the digestive system

• Digestion


  Large biomolecules hydrolysed into smaller ones that can be absorbed across cell membrane
• Amylase


  Enzyme produced in pancreas & salivary glands that hydrolyses polysaccharides (such as starch) into disaccharides (e.g. maltose) by hydrolysing glycosidic bonds
• Disaccharidases


  Membrane-bound enzymes (e.g. sucrase, lactase) found in duodenum & ileum that hydrolyse disaccharides into monosaccharides
• Protein digestion


  1. Endopeptidases hydrolyse peptide bonds between amino acids in middle of polymer chain

  2. Exopeptidases hydrolyse peptide bonds between amino acids at end of a polymer chain

  3. Membrane-bound dipeptidases hydrolyse peptide bonds between two amino acids (dipeptide)
• Protein digestion


  Starts in stomach, continuing into duodenum (first part of small intestine) & fully digested in ileum (end of small intestine)
• Absorption

  Products of digestion absorbed across cells lining ileum (small intestine)

  - covered in villi that's covered in microvilli which increases surface area, decreases diffusion distance
• Monosaccharides & amino acids


  Absorbed by co-transport, some diffused by facilitated diffusion but must be higher concentration in lumen than epithelial cell
• Bile salts


  Produced in liver, emulsify lipids into tiny droplets called micelles, increases surface area, faster hydrolysis action (more enzyme-substrate complexes)
• Lipase


  Enzyme produced in pancreas that hydrolyses ester bonds in triglycerides, forming monoglycerides & fatty acids
• Micelles


  Water-soluble vesicles made up of fatty acids, glycerol, monoglycerides & bile salts that deliver these to epithelial cells of the ileum for absorption
• Lipid absorption


  1. Fatty acids & monoglycerides simply diffuse across membrane (non-polar & lipid soluble)

  2. After entering cell, formed back into triglycerides in Golgi & ER

  3. Processed, combined with proteins to form chylomicrons

  4. Vesicle (containing chylomicrons) fuses with membrane, releases contents (exocytosis)

  5. Absorbed via lymphatic vessel