Intracellular Structure of Protozoa

Protozoa is a eukaryotic cell, therefore like other eukaryotes they contain a cytoplasm which is surrounded by a cell envelope and also contains the nucleus.

Cytoplasm

• It is a homogenous substance, which is consists of globular protein molecules. These proteins are linked together and create a three-dimensional molecular framework. 
• The cytoplasm contains different submicroscopic protein fibrils such as fibrillar bundles, myonemes, and microtubules. All of these are responsible for contractility in protozoa.
• The pigments are diffused throughout the cytoplasm of protzoa, these are responsible for the different colors of protozoa such as green, brown, blue, purple, or rose.
• The cytoplasm of protozoa is divided into two categories such as the ectoplasm and the endoplasm.
• The ectoplasm is a more gel-like structure while the endoplasma are more voluminous and fluid. Different organelles are mainly found in endoplasma.
• The cytoplasm is surrounded by a membrane.
• It contain different organelles such as ribosomes, Golgi complexes or dictyosomes (piles of membranous sacs), mitochrondria, kinetosomes or blepharoplasts (intracytoplasmic basal bodies of cilia or flagella), food vacuoles, contractile vacuoles, and nuclei.

Nucleus

• The size, shape, and number of protozoa vary based on the types of species.
• The protozoan nucleus contains the following components; chromosomes, the nucleolar substance, the nuclear membrane, and the karyoplasm (nucleoplasm).
• Protozoa contain one eucaryotic nucleus, even some of them contain multiple nuclei (e.g., almost all ciliates) throughout the greater part of the life cycle.
• The ciliates contain 2 dissimilar nuclei one of them is larger known as macronucleus and the 2nd one is small known as micronucleus.
• The macronucleus helps to maintain the metabolic activities and regeneration processes, while the micronucleus is helped with reproductive activity.
• The number of chromosome in nucleus vary based on the types of protozoa. For example, Spirotrichonympha polygyra has 2 haploid chromosomes; Spirotrichosoma magnum has 60.
• Protozoa mainly divide by both sexually and asexually.

Plasmalema and Other Cell covering region

• The cytoplasm is separated from the external environment by a semipermeable membrane known as Plasmalema.
• This Plasmalema protects the cytoplasm, helps in the exchange of various substances, and also helps in attachment with other cells.
• Although all protozoa contain a cell membrane, some of them have compound coverings of membranes transformed for protection, support, and movement. Such combinations of membranes are known as a pellicle. For example; In Euglena the pellicle is organized to ensure flexibility: in Paramecium it is quite rigid.
• The simplest form of pellicle is known as Plasmalema, for example, Amoebas are covered by a plasmalemma only.
• In some protozoa (e.g., A. proteus) a diffuse layer of mucopolysaccharides is surrounded the plasmalema. This layer helps in pinocytosis or in the adhesion of the cell to the substratum.
• The pellicle of a ciliate is thick and often variously ridged and sculptured. 
• In some protozoa and additional protective covering region has been evolved around the pellicle. These coverings are known variously asthecae, shells, tests, or loricae and occur in almost all major groups of protozoa. 
• The theca is a secreted layer directly apposed to the cell surface.
• Tests, shells, and lorica are coverings that are loosefitting. 

Pinocytosis: uptake of Nuids and soluble nutrients through small invaginations in the cell membrane that subsequently form intracellular vesicles

Feeding Structure

• The food-gathering structure of protozoa is varies based on the type of species such as; the pseudopodia of amoebas through the tentacular feeding tubes of suçtorians to the well-developed “mouth” of many ciliates.
• Amoeba uses the pseudopodial engulfment method to gather the food material.
• In ciliate the foods are ingested through the cytostome.
• Anoral groove is an indentation in the pellicle of certain ciliates. It guides food towards the cytosome and acts as a concentrating device. The interest of membranelles to the oral groove gives it a peristome.

Cysts

• In adverse environmental conditions, many protozoa form a resistant cyst to survive. This cyst can resist desiccation, low nutrient supply, and even anaerobiosis.
• The parasitic protozoa forms cysts when they transmitted from one host to another host.
• The parasitic protozoa of the intestine enter the alimentary tract as resting cysts, hatch in a suitable region, and then leave the host again as dormant cysts.
• The Asexual reproduction in some ciliates and flagellates help in cyst formation while Sexual reproduction of Sporozoa invariably results in a cyst.
• The cytoplasm is commonly attached to the cyst wall at one or several points: it is reduced in size and dormant.

Other protective Structure

Protozoa protect themselves by forming various structures or materials within the membrane-bound vesicles, such as;

• Certain ciliates release a mucilage from subpellicular vesicles known as mucocysts. 
• Several protozoa protect themselves by the expulsion of harpoonlike trichocysts. It contains a threadlike tubular structure inside of the occlusion at the distal end which contains the toxin.
• When the toxicyst is released the toxin is spread along the exterior of the thread. Toxicysts are used to paralyze and capture prey: the toxin causes paralysis and cytolysis when it contacts protozoan prey.

Locomotor Organelles of Protozoa

For locomotion, Protozoa uses three types of locomotor organelles such as pseudopodia, flagella, and cilia. There are also other protozoa which carry out their motility by the gliding mechanism.

• Pseudopodia: Those protozoa lack rigid pellicle, they mainly used pseudopodium. It is a temporary projection of the cytoplasm and a characteristics feature of the amoeba. Sometimes, they used pseudopodia to capture food substances.
• Flagella: It is a filamentous extension of the cell. The number of flagella in an individual cell varies from 1 to 8. Flagellum is made up of two important part such as; axoneme, it is an elastic filament, and the second one is a contractile cytoplasmic sheath that surrounds the axoneme.

Cilia: Some protozoa used cilia for locomotion as well as for the ingestion of foods and often function as a tactile organelle. These are fine and short threadlike extensions from the cell. The length of cilia varies based on their location on cell or they can be uniform in length. These are arranged in longitudinal, oblique, or spiral rows, inserted either on the ridges or in the furrows.