Phylum Protozoa: Characteristics & Groups/Classes with Life Cycle of Plasmodium

(Animal like Protists)

Kingdom Protoctista is a group of living organisms which includes eukaryotes and is unicellular in nature. Phylum Protozoa shows following Important Characteristics:

i.            Acellular Body

The animals of Phylum protozoa contain a cellular body i.e. the body is not divided into cells. A single cell performs all functions. They are microscopic animals.

ii.            Shape of Body in Phylum Protozoa

They are elongated, rounded oval or irregular in shape. The body is covered by plasmalema (irregular animals) or pellicle (regular animals).Phylum Protozoa

iii.            Protoplasm

The protoplasm usually consists of two parts, outer ectoplasm and inner endoplasm.

iv.            Presence of Nucleus in Phylum Protozoa

A definite nucleus is present in the body, so they are eukaryotes. The number of nucleus is one or many.

v.            Nutrition in Phylum Protozoa

They capture their food and digest it in food vacuole. The digestion is intercellular.

vi.            Respiration in Phylum Protozoa

Respiration takes place by diffusion through the external surface of the body.

vii.            Osmoregulation

They can maintain the internal salt and water balance due to pressure of osmoregulatory vacuoles in free living animals. In parasite forms, the osmoregulatory vacuole is absent.

viii.            Movement

Movement takes place by pseudopodia, cilia or flagella.

ix.            Irritability in Phylum Protozoa

The protozoaus show the response against different stimuli e.g. they show the positive response in dim light but negative response in intense light. Similarly, they show the positive and negative responses against different type of stimuli.

x.            Reproduction in Phylum Protozoa

Reproduction occurs by simple cell division, spore formation or sexual method.

All protozoa animals are unicellular. Most of the protozoa animals ingest their food by Endocytosis. There are different groups of protozoa. These are Zooflagellates, Amoebas, Actinopoda, Forminifera, Apicomplexans and Ciliates.

Classes of ProtozoaGroups of Protozoa (Table)

A.     AmoebasAmoebas

  1. This group includes all the free living freshwater, marine and soil amoebas. It also has some parasites of animals.
  2. They are unicellular.
  3. Amoeba lack flagella. They move by forming specialized cytoplasmic projections called pseudopodia (false foot).

Example: The intestinal parasite, Entamoeba histolytica, causes amoebic dysentery in humans.

The Giant Amoeba

The giant amoeba Pelomyxa palustris is the most primitive amoeba. This species has multiple membranous-bound nuclei. Such organelles are not present in any of the eukaryote. The giant amoeba obtains energy form methanogenic bacteria. These bacteria live inside the giant bacteria. Giant amoeba lives inside the mud at the bottom of freshwater ponds. They are involved in the degradation of organic molecule.The Giant Amoeba

B.     Zooflagellates

  1. These protists are mostly unicellular. A few organisms are colonial.Zooflagellates
  2. They have spherical or elongated bodies.
  3. Zooflagellates have single central nucleus.
  4. They possess one to many long, whips like flagella for locomotion. These flagella are present at anterior end. Flagellates move rapidly. They pull their body forward by lashing flexible flagella.
  5. They obtain their food by ingesting living or dead organisms. Some flagellates obtain nutrients from dead organism or decomposing organic matter.
  6. They may be symbionts, parasites or free living.

        i.            Symbionts:

Trichonymphas are complex and specialized flagellates. They have many flagella. They live as symbionts in the guts of termites. Symbionts help termites in the digestion of dry wood.

      ii.            Parasitic flagellates:

Parasitic flagellates cause diseases. For example, Trypansoma is a human parasite. It causes African sleeping sickness disease. It is transmitted by infected tsetse fly.

   iii.            Free living flagellates:

Choanoflagellates are sessile, marine or freshwater flagellates. They are attached by a stalk. Their single flagellum is surrounded by a delicate collar. These organisms are most important for evolutionary point of view as they have striking similarities with the collar cells of the sponges. It is believed that sponges have evolved from the Choanoflagellates.

C.      Ciliates

  1. Ciliates are unicellular organisms.
  2. They have flexible outer covering called pellicle. It gives them a definite shape. But this shape can be changed.Protozoa
  3. Their body is covered with cilia. The surface of paramecium is covered with several thousand cilia. These cilia are fine, short, hair-like structures. Cilia move in coordinated manner. These cilia help the organism to move forward, move backward and turn around.
  4. Some ciliates are sessile and remain attached with the rock or other surfaces. Their cilia produce water current. This water current brings the food towards the organism.
  5. Most ciliates ingest bacteria and other tiny protists. .
  6. They have special organelles called contractile vacuoles. They regulate the water movement in freshwater ciliates.
  7. All other protozoans have single nucleus but ciliates have two kinds of nuclei:
  • Micronuclei: These are small and diploid nuclei. Each ciliate have one or more micronuclei. They are involved in sexual reproduction.
  • Macronuclei: It is a large and polyploid (with many sets of chromosomes) nucleus. It controls cell metabolism and growth.
  1. Most of the ciliates undergo sexual reproduction called conjugation. During conjugation two individuals come close each other and exchange genetic material.

D.     Foraminiferans and Actinopods

  1. These marine protozoans produce shells or tests. Tests of Foraminiferans are made up of calcium carbonate. While the tests of Actinopoda is made up of silica.
  2. The shell or tests contain pores through which cytoplasmic projections come out. These cytoplasmic projections form a sticky. Interconnected net. Preys are entangled in this net.
  3. Dead Foraminiferans sink to the bottom of the ocean. They form grey mud. This grey mud gradually transformed into chalk. The old and dead Foraminiferans are changed into limestone deposits.

E.      ApicomplexansApicomplexans

  1. Apicomplexans are a large group of parasitic protozoa. Some of them cause serious diseases in man such as malaria.
  2. They lack locomotory organs, but they move by flexing (bending).They develop spore at some stage of their life. These spores are small infective agents that are transmitted to the next generation.
  3. Many Apicomplexans spend their life in two or more hosts.
  4. Example: Plasmodium.


It causes malaria. It is enters into human body of man by the biting of female Anopheles mosquito. Plasmodium first enters into liver cells. Then it enters red blood cells (RBC). It multiplies in the RBC. The RBC bursts and releases many parasites. The simultaneous bursting of millions of RBC caused the symptom of malaria. These symptoms are chill and high fever. This fever is caused by toxic substances. These toxic substances also affect the other organs of the body.Plasmodium

Life Cycle of Plasmodium:

Plasmodium is a micro-organism, belongs to the phylum protozoa. It causes a disease in the body of human beings, called Malaria. The life cycle of plasmodium is completed in the body of two hosts, hence they are known as Digenic Parasite.

  1. In body of man, it is called Primary Host.
  2. In body of female Anopheles mosquito, it is called Secondary Host.

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