Digestion and Absorption

Question 1

How is starch hydrolysed ?

Answer 1

• Salivary amylase breaks down some of it into maltose in the mouth
• Pancreatic amylase hydrolyses the rest (into maltose) in the small intestine (Duodenum)

Question 2

Where does Starch get completely hydrolysed?

Answer 2

• in the ileum,the disaccharidases maltase, lactase and sucrase hydrolyse starch into alpha glucose
• the enzymes form part of the cell-surface membrane of epithelial cells lining the ileum.

Question 3

How and where are lipids hydrolysed?

Answer 3

• the enzyme lipase is secreted by the pancreas into the small intestine and breaks them down into glycerol, fatty acids and monoglycerides by hydrolysing the ester bonds.

Question 4

How are digested lipids absorbed (extended answer)

Answer 4

• bile salts emulsify the lipids into small droplets increasing the rate of hydrolysis
• bile salts combine with fatty acids and monoglycerides to form micelles to transport the poorly soluble monoglycerides and fatty acids to the surface of the epithelial cell
• the micelles break down next to the epithelial cell to release the fatty acids and monoglycerides. They are non polar and are therefore able to diffuse through the membrane
• fatty acids, monoglycerides and glycerol are recombined in the Smooth Endoplasmic Reticulum to form triglycerides
• they are packaged in protein and phospholipids by the Golgi body to form chylomicrons
• chylomicrons are absorbed into the lacteals in the villi which eventually drain into the blood

Question 5

How are proteins hydrolysed?

Answer 5

• they are hydrolysed by endopeptidases, exopeptidases and dipeptidases to form amino acids which are then absorbed in the ileum

Question 6

How do Bile Salts speed up lipid hydrolysis?

Answer 6

• they emulsify the lipids causing them to form small droplets which increases their surface area

Question 7

Describe the hydrolysis of proteins

Answer 7

• Endopeptidases are released in the stomach and hydrolyse internal peptide bonds to form small polypeptides and peptides
• the pancreas then secretes exopeptidases which hydrolyse the peptide bonds at either end of a polypeptide so that individual amino acids or dipeptides are formed
• finally dipeptidases are membrane bound enzymes in the microvilli of the ileum and hydrolyse the bond in a dipeptide to release two single amino acids

Question 8

How is the ileum adapted for absorption? (6)

Answer 8

• large surface area due to its long length and the presence of villi and microvilli
• large number of mitochondria = more ATP for active transport
• carrier and channel proteins in the cell surface membrane for absorption of specific molecules by active transport and facilitated diffusion
• the wall of each villus has a single layer of epithelial cells providing a short diffusion pathway
• villi contain blood capillaries that absorb monosaccharides and amino acids maintaining a high diffusion gradient for further absorption
• the lacteals in the villi absorb digested lipids maintaining a high diffusion gradient for further absorption

Question 9

How are monosaccharides and amino acids absorbed?

Answer 9

• via co-transport

Question 9

Describe how monosaccharides and amino acids are absorbed via co-transport (extended answer)

Answer 9

• sodium ions are actively transported out of the epithelial cells and into the blood via carrier proteins
• a conc gradient is created for the sodium to diffuse into the epithelial cells through a co-transport protein and glucose will move through the co-transport protein with the sodium
• the glucose then moves out of the cell towards the blood capillary via facilitated diffusion through another protein