Topic 5.1 : Digestion

5.1.1 Digesting large molecules

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

5.1.2 Enzymes and digestion

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

5.1.3 Examples of digestive enzymes

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 enzyme

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).

There are other peptides that hydrolyse the individuals amino acids from the ends of the chains (exopeptidases).

Example3: 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.

5.1.4 The structure of the digestive system

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5.1.5 Function of the stomach, small intestine and large intestine

Structure: Stomach

Function

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

(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

Structure: Small Intestine

Function:

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.

(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

Structure: Colon

Function:

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

5.1.6 Absorption and assimilation

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

5.1.7 The role of the villus in the absorption of the end products of digestion

Structure of the villus: Increasing the surface area for the absorption of the end products of digestion

The inner intestine wall is folded which increases the available surface area by three times.

The villi are finger like projections into the lumen of the gut. These increase the available surface area by ten times.

The cell membrane of the epithelial cells is folded into microvilli. This increases the available surface area still further

These microscope images illustrate the same increases in surface area shown above.

This image shows other structural adaptations of 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.