Introduction to Sarcodina

Sarcodina belongs to the phylum of protozoans; these are the largest phylum of the class of protozoans. This class is classified as a subphylum under the highest order of Sarcomastigophora. The members of the Sarcodina family are known as sarcodines. It compromises amoeba and related organisms. The key feature of the organisms of this class is the presence of pseudopodium, it has a role in trapping food for the ingestion process [performed by the organisms. Pseudopod can be defined as the temporary extension of the cell protruding from the cell capsule, they can be singular as a single blunt lobo podium with simple anatomy or they can be found in the bundle as reticulopodia which protrudes from a foraminifera shell.


Features of the Sarcodines 

Sarcodines anatomy is very simple as it only consists of a single cell, with abundant protoplasm. All the metabolic processes occur in the protoplasm. Amoeba is the key organism that is used to study the physiological and biochemical features of the class Sarcodina. Amoeba is also used for the phylogenetic study of the class. Phylogenetic history and relation are generally hypothesized using the genetic and proteomic data of the organism. Some of the general characteristics and features of sarcodines are as follows. 

  1. Most of the organisms of this class are free-living carnivores.

  2. They engulf food by pinocytosis.

  3.  Some of the strains of protozoans are parasitic for example, amoebic dysentery is caused by the sarcodine family protozoa.

  4. Most of them are free living.

  5. Most of them are sessile, that is they are attached to some kind of surface and can not perform voluntary locomotion. Locomotion is rather achieved by external forces, such as water current, laminar flow, etc.

  6. Sardines are further classified into two main groups that are superclass Rhizopoda and Actinopoda.

  7. The class differentiation among sarcodines is based on the basis of the type and feature of pseudopodia. 


How Do Sarcodines Eat?

Sarcodines have a unique process of eating. They engulf the food particle by pinocytosis, pinocytosis is mediated by special appendages that protrude from the cell membrane of the protozoan. Pinocytosis can be explained as the process of engulfment of small particulate matter or liquid that is suspended in the extracellular fluid. The plasma membrane of the cell invaginates from the surface, encircling the target particle. I then completely encapsulate the particle by forming a vesicle like structure around it, the vesicle is then carried into the intracellular compartments of the cell, where it is metabolized. Pinocytosis is also referred to as fluid endocytosis. The food particle is then digested by breaking into small monomers, this done by enzymes of the protozoan cell. The major digestive enzymes are as follows acid phosphatase, acid protease, acid phosphomonoesterase, alpha-glucosidase, beta-glucosidase, and amylase. It is to be noted that pinocytosis is an active process, that is it requires ATP for the completion of the process.


Sequential Events that Take Place During the Process

There are the following steps that summarize the complete process.

  1. There is a direct contact between the inducer the food particle (example- paramecium)

  2. Inducer a liable substance of paramecium binds to the charged site of mucopolysaccharide or glycocalyx layer of the amoeba

  3. This interaction causes clumping in cilia and results in gyration toward the amoeba thus strengthening the interaction.

  4. Circumvallation starts, that is the invagination of plasma membrane around the paramecium starts.

  5. The point of attachment with cilia provides an anchor around which plasma membrane encircles forming a vacuole, this is also known as food cup.

  6. The vacuole is based on then shrinks resulting in complete loss of water.

  7. The prey dies due to dehydration. The vacuole then separates from the membrane

  8. Digestive and lysosomal enzymes are added to the vacuole 

  9. whichThe undigested part remains in the vacuole and is egested through exocytosis. 

 

Diagrammatic Representation of Pinocytosis.

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Classification of Sarcodina

It is based on classification by B.M Honigberg. Sarcodina is classified under the phylum protozoa. Protozoa are further classified into subphylum Sarcomastigophora, Sporozoa, Cnidospora and, Ciliophora. The subphylum Sarcomastigophora is further classified into three classes, Sarcodina, Mastigophora and, Opalinata. The class Sarcodina is then further classified into subclasses of Rhizopoda and Actinopoda.


Rhizopoda

The superclass Rhizopoda includes the following groups: Lobosea, Acarpomyxea, Acrasia,  Eumycetozoea, Plasmodiophorea, Filosea, Granuloreticulosa, and Xenophyophores. The most common example of this superclass is the Amoeba, which belongs to the subgroup of Lobosea. The common classification pattern is based on the similarities and differences among the pseudopodia of the specific group. The locomotion appendage is pseudopodia, it is known as lobopodia in Amoeba. The common features of this superclass are as follows

  1. They have a naked body, thus their shape is not rigid.

  2. They use pseudopodia as locomotory appendage

  3. The cytoplasm is classified into an endo and ecto cytoplasm.

  4. Contractile vesicles are present in nonpathogenic strains while they are absent in pathogenic strains. 


Lobosea

The small amoeba found in humid atmospheres such as ponds or high water content soil generally belongs to this category of the classification. Some of them are facultative parasites, causing mild infection to fatal diseases. The example of small lobosea amoeba includes Naeglena and Acanthamoeba, both of which are resistant to humid atmospheres. The presence of flagella is a unique feature of Naegleria, generally other species do not have flagellated structure. Naegleria also has some pathogenic strains that cause meningoencephalitis,  it infects humans via nasal passage then along with the bloodstream moves to the brain. In the brain, it multiplies in the meningeal sheath causing inflammation and pain. Some other commonly found species of this group are Hartmanella and Acanthamoeba. 


Actinopoda

These are the second superclass of the class Sarcodina classification. They have the protrusion appendage called axopodia.  Axopodia differs from pseudopodia in the shape and the cytoskeleton arrangement. Axopodia gas along with the slender shape, the cytoskeleton has a microtubular framework that provides the permanence to the structure relative to pseudopodia found in other protozoans. The common groups that classify under it are as follows: Acantharea, Polycistinea, Phaeodarea and Heliozoea. There are some common features that are found in this class, they are as follows,

  1. They are free-floating in nature

  2. They are classified as platonic

  3. They have a special appendage called axopodia that serves as a locomotory tool.

  4. Axopodia also performs pinocytosis

  5. Some groups of this superclass have a skeleton made up of silica, which provides them a relatively rigid structure.

  6. Asexual reproduction takes place by binary fission.

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FAQs (Frequently Asked Questions)

Q.1. How Do Sarcodines Eat Using Pseudopodium?

Ans: Sarcodines eat by the process of pinocytosis, which is a type of phagocytosis. Under which the food is engulfed in the vacuole and is subjected to digestion and assimilation in that vacuole.

Q.2. What Is the Name of the Subphylum Under Which Sarcodina is Classified?

Ans: It is classified under the subphylum called Sarcomastigophora.