The Defining Characteristics of Living Organisms

The Defining Characteristics of Living Organisms
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  • 0:05 Characteristics of Life
  • 0:40 Cellular Organization…
  • 2:06 Growth and Development
  • 3:03 Energy & Response to…
  • 4:50 Regulation &…
  • 6:51 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

A house is not a living thing, but the people, animals, and plants living inside it are. So how do we define 'life'? Learn about seven characteristics that all living organisms have in common, no matter how different they may look on the outside.

Characteristics of Life

You are a living thing. But so is the tree in your backyard, the bird that sits on one of its branches, and the worm in that bird's mouth...at least until that bird finishes its morning snack! Each of these things looks very different on the outside. But because they are all living organisms, they are more similar than you might think.

In fact, there are seven characteristics that we can use to define 'life.' All living things can be described by these properties, which help us explain in universal terms why a rock is not a living thing, but a flower, a fish, and a ladybug are!

Cellular Organization

The first thing that makes living organisms unique is that they are all made of cells, which are considered the building blocks of life. Cells are amazing, because while they are very small themselves, they can work together to form very large structures like the tissues and organs in your body. Cells are also specialized - for example, liver cells are only found in your liver, and brain cells are best kept up in your head!

Some organisms are made of just one cell, like many bacteria, while others are made of trillions of cells, like you and me. Multicellular organisms are very complex beings that have incredible cellular organization. This organization begins way down in your DNA and extends all the way to you as a whole organism.

Reproduction

How do living things on Earth get here? They don't simply appear out of thin air, but instead come from reproduction, the next common characteristic of all living things. There are two ways that offspring are produced. The first is probably what you're most familiar with, sexual reproduction. This is when organisms produce offspring by combining gametes. Humans fall into this category.

The other type of reproduction is asexual reproduction, which is when organisms produce offspring without gametes. Unlike sexual reproduction where offspring have a different genetic makeup than either parent, asexually-produced offspring are genetically identical to the parent.

Growth and Development

Once an offspring is produced, it doesn't stay like that forever. You know this all too well - you've probably changed a lot since you were first born into this world! Other living organisms experience these life changes too, and this brings us to our third characteristic of life, growth and development. Organisms grow and develop throughout their lives, but these two terms do not mean the same thing.

Growth is when features change from a small size to a larger size. Think about when you were first born - you had all the same features you do now: fingers, toes, eyes, heart, etc. The difference is they were much smaller before and then grew into larger versions as you aged.

Development, on the other hand, is when features change or transform. This is the process you went through before you were born - you started as a single cell and ended as a baby, with many different transformations in between.

Energy

Growth and development, cellular processes, and even reproduction can only occur because living organisms take in and use energy, the fourth common characteristic of life. All of life's energy ultimately comes from the sun, and this energy powers everything on Earth.

Many living organisms, such as plants and some algae, use the sun to make their own food. This process of converting sunlight into chemical energy is called photosynthesis, and these organisms that can produce their own food are called autotrophs, or 'self-feeders'. However, many organisms cannot make their own food, and therefore have to eat other living organisms to get the energy and nutrition they need. Organisms that eat other organisms are called heterotrophs, or 'other-feeders'.

Response to the Environment

The next characteristic of all living things is that they respond to environmental stimuli. What this means is that changes in the environment trigger certain responses in organisms.

For example, a Venus flytrap will snap shut very quickly when a fly lands on it, a turtle will come out to bask on a log when it's sunny, and you will go to the fridge to make a sandwich when your stomach growls at lunchtime.

Stimuli can be external (outside of the body) or internal (inside of the body), and they help living organisms maintain balance. Stimuli are detected through various senses in organisms, such as sight, taste, smell, and touch. And, the speed of response varies from organism to organism, and from stimuli to stimuli. For example, if while in the middle of leisurely making that sandwich for lunch, a bear appears in your kitchen, your response to that stimulus is likely to be much faster, and in the opposite direction!

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