For reasons that we are not getting into right now (if you are curious, is because of the second law of thermodynamics), all living organisms need energy, no exceptions. This energy is used to all the multiple processes essential to life. As the Red Queen said in Carroll’s famous story Through the Looking-Glass (you may know it as “Alice in Wonderland”):
“Now, here, you see, it takes all the running you can do, to keep in the same place.”
Even when you are still, your body is doing a lot of things! Many of those things have to do precisely with self-maintenance, effectively running as fast as it can to keep the status quo.
Depending on the specific kind of life, we will find various strategies that life uses to harvest that energy from the environment. Entities like plants and certain microorganisms are called autotrophs, which usually means that they use a process called photosynthesis, through which they capture radiant energy (light) and convert it into chemical energy. There are other types of autotrophs that gather energy from sources other than light, but we will talk about them at some other time. Then there are the heterotrophs, which must consume other organisms for survival; we humans belong in this category. Heterotrophs collect chemical energy directly from other organisms.
Green plants collect light energy through special organelles (little organs) in their cells called chloroplasts (sometimes called plastids), which have their own interesting story. Chloroplasts and a related organelle, the mitochondria, seem to be the descendants of ancient bacteria that associated themselves with other cells billions of years ago. This idea is called the Endosymbiont Theory, and again, we can talk about it some other time.
Now, in all fairness you may be getting inpatient and think, “well, what does this guy want to talk about… now?”
Well, what if I told you that there are forms of life that are both autotrophs and heterotrophs?
I like telling my students that biology is a science of exceptions. You see, besides us, no other organism has ever taken a biology class, so they do not really care about what a professor like me says about what life is, what life is not, how do we classify life or even how organisms should behave.
There are several types of critters that engage in the rather dishonest practice of kleptoplasty, which is exactly what it sounds like. These guys steal chloroplasts from other organisms that they eat (talk about adding insult to injury). One of the most beautiful is the sea slug Elysia chlorotica. Yes, I called a sea slug “beautiful”, and here’s why:
Picture credit: eol.org
These slugs manage to spare the chloroplasts in the algae that they eat from digestion. Further, they incorporate those chloroplasts within their system, effectively turning into a “solar-powered slug”. E. chlorotica is not born a photosynthetic animal; when the little slugs hatch from the egg, they are brownish with red spots. They only acquire their characteristic green color when they eat certain types of green algae.
On the other hand, there is a recently discovered organism, an aphid (essentially a bug) which is a photosynthesizing guy in its own right. Now, this is a true plantimal.
I know, not as beautiful as Elysia, but pretty cool nonetheless…
If you want to know more: