Multicellular organisms examples and characteristics

Organisms exist on earth as either unicellular or multicellular organisms. Unicellular and multicellular organisms’ examples and characteristics vary based on their structure and how they carry out their cellular activities. In multicellular organisms, life begins as a single cell until mitosis occurs, causing growth and development.

According to evolution, the development of multicellular organisms probably began with the formation of groups of cells formed in several ways. Hence, the major difference between unicellular and multicellular organisms is their cellular structure. Unicellular organisms, also known as single-celled organisms comprise just a single cell whereas multicellular organisms are made up of multiple cells. Nevertheless, there are organisms that are partially unicellular and partially multicellular such as the slime molds and the genus Dictyostelium.

This article discusses the basic facts about multicellular organisms.

Multicellularity definition

This is a condition or state of being composed of many cells or more than one cell performing differing functions. The term “multi” refers to something that is more than one. Therefore, multicellular organisms are made up of many cells. Plants and animals are multicellular organisms including humans, most fungi, and many algae.

These organisms are developed by cellular specialization and division of labor. Cell specialization in multicellular organisms allows cells to work in coordination for the proper functioning of an organism. The cells of multicellular organisms are usually efficient in a single process and then carry out various processes by depending on other cells.

The first multicellular organisms to invade the land were plants. It is supposed that plants first invaded the land and the first land plants appeared about 470 million years ago in the Ordovician period of the paleozoic era. During this era, multicellular organisms began to diversify and become abundant. It is said that multicellular organisms arose because a part of them could die or be removed but the organisms could continue to survive regardless.

Evolution of multicellular organisms

According to the records on the evolution of multicellular organisms, it’s been said that these organisms haven’t always existed. It took one billion years after the formation of the earth for a unicellular organism to appear on earth. For approximately 2 billion years, unicellular organisms existed alone on earth before the multicellular organisms manifested. The first multicellular organisms probably arose about 600 million years ago.

There are different hypotheses on the evolution of multicellular organisms:

• One hypothesis proposes that a group of function-specific cells aggregated into a slug-like mass called a grex. This mass moved as a multicellular unit and interestingly, this is essentially what slime molds do.
• Another hypothesis proposes that a primitive cell underwent nucleus division, and became a coenocyte. Then, around each nucleus (and the cellular space and organelles occupied in the space), a membrane would form resulting in a group of connected cells in one organism. This mechanism can be observed in Drosophila.
• A third hypothesis proposes that as a unicellular organism divided, the daughter cells failed to separate. Hence, resulting in a cluster of identical cells in one organism. This cluster of identical cells could later develop specialized tissues. This mechanism is what colonial choanoflagellates, plant, and animal embryos do.

Levels of organization in multicellular organisms

1. Cell
2. Tissue
3. Organ
4. Organ systems

Listed above are the four (4) levels of organization in multicellular organisms from smallest to largest. Multicellular organisms are made up of many cells which are organized into tissues. The tissues are then organized into organs, which are organized into organ systems that form the organism. The cells in multicellular organisms work in coordination for the proper functioning of the organism. These levels of organization in multicellular organisms lead to efficiency.

The majority of living things that we see without a microscope are multicellular. Humans, as an example of multicellular organisms, have a body that is a very organized and efficient system. The human body is made up of more than one type of cell and has specialized cells that are grouped together to carry out specialized functions. Therefore, the hallmark of multicellular organisms is their ability to maintain physiological homeostasis by communicating among cells, tissues, and organs.

The levels of organization in multicellular organisms as shown in the diagram above are further discussed below:

Cell (Level 1)

The cellular level is the first and most basic level of organization in a multicellular organism. A cell is known as the smallest and basic functional unit of life that is capable of reproduction. All cells have something in common even though they vary greatly in their structure and function based on the type of organism. Cells are filled with a fluid called cytoplasm and contain many structures within the cytoplasm called organelles that carry out various cellular functions. Also, cells are composed of organic molecules, nucleic acids (such as DNA and RNA), and have a membrane made out of lipids.

In organisms, cells may be prokaryotic (without a nucleus) such as in bacteria and archaea (single-celled organisms), or eukaryotic (with nucleus-enclosing DNA) such as in plants, animals, some protists, and many fungi. Basically, all multicellular organisms are eukaryotic organisms because their cells contain a nucleus and membrane-bound organelles. In multicellular organisms, most cells combine to form tissues, while some cells can be found independent of solid tissues and have their own functions. For instance, an example of an independent cell is the red blood cell found in humans that circulates in the bloodstream carrying oxygen throughout the human body.

Furthermore, a multicellular organism is made by four essential processes which include cell proliferation, cell specialization, cell interaction, and cell movement.

Cell proliferation

This involves the production of many cells from one cell. It is an increase in the number of cells due to cell growth and cell division. In multicellular organisms, life begins as a single cell until mitosis occurs causing growth and development. Mitosis is a process of cell division in organisms that has many functions.

Basically, in sexually reproducing multicellular organisms, the main functions of mitosis are the repair/replacement of damaged cells, growth, and development. Many single-celled organisms choose to reproduce asexually, whereas the majority of multicellular organisms prefer sexual reproduction. During mitosis, a single cell divides and produces two identical daughter cells (new nuclei) which contain the same shares of the parent’s cell part. This process of cell division takes place in multicellular organisms and its main purpose is for growth and the replacement of worn-out cells.

Mitosis is responsible for the multiplication of cells during embryonic development in individuals of sexual reproduction. Through this process, the zygote (which is unicellular) is transformed into a multicellular organism. Moreso, mitosis in organisms allows the formation of new cells for tissue growth and also replaces worn-out cells.

In multicellular organisms, meiosis is a different process of cell division that allows the gamete production for reproduction. This form of cell division takes place in specialized eukaryotic cells that possess two (or more) copies of all the chromosomes, DNA, and genes needed for survival. When a single cell undergoes meiotic cell division, 1-4 new cells are produced which only contain single copies of all the chromosomes, DNA, and genes needed for survival.

In the majority of multicellular organisms, meiosis is restricted to germ cells that are kept aside in early development. The germ cells stay in specialized environments provided by the sex organs or gonads. The germ cells within the gonads then proliferate by mitosis until they receive the right signals to enter meiosis.

Cell specialization

The hallmark of multicellular organisms is their ability to differentiate cells into functionally distinct cell types. This cell differentiation is also known as cell specialization which is a process whereby generic cells change into specific cells meant to carry out certain functions within the body. Cell specialization involves creating cells with different characteristics at different positions.

Cell specialization in multicellular organisms allows cells to carry out different functions that are essential for the functioning of the organism. The cells that can become any specialized type of cells are called stem cells. These stem cells are important to multicellular organisms because of their ability to differentiate into other cell types during cell division. Virtually all of the specialized cells of multicellular organisms have the same amount of DNA and the same number of chromosomes as all other cells of the organism. Moreso they develop from the same basic stem cells.

Generally, cell specialization allows cells to perform particular functions and work together to maintain homeostasis within the organism. An example of cell specialization in multicellular organisms is the blood cells within the body system which include red blood cells, platelets, and white blood cells. The red blood cells for example are specialized in carrying oxygen throughout the body. Another instance is seen in sponges, an example of multicellular organisms that are made up of many specialized cell types working together for a common goal e.g digestive cells, epidermal cells, and tubular pore cells.

Cell interaction

The coordination of the behavior of one cell with a neighboring cell is known as cell interaction. Cell-cell interaction involves the direct interaction between cell surfaces. This plays a major role in the development and growth of multicellular organisms. Cell-to-cell interaction enables cells to communicate with each other in response to changes in their immediate environment. Therefore, in multicellular organisms, the coordination of cellular activities relies on cell-to-cell signaling and signal transduction pathways.

Cell movement

This involves the rearrangement of cells to form structured tissues and organs. The body of multicellular organism examples, such as a dog, cat, or tree will definitely exhibit organization at several levels where cells that are similar would group to form tissues, then the groups of tissues form organs. Then the organs with a similar function are grouped into an organ system.

Tissue (Level 2)

The cells of multicellular organisms are capable of coming together for a specific function. A group of similar cells working together to carry out a specific function known as a tissue forms the second level of organization. More complex multicellular organisms examples such as jellyfish, sea anemones, and coral have a tissue level of organization. The jellyfish for instance have tissues that have separate digestive, protective and sensory functions. In plants, four types of tissue can be found which include meristematic, vascular, ground, and dermal tissue.

Humans and some animals as examples of multicellular organisms have four different types of basic tissues namely; Connective tissues, muscle tissue, epithelial tissues, and nervous tissue . The connective tissues e.g bone tissue are composed of fibrous cells and thus give shape and structure to organs. Muscle tissues are composed of cells that can contract and thus allow animals to move. The epithelial tissues make up the outer layers of organs like the skin or outer layer of the stomach. Nervous tissue, on the other hand, is made up of specialized cells that transmit information through electrochemical impulses such as the brain, the spinal cord, and the tissue of nerves.

Organ (Level 3)

The organ is the third level of organization in a multicelled organism. Organs are structures made up of different tissues that carry out specific bodily functions. These structures may be solid or hollow and vary considerably in complexity and size.

Some examples of organs found in fungi (an example of multicellular organisms) are haustoria (absorbing organs) and sexual organs. In plants, the roots, stems, flowers, and leaves are all examples of organs. Moreso, the lungs, heart, and brain are all examples of organs in animals and humans. Therefore, organs can be as complex as a human liver, or as primitive as the brain of a flatworm formed by a group of nerve cells, or can be as huge as the stem of a sequoia. The stem of a sequoia can be up to 300 ft in height.

Biological responsibilities such as barrier function, circulation, digestion, respiration, and sexual reproduction are delegated to specific organs in multicellular organisms. Organs such as the skin, heart, stomach, lungs, and sex organs carry out barrier function, circulation, digestion, respiration, and sexual reproduction, respectively for the functioning of the organism.

Organ systems (Level 4)

This is the fourth and final level of organization in a multicellular organism. Organ systems
are a collection of organs that work together to carry out a similar function. Complex multicellular organisms examples such as mammals, flowers, and trees have organ systems.

In humans, for instance, there are 11 different organ systems, each with its own specific functions. The digestive system is one example that is made up of organs working together to digest and absorbs nutrients from food. Each organ in an organ system focus on a part of the task. For instance, the mouth in the digestive system ingests the food, the stomach then crushes and liquefies it, while the pancreas and gall bladder produce and release digestive enzymes. The intestines then absorb nutrients into the blood.

Furthermore, in as much as most organ systems control a few specific physiological processes, some processes can be more complex and need multiple organ systems to work together. For instance, blood pressure is controlled by the combination of the circulatory system, the renal system (kidneys), and the nervous system.

Characteristics of multicellular organisms

1. One of the distinct characteristics of multicellular organisms is that they are complex and comprises of multiple cells.
2. The cells of multicellular organisms are specialized and exhibit division of labour.
3. Multicellular organisms are visible to the naked eye,thus, they are said to be macroscopic.
4. They have organs and organ systems that carryout various life functions such as digestion, blood circulation, excretion, reproduction etc.
5. Multicellular organisms must keep their cells awash in an “internal pond”because an aqueous medium is needed for the cellular exchange of nutrients, gases, and wastes.
6. These organisms contain membrane-bound nucleus and structure, thus they are all eukaryotic organisms.
7. The size of a multicellular organism increases with the number of cells in the organisms.
8. Majority of multicellular organism reproduce sexually.

Multicellular organisms examples

1. Humans
2. Stinkhorns
3. Birds
4. Puffballs
5. Reptiles
6. Giant kelp
7. Mammals
8. Truffles
9. Amphibians
10. Insects
11. Molds
12. Fishes
13. Trees
14. Mildews
15. Aquatic plants
16. Mushrooms
17. Flowers
18. Coral
19. Food crops
20. Sea anemones
21. Weeds
22. Volvox
23. Multicellular algae
24. Worms

There are so many more examples of multicellular organisms around us. The majority of living things that we see without a microscope are multicellular.

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Advantages of multicellular organisms

1. Cells can take care of each other
2. The competitive advantage of a bigger size
3. Longer lifespan

Cells can take care of each other

Multicellular organisms are made up of many cells that can take care of each other. In these organisms, the production and repair of damaged cells are achieved by employing other types of cells that serve as worker cells. These worker cells are the ones responsible for the healing of wounds, as well as the regrowth of limbs and body parts. They can also repair cellular damage from toxins and bacteria attacking the body system.

Competitive advantage of a bigger size

The advantages of multicellularity are that it allows the organism to exceed the size limits usually imposed by diffusion. Single cells with increased size have a decreased surface-to-volume ratio. Thus, they have difficulty absorbing sufficient nutrients as well as transporting them throughout the cell. Therefore, one of the advantages of multicellular organisms is the competitive advantage of an increase in size without its limitations.

Longer lifespan

Another advantage of multicellular organisms is that they have longer lifespans because they continue to live even when their individual cells die. The advantages of being multicellular permit an increase in complexity by allowing cell differentiation within an organism. Multicellularity allows the organism to develop a higher level of adaptation to its surroundings.

Disadvantages of multicellular organisms

1. Multicellular organisms require more energy to feed their multiple cells compared to a unicellular organism that is made up of just one cell.
2. The risk of infection from unicellular organisms is one of the disadvantages of multicellular organisms.
3. Being multicellular involves a complex cellular make-up, hence, it takes longer for the complex parts of the organism to develop to maturity.
4. Some groups of cells relies on each other to perform certain tasks in order to help the organism function. This can be a disadvantage because if one cell group or organ fails, the whole body system can fail. For instance, the failure of one organ, the heart, can lead to the death of the organism.

Multicellular vs unicellular organism

We can compare and contrast unicellular and multicellular organisms by analyzing their differences and similarities.

Difference between unicellular and multicellular organisms

The difference between unicellular vs multicellular organisms is majorly due to the difference in the arrangement of the cells and how it functions. The major factor used to compare and contrast unicellular and multicellular organisms is the number of cells that make up the organism. Unicellular organisms are made up of just a single cell whereas multicellular organisms are made up of multiple cells.

Another difference between unicellular and multicellular organisms is the way they carry out their cellular activities and life processes. In unicellular organisms, all their cellular activities are performed by a single cell, while the cells in multicellular organisms carry out specific cell activities by working together in groups. The arrangement of cells in a unicellular organism is much simpler whereas cell arrangement in multicellular organisms is found to be complex.

The similarity between unicellular and multicellular organisms

1. They are both made by a functional unit of life known as a cell.
2. Both the cells of unicellular and multicellular organisms have cytoplasm and a plasma membrane.
3. They both contain DNA and ribosomes for gene expression.
4. In both organisms, cell division usually occur through mitosis or meiosis.
5. Unicellular and multicellular organisms both need cellular energy to function.

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