Absorption
Biology: AQA A Level, topic 3.3.3
Co-Transporters
After the larger molecules have been hydrolysed into monomers, they must be absorbed by the cells. Amino acids and monosaccharides are absorbed through epithelial cells. This involves co-transporters.
Active transport of sodium ions
- Sodium-dependent co-transporter proteins are located in the epithelial cell membranes.
- The co-transporter proteins actively transport sodium ions into the blood.
- This causes the concentration of sodium ions in the epithelial cells to decrease.
Diffusion of sodium ions
- The decreased concentration of sodium ions in the epithelial cells causes sodium ions in the ileum to diffuse down their concentration gradient.
- The sodium ions diffuse through a co-transporter protein in the cell surface membrane.
Binding of amino acids & sugars
- When sodium ions bind to a co-transporter protein, amino acids or monosaccharides also bind to the protein.
- Binding of amino acids or monosaccharides causes the transporter protein to undergo a conformational change.
- The amino acids or monosaccharides, along with the sodium ions are transported into the epithelial cell cytoplasm.
Use
- Once inside the epithelial cells, the amino acids or monosaccharides can then be used inside the epithelial cells or absorbed into the bloodstream.
Micelles
Lipids are absorbed from the ileum through epithelial cells. This involves the formation of micelles.
Formation
- After larger, lipid molecules have been broken down into monoglycerides and fatty acids, bile salts are secreted by the liver.
- The bile salts associate with the monoglycerides and fatty acids to form micelles.
- Micelles are circular formations that are made up of phospholipid tails.
Incorporation
- The micelles move through the ileum and come into contact with the epithelial cells.
- The micelles incorporate themselves into the epithelial cell surface membrane and are absorbed.
Test yourself on Absorption
Which of the following statements about co-transporters are true?
What happens to the co-transporter proteins when amino acids or monosaccharides bind?
What substances are secreted by the liver to form micelles?
Which of the following statements about micelles are true?