The difference between unicellular and multicellular organisms? About a few million cells.
Kidding aside, it’s pretty obvious what the biggest difference between unicellular and multicellular organisms are. Unicellular organisms are those with one cell. These are organisms that belong to the kingdoms of Monera and Protista.
On the other hand, multicellular organisms have many (and we mean many – an average human body has 37.2 trillion cells) cells, some of which have different functions. These organisms belong to the fungi, plantae, and Animalia kingdoms.
But aside from the number of cells these organisms have, these are the other differences between unicellular and multicellular organisms.
Prokaryotes vs. Eukaryotes
Unicellular organisms are prokaryotes while multicellular organisms are eukaryotes. Prokaryotes are cells that do not have a cell nucleus or any organelles that are held in place by membranes. This means that their genetic material is not bound to a nucleus. Eukaryotes, on the other hand, are cells that have a membrane-bound nucleus that holds genetic material and its membrane-bound organelles.
Because unicellular organisms only have one cell, they are almost always microscopic and invisible to the naked eye. They will require a microscope or other tools just to be seen. On the other hand, multicellular organisms can vary in size. Organisms like some members of the kingdom fungi will require a microscope to be visible, but organisms can grow as large as the blue whale.
Complexity and Body Functions
As a result of size, multicellular organisms are often much more complex than unicellular organisms. Unicellular prokaryotes do not have internal organs, while multicellular organisms have different types of cells to function. Take humans, for example, with certain cells for our skin, hair, and organs. In comparison, unicellular organisms’ bodily functions are limited to its organelles while multicellular organisms have multiple functions (e.g. reproduction, digestion, respiration for humans and other animals).
A unicellular organism carries out all its life processes in that cell alone. While it can reproduce, if it enters an environment it cannot survive in, it is unlikely to survive as everything it needs to process and stay alive is in that one cell. On the other hand, a multicellular organism has millions of cells to replace dead cells, so the organism can continue to function.
Let’s say, for example, you get a cut on your arm. On a microscopic level, your skin cells are damaged and your cut exposes the inside of your body to external factors like bacteria to enter your body and infect it. Even though some of your skin cells are damaged, the rest of your cells are at work. You have cells clotting the cut, cells beginning to replace the old cells, and cells that are killing any external bacteria that may have entered the cut. A unicellular organism cannot do this, and any injury could mean its death since it is not equipped to survive.
As mentioned above, it is easy for a unicellular organism to take on damage and die compared to multicellular organisms built to withstand bodily damage. Since unicellular organisms are much less complex and have organelles focused on one function, it can die easily, giving it a shorter lifespan.
For example, bacteria can be common and millions of cells are found all around us (including our own bodies), but they only have an average lifespan of 12 hours before it dies out. However, in its lifespan, it’s likely to have already generated over two million other bacteria to take its place.
In contrast, an average human has a lifespan of 72 years even if they take minor injuries at some point in their life. This is because cells are continuously regenerating and can slowly heal any injuries. Sure, cell regeneration won’t stop fatal wounds because cells cannot grow fast enough to stop the blood loss and regenerate skin and muscle cells fast enough, but this ability to heal from certain injuries cannot apply to unicellular organisms.
Outer Cells vs. Inner Cells’ Functions
Unicellular organisms do not have cells outside of its organelles to protect its internal parts, so it is exposed to its entire environment. Multicellular cells, however, can have external cells specialized to protect its internal parts from the environment while the internal cells focus on other functions. It’s why you’ll notice that most plants and animals have a much tougher exterior compared to the texture of its inner organs.
Cells from both unicellular and multicellular organisms can only reproduce asexually. However, because a unicellular organism has everything inside of it, the process of reproduction consumes the entire cell. In comparison, not all cells from multicellular organisms are dedicated to reproduction. In a human body, for example, somatic cells have the purpose of forming the body and do not reproduce.
Because of the size difference, a unicellular organism is operating at a heavy workload as everything in its cell needs to perform to maintain the cell’s lifespan. A multicellular organism, however, have cells with less workload because it is working with other cells to perform certain functions. This affects the way unicellular organisms have a much shorter lifespan than multicellular organisms.
The differences between a unicellular organism from a multicellular organism has more to do than just numbers. However, it is these numbers that affect their differences. A unicellular organism must depend on everything it has inside of it to survive, and because it is exposed to harsh environments with no form of protection, it has a shorter lifespan and can easily die from the slightest form of trauma.
On the other hand, because multicellular organisms have over millions of cells to perform various functions, they’re more likely to survive the same form of trauma that can easily eliminate its unicellular counterpart. And out of these cells, not all of them provide the same function to the organism as each plays a specific role to ensure the organism’s survival for as long as possible.
However, that’s not to say that a unicellular organism is weaker than a multicellular organism. While bacteria have a short lifespan, they can still grow rapidly in numbers – enough to infect multicellular organisms like humans. This is why it’s important to study both unicellular and multicellular organisms to understand the implications of their characteristics and how they affect other organisms.