Digestion And Absorption

• Define Digestion


  The process in which large biological molecules are hydrolysed to smaller molecules that can be absorbed across cell-surface membranes and assimilated.
• Outline the structure of the alimentary canal


  - Salivary glands.

  - Oesophagus.

  - Stomach.

  - Ileum.

  - Large intestine.

  - Rectum.

  - Anus.

  - Pancreas.

  
• Describe the salivary glands

  Situated near mouth they pass the amylase enzyme in their secretions.
• Describe the ileum

  - Long muscular tube ---> food further digested in ileum by enzymes produced by its walls and by glands that pour secretions into it.

  
  

  - Has an adapted structure for absorption.
• Monomers of sucrose

  Glucose + Fructose
• Outline the basic process of digestion

  1. Physical Digestion:

  - Food (if large) broken down into smaller pieces by chewing - then ingested - and then by churning muscles in stomach wall.

  - Provides a large SA for chemical digestion.

  
  

  2. Chemical Digestion:

  - All digestive enzymes function by hydrolysis ---> several different enzymes all work together to hydrolyse a molecule.
• Explain how carbohydrates are digested in mammals?

  1. Saliva from salivary glands mixed with food in mouth during chewing.

  
  

  2. Salivary amylase starts hydrolysing glycosidic bonds in starch ---> maltose

  
  

  3. Food then enters stomach. Amylase denatured, preventing further starch hydrolysis.

  
  

  4. Food passed into small intestine (churning) ---> mixes with pancreatic juices.

  
  

  5. Pancreatic amylase continues starch hydrolysis to maltose. Alkaline salts produced by pancreas/intestinal wall keep pH neutral ---> amylase can still function.

  
  

  6. Intestine wall muscles push food along ileum. Epithelial lining produces the membrane-bound disaccharidases maltase (part of epithelial cell membrane), sucrase and lactase.
• How are lipids digested?

  1. Lipids hydrolysed by lipase enzymes, produced in the pancreas, that hydrolyse the ester bond found in triglycerides ---> fatty acids and monoglycerides.

  
  

  2. Lipids (fats + oils) firstly split up into tiny droplets called micelles by bile salts (produced by liver) ---> emulsification ---> increases SA of the lipids so speeds up the action of lipases.
• How are proteins digested?

  1. Endopeptidases - hydrolyse peptide bonds between amino acids in the central region of a protein molecule ---> forms a series of shorter peptide molecules.

  
  

  2. Exopeptidases - hydrolyse peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases ---> forms single amino acids and dipeptides.

  
  

  3. Dipeptidases - hydrolyse peptide bond between the two amino acids of dipeptide ---> membrane-bound - part of epithelial cell-surface membranes.
• How is the ileum adapted for absorption of digestion products?

  1. Thin-walled ---> short diffusion distance.

  
  

  2. Contain muscle ---> can move to mix with the ileum contents to help maintain favourable concentration gradients.

  
  

  3. Many capillaries/rich network on other side of epithelial cells ---> blood can carry away absorbed molecules

  
  

  4. Many channel/carrier proteins.

  
  

  5. Microvilli ---> finger-like projections of the cell-surface membrane that further increase the SA for absorption.

  
  

  Epithelial cells also have many mitochondria to provide ATP for active transport.
• How are triglycerides absorbed?

  1. Micelle structures formed ---> monoglycerides remain in association with the bile salts that initially emulsified the lipid droplets.

  
  

  2. Micelles come into contact with villi epithelial cells as they move through ileum. Micelles break down, releasing the monoglycerides and fatty acids ---> non-polar molecules - easily diffuse across the cell-surface membrane into the epithelial cells.

  
  

  3. Once inside epithelial cells, monoglycerides and fatty acids transported to the ER where they are recombined to form triglycerides.

  
  

  4. Starting in ER ---> golgi apparatus, triglycerides associate with cholesterol and lipoproteins to form chylomicrons (particles adapted for lipid transport).

  
  

  5. Chylomicrons move out of the epithelial cells by exocytosis ---> enter lymphatic capillaries called lacteals, found at centre of each villus.

  
  

  6. Chylomicrons then pass into blood via lymphatic vessels ---> triglycerides in chylomicrons hydrolysed by enzyme in blood capillaries' endothelial cells, from where they diffuse into cells.