Topic 6: Absorption

What enzymes does the pancreas secrete into the lumen of the stomach?

What enzymes does the pancreas secrete into the lumen of the stomach?

1. Amylase

2. Lipase

3. phospholipase

4. protease (endopeptidases and exopeptidases)

What enzymes do the epithelial cells of the Small Intestine secrete?

1. Nucleases

2. Lactase

3. Sucrase

4. Matlase

5. Dipeptidases

How are proteins digested?

1. The pancreas secretes endopeptidases and exopeptidases which hydrolse peptide bonds within polypeptide chains and at the ends of polypeptide chains. This creates dipeptides.

2. The epithelial cells of the Small Intestine secrete dipeptidases which hydrolyse dipeptide bonds, creating amino acids.

How are lipids digested?

1. Pancreas secretes lipase, digestion occurs in small intestine lumen. Lipase breaks down lipids into glycerol and fatty acids or monoglycerides and fatty acids.

What process aids lipid digestion?

emulsification via bile salts which are secreted by the gall bladder and bind to fatty acids, increasing their surface area.

What enzymes digest starches?

1. Amylase hydrolyses the 1,4 glycosidic bond found in amylose and amylopectin.

   Amylose —> maltose

   Amylopectin → dextrins

2. Immobilised dextrase and maltase are found in membrane of epithelium cells.

Dextrase: hydrolyses the 1,6 glycosidic bond in amylopectin.

Maltase: hydrolyses maltose →glucose

How are nucleic acids digested?

1. nucleases in the SI break the phosphodiester bonds between the nucleotide bases.

Nucleic acid → nucleotides

How are amino acids absorbed by the villus?

• amino acids diffuse into the epithelium cell

• amino acids enter the capillaries via facilitated diffusion.

• amino acid co-transport proteins pump one sodium ion and one amino acid into the capillary.

• sodium-potassium pumps maintain the concentration gradient of Na+ ions, using active transport.

How is glucose absorbed by the villus?

1. Sodium ions and glucose molecules are co-transported into the epithelial cells via facilitated diffusion

2. This is a passive process but depends on the concentration gradient of sodium ions from the lumen of the ileum into the epithelial cell

3. The gradient is maintained by the active transport of sodium ions out of the cell and into the blood via a sodium-potassium pump at the capillary end of the cell

4. The glucose molecules diffuse across the epithelial cell and enter the capillary by facilitated diffusion through a glucose channel protein

How are lipids and monoglycerides absorbed by the villus and into the capillaries?

1. Fatty acids and monoglycerides can enter the epithelial cell by simple diffusion

2. They are non-polar molecules so they can diffuse through the phospholipid bilayer of the cell surface membrane

3. Fatty acids also move by facilitated diffusion through fatty acid transport proteins

4. Inside the epithelial cell, fatty acid chains recombine with monoglycerides or glycerol to form triglycerides, which are unable to diffuse back into the lumen

5. The triglycerides are packaged up with cholesterol and encased in phospholipids and proteins to form lipoproteins

6. These droplets then enter the lacteal or capillaries via exocytosis