What do a bacterium and a frog have in common?

Do they share characteristics with us? All living organisms, including the smallest bacterium and a frog, share certain characteristics of life. Without these characteristics, there is no life.

Characteristics of Life

Look at the duck decoy in the 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?

Not all scientists agree exactly about what makes up life. Many characteristics describe most living things. However, with most of the characteristics listed below, we can think of one or more examples that would seem to break the rule, with something non-living being classified as living or something living being classified as non-living. There is not just one distinguishing feature that separates a living thing from a non-living thing. A cat moves but so does a car. A tree grows bigger, but so does a cloud. A cell has structure, but so does a crystal. Biologists define life by listing characteristics that living things share. Something that has all of the characteristics of life is considered to be alive.

There are many characteristics that living organisms share. All living organisms:

• Respond to their environment.
• Grow and develop.
• Produce offspring.
• Maintain homeostasis.
• Have complex chemistry.
• Consist of cells.

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. Bacteria grow by increasing in cell size and volume. Animals also grow and develop. How will the tadpoles change as they grow and develop into adult frogs?

Reproduction

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 frog or a microscopic bacterium, it is capable of reproduction.

There are two types of reproduction, asexual and sexual reproduction. Sexual reproduction is a type of reproduction that involves two parents and produces genetically different offspring. Asexual reproduction is a type of reproduction that involves a single parent and results in offspring that are all genetically identical to the parent. For example, bacteria reproduce by asexual reproduction creating an exact copy of themselves.

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. Some species of tree frogs adjust their skin color as temperature changes in their environment.

Complex Chemistry

All living things—even the simplest life forms—have complex chemistry. Living things consist of large, complex molecules, and they also undergo many complicated chemical changes to stay alive. Thousands (or more) of these chemical reactions occur in each cell at any given moment. Metabolism is the accumulated total of all the biochemical reactions occurring in a cell or organism.  All living things need a continuous input source of energy to sustain life processes. Complex chemistry is needed to carry out all the functions of life.

Cells

All forms of life are built of at least one cell. 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 illustrations of the frog cells on the left and bacteria cells on the right in Figure below. How are they similar?

If you zoom in very close on a leaf of a plant, or on the skin on your hand, or a drop of blood, you will find cells. Most cells are so small that they are usually visible only through a microscope. Some organisms, like bacteria, plankton that live in the ocean, or paramecium, are unicellular, made of just one cell. Other organisms have millions, billions, or trillions of cells.

Summary

• 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 have complex chemistry.
• All forms of life are built of cells. A cell is the basic unit of the structure and function of living things.

Review

1. List the characteristics of life.
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?