Food is the most important requirement for the survival of living organisms. All living things use organic substances like lipids, carbohydrates and proteins as their food. Plants prepare the food (carbohydrates) by the process of photosynthesis in their body (autotroph). But in order to fulfill the food requirements heterotrophic organisms face different problems.

First of all, they face the problem of obtaining and entering the food in their body. The obtaining and entering the food in the body of an organism is known as Ingestion. Multicellular organisms have different structures for this purpose. For e.g. Pseudopodia, tentacles, mandibles, claws and teeth etc.

The obtained form must require another process of conversion of its complex nature into simple form. Two types of products are obtained as a result of this process. One of which is necessary for the body i.e. digested food particles. While the other one is unnecessary waste products, expelled from the body by the process of Egestion.

Digested food particles distributed to all cells of the body by means of Transportation. In cells they perform two functions. They built up into complex materials by the process of Assimilation or they produce energy by the process of Metabolism.

As there is no mouth in Amoeba, ingestion occurs at any point of the body and pseudopodia assist in engulfing the food. Ingestion occurs by any of the following manners:

The food organisms are killed by toxic secretions of pseudopodia and is engulfed by the invagination of plasma membrane into the cytoplasm and a food vacuole is formed.

The digestion is intracellular and food vacuoles act as a temporary stomach for digestion. The surrounding protoplasm secretes enzymes into the food vacuoles which digest the food particles.

Egestion occurs through a temporary opening formed at the time of egestion. The undigested food rasidue gradually moves backward to the periphery and is lagged behind as the animal moves forward.

Paramecium has a depression on its ventro lateral side called Oral Groove. It runs obliquely in the body forming a tube called Vestibule, which leads into an opening called Cytostome. Extending from the cytostome towards the centre of the body is Cytopharynx. Oral groove contains cilia. Due to their vibratory movements’ fluid containing food particles enters in it. In vestibule some particles are rejected and thrown out, but other pass into the cytostome. At the end of cytopharynx a food vacuole is formed which gets fitted with food particles.

The cytoplasmic movement in the body of paramecium is known as Cyclosis. When the food vacuole is detached from the cytopharynxm, they move throughout the length of the body due to cylosis. During cyclosis digestion occurs by enzymes secretd by protoplasm in the food vacuoles.

The undigested matters are egested through the cyto with some force.

As we know that Hydra is a dipiobtastic animal, containing a double layered structure with central cavity. Mouth is present on the upper surface while lower surface attached to the hard objects. Mouth is surrounded by Tentacles. Outer layer, Ectoderm contain following cells:

1. Epithelio muscular cells
2. Cnidoblasts (contain Nematocysts organs of offense and defence). They are more on tentacles.
3. Sensory cells
4. Interstitial cells
5. Nerve cells

Inner layer, endoderm contain following cells:

As soon as any aquatic animal comes in contact with the tentacles, the barbs of the Nematooyst puncture the body of the prey and threads penetrate it. The poisonous secretions, then enters the body and paralyze the prey. Tentacles along with the prey bent toward the mouth and ingestion completed after the entrance of the prey into body cavity through mouth.

As food enters the body cavity, the secretory cells pour their secretions into it. The enzymes are mixed with the food by the contraction and expansion of the body.

The Nutritive or Endothelio-Muscular cells of endoderm with the aid of their pseudopodia engulf food particles from the cavity and form food vacuoles . The digestion completed as in unicellular organisms.

The undigested food particles are egested through the mouth.

Planaria is the multicellular animal belonging to the Phylum Platyhelminthes. It is a triplobtastic animal with a digestive tract in the body. On ventral side mouth is present which leads into a cylindrical Pharynx. The Pharynx can be everted through the mouth as a Proboscis. The proboscis is mobile and can be extended greatly. The attached end of Phraynx leads into an intestine which divides at once into three branches. One extending forward in the middle up to the head. While the other two going backward to the posteror end on o\either sides of the pharyngeal cavity. All branches give numerous branching diverticuli, all ending blindly their being no anus.

The food is taken in through the proboscis in which digestive juices are poured onto the food. The food is broken by a pumping action of the pharynx and acted upon by the extruded juices for extracellulaar digestion. Through the intestine food reached the cells of the body where intracellular digestion takes place. The undigested matter is egested through the mouth.

Now we are going to study the process of digestion in Phylum Annelida. In this, there is a tube like tract present in the entire length of the body formed by different organs of digestion and absorption. These animals show only extracellular digestion. This system consists of a one way tube in which food enters at one end (mouth) and the wastes exit through the other end (anus). This tube is known as Alimentary Canal which consist of:

1. Mouth
2. Buccal Cavity
3. Pharynx
4. Oesophagus
5. Gizzard
6. Stomack
7. Intestine

Buccal cavity is present in the first three segments. Pharynx is a muscular structure up to the 5^th segment. Oesophagus is a small tube up to the 8^th segment. In this segment it forms a muscular pear shaped structure Gizzard which perform the function of crushing of food. Stomach is presentfrom 11^th to 14^th segment. After this a portion of alimentary canal known as intestine. It has a paired outgrowth, intestinal ceaca in the 26^th segment. From this segment, the inner layer of intestine shows finger like projections on the dorsal side in each segment called Typhlosole. The region above the 26^th segment of intestine known as Pre-typhlosolar region. Last 23 segments of the body re devoid of any typhlosole called Rectum or Post typhlosolar region. While those segments, which contain typhlosole known as Typhlosolar region. Typhlosole increases the surface area for absorption of the digested food particles. Different Enzymes like Amylase. Tryptase Maltase, Diastase, Lipase etc are secreted in different organs of Alimentary canal. They digest the food particles and absorption takes place in the intestine. The undigested waste products are egested through the anus, which is present in the last segment.

Like Earthworms, the higher heterotrophs (Humans) also shows extracellular digestion and consists of Digestive system. This system composed of specialized one way tact called Gastro-intestinal tract (GIT) and Digestive Glands. These glands secrete enzymes specific for different food particles. They docompose the food particles by the process of enzymatic hydrolysis. Enzymes, carbohydrase are specific for Carbohydrates, Proteases or Proteotytic enzymes for Proteins and Lipases for Lipids.

Gastro-intestinal Tract of human being is composed of:

It is also divided into three parts:

Ileum opens into a small part of large intestine called Caecum. On its opening Heo-ceacal valve is present.

It is the largest part of Large intestine, which further divides into four parts:

1. Ascending Colon – running upwards on right side
2. Transverse Colon – running from right to left side
3. Descending Colon – running downwards on left side
4. Sigmoid Colon – running backwards to reach the midline position and then downwards.

Small part of large intestine serves the function of storage of Fecal matters.

It is the exit of GIT through which egestion takes place.

Following are those Digestive Glands, which are present along with the GIT. They are of Endocrine type and release their secretions into the GIT by means of duets.

There are three types of paired Salivary Glands present in the body, namely:

1. Parotid Glands
2. Sub-Lingual Glands
3. Sub- Mandibular Glands

All these glands secrete Saliva in the oral cavity. Saliva contains an enzyme called Ptyalin.

It is a small gland present with the Duodenum. It secretes Pancreatic Juice in the duodenum through pancreatic duct.

As food enters the oral cavity, different salivary glands release Saliva on it.

Different digestive glands present in the walls of stomach release their secretions on the food.

Secrete HCl which performs the following functions:

• It provides Acidic medium. (pH = 1.5 – 2.5)
• Acidic medium activates the enzyme pepsin.
• It kills the germs present in the food particles.

They secrete enzyme Pepsin and Renin. Pepsin broken down Proteins into Peptides.

Renin coagulates the Casein.

Stomach is a muscular organ, due to its contractions and relaxation food turned into a paste like substance called Chyme. The muscular action of stomach is known as Peristalsis. Stomach also performs the function of storage of chyme for sometime and it passed the chyme with some intervals into the duodonum.

Absorbed food particles brought to the liver by means of Hepatic portal veins and Lymphatics. Liver is the largest organ of the body.

It performs different functions, which are as follows:

• It maintains the appropriate quantity of nutrients in the body.
• In the liver absorbed Glucose is converted into Glycogen which can be used for energy.
• It removes the excess nitrogen from the Amino Acid producing from them carbohydrates that can be used for energy.
• Action on fatty acids results in the formation of ketone bodies which later released as nutrients for active muscles.

Poisonous substances broken into harmless compounds in the liver.