Isn't it crazy that we go from a bunch of random stuff put together in the right way
to becoming living things? Well, what does it even mean to be alive?
When is something no longer considered non-living but alive? Hello there, I'm Caroline, and welcome
back to Biology, where today we are going to jump into the topic of what makes something alive.
Let's take a look at a few examples before we really get into this.
Me? I am alive? Yes, definitely alive right now. Chair made from a tree? I've heard that
trees are living things, so... alive? No. Well, why not? Wasn't it alive at some point?
As you may have guessed, this lesson is all about what it takes for something to
be considered alive. By the end of our time here, you should be able to describe seven
characteristics that we use to determine life and determine when something is living or non-living.
We'll practice with an example to check your understanding, don't worry. Ready? Let's go!
So, to begin with, we said that we're going to use seven criteria to determine if something is alive.
It's important to note here that something has to meet all seven of those criteria to
be considered alive – not just one, not even six out of seven, but all of them.
As we go, I want you to think about something that may or may not be alive that you can check against
these criteria. Seriously, pause the video and write your idea down. It could be a rock,
a leaf, or even something like the water in your water bottle. We will come back to it at the end.
First up in our criteria for life is organization. This is the idea that living things are organized
into coordinated structures that are made up of one or more cells. Note,
a living thing has to have at least one cell. That means that the smallest things alive are
single-celled organisms. Even those have their own organization too. Multicellular organisms,
organisms that are made of more than one cell, tend to show similar patterns of organization.
Cells in a multicellular organism join together into groups, eventually building
up to make the whole living thing. It's a pattern we see in living things in nature.
The second criteria for life is responding to stimuli. Again,
this is shared from the lowliest single-celled organisms to the most complex multicellular.
All living things respond to stimuli in one way or another. For example, take a look at this.
The leaves of the Mimosa pudica plant fold up in response to even the lightest touch.
A stimulus, by the way, is anything that causes a reaction.
Different living things have different ways to respond to stimuli, but they all do it. Like,
for example, a plant that opens flowers to light. Some plants can actually move in order to take
better advantage of light, capturing more and thereby producing more energy for themselves.
Next up in our criteria for life, living things make more living things. That's right,
the third characteristic of living things is reproduction. Single-celled organisms,
like bacteria, tend to go through a process called asexual reproduction, making exact
copies of themselves without the help of a second cell, whereas more complex multicellular organisms
use sexual reproduction, mixing their traits and creating new combinations.
In both cases, however, DNA, the instructions for life, must be copied and passed on.
Speaking of reproduction, the fourth of our criteria for life is what determines
if an organism even can take part in reproduction. Criteria number four is adaptation and evolution.
Simply put, an adaptation is a trait that makes an organism better suited to its environment.
These are really features that help organisms survive long term,
like camouflage in an octopus or like extra layers of fat on organisms that live in cold climates.
Organisms that are best adapted to their environment survive and reproduce better.
As this happens over and over, the organism may change, or evolve, hence the term "evolution."
Now, let's say for a second that you have a pet rock that you sit on your desk at home.
Day after day, that little rock sits there. It doesn't need food or water, it doesn't change,
and it also doesn't grow. Is it living or non-living? Non-living, of course, because it
doesn't grow. Growth and development is actually the fifth characteristic in our criteria for life,
by the way. Living things don't stay the same forever. Even single cells have a life cycle.
You're bigger now than when you were born, right? Well, if you're a living thing, you're going to
grow and change throughout your life, according to the instructions in that DNA we mentioned earlier.
As you grow, you also need to make sure that your body keeps on running right.
That's where the sixth characteristic comes in, regulation and homeostasis.
Living things self-regulate through a process called homeostasis, the tendency to maintain a
balanced state. They do this in order to cope with stressors in the environment, like hot
or cold temperature, and keep the conditions of their bodies within a specific range of function.
Now, in order to do all this cool stuff, like grow,
reproduce, and keep on being your same old awesome self, can you guess what you need?
Well, if you said energy, you would be right. The final characteristic of a living thing is
the processing of energy, and the reactions in our body that process energy are called metabolism.
Plants can make their own food to metabolize by capturing energy straight from the sun.
Other types of organisms, like animals, get their energy from the food they eat.
Our bodies, and plant bodies, break foods down into specific types of energy
that are used to carry out processes to survive.
Okay, how we doing? Still with me? Good, because we are wrapping it up here.
We covered seven characteristics of life, namely organization, responding to stimulus,
reproduction, adaptation and evolution, growth, regulation and homeostasis,
and the processing of energy. A key point that I stated at the beginning of the lesson
was that in order to be considered alive, something has to have all seven characteristics.
So, let's practice. Would you consider a piece of coral in the ocean alive?
Let's list a few things that we can easily find out about coral through a bit of research. It is
made of cells, grows into huge reefs, has built-in sunblock to resist bleaching, has colonies made
of hundreds to thousands of polyps, singular column-shaped bodies together can regulate its own
pH levels inside to resist ocean acidification, feeds on plankton and other small reef life to
collect energy, and lastly, the polyps themselves pull back when disturbed or damaged. Now, pause
the video for a second and compare these with the criteria for life. Can you fit them all in?
Back? Well, what do you think? Coral does, in fact, fit the criteria for
life. Coral shows organization in the fact that it is made of multiple cells,
which in turn form a larger organism. It demonstrates a stimulus response in that
the individual polyps of coral will pull back when disturbed or exposed to air. It's
easy to see that coral reproduces because each individual coral is actually a colony of polyps.
An example of adaptation in coral is the fact that the pigments that they produce
act as a natural sunblock, protecting them from UV in the shallow areas.
Coral also exhibits growth by nature of the immense coral reefs that exist.
Homeostasis in coral is seen in the fact that many coral can regulate their own
pH to deal with ocean acidification. Lastly, coral needs to feed. Coral demonstrates the
need for energy by the fact that it feeds on plankton and other small life found in the reefs.
Anyhow, now that you're an expert on deciding if something is alive or not, it would be a great
time to think back to that thing I asked you to write down at the beginning of the lesson.
If you run through the criteria like we did with coral, does it make the cut?
Let me leave you with one to think about that even scientists don't all agree on:
Are viruses alive? Sometimes it's really hard to tell, huh? For now, make sure you take a
peek at the practice to review and the lesson PDF for an activity on what we learned here.
Until next time, remember, biology isn't just science – it's the way of life!