requirements for digestion in animals are similar. The animals require water, oxygen, simple sugars, amino acids, fatty acids, vitamins and many other inorganic and organic substances. Except water and oxygen, all other substances are rare in the natural environment. So these substances are not available to the organisms directly.
These substances are available naturally in the form of proteins, starch, fats, vitamins and minerals. These molecules are useless as such. So they must be broken down or digested into simple molecules like amino acids, sugars and fatty acid. These simple molecules can easily pass through the cell membranes and used by the body.
5 Characteristic Processes involved in Animal Digestive System:
The taking in of the complex food is called ingestion.
The breakdown of complex organic compounds of food into simple diffusible molecules by the action of enzymes is called digestion. For example proteins like meat, fish, and egg are broken into amino acids. There are two types of digestion:
- Intracellular digestion: The breakdown of food within the cell is called intracellular digestion.
- Extracellular digestion: The breakdown of food outside the cell within the gut cavity or lumen is called extracellular digestion. In this case, the enzymes for digestion are secreted in the digestive cavity for digestion.
The uptake of diffusible food molecules from the digestive cavity into the cells or bloodstream is called absorption.
The utilization of the products of digestion for the production of energy or synthesis of cellular material is called assimilation.
The elimination of undigested matter from the body is called egestion.
Digestion in Man
The digestive system of man is composed of long coiled tubes. This tube extends from the mouth to the anus. The main parts of the digestive system are the oral or buccal cavity, esophagus, stomach, small intestine (duodenum, jejunum, and ileum), large intestine (ascending colon, transverse colon, descending colon, caecum, and rectum).
There are various glands associated with these regions. These are the salivary gland, liver, and pancreas. There are three sites of digestion in the digestive system of man. These are the oral cavity, stomach, and small intestine.
The digestive system of Amoeba
Amoeba proteus has intracellular digestion. It feeds on many kinds of tiny organisms. The organisms live with it in freshwater ponds and shallow lakes. Amoeba also feeds on particular organic matter. Its process of digestion is divided into the following steps:
Food may be ingested at any point on the surface of the body. When amoeba comes in contact with the food particle, it at once puts out its pseudopodia around it.
These pseudopodia fuse together around the food particle and form a food vacuole. Sometimes, the food particle is too big like ingesting paramecium. In this case, the amoeba encircles it and forms a large food vacuole.
Many changes take place in the food vacuole during the process of digestion. First, it becomes small than large and again becomes small. The lysosomes fuse with this food vacuole. These lysosomes secrete hydrolytic (breaking) enzymes into the food vacuole. The first phase of digestion is the killing and softening of food. It takes place in an acidic medium with a pH of 5.6. Later it becomes alkaline with a pH of about 7.3. Digestion is completed in this phase.
After the completion of digestion, the membrane of the lysosome forms numerous fine canals. The products of the digestion are passed into these canals. Then this digested material is passed into tile surrounding cytoplasm and utilized in different metabolic reactions.
Undigested material is removed from the organism into the surrounding through any point on its surface.
The digestive system of Hydra
Hydra is an aquatic, diploblastic coelenterate. It has a vase-like body. Its body is composed of two layers of cells.
Gastro Vascular Cavity
The central cavity of the body functions as a digestive cavity. The animal has only one opening to the outside. It is called mouth. Such a digestive system having a single opening is called a sac-like digestive system. The mouth is surrounded by mobile tentacles. The digestive cavity of this kind is called the gastrovascular cavity or coelenteron.
There are numerous stinging cells or nematocysts in hydra. These nematocysts are embedded in the tentacles. Each nematocyst consists of a hollow thread. This thread is coiled within a capsule. A tiny hair-like trigger projects outside.
The following steps take place during the process of digestion:
The preys of hydra are animals like Daphnia or Cyclops. When prey comes in contact with the trigger the thread of the nematocyst comes out. It ejects poison and the prey is paralyzed or sometimes killed. Hydra then grasps its prey with its tentacles. These tentacles then push the prey into the digestive cavity through the mouth.
Both extra and intercellular digestions take place in hydra.
- Extracellular digestion: The glandular cells in the gastrodermis secrete enzymes. These enzymes start extracellular digestion. The flagellated gastrodermis cells and contraction of the body help in the mixing of food with enzymes. The enzymes break up the food into fine particles.
- Intracellular digestion: The fine particles are then engulfed by phagocytic gastrodermal cells. These cells form digestive vacuoles. The process of digestion is completed in these digestive vacuoles.
The digested food is then absorbed from the digested vacuoles.
The indigestible food is expelled from the gastrovascular cavity through the mouth.
Digestion in Planaria
Planaria is a free-living flatworm. It is found in freshwater streams and ponds. It has a sac-like digestive system.
It has a single opening called a mouth. This mouth is located on the ventral surface near the middle of the animal. The mouth opens into the pharynx. This pharynx is muscular and tubular. The pharynx leads into the intestine. The intestine immediately divides into three branches. One branch is anterior and two are lateral branches. The anterior branch extends forward. Each main branch gives off numerous small branches. Each small branch that ends blindly is called a caecae.
This process is divided into the following steps:
- Ingestion: Planaria protrudes its pharynx through its mouth and engulf the prey. It then pushes the prey into the gastrovascular cavity.
- Digestion: Food is digested in the intestine. Both extra and intracellular digestion is present in planaria. During extracellular digestion, glands release enzymes in the intestine. These enzymes break the food into small particles. These small particles are finally engulfed by phagocytic cells. Intracellular digestion takes place in these cells. So the process of digestion is completed.
- Absorption: The digested food is then transferred to the remaining body by diffusion. The branched intestine helps in the diffusion of materials to the body cell.
- Egestion: Undigested food is egested through the mouth.
Digestive System of Cockroach
The cockroach has a tubular digestive system. It has a mouth for ingestion and an anus or cloacal aperture for egestion. It is a more efficient system than a sac-like digestive system. Cockroaches have specialized organs for the efficient digestion and absorption of food. It can be divided into foregut, mid gut, and hind gut.
The foregut is composed of mouth cavity, pharynx, crop, and gizzard. A pair of salivary glands are present in the thorax region of the body. These salivary glands secrete saliva and pour this saliva into the mouth cavity.
2. Mid Gut
The midgut is a short narrow tube called the stomach. Short finger-like hollow tubes open into the anterior end of the midgut. These tubes are called hepatic cancer.
3. Hind Gut
The hindgut is a long coiled tube. The terminal (last) part of which forms a thick-walled chamber called the rectum. The rectum opens to the exterior through the anus.
The cockroach feeds on all types of foods. It cuts the food into small pieces with the help of mandibles. These small pieces of food are mixed up with saliva in the mouth cavity.
The enzymes of saliva hydrolyze the starch present in food. The partly digested food is stored in the crop. A small amount of food is passed from crop to gizzard. This food is grounded in the gizzard and ¡s transferred into the midgut. The hepatic caeca produce enzymatic secretions.
These enzymes digest the food completely. The indigestible food is temporarily stored in the rectum. The fecal matter is then egested out through the anus.