For those of you who have stumbled across this post thinking this is about life in general. It is, just like the scientific version of life in general. So stick with me as we cover a few things a lot of people struggle with. If you are a teacher, this post is specifically for you. We are talking about things students often misunderstand when it comes to the characteristics of life. So without further adieu, and in no particular order, 5 common misconceptions about what scientists consider living and non-living, biotic and abiotic.
Many living things respire. They need oxygen so they can do aerobic cellular respiration. We know aerobic respiration is much more efficient than anaerobic respiration. We also know it evolved later. This is why many organisms are unable to use oxygen. Some organisms cannot even be around oxygen. Oxygen is a poison. While on paper oxygen looks stable, it actually likes to pull electrons off and send them flying in new directions. In other words, oxygen produces free radicals. Loose electrons, also called free radicals, can combine with proteins and other molecules forcing them to change shapes disrupting their function. Our cells use a group of special molecules that can accept free radicals no problem. These molecules are called antioxidants because they prevent oxidation/reduction reactions. Organisms without a system for preventing oxidation/reduction reactions die quickly in the presence of oxygen. We call these organisms obligate anaerobes because they cannot live in the presence of oxygen.
A famous example of an obligate anaerobe are botulinum. They produce Botox. Useful now adays but early settlers in the US found it deadly. Botulinum can live in canned food. If it isn’t canned properly The start growing (cause there isn’t oxygen in the can) and produce the Botox. Eating it causes cramps and prevents digestion. Many people died from improperly canned food. You may be wondering how the spores could survive. If you are that is a good question. Botulinum, as well as many other kinds of bacteria are able to form specialized protective coverings when conditions are tough. When the bacterium is in this state they are inactive and waiting for better conditions. Come to think of it this would be a good thing to discuss with students: are cells in stasis, that is inactive cells, are they alive? But I digress… bacterial cells that form spores can survive incredibly harsh conditions. That’s why botulinum spores get in our cans. They float through the air, landing on stuff all around us, but cannot leave stasis or they die.
Obligate anaerobes aren’t the only ones who can live without oxygen. Some organisms can switch between aerobic and anaerobic metabolism. We can even do this, but only for a very very short time. Other organisms can survive just fine indefinitely in aerobic as well as anaerobic conditions. Yeast is one of my favorite examples. Not only are they useful for making bread, if you cut them off of oxygen they make ethanol as a byproduct of anaerobic respiration. In other words they do alcoholic fermentation. We are not so fancy. Without oxygen humans just make lactic acid. A byproduct believed to be involved in the sensation of sore muscles, who wants that? Alcohol on the other hand had many uses. Not only is it a delight to consume (in moderation of course), it is also useful for cleaning and in the lab.
So with two fairly well known examples we see life doesn’t need oxygen to live. But there is more to this story. At the earliest stages of life, earths atmosphere was anaerobic. There was air, but no oxygen. Oxygen is highly reactive and would have bound itself to rocks and other atoms floating around. It wasn’t until photosynthesis evolved that the earth began to become oxygenated. Before that chemosynthesis is believed to be the primary source of energy for life. Once the earth began being oxygenated many species were likely killed off. A great and silent mass extinction. Bacteria doesn’t fossilize very well. There are countless unknown organisms from that time we will never get to learn about. But their descendents might still be around. A group researching the oceans found life in the muck at surprising depths. Little is known about these organisms, but they seem to be anaerobic. The point is there is a whole box nch of life that doesn’t need oxygen.
If your students are insisting all living things need oxygen, maybe you talk about some of the things above. You can also do a soft correction. When students say everything needs oxygen, many times they are hinting that organisms have a metabolism. And that is something all living things truly have in common.
All living things, including plants respond to the changing conditions around them. Plants are just slow to respond. Many of plant responses involve growth responses or tropisms. Take phototropism for example. Plants grow toward the light. Ever seen your plant turning toward the window? That is phototropism. Here is another example students might be familiar with: leaves falling off of trees. I mean, why do they do that? Do they just know, “oh, its time to quit photosynthesis”? No, they recoginize the change in temperature. They are responding to the environment.
Plants have other more subtle responses as well. Ever notice morning dew? The dew is a collection of water that comes from evapotransporation. A big facy word meaning water coming out of plant leaves. Plants have small openings in their leaves called stomata. Stomata are surrounded by specilized cells that can open and close. The cells are called guard cells because they “guard” the entrance. But they aren’t really preventing unwanted guests. It is more like guard cells are stoping wanted guests from leaving. That wanted guest is water. When there isn’t enough moisture in the air guard cells close. You might wonder then why guard cells would bother opening at all. Don’t plants need all the water they can get? Yes, they do! But they also need carbon dioxide and they need to get rid of oxygen. So they open their guard cells to perform gas exchange. They loose a bit of water at the same time, but it is worth it to keep doing photosynthesis. Plants give morning dew because it is the coolest time of day. Plants loose the less water when it is cool. Making it the best time to perform gas exchange.
Ok, you might fight me on this one. If you do, I will already say, “you have a good point”. But first let me explain my position. Viruses do not qualify for all of the characteristics of life. They do not have cells. They do not respond to their environment. They do not have a complex metabolism. They do not maintain homeostasis. However, they do reproduce, pass on traits to their offspring, and evolve. A group of scientists want to change our definition of life to include viruses because they are capable of evolution. A thing unique to life and viruses. So why not combine them? This is a great question to discuss with your students. If your up for it, you might even have a little class debate. I will go over a few arguements you might see (or inject if students are coming up empty on ideas).
Virions (the little viruses outside of the cell) do not hit all of the characteristics of life. Neither do organisms in stasis (i.e. bacteria that have formed spores or water bears in space). A virus inside a cell can potentially hit all the characteristics of life. We already noted that viruses reproduce, pass on traits to their offspring, and evolve. That part is not controverial and is generally accepted by the scientific community. Here are some controversial arguments. Viruses in a cell have a cell. It has the host cell. It has the hosts metabolism. It does respond to the environment. Some viruses will incorporate themself into the hosts genome, move around the genome, and excise themselves from the genome when the time is right. Viruses absolutely respond to the environment. If that environment is the cell. You might not consider a cell an environment, but I ask you to remember endosymbiosis. Endosymbiosis says that one cell can live inside of another cell. This is how we think mitochondria and chloroplast evolved, but there are other examples of similar endosymbiotic events. So for the case of a bacterium living inside another cell, their environment would be the host cell.
Now a good counter argument to this whole “viruses respond to their environment because they respond to the cell” is to remember another idea put forth in the paragraph above, “viruses have a cell, the host cell”. So is it possible for the host cell to count as a viruses cell as well as a viruses environment? Who knows! Maybe that is for the scientific community to decide. But it does make for a good classroom discussion. No matter where you go with this discussion, be sure that you are clear about how you want students to answer this question on the test. As for the state of Texas, they say viruses are not alive. That is the current scientific consensus. Maybe one day that will change.
Soil is poop. Excuse my four letter word there. Soil is the remains of organisms. Soil is not found in areas without life. (I’m looking at you Mars.) You need life to get soil. Because soil comes from living things it is considered a biotic factor. “But Mr. Wilkinson, soil isn’t alive!” That is right my imaginary interlocuter. Things do not need to be alive to be considered biotic. They only need to originate from living things. While the root of biotic comes from bio which means life. The term biotic does not mean it is currently alive. A dead bird is still a biotic factor. It is an important disticution because biotic factors influence the environment different than abiotic factors. Other organisms can eat the bird and still be heterotrophic, while organisms that eat rocks (abiotic factors) would be chemoautorophs. They obtain energy very differently. And yes, before you say anything imaginary interlocutor, there are organisms that eat soil. They are called worms. Who get eaten by birds. Who are turned into soil. That is eaten by worms… The circle of life!
If you do a quick search, and at least one of your students is likely to do this, you will find several different lists of the characteristics of life. Some with only 5 characteristics and others with 8. It is important to remember that the number does not matter. The point of the characteristics of life is to describe a cumulative property. Oh, you don’t know that word, “cumulative property”? Let me expound. Whenever there are a bunch of things that all come together to make a new thing that would not be there otherwise. That, that is a cumulative property. Like a car. You can have an engine and no car. You can have wheels and no car. Seats and no car. But when you put wheels, engines, and seats together… sometimes… you get a car! Othertimes an airoplane, or a train, or an electric bicycle. The point is, all of those things on their own do not make a car. Cars are formed when a bunch of other things create something new. It is the same with life. Maybe cars aren’t the best example, but it is something most students are familiar with and something they understand has parts. It is what I use in my class to explain it anyway.
Well, you might ask, why do the lists vary in number? That one is easy. Because people vary in thought. Not everyone thinks the same way. Some people say, “evolves” and they already understand that evolves means it happens at the population level, involves reproduction, and passing down traits to offspring. So evolves is the only characteristic needed. Some people say, “maintains homeostasis” and understand that to maintain homeostasis one must be able to respond to the environment and to respond to the environment one must have a complex metabolism. So maintains homeostasis covers two other characteristics.
So we can, if we wanted to, reduce the characteristics of life to three things: made of cells, maintains homeostasis, and evolves. But most people, and thus most students, will not understand the implied characteristics that go with those terms. I mean the whole point of describing the characteristics of life was to expand the definition of life in the first place. We could reduce it to one characteristic: alive. But that doe not help us understand how living things are different from their environment. So don’t do that to your students. Give them more information than they think they need. Cause we all know most of them won’t look it up even when they don’t have all the information they need. Ah, the joys of teaching.
There you have it. Five common misconceptions about life. At least one of them will give you a nice discussion in class. If you want more resources to help you have riveting discussions and excellent experiences check out my TpT store. You might also enjoy this FREE resource.
