Characteristics of Life

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Have you ever seen those toys that you can put in water for a few days and then they increase in size? They always have this awesome thing on the package like, “Grows a million times larger in water! ”---ok I’m exaggerating---but they do grow impressively larger.

I had one once. It was a pony. I had dreams for it to grow especially large, so to give it plenty of room, it spent several days in a bathtub.

And while it did technically grow larger, the dreams I had imagined for it did not really come to be. The fact that it could increase in size sort of gave me some kind of thought in my mind that it just might be alive. Because things that are alive do grow, right?

Combine that with my overactive imagination and it was difficult for me to come back to reality that this toy pony may increase in size but it’s not alive. When we talk about biological growth and development, this type of increase in size is not the same thing. And if we’re going to truly explore biology, the study of life, it makes it essential that we understand characteristics of things that are alive.

Life is difficult to define, but the characteristics of living things can be explored. We want to mention right here, before we get started, that what makes an organism “alive” still results in a lot of questioning by biologists. Hey, remember, there are many organisms undiscovered on our own planet---and new information revealed in biological sciences all the time---so this is our disclaimer that this information can change and is still debated.

Kind of like what we mentioned in our classification video. This is why we are not going to give a certain numerical value for how many characteristics of life there are, because we do not want to imply they are in a specific order or that what we are listing cannot be expanded upon---- or that this can’t include exceptions that we may not mention. Also, the characteristics we talk about can certainly be titled differently.

See, we’re not trying to talk about a list that one would memorize here. Our goal is actually to get you thinking about what it means for an organism to be alive and characteristics of life that can be explored when trying to study living things. Organization.

We can look at the cell theory here and see that, according to this theory, living organisms are made of cells. Some organisms are unicellular- so they can be made up of just one cell. But more complex organisms, like this pony here, are multicellular and therefore made up of many cells which can be arranged into tissues---the tissues can make up organs----the organs can be a part of organ systems.

These are biological levels of organization! As for our bathtub-grown pony? No cells.

Homeostasis. Maintaining a regulated balance is so important for many biological processes to even happen---for example, enzymes typically need a certain pH range to even work correctly. Maintaining homeostasis can mean maintaining a certain temperature and a certain percentage of water concentration.

Ponies and your human body have all kinds of feedback systems in place to maintain homoeostasis, but even a single-celled organism---like an amoeba---relies on its cell membrane to maintain homeostasis internally. Living organisms have regulation of homoeostasis---which is not going on here in bathtub-grown pony. Metabolism.

. If you’re alive, you need to have some way to capture energy and use energy for processes, including some processes that keep homeostasis like mentioned previously. Chemical reactions happening in living organisms are part of metabolism.

So just to give some examples of reactions that are part of metabolism in living organisms--- plants are autotrophs and can capture light energy to make glucose in a process known as photosynthesis. Animals are heterotrophs that typically need to eat something and then digest it in order to obtain glucose. Both of these example organisms break down glucose in cellular respiration to make ATP energy.

Metabolism is happening in this pony. But not in bathtub pony. Reproduction.

This can be very simple, like unicellular bacteria which can copy DNA and split—into 2---and now you have two bacteria. Or it can be more complex, like this living pony, which involves sperm and egg cells uniting to make a fertilized egg known as a zygote which will eventually develop into a baby pony. No reproduction for this bathtub-grown pony.

Growth and Development. Living organisms have genetic material which codes for its development and growth. A baby pony will develop and grow up into a grown up pony because its genetic material contains the instructions for this development and growth.

Bathtub-grown pony may increase in size due to water entering it, but it’s not growing and developing based on genetic instructions. Response to Stimuli. Responding to stimuli is often considered a characteristic of life.

There can be internal and external stimuli. If this pony feels the need to eat, this can be due to coordination of many body systems internally alerting the pony that it is hungry. If this pony senses danger, this external stimulus may cause it to flee.

Not so for the bathtub pony. But you know, a response to a stimulus might not be obvious. I used to have a lot of cacti when I was a kid.

And while I wouldn’t call them especially exciting when I was a kid, I did notice that if I left them in a windowsill and didn’t rotate them, they would bend---at least, over a period of a few weeks. That’s AMAZING, because they are responding to light. Plants responding to light is a response to a stimulus.

Evolution. A working definition of life summarized by Dr Gerald Joyce as part of a panel of exobiologists describes life as “a self-sustaining system capable of Darwinian evolution. ” Fascinating quote.

Many scientists view the process of evolution as a characteristic of life, although this would occur over a period of time. The gene frequencies in a population of living organisms can change over time due to mechanisms, such as natural selection. Some genes can code for certain traits that result in higher reproductive fitness, while some genes may code for traits that lower reproductive fitness---and therefore may be selected against.

Over time, these can result in adaptations. Because the bathtub pony cannot even reproduce in the first place…you won’t see that. Characteristics of life are definitely intriguing to explore.

Remember that these characteristics can be adjusted-----or potentially have exceptions. Some things are very hard to classify. Take viruses.

In our virus video, we talk about how viruses are considered, by the majority of scientists, to not be living. Viruses, with their genetic material, can reproduce---although they need a host to reproduce. They can evolve.

But overall they seem to lack a lot of the other characteristics of life, and aren’t typically classified as living. It’s also interesting to think about the potential of extra terrestrial life, that is, life that is not on Earth. Would it still have these characteristics of life that we have discussed?

Sometimes with science, we find ourselves with even more questions. Well…that’s it for the amoeba sisters and we remind you to stay curious.