltase -> alpha glucose

Question 24

Process of carbohydrate digestion in humans

Answer 24

1. Salivary amylase in mouth hydrolyses starch -> maltose
2. Food enters stomach. HCl in stomach denatures amylase
3. Food enters small intestine: mixes with pancreatic juice (contains pancreatic amylase)
4. Muscles in intestinal wall push food along ileum (peristalsis).
5. Maltase is membrane bound in the intestine. Maltose -> alpha glucose

Question 25

Other common disaccharides that are digested:

Answer 25

• Sucrase -> hydrolyses single glycosidic bond in sucrose -> glucose + fructose
• Lactase -> hydrolyses single glycosidic bond in lactose -> glucose + galactose

Question 26

Lipid digestion

Answer 26

• Lipases - hydrolyse ester bond in triglycerides -> fatty acids + glycerol

Question 27

Monoglyceride

Answer 27

Glycerol + 1 fatty acid

Question 28

Emulsification

Answer 28

• Bile salts split up lipids into micelles (tiny droplets)
• Surface area -> for lipase action

Question 29

Protein digestion

Answer 29

Peptidases (proteases)

• Endopeptidases
• Exopeptidases
• Dipeptidases

Question 30

Endopeptidases

Answer 30

Hydrolyse peptide bond between amino acids in the central region of a protein

Question 31

Exopeptidases

Answer 31

Hydrolyse peptide bond on the terminal amino acids of the peptide molecule formed by the endopeptidases

Question 32

Dipeptidases

Answer 32

Hydrolyse peptide bond between 2 amino acids of a dipeptide

Question 33

Characteristic of dipeptidases + disaccharidase

Answer 33

Membrane-bound: part of the cell-surface membrane of the epithelial cells lining the ileum

Question 34

Villi in the ileum

Answer 34

• Increase S.A. for diffusion
• Thin walls: short diffusion distance
• Contain muscle: so can move, which maintains diffusion gradient
• Good blood supply: maintains diffusion gradient

Question 35

Absorption of amino acids + monosaccharides

Answer 35

Co-transport

Question 36

Absorption of triglycerides

Answer 36

1. Micelles (contain bile salts and fatty acids) come into contact with epithelial cells (lining the villi)
2. Micelles break down -> releasing monoglyceride + fatty acids
3. These are non-polar so easily diffuse across the cell-surface membrane into the epithelial cells by simple diffusion
4. Inside the epithelial cells, the monoglycerides + fatty acids are transported to E.R. where they recombine to form triglycerides
5. Triglycerides associate with cholestrol + lipoproteins to form chylomicrons
6. Chylomicrons move out of the epithelial cells by exocytosis and move to cell membrane as vesicles
7. They enter lacteals
8. They pass into blood system
9. Triglycerides are hydrolysed by an enzyme in the endothelial cells of blood capillaries

Question 37

Lacteals

Answer 37

Lymphatic capillaries found at the centre of a villus