(Full Nutrition in Amoeba) – How does Amoeba obtain its food?

Introduction To Nutrition In Amoeba

Amoeba is carnivorous and their mode of nutrition is known as holozoic nutrition. Holozoic nutrition is characterized by the ingestion and internal processing of food particles in the body of Amoeba.

When the amoeba gets near to its food, it engulfs its food particle through the process of phagocytosis by forwarding its pseudopodia and covering the whole food particle to take the food inside its body. It always takes the food in solid form.

The phagocytosis process in amoeba includes intake, digestion, absorption, assimilation, and egestion process as a whole.

Amoeba doesn’t feed on everything. The food particles of Amoeba consist of bacteria, diatoms, desmids, flagellates, ciliates, and rotifers.

And, they can only display its preference to feed for certain kinds of organisms whom it approaches and avoids others. It is also able to distinguish organic food particles from the inorganic ones.

It is also to be noted that amoeba has no special sense organs other than its cell membrane that responds to the various stimuli in the environment.

So, they are almost able to select their food in the absence of the various sense organs. They use chemotaxis as the most essential mechanism to detect the food particles and locomote.

How does Amoeba obtain its food? Nutrition in Amoeba takes place by 5 steps

1. Selection of Food: It is the process through which the amoeba will choose and select the food to feed on by responding to the various stimuli in the environment.
2. Ingestion of Food: It is the process through which the amoeba will intake the food within its food vacuole in the cell through phagocytosis or other mechanisms.
3. Digestion of Food: It is the process through which the amoeba will kill and break down the intaken food by various enzymatic actions inside the food vacuole.
4. Absorption & Assimilation of Food: It is the process through which the amoeba will absorb the various nutrients from the food and will utilize it for its various cellular metabolic uses.
5. Egestion of Food: It is the process through which the amoeba will egest or take out the undigested part of the food from the food vacuole and then will eliminate it completely out of the cell.

Let’s understand each of these steps in detail…

1. Selection of Food: How amoeba selects the food to feed on?

An organism must know what it will feed on, and so does the amoeba. Amoeba finds their prey by identifying it in their environment through chemoreceptors on its cell membrane.

It takes the help of chemotaxis due to extracellular cues to identify that a potential food source is nearby or not. It does so with the help of its cell membrane that makes it able to sense the extracellular signaling molecules called chemoattractants (e.g. cAMP) to extend pseudopodia to engulf the food particle.

As soon as the G-proteins-coupled receptors on the cell membrane are excited the Amoeba receives its cell signals due to which the food enters the amoeba’s cell through the process of endocytosis or phagocytosis.

Amoeba only feeds on solid organic particles which it can detect using its cell membrane. They feed on a variety of organisms, such as bacteria, algae, and other protozoans like diatoms, desmids, flagellates, ciliates, etc.

They simply eat by surrounding tiny particles of food with pseudopods and ingesting that particle inside, forming a bubble-like food vacuole. The food vacuole will soon digest the food in further steps.

DO YOU KNOW: There are some Amoeba species that can engulf photosynthetic bacteria within its cell to become photosynthetic itself. These amoebae can engulf a bacterium and keep it within its cell alive and will eventually utilize the bacterial genes for the purpose of photosynthesis. They act as both heterotrophic mostly and autotrophic sometimes. For example: Dicty (Dictyostelium discoideum)

2. Ingestion of Food: How does amoeba ingest the food?

Amoebae use their pseudopods to ingest the food particle in contact with the amoeba’s cell membrane by a method called phagocytosis. Phagocytosis is one type of endocytosis mechanism for ingesting food.

Phagocytosis is the process by which a cell uses its plasma membrane (here it takes the form of pseudopodia) to engulf and fully cover a large food particle, giving rise to an internal compartment called the phagosome where that food gets trapped.

The phagosome thus formed will take the engulfed food into the cell through a minute opening in the cell membrane and will pass it inside the food vacuole where the further digestion, absorption, and assimilation of the food particles will occur.

According to Rhumbler (1930), Amoeba can ingest food in 4 possible ways depending on the nature of the food. These ways are Import, Circumflence, Circumvallation, Invagination, and Pinocytosis.

1. IMPORT: This type of ingestion occurs when any food like the algal filament comes in direct contact with the amoeba. In this type, the amoeba remains motionless and doesn’t extend its pseudopodia but in fact, the food passively sinks inside the body by rupturing the cell membrane and gets inside the body.

2. CIRCUMFLENCE: In this process, Amoeba ingests the immobile food materials that are in direct contact with the cell membrane by forming a food cup-shaped surface. Then it stretches its pseudopodia to bring the enclosed food in direct contact inside the food vacuole.

3. CIRCUMVALLATION: In this process, no direct contact of amoeba with the food particle is required. Amoeba uses this process to ingest an active prey like flagellate or ciliate by extending its pseudopodia like a food cup with which it will surround the food without touching it. Without touching the food material, the edges of the pseudopodia come close together, and with the ectoplasmic membrane forms the food vacuole along with the water which encloses the prey.

4. INVAGINATION: In this method of ingestion, the food is adhered to by the toxic and sticky secretion of ectoplasm. The food organism is sucked in, upon contact with the ectoplasm, by the formation of an ectoplasmic tube. This tube, upon engulfment, takes the form of a food vacuole.

5. PINOCYTOSIS: It is seen that amoeba can form buds from its cell membrane. This is seen during the ingestion of liquid into the amoeboid cell by the budding of small vesicles from the cell membrane. These buds with vesicles are formed as the amoeba moves. Each of these buds has channels that run through the bud to deep into the body of the amoeba. These vesicles can ingest the food particles inside of the bud and pass it into the cytosol.

3. Digestion of Food: How does amoeba digest the food?

Digestion of food in amoeba occurs as soon as the food gets ingested inside the cell.

All of the digestion in amoeba takes place inside the food vacuoles only. It’s like the gut of higher animals but at the cellular level.

These food vacuoles are formed by the extension and joining of the pseudopodia that captured the prey with a drop of water. The food particle that was trapped by the pseudopodia can now be seen inside the food vacuoles.

There are many food vacuoles inside an actively feeding amoeba. These food vacuoles represent a number of spherical, small and large vesicles inside the cytoplasm. These food vesicles contain water and food in various stages of digestion.

The food vacuoles contain lysosomes that contain the various digestive enzymes that help in the digestion of the trapped food particle slowly and eventually. During the digestion process, the reaction inside the food vacuole is first acidic and then alkaline.

When the food vacuole is acidic, the organism that is trapped inside gets killed due to the high pH effect.

And when the food vacuole is alkaline, the digestion of starch, fats, proteins, carbohydrates, etc. takes place from that dead food particle by the enzymes amylase, protease, lipase, etc.

Actually, the process of digestion is very much simple. The amoeba extends its pseudopodia around the food and engulfs it. The food so gets trapped in the food vacuole where it is digested by the digestive enzyme and finally the food is absorbed and distributed all throughout the body of the amoeba.

4. Absorption & Assimilation of Food: How absorption and assimilation occur in amoeba?

Absorption is the process of absorbing nutrients like proteins, lipids, starch, etc. from the digested food material into the cytoplasm leaving behind the undigested food material in the food vacuole of amoeba.

Actually, the real process of obtaining energy from the absorbed food nutrients in order to make its use in the body is termed assimilation. In amoeba, absorbed food molecules are utilized for producing the energy required to carry out different life processes within the cell.

It has been noticed that as the digestion goes on, the food vacuoles gradually shrink in size and this is due to absorption. The nutrients of that digested food are absorbed by the cell through simple diffusion through the membrane of the food vacuole.

Absorption of nutrients takes place in each and every part of the cytoplasm as the food vacuoles due to its ability of cyclosis to keep on moving in the endoplasm. The total completion of digestion and absorption can take about 30-32 hours.

In some cases, it has also been seen that the amoeba absorbs large quantities of food at a time. This results in an excess of food to be absorbed and so, the excess food gets stored in the form of glycogen as well as lipids as a reservoir of future energy.

After the successful absorption of nutrients from the digested food, the absorbed nutrients are now assimilated to form new protoplasm of the cell. The absorbed food nutrients can now also be utilized for energy production, growth, repair as well as for multiplication.

In the body, the metabolism of fats and carbohydrates produces CO₂, whereas the metabolism of proteins produces ammonia (NH₄⁺).

5. Egestion of Food: How does egestion occur in Amoeba?

Egestion in Amoeba is the process of excretion or getting rid of those undigested food materials that remained in the food vacuoles and weren’t absorbed.

In amoeba, this process is carried out by rupturing the cell membrane to remove the undigested food material from its body by a variety of mechanisms.

It has been seen that the egestion of the undigested food residue takes place at any point of time from the surface of the body. And in amoeba, there is no definite exit for egestion to take place.

So, in actively moving amoeba the much-reduced food vacuoles, containing the undigested food residue are shifted backward as the amoeba moves forward due to the streaming movement of the food vacuole.

After the backward shifting of the food vacuole, the discharge of the wastes takes place through any point of the posterior end by rupturing the cell membrane, as the animal moves on.