Digestion (HP)

• Peristalsis:

  It moves food through the alimentary canal. In the small intestine, peristalsis mixes food w enzymes & forces products of digestion into contact w the wall of the intestine ∴ food is moved very slowly to allow time for digestion.

  General process is :

  1. Contraction of longitudinal muscle expands the lumen in front of the food giving it space to move into.
  2. Contraction of circular muscles behind the food propels it forwards.
• Large food molecules need to be digested before they can be absorbed. Large food molecules are ≈ insoluble, too large for diffusion across membranes into blood. Products of digestion are ≈ soluble, small enough for absorption into the blood & ltr assimilation into the tissues. 
• 3 main types of digestive enzyme synthesised by pancreas:

  • amylase to digest carbohydrates, e.g. starch
  • lipases to digest lipids, e.g. triglycerides
  • endoproteases to digest polypeptides
• Pancreatic juice containing enzymes is released into duodenum via the pancreatic duct. The small intestine is where final stages of digestion occur.
• Small intestine completes digestion of food molecules. Chyme enters the duodenum. Bile from gallbladder & liver is emptied into duodenum, neutralising acid & emulsifying fats. Pancreatic enzymes are released (amylase, trypsin). Enzymes are further released into the jejunum. The ileum is where food absorption occurs. Villi (finger-like projections) ↑ SA for absorption & has a rich blood supply. Peristalsis keeps the mixture of digested & undigested food moving through the intestine. 
• The small intestine contains 4 distinct tissue layers from the lumen. Mucosa – inner lining, includes villi. Submucosa – connective tissue (btw mucosa & muscle). Muscular layer – inner circular & outer longitudinal muscle perform peristalsis. Serosa – protective outer layer. Epithelial cells – single outer layer of cells on each villus.  
• Many villi protrude into the lumen, ↑ SA for absorption. Single-cell layer of epithelial cells on villi allows short path for diffusion. Epithelial cells also have microvilli on their surface to further ↑ SA.
• Digestion breaks down large food molecules into smaller molecules. Absorption is the uptake of these molecules into the blood. Once in blood, they are carried to tissues for assimilation (taken in to be used). Along w vitamins & minerals, all products of digestion (monosaccharides, amino acids, fatty acids, glycerol, nucleotides) are absorbed by the villi. 
• Starch consists of amylose (1,4 bonds) & amylopectin (1,4 bonds & 1,6 bonds). Amylase breaks 1,4 bonds in chains of ≥ 4 monomers, producing maltose. Maltase digests maltose into glucose monomers. Dextrinase breaks 1,6 bonds that amylase cannot deal with, forming glucose monomers. Note: cellulose remains undigested.
• Glucose process:

  1. Glucose is co-transported* w Na ions into epithelial cells of villus.
  2. Glucose moves by facilitated diffusion into the lumen of the villus.
  3. Glucose then diffuses short distance into adj capillaries where it dissolves into the blood plasma.
  4. Blood in capillaries moves to venules, then to hepatic portal vein which transports glucose to liver.
  5. Liver absorbs excess glucose which will be converted into glycogen for storage.
• Dialysis (visking) tubing modelling absorption:

  It initially contains a mixture of starch & glucose inside the condom looking part. The tubing is semi-permeable & contains pores ≈ ranging 1 – 10 nm in diameter.