Why bacteria and viruses are so much smaller than animal cells?

Why bacteria and viruses are so much smaller than animal cells?

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Why are bacteria and viruses so much smaller than animal cells? - I don't have more information about the question, sorry if this is too vague.

Cells/viruses vary in size mainly according to their function, components & requirements imposed by cell division/virus replication.

Bacterial cells are so small because they need a large surface area to cell volume to take in nutrients. Bacteria accumulate nutrients from the environment by diffusion alone, and so adopt certain sizes and shapes so they can import nutrients most efficiently. other evolutionary advantages are also existent in response to the demands of the environment and predators

Viruses function is pretty minimalistic. they pretty much consists of nucleic acid surrounded by a protective coat of protein called a capsid. this is the primitive virus, all it nedds is to infect cells and pass the information stored inside them.

The size of animal cells however depends partially on the type of cell and its function. Red blood cells are pretty small in comparison to other animal cells, they don't need to divide (so do not own a nucleus) and need a large surface area in order to diffuse Oxygen efficiently. Egg cells for example serve to store nutrients and don't need to carry out active metabolism, and so they are much bigger.

I took some of the examples from here, and for more technical information about the upper limits of cell size and plant cells click here.

The big and the small

Getting your head around mind-bendingly big and unimaginably small things is really hard - our brains just weren't built to do it.

Bacteria are about one tenth the size of our cells, but ten times bigger than viruses. (Source: CDC/ Dr. Ray Butler)

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To get by as a human, all evolution required of our ancestors was the ability to outrun or outwit anything that could kill them before they made a baby. Being able to mentally picture all the mammoths on the planet wasn't high on the essential skills list.

Thanks to money, or what we can buy with it, most of us can wrap our heads around the relative size of a thousand, a hundred thousand and a million. But once we get past a few million it all starts to get harder to picture. The same goes for the small stuff — we know viruses and bacteria are small, and atoms are even smaller, but just how small is as hard to grasp as Rupert Murdoch's bank balance.

But it's easy to get a grip on just how huge or tiny something is: you just compare its size to the size of things we're familiar with, like us.

On a scale of the smallest to the largest things we can see in the solar system, we sit pretty much dead middle. Or Danny DeVito does — at 1.50 metres he's 10 billion times bigger than an atom and one billion times smaller than the diameter of the sun.

The small stuff

Our bodies are made up of cells, and compared to us they're tiny. The average human cell is about 10 -15 micrometres (µm), which means we're about 100,000 times bigger than our cells. If your cells were the size of a five cent piece, you'd be two kilometres tall.

Not all cells are that tiny. Human egg cells are about 130 µm, which is wider than a human hair (100 µm) and 30 times wider than the head of even the most ambitious sperm. But they're nothing compared to the giants of the cellular world. The nerve cells in your sciatic nerve are about a metre long - they run all the way from your spine to your foot.

Bacteria are cells too, but they're only about one tenth the size of our cells. And viruses are smaller again — they're about a hundredth the size of our cells.

So we're about 100,000 times bigger than our cells, a million times bigger than bacteria, and 10 million times bigger than your average virus!

If a virus was the size of a five cent coin, a bacterium would be the size of a dinner plate, and you would be 200 kilometres tall!

Viruses are tiny compared to all other living things, but they're giants compared to atoms and molecules.

The really small stuff

The flu is a pretty typical virus. It's just a chunk of RNA wrapped in a bit of protein, measuring about 120 nanometres (nm) across, which makes it about a thousand times bigger than an atom.

Atoms are the basic units of matter. They're like the Lego bricks that all things are made of, from stars to steel and from oxygen to oxen. They're tiny. But even tinier than atoms are the protons, neutrons and electrons that they're made of. A hydrogen atom is 100,000 times bigger than the proton that sits in the middle of it! That's the same size difference as between you and one of your cells. So protons are tiny, and electrons and neutrons are too.

But what about at the other end of the scale?

The big stuff

When we think of the big things in the universe, we think planets, stars and galaxies. And while Earth seems big from where we sit, it's pretty puny as far as the solar system goes: you could fit more than a million earths inside the sun.

But the sun's no galactic giant either. It's nothing compared to the really big stars. The biggest known star, VY Canis Majoris, is a hypergiant around 2000 times the size of the sun. So if the sun was the size of a ping pong ball, VY Canis Majoris would be almost as big as a footy field.

But it's not just the size of stars that's big, it's the number of them as well. No one's done a head count yet, but best estimates put the number of stars in the Milky Way at around two hundred billion. That's 200,000,000,000 stars in our galaxy alone!

And there are plenty more galaxies out there. In fact, it looks like there are around two hundred billion galaxies as well. So there is at least one galaxy out there for every star in the Milky Way — it's brain hurty time!

If you multiply the number of galaxies by the average number of stars in them, you get the number of stars in the universe, which is something like a million billion billion stars. Forget about Rupert's riches — that's a seriously big number! It looks even bigger when you write it out in full:

There are 1,000,000,000,000,000,000,000,000 stars in the universe.

It's hard to imagine a number that big, but you'd be surprised how often you come across things that size. Every time you have a glass of water you're swallowing 10 million billion billion water molecules. So there are ten times more molecules in a glass of water than there are stars in the entire universe.

Why Are Cells so Small?

Brooklyn College explains that cells are small because they must have a large surface area relative to the amount of volume they contain to function properly. As a sphere grows larger, its volume increases much more rapidly than its surface area does. This presents logistical problems for the cell, as it tries to transport resources and products through a large volume without the resources available via a large surface.

As an example, a typical animal cell requires oxygen to survive. The size of the cell partially dictates the amount of oxygen it needs, while the surface area of the cell limits the amount of oxygen that can be absorbed at a time. Accordingly, when the size of a cell grows, its demand for oxygen and other resources rises at a rapid rate, while its capacity for absorbing oxygen increases more slowly. At some point, the size of the cell will cause the cell to divide or die, according to Brooklyn College.

Despite the limitations on cell size that are imposed by the surface-to-volume ratio, a 2013 study published in the journal Nature Cell Biology demonstrates that gravity also limits cell size. While gravity is a negligible force at very small scales, cells that attain about 1 millimeter in diameter must include structural elements to keep some organelles stable under the influence of gravity. Without such elements, cell components can lose their structural integrity.

Characteristics of the bacterium

Most bacteria, apart from the cocci variety, move around with the aid of small lashing tails (flagella) or by whipping their bodies from side to side. Under the right conditions, a bacterium reproduces by dividing in two. Each ‘daughter’ cell then divides in two and so on, so that a single bacterium can bloom into a population of some 500,000 or more within just eight hours.

If the environmental conditions don’t suit the bacteria, some varieties morph into a dormant state. They develop a tough outer coating and await the appropriate change of conditions. These hibernating bacteria are called spores. Spores are harder to kill than active bacteria because of their outer coating.


The biggest difference between these two groups is that bacteria are considered living things and are made of cells, whereas viruses are not (and aren't made of cells).

Expanding on that, bacteria are unicellular organisms belonging to the domain Eubacteria but now called Bacteria and Archaea ,

They are ubiquitous on Earth and while they often have a bad reputation for causing disease, many are harmless to humans and some are very helpful to us (for example, the E. coli in your gut helps you digest food).

Viruses, on the other hand, are not in the organism classification scheme as they are not considered living things. They are not made of cells like organisms, instead they are generally composed of a protein coat surrounding genetic material (DNA or RNA). They do not respire the way organisms do, nor can they reproduce independently. Some have a lipid coat while others do not.

Here are a few other differences:

  1. Though bacteria are small, viruses are much smaller still. They are said to be ultramicroscopic. Bacteria can be more than 10 to 100 times bigger than viruses!
  2. Viruses are generally parasitic in some way. Most tend to damage their hosts. Some bacteria do this, but many do not.
  3. Bacteria reproduce by binary fission, while viruses must use host cells to create more viruses.

One more note-- viruses don't just attack large organisms. Viruses will even attack bacteria!

Here is an image that helps compare the structure of the two (note that it is not to scale- the virus should be much smaller):

Difference Between Bacteria and Yeast

Microorganisms are a taxonomically diverse group of organisms. Microbes include bacteria, cyanobacteria, protozoa, some algae, fungi and viruses.

Bacteria were first observed in 1674. The name originated from the Greek word “small stick”. Bacteria are unicellular and typically few micrometers long. They have a diversity of shapes. They may occur as attached to surfaces. They form biofilms having different species. Their thickness can be a few micrometers to several centimeters. There are many shapes such as cocoid, bacilli, spiral, comma and filamentous. There is no membrane bound organelles. They lack a nucleus, mitochondria, chloroplasts, golgi bodies and ER. DNA is present in the cytoplasm, in an area called nucleoid. DNA is highly coiled. 70+ type ribosomes are present. Cell wall consists of peptidoglycans. Gram positive bacteria possess a thick cell wall with several layers of peptidoglycan. Gram negative bacteria cell wall has few layers surrounded by a lipid layer.

A smaller DNA molecule may also be present. It is called a plasmid. The plasmid is circular and contains extra chromosomal material. It undergoes self replication. They carry genetic information. However, the plasmid is not essential for the survival of the cell. Flagella are rigid protein structures used in motility. Fimbriae are fine filaments of the protein involved in attachment. Slime layer is a disorganized layer of extra cellular polymers. Capsule is a rigid polysaccharide structure. It is also called the glycocalyx. The capsule provides protection. It contains polypeptides. Hence it resists phagocytosis. Capsule is involved in recognition, adherence and formation of biofilms. Capsule is associated with the pathogenesis. Some produce endospores which are highly resistant dormant structures.

Yeast is a fungus. Fungi are eukaryotes. Most of which are multicellular with a vegetative body forming a mycelium, but yeast is unicellular. Fungi are always heterotrophic, and they are the major decomposers living on dead organic matter. Decomposers are saprophytes. They secrete extra cellular enzymes to digest organic matter and absorb the simple substances formed.

Classification of fungi is based on 2 main characteristic features. Those are morphological features of vegetative mycelia and characteristics and organs and spores produced in sexual and asexual reproduction. Fungi are classified into 3 main divisions Zygomycetes, Ascomycetes and Basidiomycetes. Yeast is a unicellular Ascomycetes fungus. It is a saprophytic fungus growing in sugary media. It is round or spherical or oval in shape. It contains a single nucleus. At the centre of the cell is a well marked vacuole with granular substances suspended in it. Normal eukaryotic organelles except chloroplasts are found within the cells. Lipid and volutene granules too are present. Surrounding the cell is a cell wall. No chitin is found in the cell wall. The common mode of asexual reproduction is budding. During sexual reproduction ascusspores within asci are formed, but no ascocarps are formed.

What is the difference between Bacteria and Yeast?

Bacteria are prokaryotes and yeasts are fungi which are eukaryotes. The 2 types of organisms are fundamentally different.

• In bacteria there is no organized nucleus and in yeast there is an organized nucleus.

• In bacteria there is only a single circular DNA. In yeast, there are several linear DNA.

• In bacteria nucleolus is absent and in yeast nucleolus is present inside the nucleus.

• In bacteria 70s ribosomes are present. In yeast 80s ribosomes are present.

Content: Bacteria Vs Virus

Comparison Chart

The cell wall is made up of lipopolysaccharide or peptidoglycan.As they do not have the cell, so the cell wall is also absent, instead have a protein coat (capsid), which protects the genetic material.

Definition of Bacteria

Bacteria are a prokaryotic, unicellular organism, that is found in the deep earth, ocean, air and inside the human’s body. The whole body of the bacteria is made up of the single cell and is very simple to understand in comparison to the eukaryotic one.

They are not always harmful as viruses, and they proved to benefit economically, as well as scientifically. The outer structure of the bacterial cell is made up of two layers, the outer one and the inner one, the cell organelles are not well developed, the nucleus is free floating. Certain bacteria have extra circular DNA also, known as a plasmid.

There are various ways to classify bacteria, which can be on the basis of the cell wall, their size, shape. The bacteria multiply asexually by the process of binary fission or budding. There are three methods of gene transfer, which are transformation, transduction, and conjugation.

Bacteria are environment-friendly too, they help in nitrogen fixation for plants, and in cellulose and biodegradation. Bacteria are also used in food and chemical preparation. They are used in preparing antibiotics too.

Definition of Virus

The virus is a Latin word, which means “poison” or “slimy liquid” and is known as a very small infectious agent, visible through an electron microscope. They do not have any cellular structure but contain genetic material within a protein coat. The study of the virus is called as ‘virology’.

The virus is being placed under special taxonomic position and has their own kingdom because these are not animals, nor plants and neither comes under any category of prokaryotic cells. They do not have the capability to reproduce and divide by their own, hence cannot be said as organisms.

The only thing they have is the DNA or RNA and protein as their genetic material. Almost all the virus is susceptible to infection, and different types of virus invade different type of host cell.

Virus penetrates into the host cells inside the body, and use their machinery to get replicate and spread, and further, they destroy the invaded or infected cell. The infection caused by a virus is called as viral, and these can attack any type of cell, that is it can be a plant cell, a human cell, a bacterial cell, etc. and thus resulting in different types of viral infection. These viral infections can last up to a certain time, or they can be permanent and life-threatening too.

The Dutch scientist ‘Martinus W. Beijrinck‘ was the first to inform that virus are the new infectious agent and named it as ‘vivum fluidum‘, which means a new live reproducing organism, which is different from other organisms. Before this, the Russian scientist ‘Dmitry I. Ivanovsky’ in the year 1892, noticed the activity of the viruses. Later on, both the scientist came to know about the disease of tobacco plants called as tobacco mosaic virus.

The protein coat is called the capsid, protects the nucleic acid. The nucleic acid is the main constituent, which can be DNA (deoxyribonucleic acid) or RNA or ribonucleic acid). The virus attaches to the cell’s receptor present on the surface of the cell, these are very specific for every surface, and then the virus infects and gets attached to cells machinery. Here they (virus) start replicating, and later the virus kills the host cells and gets itself release and further attacks a new cell.

Do microbes interfere with each other?

The networks and interplays between different species of microbes are incredibly complicated.

Then there’s how external factors come into play. We know, for example, that the balance of two groups of bacteria – the Firmicutes and Bacteroidetes – affect obesity, but the link still isn’t clear or consistent enough to know how we might influence it.

And even if we were to find a potential treatment, there’s no telling if the body would accept it.

It takes guts

Bacterial residents of the intestines may influence neurons and the brain through several routes.

Substances secreted by microbes into the gut may infiltrate blood vessels for a direct ride to the brain.

Microbes prompt neuropod cells in the gut lining to stimulate the vagus nerve, which connects directly to the brain.

More indirectly, microbes activate enteroendocrine cells in the gut lining, which send hormones throughout the body.

Even more indirectly, gut microbes influence immune cells and inflammation , which can affect the brain.


Common forms: Viruses cause colds and flus, as well as more serious conditions such as HIV/AIDS, Ebola and COVID-19.

How viruses make us sick

A virus is the simplest of germs—it is nothing but genetic material encased in protein. Researchers debate whether a virus is even "alive."

By itself, a virus can accomplish nothing—it needs to enter a living thing to perform its only function, which is to replicate. When a virus gets inside a human body, it can hijack a person's cellular machinery to produce clones of itself, overtaking more cells and continuing to reproduce.

Viruses also are capable of infecting any living thing, including bacteria and fungi.

When the virus reproduces faster than the immune system can control it, it begins to destroy cells and harm the body.

Viruses are also the smallest germ, making them generally the easiest to contract—they're so tiny they can spread through the air in a cough or a sneeze. Some viruses also are spread by mosquitoes or through bodily fluid.

How to treat viruses

Since each virus is very different, no one drug exists to attack whichever virus is in your body. Vaccines give preemptive protection from certain viruses by training the body's immune system to recognize and attack a specific virus.


  1. Forsa

    people, what news from the front?

  2. Zulkijin

    What an entertaining answer

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