3.3.3 Digestion and absorption

• The digestive process:

  • Ingestion- food enters mouth of the organism
  • Digestion- physical process to increase the SA:V ratio of the food
  • Absorption- small, monomer units are absorbed into the blood, where it is carried away
  • Assimilation- rebuilding/ utilising nutrients
  • Defecation- removing materials that weren't broken down, different to excretion
• Endopeptidase- hydrolyses bonds in the middle of a peptide
• Exopeptidase- breaks terminal peptide bonds
• Maltase- hydrolyses maltose into two alpha glucose molecules
• Sucrase- hydrolyses sucrose into fructose and glucose
• Lactase- hydrolyses lactose into galactose and glucose
• Amylase- hydrolyses starch (polysaccharide) into maltose (disaccharide)
• Maltase, sucrase and lactase are membrane-bound disaccharidases.
• Bile- produced in the liver, stored in the gall bladder
• Emulsification is when bile breaks up large droplets if fat into smaller droplets of fat, increasing the SA:V ratio for digestion, making it easier for lipase to hydrolyse the triglyceride into smaller molecules (e.g. glycerol, fatty acids, monoglycerides).
• The mouth releases salivary amylase, which hydrolyses starch into maltose.
• The stomach (pH 1-2) releases pepsin, which hydrolyses large polypeptides into amino acids, and smaller polypeptides.
• The pancreas releases proteases, amylase and lipase, which hydrolyses starch, large polypeptides and triglycerides into amino acids, maltose, fatty acids, monoglycerides and glycerol.
• The ileum (on microvilli which are found in the small intestine) releases sucrase, maltase and lactase which hydrolyses sucrose, maltose and lactose into their monosaccharides.
• Ileum of a small intestinal cell:

  • Thin layer of cells
  • Large surface area (many found on microvilli)
  • Glucose and amino acids are absorbed by the bloodstream which is part of the circulatory system
  • Fatty acids are absorbed by the lacteal which is part of the lymphatic system

  
• Absorption of amino acids:

  1. Sodium ions (Na+) are actively removed from the cell via the sodium-potassium pump which requires ATP.
  2. Amino acids are removed from the cell by via facilitated diffusion into the capillaries.
  3. This maintains a concentration gradient.
  4. Na+ and amino acids in the lumen (outside the cell) both bind to the co-transport protein.
  5. The co-transport protein changes shape, transporting the molecules across the membrane. Na+ ions go down the conc. gradient and amino acids either go against or down the conc. gradient.
• Lipid absorption:

  1. Fatty acids and monoglycerides (from fat digestion) leave micelles and enter the epithelial cell.
  2. Fatty acids link to form triglycerides.
  3. Fatty globules combine with proteins to form chylomicrons (inside the golgi apparatus).
  4. Chylomicrons are extruded from the epithelial cell (via exocytosis of vesicle) and enter a lacteal.
  5. Lymph in the lacteal (part of the lymphatic system) transports chylomicrons away from the intestine.