What do a bacterium and a whale have in common?
Do they share characteristics with us? All living organisms, from the smallest bacterium to the largest whale, share certain characteristics of life. Without these characteristics, there is no life.
Characteristics of Life
Look at the duck decoy in Figure below . It looks very similar to a real duck. Of course, real ducks are living things. What about the decoy duck? It looks like a duck, but it is actually made of wood. The decoy duck doesn’t have all the characteristics of a living thing. What characteristics set the real ducks apart from the decoy duck? What are the characteristics of living things?
This duck decoy looks like it’s alive. It even fools real ducks. Why isn’t it a living thing?
To be classified as a living thing, an object must have all eight of the following characteristics:
- It responds to the environment.
- It grows and develops.
- It produces offspring.
- It maintains homeostasis.
- It undergoes metabolic changes.
- It consists of cells.
- It contains DNA.
- It evolves (as a population).
Response to the Environment
All living things detect changes in their environment and respond to them. What happens if you step on a rock? Nothing; the rock doesn’t respond because it isn’t alive. But what if you think you are stepping on a rock and actually step on a turtle shell? The turtle is likely to respond by moving—it may even snap at you!
Growth and Development
All living things grow and develop. For example, a plant seed may look like a lifeless pebble, but under the right conditions it will grow and develop into a plant. Animals also grow and develop. Look at the animals in Figure below . How will the tadpoles change as they grow and develop into adult frogs?
Tadpoles go through many changes to become adult frogs.
All living things are capable of reproduction. Reproduction is the process by which living things give rise to offspring. Reproducing may be as simple as a single cell dividing to form two daughter cells. Generally, however, it is much more complicated. Nonetheless, whether a living thing is a huge whale or a microscopic bacterium, it is capable of reproduction.
Keeping Things Constant
All living things are able to maintain a more-or-less constant internal environment. They keep things relatively stable on the inside regardless of the conditions around them. The process of maintaining a stable internal environment is called homeostasis . Human beings, for example, maintain a stable internal body temperature. If you go outside when the air temperature is below freezing, your body doesn’t freeze. Instead, by shivering and other means, it maintains a stable internal temperature.
It Undergoes Metabolic Changes
All living organisms have a complex chemistry. A flower has a complicated and beautiful structure. So does a crystal. But if you look closely at the crystal, you see no change. The flower, on the other hand, is transporting water through its petals, producing pigment molecules, breaking down sugar for energy, and undergoing a large number of other biochemical reactions that are needed for living organisms to stay alive. The sum of all the chemical reactions in a cell is metabolism .
All forms of life are built of cells. A cell is the basic unit of the structure and function of living things. Living things may appear very different from one another on the outside, but their cells are very similar. Compare the human cells on the left in Figure below and onion cells on the right in Figure below . How are they similar? If you click on the animation titled Inside a Cell at the link below, you can look inside a cell and see its internal structures. http://bio-alive.com/animations/cell-biology.htm
Human Cells (left). Onion Cells (right). If you looked at cells under a microscope, this is what you might see.
Photo credit: © WGBH Educational Foundation
It contains DNA
All forms of life contain DNA, deoxyribonucleic acid, which is the genetic material in cells. It was passed on to offspring from their parents and determines their characteristics.
It evolves (as a population)
All forms of life evolve, or change over time. This means that descendents may be different from their ancestors.
Is It Alive?
Life occurs in many incredible forms in many unexpected places. It isn't always obvious when something is alive so what do you need to look for to make that determination?
Use the below resources to answer the following questions:
- What characteristics do you feel are essential to consider something alive?
- How does a “snottite” differ from organisms you are more familiar with?
- All living things detect changes in their environment and respond to them.
- All living things grow and develop.
- All living things are capable of reproduction, the process by which living things give rise to offspring.
- All living things are able to maintain a constant internal environment through homeostasis.
- All living things undergo metabolic changes.
- All forms of life are built of cells. A cell is the basic unit of the structure and function of living things.
- All forms of life contain DNA. DNA is the genetic material passed from parent to offspring.
- All forms of life evolve as populations.
Use this resource to answer the questions that follow.
- http://www.hippocampus.org/Biology . Non-Majors Biology Search: Defining Biology
- What does "biology" encompass?
- What characteristics define life?
- Define metabolism.
- Are viruses living? Explain your answer.
1. List the six characteristics of all living things.
2. Define homeostasis.
3. What is a cell?
4. Making the next generation is known as ____________.
5. Assume that you found an object that looks like a dead twig. You wonder if it might be a stick insect. How could you determine if it is a living thing?