6.1.1 Digestion of macromolecules
6.1.2 Enzymes and digestion.
6.1.3 Types of digestive enzyme
6.1.4 Structure of the digestive system
6.1.5 Functions of the stomach. small intestine and large intestine
6.1.6 Absorption and assimilation.
6.1.7 Structure and function of the villus.
Most food molecules are large polymers and insoluble
They must first be digested to smaller soluble molecules before they can be absorbed into the blood
Enzymes are biological catalysts that increase the rate of reaction
Digestive enzymes are secreted into the lumen of the gut
Digestive enzyme increase the rate of reaction of the hydrolysis of insoluble food molecules to soluble end products
Digestive enzymes increase the rate of reaction at body temperature
This image illustrates the reduction in activation energy that is achieved by the use of an enzyme
Notice that the normal reaction requires a higher activation energy which would correspond to a high body temperature. This is usually not possible in living organisms.
The enzyme-catalysed reaction has a lower activation energy. This lower activation energy would correspond to body temperature but is only possible in the presence of an enzyme
Example 1 Pancreatic amylase:
Conditions:
Source the Pancreas
Optimal pH 7.5-7.8
Substrate is starch (amylose)
End product is the disaccharide maltose
Action: hydrolysis of 1-4 glycosidic bonds
Example 2: Pepsin is a protease produced in the stomach
Conditions:
Source is the stomach
Optimal pH is 2
Substrate is a polypeptide chains of amino acids
End product is small polypeptides
Action is the hydrolysis of peptide bonds within the polypeptide chain (endopeptidase).
Example 3: Pancreatic lipases:
Source is the pancreas
The optimal pH is 7.2
The substrate is a triglyceride lipid
The product is glycerol and fatty acid chains
The action of pancreatic amylases also requires the presence of bile salts that emulsify the lipid. This emulsification has two effects:
Increases the surface area of the lipid for the digestion of fat
Exposes the glycerol 'head' structure to the enzyme
Action: hydrolysis of ester bonds between the glycerol molecules and the fatty acid chains.
1. Stomach:
The stomach stores the food from a meal and begins protein digestion.
(a) Lumen of the stomach which stores the food from a meal
(b) Gastric pits from which mucus , enzymes and acid are secreted
(c) Mucus secreting cells. Mucus protects the surface of the stomach from auto-digestion
(d) Parietal cells that produce HCL which kills microorganism that enter the digestive system (food & tracheal mucus). This also converts inactive pepsinogen to active pepsin
(e) Chief cells: produces pepsinogen a protease enzyme
2. small Intestine
In the small intestine digestion is completed.
The products of digestion are absorbed into the blood stream.
(a) Villus which increase the surface area for absorption of the products of digestion
(b) Microvilli border of the epithelial cell increases the surface are for absorption.
(c) Lacteals are connect to the lymphatic system for the transport of lipids.
(d) In the wall of the small intestine are the blood vessels to transport absorbed products to the general circulation, There are also the muscle to maintain peristalsis
3. Large Intestine or colon:
The colon is responsible for the reabsorption of water from the gut.
(a) The lumen of the colon
(b) The mucus producing goblet cells
(b) Muscular walls to maintain peristalsis
Insoluble food molecules are digested to soluble products in the lumen of the gut.
Absorption:
The soluble products are first taken up by various mechanisms into the epithelial cells that line the gut.
These epithelial cells then load the various absorbed molecules into the blood stream.
Assimilation:
The soluble products of digestion are then transported to the various tissues by the circulatory system.
The cells of the tissues then absorb the molecules for use within this tissues
The structure of the villus increases the surface are for the absorption of digested food molecules.
(a) folds increase SA:VOL ration by X 3
(b) Villi project into the lumen of the gut increasing the surface area by X 10
(c) Microvilli are outward folds of the plasma membrane increasing the surface area another X10
This sequence of light microscope and electron micrograph images show the same sequence as the diagram above.
Histological adaptations within the villus.
Blood supply in the villus which absorb the end products of digestion from the epithelial cells
The lacteals (green) that receive the lipoproteins before transporting them to the circulatory system.
Muscular walls that maintain the movement of chyme by peristalsis.