3.3 Digestion and Absorption

• Why can't large biological molecules be absorbed from the gut?

  They are too big to cross cell membranes
• What happens to large molecules during digestion?

  They are broken down into smaller molecules
• What are most large biological molecules classified as?

  Polymers
• What process breaks down polymers into monomers?

  Hydrolysis reactions
• What do hydrolysis reactions do?

  Break bonds by adding water
• What are carbohydrates broken down into during hydrolysis?

  Disaccharides and monosaccharides
• What are fats broken down into during hydrolysis?

  Fatty acids and monoglycerides
• What are proteins broken down into during hydrolysis?

  Amino acids
• What do digestive enzymes do?

  Break down biological molecules in food
• Where are digestive enzymes produced?

  Specialised cells in the digestive system
• Why are different enzymes needed for digestion?

  They work with specific substrates
• What does amylase catalyse the conversion of?

  Starch into maltose
• What type of bonds does amylase hydrolyse?

  Glycosidic bonds in starch
• Where is amylase produced?

  Salivary glands and pancreas
• What do membrane-bound disaccharidases do?

  Break down disaccharides into monosaccharides
• What are the products of maltose hydrolysis?

  Glucose + glucose
• What are the products of sucrose hydrolysis?

  Glucose + fructose
• What are the products of lactose hydrolysis?

  Glucose + galactose
• How are monosaccharides transported across cell membranes?

  Via specific transporter proteins
• What do lipase enzymes catalyse?

  Breakdown of lipids into monoglycerides and fatty acids
• Where are lipases produced?

  In the pancreas
• What do bile salts do during lipid digestion?

  Emulsify lipids into smaller droplets
• Why are smaller lipid droplets more effective for digestion?

  They have a bigger surface area
• What do micelles do after lipid digestion?

  Help transport fatty acids and monoglycerides
• What are endopeptidases responsible for?

  Hydrolysing peptide bonds within proteins
• Where are endopeptidases synthesized?

  In the pancreas
• What is the function of exopeptidases?

  Hydrolyse peptide bonds at protein ends
• What do dipeptidases specifically act on?

  Dipeptides
• How is glucose absorbed in the ileum?

  By active transport with sodium ions
• Where are dipeptidases located?

  In the cell-surface membrane of epithelial cells
• How are the products of digestion absorbed?

  Across the ileum epithelium into the bloodstream
• How is fructose absorbed in the ileum?

  Via facilitated diffusion
• What role do micelles play in absorption?

  Help move monoglycerides and fatty acids
• How do monoglycerides and fatty acids cross the epithelial cell membrane?

  By simple diffusion
• How are amino acids absorbed in the ileum?

  Similar to glucose and galactose
• What happens to sodium ions during amino acid absorption?

  They are actively transported out of cells
• What maintains the diffusion gradient in the ileum?

  Rich network of capillaries
• What are chylomicrons?

  Modified triglycerides with protein and cholesterol
• What happens to triglycerides after absorption?

  They are reformed into triglycerides in the smooth ER
• How do chylomicrons enter the bloodstream?

  Via lacteals through exocytosis