I can describe the characteristics of life that are common to all living things.
Think about how the things pictured above are different. You could probably make a list that is pages long of those differences. You could start with pictures a, b, and c being microscopic. You can't even see them with the naked eye. You may say that the raccoon is the only one that can see or has fur. The differences are almost endless. Now, think about what mushrooms, raccoons, pine trees, and bacteria have in common. This list may be a bit more difficult to start.
Characteristics of Life
Although they seem so different, all of these are living things, or organisms...just like you! But, how do you define a living thing? It might seem hard to think of similarities among such different organisms, but they actually have many things in common. The research of scientists indicates that all living things evolved from the same common ancestor that lived billions of years ago. This is why we are able to still find similar characteristics amongst them all today. See http://vimeo.com/15407847 for a powerful introduction to life. Below is a list of characteristics that are shared by all organisms:
- They need energy and resources to carry out life processes.
- They are composed of one or more cells.
- They respond and adapt to their environment.
- They reproduce.
- They grow and develop.
Living Things Need Resources and Energy
Why do you eat everyday? As you probably heard someone say as you were growing up, you eat to get energy. You also use the food you eat to build new parts of your body and replace or repair worn out parts. The work you do each day, from walking to thinking to growing taller, is fueled by energy from the food you eat. But you are not the only one. In order to grow, reproduce and merely maintain themselves, all living things need energy. But where does this energy come from?
The source of energy differs for each type of living thing. In your body, the source of energy is the food you eat. Here is how animals, plants and fungi obtain their energy:
- Consumers, such as animals, must eat plants or other animals in order to obtain energy and building materials.
- Other consumers, such as mushrooms and other fungi obtain energy by absorbing nutrients from dead or decaying matter. That’s why you often see fungi growing on a fallen tree; the rotting tree is their source of energy (Figure below).
- Producers, such as plants, don’t eat like animals do. Instead, as you learned in the previous unit, they use energy from the sun to make their food, or sugars, through the process of photosynthesis.
Since plants harvest energy from the sun and other organisms get their energy from plants, nearly all the energy of living things initially comes from the sun.
Orange bracket fungi on a rotting log in the Oak Openings Preserve in Ohio. Fungi obtain energy from breaking down dead organisms, such as this rotting log.
Each of the types of organisms described also exchanges gases with its environment in an effort to use the energy it takes in. For example, to burn the food you eat, you take in oxygen and release carbon dioxide gas as a waste product. Plants, on the other hand, take in carbon dioxide to perform photosynthesis and build their sugars. They release oxygen as a waste product of that process.
Living things also are made of water, making it another resource, in addition to energy and gases, they need to survive. The reactions that take place within living things occur within that watery medium. Many living things even use water as their environment.
Living Things Are Made of Cells
If you zoom in very closely on a leaf of a plant, or on the skin on your hand, or a drop of blood, you will find cells (Figure below). Cells are the smallest unit of living things. They are like tiny factories where virtually all life processes take place. 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 the paramecium shown in Figure below are made of just one cell. They are known as unicellular organisms (the prefix "uni-" means one). Other organisms, like oak trees and jaguars, have millions of cells. They are known as multicellular organisms (the prefix "multi-" means many). Although the cells in these organisms are microscopic, other cells, like eggs, are some of the biggest cells around. A chicken egg is just one huge cell!
Regardless of the type of organism, all living cells share certain basic structures. For example, all cells are enclosed by a membrane. The cell membrane separates the watery cell from its external environment. It also controls what enters or leaves the cell. Although the cells of different organisms are built differently, they all function much the same way. Every cell must get energy from food, be able to grow and reproduce, and respond to its environment.
Reptilian blood cell showing the characteristic nucleus. A few smaller white blood cells are visible. This image has been magnified 1000 times its real size. Multicellular organisms, such as reptiles, have many different kinds of cells that make them up (for example, bone cells, skin cells, nerve cells and blood cells).
This paramecium is a unicellular, or single-celled organism. Notice it is composed of just one cell.
Living Things Respond and Adapt to Their Environment
Have you ever seen a turtle retract its head into its shell when it senses danger? Did you know that sunflowers actually move toward the sun throughout a day? Or, maybe you've noticed your pupils constrict when you move from a dark room to a bright one. All living things are able to respond to stimuli, which are changes in their environment that cause them to react. These stimuli could include changes in light, heat, sound, and chemical and mechanical contact. Organisms have means for receiving information, such as eyes, ears, and taste buds. In general the organism's response helps it to survive and reproduce.
A bird fluffs its feathers to stay warm.
Watch this amazing time-lapse video to see how a plant responds to the stimuli of light and gravity as it grows. Why do you think it is important for a plant to respond appropriately to these stimuli for proper growth?
Organisms are also well adapted to their environment. For example, polar bears would not survive very well in the Sahara Desert, but their thick fur allows them to thrive in cold, polar regions. Charles Darwin, a scientist who studied organisms in the Galapagos Islands off the coast of South America in the 1800s, led many scientists to understand that the birds on the different islands had differences in their beaks. Scientists have learned that the different shaped beaks allow the birds to eat the different types of seeds on the various islands. Again, the birds are adapted to the conditions in their differing environments.
Living Things Grow and Reproduce
Like plants in the video you watched in the previous section, all living things have the potential for growth. The ducklings in Figure below have a lot of growing to do to catch up in size to their mother. Multicellular organisms like ducks grow by increasing the size and number of their cells. Their cells divide and enlarge, which causes the organims to increase in size. Unicellular organisms, since they are made of just one cell, don't rely on their cells to divide. Instead, their one cell just grows in size.
Because their cells will divide over and over again, these ducklings will grow to become as big as their mother by the time they are about a year old.
As the ducklings grow, they will develop and mature into adults. By adulthood, they will be able to reproduce. Reproduction is the production of offspring. The ability to reproduce is another characteristic of living things. You may know that zoos across the world are making an effort to create breeding programs to prevent populations of organisms from going extinct. The zoos then release the organisms back into their native habitat to increase the population. Populations of organisms that do not reproduce will go extinct. As a result, species like that ultimately failed to reproduce are not alive today. (Figure below).
The key to preventing extinction is for an organism to reproduce before its life span runs out. An organism's life span is the amount of time it is expected to live. Because organisms are living, another characteristic they possess is that they will also die. The life spans of organisms vary greatly. For example, the American box turtle may live for 123 years, while a water insect known as the mayfly lives no longer than one day! It merely lives to reproduce and carry on the species in that short time.
cell: smallest unit of structure and function in living things
organism: a living thing
1. Define organism.
2. Describe three characteristics of living things.
3. Identify three ways organisms can get the energy they require.
4. What is a cell?
5. Think about fire. Can fire be considered a living thing? Why or why not?
6. Why must organisms reproduce?
7. How is growth in a unicellular organism different from growth in a multicellular organism.
3.1.A.a: Describe the common life processes necessary to the survival of organisms (i.e., growth, reproduction, life span, response to stimuli, energy use, exchange of gases, use of water, elimination of waste)
3.1.E.b: Identify examples of unicellular (e.g., bacteria, some protists, fungi) and multicellular organisms (e.g., some fungi, plants, animals