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Unicellular Organisms Definition

In biology, the term 'unicellular organisms' itself defines what type of living entities they are. They are single celled organisms, wherein the functions like feeding, locomotion, expelling wastes, reproduction, etc. are carried out by the single cell. In most cases, they are minute sized and require microscopes for viewing. In contrary to this, organisms consisting of more than one cell are known as multicellular organisms. All plants and animals which are viable with naked eyes are examples of multicellular types.

Based on the complexity of the cell, unicellular organisms are classified into two types, namely, prokaryote and eukaryote. The former has simple cellular structure, when compared to the latter type. Also, the prokaryotic unicellular organism (e.g. bacteria) is devoid of cell nucleus; whereas the eukaryotic unicellular organism possesses nucleus in the cell. Speaking about the unicellular organisms functions, they acquire specific methods to move from one place to another, assimilate nutrients, grow and multiply their population.

Unicellular Organisms Examples
Majority of the microbes (excluding virus) are unicellular in organization. According to the theory of evolution, unicellular organisms were the the first to evolve on Earth. Their origin date back to 3.8 billion years ago. Each of them possesses specific characteristic features, which help in adaptation to a wide range of environmental conditions. You can find unicellular organisms in every habitat, even in the least hospitable conditions. Following are some of the common unicellular life forms:

Amoeba is also a unicellular, eukaryotic protozoan, which is found in almost all freshwater habitats. Well known for its unique mode of locomotion, it does not have a particular shape. In fact, its cell shape depends upon the prevailing condition. Whenever required, an amoeba extends false feet (pseudopodia) and uses it for phagocytosis and locomotion. Know more on classification of amoeba.

A slipper shaped eukayotic protozoan, paramecium consists of a single cell. Its body is lined by minute hair like cilia, which help in locomotion and feeding. Paramecium reproduction is studied in detail, so as to understand the multiplication rate. Under favorable conditions, it reproduces by asexual method; while in stress, reproduction take place sexually.

All of us have a brief idea about bacteria. Right from formation of curd to infectious diseases, bacteria are present anywhere in the environment. They are minute and have different shapes (rod, spherical, spiral, etc.). Some of the bacterial strains are adapted in harsh conditions such as deep inside the earth's crust and hot springs. They play a crucial role in recycling of nutrients.

Also known as blue green algae (BGA), cyanobacteria is a unicellular organism. It possesses the characteristics of both bacteria and algae, hence the name. Cyanobacteria resembles algae as both undergo photosynthesis for food production. While the prokaryotic nature of BGA makes it similar to bacteria. 

You may be interested in knowing euglena facts and chlorella algae.

Besides these, examples of unicellular organisms include diatoms, euglena, chlorella and chlamydomonas. In order to get an idea on how do unicellular organisms look like, you can study themicroorganisms in pond water. For this biological experiment, collect a fresh water sample from a garden pond in a small bottle. Using an eye drop, put a small drop of the water sample in a microscope slide, gently place coverslip over it and observe under the microscope. You will find minute organisms moving randomly, which are nothing but unicellular organisms.

Types of unicellular organism
These organisms have simple body mechanism. Being single celled they lack tissues and specific organs.
  1. Unicellular prokaryotic organisms: All prokaryotes are unicellular in nature, like different types of bacteria and cyanobacteria (blue green algae). They do not have membrane bound true nucleus. Genetic material present in prokaryotes are known as nucleiod. Their genetic material also lack presence of intron (nonfunctional part of gene). For example E.coli, Salmonella typhi(bacteria) and Nostoc (cyanobacteria).
  2. Unicellular eukaryotic organisms: Mainly organisms of kingdom protista are considered as unicellular eukaryote. These are mainly free living, aquatic parasites and are categorized as 3 types a) protozoa-known as primitive animal b) fungi-mainly chemo-heterotroph and c. algae or plant like protista.
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