Heterotrophic Nutrition – Digestion in Man.

Concept and importance of Digestion:

The heterotroph obtains its food as ready-made organic compounds. These compounds are complex and of large molecules (Proteins / Starch / and Fats) that they can not pass through the cell membranes of the living organisms. These large molecules have to be broken down into molecules of smaller size and simpler structure (Amino acids / Glucose / and Fatty acids and glycerol). As these molecules are small and soluble in water, they can easily be absorbed by the cell, either by diffusion of by active transport. Cells will then use these simple compounds as a source of energy, or in growth of new tissues.

Digestion:

Digestion is the conversion of large food molecules (polymers) into smaller ones (monomers) by means of hydrolysis. This process is helped by enzymes.

Enzymes.

The enzyme:

A protein substance that has the properties of a catalyst, as it has specific ability to activate a particular chemical reaction.

Enzymes are specific in their actions. The reaction which is catalyzed by a certain enzyme depends on: a. The structure of the reacting molecules.

b. The nature of the enzyme.

An The reacting Unstable The Products of

enzyme + substance intermediate enzyme + the reaction compound

Enzymes only accelerate the rate of a reaction until it reaches a case of equilibrium. They never affect the product of the reaction. Some enzymes may have a reversible effect: this means that the same enzyme that catalyzes the decomposition of a complex molecule into two simpler ones may recombine the two simple molecules to give the same complex molecule once more. Some enzymes are secreted by the cell in an inactive state. So, they need certain substances to be present to activate them. Ex.: Pepsin enzyme is secreted by the stomach as inactive Pepsinogen, that is changed into active Pepsin in the presence of Hydrochloric acid in the stomach. The intensity of an enzymatic reaction depends on the temperature and the PH value.

Digestion in Man

Human digestive system is built up of a long tube extending from the mouth to the anus. It comprises the mouth, the pharynx, the esophagus, the stomach, the small intestines, the large intestines, and the anus.

1. Buccal Digestion (Digestion inside the mouth):

The Human digestive system begins with the mouth opening. Mouth contains:

a. The teeth:

Teeth in one jaw are differentiated into:

• 4 incisors: at the front of the jaw to cut food.

• 2 canines: one at each side of the incisors to tear food.

• 4 premolars: two at each side to crush and grind food.

• 6 molars: three at each side to crush and grind food.

b. The tongue:

- Acts as the taste organ.

- Manipulates food and mixing it with saliva.

c. Saliva:

Is secreted from 3 pairs of salivary glands that open in the mouth cavity through ducts that pour saliva over food. Saliva contains mucous that softens food and lubricate it to facilitate its swallowing. Saliva contains Amylase (Ptyalin) enzyme that acts in a weak alkaline medium. Amylase enzyme catalyzes the hydrolysis of starch to the disaccharide maltose.

d. The pharynx:

Pharynx exists at the back of the mouth. It is a common passage for both food and air. Two tubes extend downwards from the pharynx: the esophagus (that leads to the stomach) and the trachea (which is a part of the respiratory system). Swallowing process is an organized reflex action: as food is pushed from the mouth to the esophagus. Top of the trachea together with the larynx are elevated, so the epiglottis closes over the glottis entrance to the air passage.

e. The esophagus:

A tube of 25 cm long, that extends from the pharynx downwards through the neck and into the chest cavity. It lies parallel to the vertebral column. The esophagus is lined with glands secreting mucus. Food is carried through the esophagus to the stomach by a series of rhythmical muscular contractions and relaxations which is called peristalsis. This movement is continuous along the length of the digestive system. It is responsible for pushing the food forwards, churning it, and mixing it with the digestive juices, and the absorption of the digested food.