3.3 Digestion and Absorption

Question 1

What is digestion?

Answer 1

The process by which large insoluble molecules are hydrolysed into smaller, soluble molecules that can be absorbed across the cell membranes into the bloodstream and delivered to cells in the body.

Question 2

Describe the human digestive system.

Answer 2

1. Mouth - chewing, mechanical digestion
2. Salivary glands - produces salivary amylase to break down the food
3. Oesophagus - peristalsis moves food down to the stomach
4. Stomach - digestion begins here, churning of food (mixed with enzymes)
5. Small intestine - site of food absorption
6. Large intestine - site of water reabsorption

• Pancreas (produces and secretes enzymes → amylase, protease, lipase)
• Liver (produces bile)
• Bile (neutralises stomach acid and emulsifies fats)

Question 3

Outline the digestion of carbohydrates.

Answer 3

Takes place in the mouth and small intestine
- amylase hydrolyses starch into maltose
- the disaccharidase maltase catalyses the hydrolysis of maltose into glucose (membranes of the epithelial cells lining the small intestine as maltase is membrane bound)

Question 4

Outline the digestion of proteins.

Answer 4

1. Protein digestion begins at the lumen of the stomach.
2. Endopeptidase hydrolyses the peptide bonds in the middle of the polypeptide to make smaller protein “chunks”.
3. Exopeptidase hydrolyses the peptide bonds between amino acids at the end of a polypeptide
4. Membrane bound dipeptidase which are found in membrane of epithelial cell in ileum hydrolyse the peptide bond between 2 amino acids

Question 5

Outline the digestion of lipids.

Answer 5

Emulsification:
1. Lipids are coated in bile salts to create an emulsion
2.Emulsions causes lipids to split into small droplets, forming micelles (provides a large SA to enable faster hydrolysis action by lipase
Digestion:
- takes place solely in the lumen of the small intestine
- lipase hydrolyse lipids into glycerol and fatty acids by breaking ester bonds

Question 6

Describe the co-transport mechanisms for the absorption of amino acids andmonosaccharides.

Answer 6

• glucose and Na+ are cotransported into epithelial cell as there is higher conc in cell, then actively transported out of epeithelial cell into bloodstream
• amino acids are cotransported into epithelial cell with Na+ and is then actively transported out of epithelial cell into blood stream

Question 7

Outline lipid absorption

Answer 7

1. Micelles containing bile salts and fatty acids
2. Make fatty acids more soluble in water and bring them to cell lining of ileum
3. Fatty acids absorbed by diffusion
4. Triglycerides reformed in cells
5. Vesicles move to cell membrane

Question 8

Where is Amylase produced and released?

Answer 8

Produced: pancreas and salivary glands
Released: mouth and small intestines

Question 9

Where is Maltase produced and released?

Answer 9

Produced: small intestine
Released: small intestine

Question 10

Where is endopeptidase/exopeptidase produced and released?

Answer 10

Produced: pancreas
Released in: small intestine

Question 11

Where is dipeptidase produced and released?

Answer 11

Produced: small intestine
Released: small intestine

Question 12

Where is lipase produced and released?

Answer 12

Produced: pancreas
Released: small intestine

Question 13

How do micelles help lipid absorption

Answer 13

• make the fatty a cuds more soluble in water
• carry fatty acids to the epithelial cells of ileum
• help maintain higher concentration gradient of fatty acids compared to inside the epithelial cells of ileum

Question 14

Advantages of the ileum for efficient absorption

Answer 14

• covered in microvilli → which have thin walls surrounded by a network of capillaries
• epethial cells have even smaller microvilli
• ADV: increases the surface area so shorter diffusion distanc