Characteristics of Life - Online Tutor, Practice Problems & Exam Prep

Living organisms exhibit eight key characteristics that distinguish them from nonliving entities: they are composed of cells, maintain order, respond to stimuli, sustain homeostasis, reproduce, possess a dynamic metabolism, have heredity through DNA, and can evolve. Viruses, however, lack cellular structure, the ability to reproduce independently, and a dynamic metabolism, though they can maintain some internal stability. Understanding these characteristics and the biological hierarchy—from multicellular organisms to atoms—highlights the biochemical unity of life and the evolutionary connections among all living things.

Characteristics of Life

We're going to talk about what makes things alive. So there are actually 8 characteristics that are shared by all living organisms, and these eight characteristics distinguish them from the nonliving things that surround them. The lack of just one of these 8 characteristics would render something nonliving. You guys have talked about these characteristics in your previous biology courses. We're going to do a quick review on each of these characteristics.

For the first characteristic, we have that all living things are composed of cells. That's pretty straightforward, right? It's just saying that the most basic fundamental unit of life is the cell. The second characteristic of life is that all living things have order. This means that they are not random. In fact, what they do is they take smaller, simpler molecules and strategically build them into larger, more complex molecules and structures to survive.

The third characteristic is that living things respond to stimuli. This means that they can respond to specific triggers in the environment. The fourth is that living things maintain homeostasis. You've learned in the past that homeostasis refers to the ability of organisms to maintain internal conditions despite the fact that external conditions are constantly changing.

The fifth characteristic of life is that all living things can reproduce. This means that they can create more life, but they can do that either sexually or asexually. The sixth characteristic is that all living things have a dynamic metabolism. Metabolism refers to the sum of all chemical reactions. Dynamic is referring to the ability of this metabolism to change and be flexible under different conditions. Remember that the metabolism of an organism is focused on extracting and transforming environmentally acquired energy so that they can use that energy to survive.

The seventh characteristic is that all living things have heredity. We know that the hereditary information of a cell is DNA, and DNA can be passed down from older generations to future and newer generations. The last characteristic of life is that living things can evolve. We've learned in the past that evolution has to do with changes to the DNA or mutations to the DNA over long periods of time. In a population, sometimes that leads to different physical traits and sometimes those physical traits allow organisms to be better adapted and improved fitness or survival in an environment. We'll talk more about evolution later on in our course.

Now we know viruses are not alive, but in our next video, we're going to talk about which of these characteristics above viruses lack.

Characteristics of Viruses

So we already know from our previous biology courses that viruses are not considered alive and, of course, that's because they lack some of the characteristics of life. But the question is, which ones do they lack? And so, here what we're going to do is list the corresponding number of each characteristic that viruses tend to lack. And so for the first one, we know that viruses are not cellular. So we'll go ahead and cross that off the list and list characteristic number 1 as being one that viruses lack. When we look at the typical structure of a virus, recall that they have a protein shell that's known as a capsid, and the capsid encapsulates a nucleic acid molecule that could be either DNA or RNA. Some viruses contain a lipid layer that surrounds the capsid, known as an envelope. Viruses are much smaller than even the components of cells, so they're definitely not cellular.

Now, for the second characteristic order, viruses are ordered, and in fact, they are an assembly of macromolecules that come together in an ordered fashion. So we'll go ahead and give them a green checkmark for this one. Now, for response to stimuli, there's not enough research and evidence out there that shows that viruses can respond to stimuli. So we'll go ahead and cross this off the list and list it as one that they lack. For maintaining homeostasis, viruses don't have complex mechanisms like living organisms do to maintain homeostasis. However, many scientists say that the envelope of some viruses and even the capsid of viruses can help to maintain and stabilize internal conditions. And for that reason, we'll go ahead and give them a green checkmark for this one.

For reproduction, viruses are on this blurry line because viruses cannot reproduce sexually or asexually. And in fact, we know that viruses can only reproduce by hijacking the machinery of other living cells. And because viruses don't have the machinery themselves to reproduce, we will go ahead and cross this off and list it as one that they lack. Now, for characteristic 6, viruses do not have a dynamic metabolism like that of living organisms. And so we'll go ahead and cross this one off and list it as one that they lack. Now for heredity, we know that viruses can contain and also evolve over time. And what you'll notice is that viruses have a collection of some of these characteristics but they lack other characteristics. And some scientists are pushing to redefine our definitions of life, but that hasn't happened yet and viruses are not considered alive. Now, in our next video, we're going to change gears a little bit and talk about how living organisms order themselves. So we'll talk about the biological levels of organization, and I'll see you guys in that next video.

Biological Organization

Living organisms are incredibly ordered and contain what's known as a biological hierarchy of organization. And we've talked about this plenty in our previous bio courses, so we're just going to do a quick review here. In our example of the biological levels of organization, notice that on the far left, we have larger and more complex organisms and structures, and as we move from the left towards the right, we get smaller and less complex organisms and structures. Now starting over here on the far left, we have multicellular organisms which includes ourselves, obviously. Humans are composed of multiple organ systems, where each organ system is a collection of individual organs that work together. The organs are made up of a bunch of tissues, and the tissues are a collection of the same types of cells. We've learned that cells are made up of smaller components called organelles, and the organelles are made up of macromolecules, where the term here macro means large. So these are large molecules.

Macromolecules, moving forward in our biochemistry course, we'll talk a lot more about the macromolecules and the organelles. The macromolecules are made up of smaller molecules, and the molecules are made up of individual atoms that are bound together chemically. These include the atoms that we see on the periodic table. The way that this hierarchy works is that the lower levels are used to build the immediate higher levels. And notice that we have this big red box that is going around the macromolecule, molecule, and atoms levels. What this is referring to is what's known as the biochemical unity of life. And all that's saying is that all living organisms have a resemblance, and you can see that when you compare these bacteria to me. No. I'm just kidding. The biochemical unity of life does say that there's a resemblance between all living things, but that resemblance is seen at the atomic and molecular levels of life. There's a lot of evidence at the atomic and molecular levels that support what's known as a common universal ancestor, where all life today is theorized to have evolved from a single universal common ancestor. And hopefully, these bacteria don't look like me, but at the biochemical and molecular levels, we have a great resemblance, and that's pretty fascinating.

Which of the following is not a characteristic of life?

• A. Ability to reproduce sexually or asexually.
• B. Composed of cells.
• C. Ability to metabolize oxygen.
• D. Maintain homeostasis.

Indicate which of the following statements is false:

• A. Viruses contain hereditary information.
• B. Viruses do not have the ability to reproduce without hijacking a host cell.
• C. Viruses are composed of cells.
• D. Viruses do not need to extract/consume energy from the environment to exist.

Here’s what students ask on this topic:

What are the eight characteristics of life that distinguish living organisms from nonliving entities?

The eight characteristics of life that distinguish living organisms from nonliving entities are: 1) composed of cells, 2) maintain order, 3) respond to stimuli, 4) sustain homeostasis, 5) reproduce, 6) possess a dynamic metabolism, 7) have heredity through DNA, and 8) can evolve. Each of these characteristics is essential for defining life. For instance, cells are the basic unit of life, and maintaining order involves organizing smaller molecules into complex structures. Responding to stimuli and sustaining homeostasis are crucial for survival in changing environments. Reproduction ensures the continuation of species, while dynamic metabolism allows organisms to adapt their energy use. Heredity through DNA enables genetic information to be passed down, and evolution allows species to adapt over time.

Why are viruses not considered living organisms?

Viruses are not considered living organisms because they lack several key characteristics of life. Specifically, viruses are not composed of cells, cannot reproduce independently, and do not have a dynamic metabolism. Instead, viruses consist of a protein shell called a capsid that encloses nucleic acid (DNA or RNA). They can only reproduce by hijacking the cellular machinery of a host organism. Additionally, viruses do not exhibit a dynamic metabolism, meaning they cannot perform metabolic processes on their own. While they can maintain some internal stability and evolve over time, these characteristics alone are insufficient to classify them as living entities.

What is homeostasis and why is it important for living organisms?

Homeostasis is the ability of an organism to maintain stable internal conditions despite changes in the external environment. This regulation is crucial for the survival and proper functioning of living organisms. For example, humans maintain a constant body temperature, pH level, and glucose concentration. Homeostasis involves various feedback mechanisms that detect changes and initiate responses to counteract those changes. Without homeostasis, organisms would be unable to sustain the optimal conditions necessary for cellular processes, leading to impaired function or death. Thus, homeostasis is a fundamental characteristic that distinguishes living organisms from nonliving entities.

How do living organisms exhibit a biological hierarchy of organization?

Living organisms exhibit a biological hierarchy of organization, which ranges from larger, more complex structures to smaller, simpler ones. This hierarchy includes multicellular organisms, organ systems, organs, tissues, cells, organelles, macromolecules, molecules, and atoms. For example, humans are multicellular organisms composed of various organ systems, such as the circulatory and respiratory systems. Each organ system consists of individual organs, which are made up of tissues. Tissues are collections of similar cells, and cells contain organelles like the nucleus and mitochondria. These organelles are composed of macromolecules, which are large molecules made up of smaller molecules and atoms. This hierarchical organization highlights the complexity and interdependence of biological structures.

What is the biochemical unity of life and how does it support the theory of a common universal ancestor?

The biochemical unity of life refers to the similarity in the molecular and atomic composition of all living organisms. This concept suggests that all life forms share common biochemical building blocks, such as DNA, RNA, proteins, and lipids. These similarities provide evidence for the theory of a common universal ancestor, which posits that all current life forms evolved from a single ancestral organism. The resemblance at the molecular level, such as the use of the same genetic code and similar metabolic pathways, supports this theory. The biochemical unity of life underscores the interconnectedness of all living organisms and their shared evolutionary history.