Achoo!
Excuse me... Oh, hey there. Bea here, and welcome back to Biology.
Ugh, I am feeling awful today! I went to the doctor, but she just said it was a cold and
that all I could do was come home and rest. Last year, when I had strep throat, she gave me some
antibiotics that made me feel better right away. I sure wish there was a magic medicine for colds!
Do you know why antibiotics won't work against a cold? Colds are caused by viruses, not bacteria.
Antibiotics kill bacteria by disrupting processes such as growth and reproduction.
Some antibiotics also target the cell wall of a bacteria, and others target their ribosomes.
Unfortunately, none of these strategies will work on a virus. Viruses don't grow and reproduce,
and they don't have a cell wall or ribosomes. Are they even alive?
Remember those seven criteria for life that we talked about at the very beginning of
the course? We'll go through the list today to see if viruses make the cut.
As we go through the lesson today, we'll review the seven criteria for
life and use those seven criteria to determine whether viruses are alive.
The first criteria for life is that the object in question must be made of cells.
Viruses are most commonly compared to bacteria cells. Can you even tell which
is which out of these? By the end of our lesson today, you probably will be able to.
Let's look at the structure of a virus particle to see if it at least resembles a cell.
It does have either DNA or RNA, which is usually surrounded by a protein coat called the capsid.
The outside of the particle is surrounded by an envelope. This envelope is made out of
pieces of your own cell membranes. Yup, these little viruses are thieves! On the envelope,
there are many different types of proteins. These help the virus recognize and attach to your cells.
Viruses come in many different shapes. Spherical is the most common, and these viruses include
colds, flus, and even the coronavirus. But they can sometimes have more unusual shapes too,
like helical, polyhedral, or even a complex shape that resembles some kind of spider robot.
Notice that one thing you did not see on any of the virus structures was a cell membrane,
unless we count the pieces that they stole from yours. They have no cell
wall and definitely no ribosomes or other organelles. So, no,
antibiotics will not have any effect on a virus and are therefore not helpful if you have a cold.
It seems pretty clear that viruses fail our tests for the first criteria of life. They
are definitely not made of cells. Remember that to be considered alive, we generally say that
all seven criteria must be met. But just to be sure, let's go through the rest of the list.
Can viruses have little baby viruses?
If viruses are alive, then they should be able to reproduce. That's the second criteria for life.
Otherwise, we'd run out of viruses pretty quickly... and I would be glad for that today.
Our answer gets a little murky here because scientists generally say that viruses replicate,
not reproduce. The reason for that distinction is that they are unable to reproduce on their own.
When a virus infects a host cell, which could be a cell in your body, it takes over the cell and
forces it to make more copies of the virus. It's like viruses turn your cell into a copy machine
and use it to make copies of themselves. So while they do have a way of making more virus particles
with the same DNA, viruses don't actually make new viruses themself – they make you do it for them.
Does that count as reproduction? Many say no, but you could possibly make a case for maybe here.
Do you remember what a stimulus is? It is anything that causes a physical or behavioral change.
What happens when you get cold? You shiver. When you get hot, you sweat. Humans respond to stimuli,
like being hot or cold. Responding to stimuli is the third criteria for life.
We can only give this one a maybe for viruses because while scientists haven't
directly observed it, they don't have enough information to rule it out either.
It's possible that chemical reactions in the virus particle would cause it to
behave differently in different temperatures or conditions. Maybe scientists will observe this
in your lifetime or completely rule it out. For now, we'll leave it as a question mark.
Remember that adaptations are inherited traits that make an organism more likely to survive
and reproduce, and the ability to adapt is our fourth criteria for life. In the case of a virus,
we could include being more likely to replicate, since we don't use the word "reproduce."
Do viruses have adaptations? If the genetic makeup of a virus makes it successful at replicating
and infecting host cells, it is more likely to continue infecting cells and being replicated.
Notice I didn't use the word "survive," because we haven't established that viruses are alive,
but it's the same idea. Viruses have genetic diversity due to different DNA,
just like cells, so one virus particle may be more successful than another. And if it is more
likely to be replicated and pass on the good DNA, then we can say that it has an adaptation.
This means that viruses do evolve over time. You may even be aware of this if you get a
flu shot each year. The flu is caused by a virus, and the virus is constantly changing,
so last year's flu shot won't be as effective against the strains of flu going around this year.
We have to keep updating the vaccine to keep up with the evolution of those pesky little viruses.
So, this is one test that viruses do actually pass. It's a little unfortunate for us humans,
as we constantly try to evade them, but yes, viruses do adapt and evolve.
To grow means to increase in size or complexity over the course of one's lifetime,
and growth is a requirement for being considered alive.
For viruses, this one is a simple "no." As a virus invades host cells and replicates,
it does not change in any noticeable way, so it fails this portion of the test
As humans, we obtain energy from the food we eat, and we use that energy
to do all of our daily activities, like schoolwork, sports, gaming, and so on.
Behind the scenes, our bodies are also using energy to digest our food, make our heart beat,
and many other processes that keep us alive. Being able to obtain and use energy is the sixth
criteria for life, and that doesn't just mean humans. You can probably guess that viruses don't
eat. They are also not photosynthetic, so unlike plants, they don't make their own food either.
But replicating virus particles does require energy. If viruses are not obtaining energy,
how are they using it? Remember that it's not the virus itself doing the
replicating — it's you! The energy to replicate viruses comes from YOUR cells.
Does commandeering the energy of another organism for your own purposes count as using energy?
This one is debatable. Most scientists would probably say no, but it is a pretty ingenious
way of making someone else do the work for you, so we'll call this one a maybe.
The last criteria for life is the ability to maintain homeostasis. We'll talk more
about homeostasis in the next unit, but for now, it's enough to know that it refers to our body's
ability to maintain a stable internal environment. It's how our bodies are able to maintain the right
temperature, water content, pH, and so much more, regardless of what is going on around us.
Our biggest clue as to whether viruses can maintain homeostasis has to do with energy.
For all animals, keeping our bodies balanced requires a constant source of energy.
Is that why my dog is always so hungry?
Probably. Viruses may commandeer our own energy for the purposes of replication,
but they have no way of using energy to change or maintain their own internal state. They are at the
mercy of whatever conditions they find themselves in. So, our last criteria for life earns a
definitive "no." Viruses have no internal ability to regulate their status or achieve homeostasis.
To be considered alive, we have already learned that all seven criteria for life must be met.
Let's see how viruses did. Organized into cells – no. Able to reproduce – maybe.
Respond to stimuli – maybe. Adapt and evolve – yes. Growth – no. Obtain and
use energy – maybe. Regulation and homeostasis – no. They definitely don't meet all seven,
so it is generally accepted that viruses are not alive. They can be better thought of as little
packets of DNA that can invade cells, causing those cells to make more little packets of DNA.
If they are not even alive, how do viruses make us so sick?
Remember that a virus replicates itself by injecting its DNA into our own cells, causing our
cells to make more copies of the virus. Before that invasion happened, our cells were hard at
work. They were working as blood cells, muscle cells, nerve cells, and so on. They had important
jobs to do. Once they were turned into a viral copy machine, they could no longer do their job.
Our body is smart enough to know that this is going to be a problem if it continues,
so our immune system launches into war mode in an attempt to fight the virus. We'll learn
more about the immune system in a later unit, but know that it's not an easy battle to fight. Many
of the symptoms you feel when you are infected with a virus are actually not a direct result of
the virus itself but rather the result of your own immune system working against the virus.
Our temperature goes up, giving us a fever, and our sinuses produce mucus.
These are side effects of the work our immune system is doing. But without this defense system,
the virus would infect and kill all of our cells, and you can probably guess how that
would turn out for you. So... I guess I'm supposed to be grateful for this cough.
It's actually the result of my body working to keep me alive!
Of course, bacteria can make us sick too. They make us sick by producing toxins that damage our
cells. Our bodies tend to respond in much the same way as they do to a virus. We get fevers,
coughs, runny noses – this is why sometimes it is hard to tell which is making us sick. Well,
that wraps up our lesson on viruses. Today we reviewed the seven criteria
for life and used those seven criteria to determine that viruses are not alive.
Do you think you can pick out the viruses from the bacteria now?
The easiest way to identify a virus, as opposed to a bacteria, is to look for the
proteins on the outside of the envelope. They look like a spiky ball. But remember,
they are microscopic – even tinier than bacteria and much smaller than plant and animal cells.
Well, my friends, this is your last lesson on the Cellular Structure and Function Unit. Next time,
you'll have a chance to review what you've learned here and take the unit assessment.
I'm gonna go take a nap. Until then, remember that biology isn't just science – it's the way of life!