Digestion and absorption 3.3.3

• What is digestion 


  Hydrolyses large insoluble molecules into smaller soluble ones
• How are proteins digested 


  Proteins are large complex molecules hydrolysed by group of enzymes

  Endopeptidases - hydrolyses peptide bonds between amino acids in the middle of polymer chain

  Exopeptidases - hydrolyses peptide bonds between amino acids at the end of polymer chain

  Dipeptidases (membrane bound) - hydrolyses peptide bonds between 2 amino acids.
• How are lipids digested


  Lipids hydrolysed by enzyme lipase. Lipase is a enzyme produced in the pancreas that hydrolyses ester bond found in triglycerides to form fatty acids and monoglycerides.
• Lipids digestion 


  Split into tiny droplets by bile salts into micelles. This process is called emulsification which increases surface area of droplets for faster hydrolyses. Micelles are water soluble vehicles that contain bile salts, fatty acids and monoglycerides. Micelles release fatty acids and monoglycerides to the lining of the ileum. Fatty acids and monoglyceries are absorbed by diffusion due to non polar nature.

  Triglycerides reformed inside of the endoplasmic reticulum and golgi body.

  Vesicles move to cell membrane.
• Describe the mechanism for absorption of monosaccharides.


  Sodium ions and glucose molecules are co-transported into epithelial cell by facilitated diffusion.

  Glucose molecules diffuse across the epithelial cell and enter the capillary at the other end of cell by facilitated diffusion.

  Concentration gradient of sodium ions is maintained by actively transporting the sodium ions out of epithelial cell into the blood.
• Mechanism for absorption of amino acids


  1. Specific amino acid co-transport proteins found within cell surface membrane of epithelial cell in ileum

  2. Amino acid transported when sodium ions present

  3. For every sodium ion transported into cell, an amino acid transported

  4. Occurs via facilitated diffusion down concentration gradient

  5. Amino acid diffuse across epithelial cell and then enter capillary via facilitated diffusion

  6. Concentration of sodium ions maintained by actively transporting out of epithelial cells into blood
• Use knowledge of lipid digestion to explain differences in results (3)

  Decrease in triglycerides because ester bond hydrolysed by action of lipase

  Triglycerides decrease because of hydrolysis of triglycerides

  Fatty acids increase because of digestion of ester bonds
• SDs question

  No significant difference in protein absorption

  Because standard deviations overlap

  So mean % absorption is not affected by % of protein in diet so something else is a limiting factors e.g. protease
• How is golgi apparatus involved in the absorption of lipids

  Modifies triglycerides

  Combines triglycerides with proteins

  Packaged for release/excytosis or forms vesicles
• Benedicts solution - amylase 


  Starch hydrolysed into maltose

  Maltose hydrolysed into glucose

  By maltose

  To be absorbed into the bloodstream
• Explain why maltase catalyses only one reaction 


  Maltase has a specific tertiary structure/complementary in shape, so only maltose can bind to to active site

  To form an ES complex
• Where is maltase found


  Maltase is a membrane bound dissacharide found in the lining of the ilem/small intestine.