Difference between unicellular and multicellular organisms is primarily due to the difference in cell composition, cell function and cell arrangement.
Cell composition: The unicellular and multicellular organisms incorporate a single cell and multiple cells, respectively.
Cell’s function: Unicellular organisms mediate all their cellular activities by a single cell itself, while multicellular organisms perform specific cell activities through a distinct group of cells.
Cell arrangement: It is much simpler in unicellular organisms, whereas multicellular organisms have a complex cell arrangement.
Both unicellular and multicellular organisms are the two major categories of the cell-type, depending on cell number, shape, size etc.
Content: Unicellular vs Multicellular Organisms
|Properties||Unicellular Organisms||Multicellular Organisms|
|Cell number||Contains single cell||Contains multiple cells|
|Cell size||Small||Comparatively large|
|Cell type||Includes organisms having both prokaryotic and eukaryotic cell type||Includes organisms comprising only eukaryotic cell type|
|Cell differentiation||Generally absent, but unicellular yeasts may undergo differentiation||Specialized cell differentiation occurs|
|Evolution||Oldest life forms evolved 3.8-4 billion years ago||These evolved from the prokaryotes|
|Division of labour||Limited to the organelle level||Specified to cellular, tissue and organ system level|
|Regeneration ability||Shows greater tendency to regenerate||Shows low regeneration ability|
|Reproduction||Occurs via budding and binary fission||Occurs via gamete fusion|
|Transport mechanism||The transport mechanism for food and water occurs through simple diffusion||The transport mechanism for food and water occurs by the diffusion, active and passive transport methods|
|Examples||Bacteria, Protozoans, unicellular amoeba etc.||humans, animals, plants etc.|
Unicellular organisms: These can define as the living-organisms, which possess a single cell only to perform different life processes or cellular activities in the cytoplasm itself. It includes prokaryotic organisms like bacteria and archaea, and eukaryotic organisms like protozoa, unicellular algae and unicellular fungi.
One of the most common features of a unicellular organism is that these are microscopic, i.e., easily observable through the naked eye. These commonly reproduce via asexual methods like budding, fragmentation and binary fission, but few can also reproduce sexually via conjugation (like bacteria and protists). Because of single-cell composition, the cell arrangement is quite incomplex. These organisms can thrive in extreme heat, acidity, salinity etc.
Multicellular organisms: These can define as the living-organisms, which possess multiple cells with distinct cell organelles to perform separate cellular activities or different life processes inside a body. It only includes eukaryotic organisms like insects, animals, birds, human etc.
One of the most common features of a multicellular organism is that these are macroscopic or easily noticeable through the open eye. These commonly reproduce via sexual methods (by the formation of zygote), but few members can grow via asexual means like budding, spore formation etc. Because of multiple cell composition, it has a composite cell arrangement. Size of the organism increases as the cell number increases.
Key Differences Between Unicellular and Multicellular Organisms
- Unicellular organisms are single-celled, having a small size, whereas multicellular organisms are large-sized, containing multiple cells.
- The cell arrangement of unicellular organisms is incomplex than the multicellular organisms.
- Unicellular organisms are microscopic only evident under the microscope, whereas multicellular organisms are macroscopic, i.e. detectable with the naked eye.
- Unicellular organisms include microorganisms of both prokaryotic (bacteria, archaea, etc.) and eukaryotic (Protozoa, unicellular algae, unicellular fungi, etc.) cell type. In contrast, human, animal, plant etc. are the members of multicellular eukaryotic cell type.
- The division of labour in unicellular organisms is only limited to the organelle level, while in multicellular organisms it is specified to cellular, tissue and organ system level.
- Due to single-cell and high workload, the unicellular organisms have a shorter life span and lower operational efficiency in comparison to the multicellular species.
- Unicellular organisms generally do not undergo cell differentiation, except unicellular yeasts. Oppositely multicellular organisms undergo specific cell division to form differentiated cells, which can mediate particular tasks.
- The members with multiple cell-type, lose the regeneration ability over a period of cell growth and differentiation or cells ageing. Oppositely, unicellular organisms are immortal by showing a considerable tendency of regeneration.
- Both single and multi-celled organisms comprise of a functional unit of life, known as “Cells”.
- The presence of plasma membrane and cytoplasm occurs in both types.
- Both the organisms share a similar feature by containing DNA and ribosomes for the gene expression.
- The process of cell division typically occurs in both the types commonly through mitosis or meiosis.
- Both organisms need cellular energy to function.
Therefore, we can conclude that a cell can be unicellular and multicellular type, which undergoes the same life processes like respiration, digestion, excretion etc. to sustain life. A significant difference is in a cell number that, in turn, creates a difference in cellular function, work efficiency, cell arrangement etc.
The unicellular organisms compose of a single cell due to which it performs all the work by its own and the working efficiency, and life span becomes low due to heavy workload. In contrast, multicellular species are composed of multiple cells that perform distinct functions by coordinating with themselves and the working efficiency and the life span becomes high as the cells are specialized to perform different tasks.