Digestive System (Feeding and Digestion)
Feeding and Digestion
Feeding is the process by which organisms obtain their food. Taking in of food is ingestion. Various methods of feeding are observed in animals such as:
1. Engulfing. This is resorted by the amoeba. Food is engulfed by the used of pseudopodia which simply flow over and around the food particle until it is enclosed in a food vacuole. Food is digested within the vacuoles.
2. Use of Cilia. Ciliates use their cilia in creating currents of water that draw the food into the oral groove leading into a gullet where a food vacuole is formed to receive the good.
3. Filter Feeding. Some animals like crustaceans, clams, and a few birds are filter feeders. Suspended food particles in the water are strained out by filtering devices used to trap food. Food is then passed along the mouth for ingestion.
4. Ingestion of Large Food Masses. Earthworms ingest large quantities of soil when feeding. The organic matter in the soil is digested as the soil passes the intestines. The earthworm thus "eats its way" through the soil leaving behind a trail of casts.
Many plant-eating invertebrates feed by scraping, chewing or boring through large plant masses. Examples are grasshoppers, crickets and termites.
5. Use of Specialized Structures for Capturing Prey. Coelenterates make use of nematocysts found in the tentacles to paralyze their prey which the tentacles seize and carry to the mouth. Frogs use their flexible tongues which could easily be flicked out to capture prey. This is also observed in some reptiles.
6. Fluid feeding. Mosquito and spider are examples of fluid feeders. The mouth of mosquito include a needle-like tube that can penetrate through plant or animal tissue. This serve as "straw" through which it draws body fluids from its victim or host. Once a mosquito has penetrated the skin, an anti-coagulant is injected to prevent blood clotting. Spider injects digestive juices into the body of their prey which liquify the tissues injected. The resulting fluid is then sucked by the spider. The tapeworm merely feeds on digested food which is in liquid form, by "bathing" on the food. The food passes through the body surface.
Digestion is the process by which large food particles are broken down into simple absorbable forms.
Intracellular and Extracellular Digestion
Intracellular digestion is the process by which a food particle is acted upon and changed into simple forms within the cell. Digestion takes place within the cell. Example, in Amoeba and Paramecium, ingested food is enclosed within a food vacuole where digestive enzymes chemically break it down.
Extracellular digestion is the process by which food is broken down in a digestive cavity. The resulting basic food units are then absorbed by the body. Example, in Hydra, large food particles are ingested and then brought into the gastrovascular cavity where digestive enzymes break down the food particles.
Higher animals have a one-way digestive tube or alimentary canal system which have 2 openings, the mouth (for ingestion of food) and the anus (for egestion of wastes).
The digestive tube may be subdivided into different regions for different functions. Like in earthworm, the first region of the digestive tube is the mouth, then the pharynx, esophagus, which have an enlargement called crop where food is temporarily stored, gizzard (muscular stomach) where food undergoes physical digestion, intestines where food undergoes chemical digestion. Food passes through the walls of the intestines and carried to all parts of the body. The undigested food is expelled through the anus.
Other higher animals like mollusks, arthropods and echinoderms also have alimentary canal system.