❌Exchange: Enzymes And Digestion

Question 1

What are the uses of physical breakdown in digestion?

Answer 1

To make the food smaller so it can be ingested and to give it a larger surface area for chemical digestion to occur more efficiently

Question 2

Name the three important digestive enzymes and what the hydrolyse

Answer 2

• carbohydrases hydrolyse carbohydrates into monosaccharides
• lipases hydrolyse lipids into glycerol and fatty acids
• proteases hydrolyse proteins into amino acids

Question 3

Go over the hydrolysis of starch

Answer 3

1. saliva enters the mouth from the salivary glands and is throughly mixed with food during chewing
2. salivary amylase in the saliva starts hydrolysing starch into maltose and contains mineral salts to help maintain the pH at around neutral
3. the food enters the stomach where the acid denatures the amylase and prevents further hydrolysis of starch
4. the food then passes onto the small intestine where it mixes with pancreatic juice which contains pancreatic amylase
5. the pancreatic amylase continues the hydrolysis of any remaining starch into maltose as alkaline salts are produced by the pancreas and intestine wall to maintain the pH at neutral for amylase to function
6. The epithelial lining then produces the membrane bound disaccharidase maltase which is part of the cell membrane of the epithelium and the maltase hydrolysed the maltose into alpha glucose

Question 4

Other than maltase, give two other membrane bound disaccharidases and what they hydrolyse

Answer 4

• sucrase hydrolyses the single glycosidic bond in sucrose to produce glucose and fructose
• lactase hydrolyses the single glycosidic bond in lactose to produce glucose and galactose

Question 5

Outline the digestion of lipids

Answer 5

1. The lipids undergo physical digestion and are split up and dispersed into micelles by bile salts produced in the liver. The bile emulsifies the lipid and produces a larger surface area for the lipase to act on
2. The micelles then undergo chemical digestion as the lipases in the pancreatic juice in the duodenum hydrolyse the ester bonds in triglycerides to produce glycerol, fatty acids and monoglycerides

Question 6

Outline the digestion of proteins through the three peptidases that hydrolyse them

Answer 6

1. Endopeptidases like pepsin and trypsin hydrolyse peptide bonds in the middle of the polypeptide chain to produce a series of shorter polypeptides
2. Exopeptidases hydrolyse peptide bonds at the ends of the polypeptide to produce dipeptides or single amino acids
3. Dipeptidases are membrane bound being part of the cell surface membrane of epithelial cells lining the ileum and hydrolyse any remaining dipeptides into single amino acids

Question 7

How do the properties of the villi which are situated at the interface between the lumen of the intestines and the blood and other tosses inside the body increase the efficiency of absorption?
(There are 5 ways)

Answer 7

• they increase the surface area for diffusion
• they are very thin walled so reduce the distance over which diffusion takes place
• they contain muscle so are able to move which maintains the diffusion gradient as the movement mixes the contents of the ileum ensuring the products of digestion which are absorbed from the food are being replaced by new material rich in the products of digestion
• they are well supplied with blood vessels so blood can carry away absorbed molecules and hence maintain a diffusion gradient
• the epithelial cells lining the villi have microvilli which further increase the surface area for absorption

Question 8

Outline the absorption of triglycerides from the formation of micelles

Answer 8

1. The micelles which were formed by the bile salts come into contact with the epithelial cells lining the villi of the ileum.
2. The micelles then break down releasing the monoglycerides and fatty acids and as they are polar molecules, they easily diffuse across the cell surface membrane, into the epithelial cells
3. The monoglycerides and fatty acids are then transported to the endoplasmic reticulum where they are recombined to form triglycerides
4. Starting in the er, and into the Golgi, the triglycerides associate with cholesterol and lipoproteins to form chylomicrons which are particles adapted for the transport of lipids
5. The chylomicrons them move out of the epithelial cells by exocytosis and enter lacteals which are lymphatic capillaries found at the centre of each villus
6. The chylomicrons then pass via lymphatic vessels into the blood system and the triglycerides in the chylomicrons are hydrolysed by an enzyme in the endothelial cells of blood capillaries from where they diffuse into cells