Inside:  There are six main characteristics of living things used to decide if something belongs in the tree of life.  Most of the time its pretty easy to decide.  There is at least one exception.  Do you know what it is?

Understanding the characteristics of living things usually makes it easy to decide if something should belong in the tree of life.  If it has all 6 characteristics of a living thing, then scientists consider it as part of the tree of life.   The tree of life can get pretty complicated.  Millions of organisms are found on it.  We are found in the last group, Eukaryota.  Our cells are highly organized and we have our DNA in a nucleus.

All of the organisms in this tree share the 6 characteristics of living things outlined below.  Bacteria and Archaea don’t have tiny cell ‘organs’ or organelles like in Eukaryote cells but they still have a very definite pattern of organization.  They reproduce on their own, they grow and they respond to stimuli.

Let’s look in more detail at the six main characteristics of living things.  If you come across something you’re not sure of, you’ll have a list of characteristics to compare it to.  Child’s play, right?

: Eric Gaba (Sting, fr:Sting), Cherkash, PD, via Wiki Commons


6 Main Characteristics of Living Things

  1. Living things must maintain balance in all of their processes.
  2. They must have levels of organization in their structures.
  3. Living things must reproduce.
  4. Organisms must grow.
  5. They must use energy.
  6. They must react to stimuli like light, heat or touch.

Living things must maintain balance in all of their processes

All organisms must maintain balance in their processes.  This ability is known as homeostasis.  Living things have feedback mechanisms that keep things like their temperature, pH and chemical levels within a range that allows them to survive and function the best.  Humans keep their temperature at about 37ºC.  When our temperature starts going above that we sweat.  Sweating is a feedback mechanism.  When we are too cold we shiver to create heat in our bodies.  Shivering is also a feedback mechanism that keeps our temperature from changing too much.

Image by Linus Schütz from Pixabay

They must have levels of organization in their structures

The simplest building block of living things is the cell.  There are many single-celled organisms.  Inside the cell there is organization.  Proteins and fats work together to form different parts of the cell.  The cell contains smaller parts called organelles that have specific jobs.  Mitochondria burn the food energy you eat and release energy you can use.  Even single-celled organisms need energy and get usable energy from their mitochondria.

In more complicated organisms, multiple cells work together.  Similar cells work together as tissues, like lung tissue.  Tissues work together to form organs.  The heart has muscle tissue, nerve tissue, connective tissue.  Organs work together in even more complex organisms, like ourselves, to form organ systems, like the circulatory system.  It is made up of the heart, lungs, blood vessels and blood.

bacteria have organization as living things

Image by Arek Socha from Pixabay


Living things must reproduce

You might not want to hear this, but living things are designed to pass on their genetic information in their DNA.  Sex is one way organisms make offspring.  Most higher level organisms use sex as a way to pass on their DNA to offspring.  Most single-celled organisms simply split into two cells once they get big enough.  Clones are the result of this splitting.  Sometimes there is a genetic mutation that gets passed on to the new cells but those are rare.

It is generally accepted among scientists that organisms must be able to use their own mechanisms to reproduce.  They must have all the stuff within themselves and their sexual partner if using sexual reproduction, to create offspring.

As living things dragonflies reproduce

Image by jggrz from Pixabay

Organisms must grow

Living things need to eat or absorb nutrients like carbohydrates and proteins.  By eating they make themselves bigger.  Single cells become bigger cells.  Living things made up of more than one cell add more cells to their body.  By adding more cells, complex organisms get bigger – they grow.

Living things grow

Image by 경복 김 from Pixabay

They must use energy

Living things ‘eat’ to get nutrients.  Proteins make more organism and help it to grow.  Energy is stored in carbohydrates like sugar and in fat.  Remember I mentioned that little organ in all cells called the mitochondria?  Sugar is ‘burned’ in this structures to release a form of energy the cell can use.  Mitochondria can use parts of fat to make energy when there is not enough sugar.   So, mitochondria create this special cell energy known as ATP from fat and sugar.

When horses run they use energy.

Image by Rencie Horst from Pixabay

They must react to stimuli like light, heat or touch

All living things react to stimuli like light and touch.  Even single-celled organisms have spots on their cell that react to things like light and touch.  Sometimes its good to move towards light to find a food source.  Sometimes its better to stay in the dark to hide from predators.  More complex organisms like us have many special cells and organs that help us react to our surroundings.  Eyes make sense of things we see.  So we can find food or shelter.  We can avoid other things that would eat us.  Heat sensors on our hands and feet tell us to move if something is too hot and will damage our skin.

Living things like sunflowers react to stimuli like light.

Photo by David Dibert from Pexels

Is it Always Easy Using Characteristics of Living Things to Tell if Something is Alive?

Most of the time, it is pretty easy to tell if something is alive.  In fact, its usually child’s play.   Check out this post, Actually, What Makes Things Alive? :Easy Experiment,for a way to teach even very young kids how to tell if something is alive.

However, there are scientists who disagree on at least one thing.  It is something that causes sickness in other living things.  Viruses cause disease in all kinds of living things including some single celled organisms like bacteria.  There are many that cause deadly disease in humans.  Ebola, the common cold and the flu as well as the current Covid 19 pandemic are all because of viral infections.  Most scientists feel that viruses are not living things.  But, there are now some scientists who disagree.  Some now want viruses classified as living things.

Viruses do have DNA or RNA.  They do have a structure of sorts.  They make copies of themselves but they need another living cell to make their parts and put them together.  Outside of another living cell, they don’t do anything.  They don’t eat or sleep or react to light or heat or pressure.  But, there are scientists who think they are not living and other scientists who believe they should be part of the organization of life.

I’ve created a cool reading activity to let you or your middle to high school level students decide for yourselves.  Are viruses a living thing or not? Check out the activity in my TPT store by clicking the picture below.