What is Life?

Based on what happens inside of cells, the characteristics of life include: homeostasis, transfers of energy, transportation of molecules, disposal of wastes, creation of new molecules

Characteristics of Life

One of the most popular questions that students have in biology class is, “Are there really aliens?” Although most scientists will say that no aliens have been discovered yet, this is actually a much more difficult question to answer than you might think. What really is an alien? A little green man with antennae? A slimy mass that inches around?

Helicoprion, a fish genus that lived about 250 million years ago.

An alien is any form of life that does not come from Earth. Unfortunately, that doesn’t make it much easier to be able to tell what an alien is. There are millions of undiscovered species of life here on Earth, and many of them look like they might come from other planets. In fact, scientists estimate that over 100 undiscovered species go extinct every day just in the world’s rain forests!

Even as we’re still just looking on Earth, how do we figure out what is alive and not alive? Is something alive because it moves? By that definition, air and water would be alive. The real answer is much more complicated, and not all scientists agree on what things are alive and which are not living.

The first way that we can tell something is alive is by whether it creates new molecules. Just as you are growing, making new blood, hair and skin all of the time, all living things make new molecules. Even single-celled organisms are renewing parts of their cell and growing so that they can reproduce. In fact, just think what would happen if organisms didn’t make new molecules all of the time – how would sperm and eggs ever form, how would one cell split into two cells, how would the organism ever grow? In other words, for an organism to use the food and water around it, it must somehow make new molecules.

The "Dumbo" octopus, which lives 1000 feet below the surface of the ocean

When those new molecules aren’t needed any more, all organisms must have some way of getting rid of wastes. Humans do this in many ways: we breathe out carbon dioxide through our mouths, we often sweat out urea (which is also in our urine) and we defecate solid matter that our bodies can’t use. On a smaller level, however, each one of our cells is getting rid of wastes. When our cells let certain materials in, they also release other materials that aren’t needed any more. Every cell produces its own waste, from the simplest bacteria to the strangest deep sea creature (like the “dumbo” octopus to the right). Again, imagine what would happen if our cells were able to make new molecules, but not get rid of wastes: We’d get full of carbon dioxide and explode very quickly!

The “trash” that we make inside of our bodies has to be transported to places like the nose, bladder and anus. If it weren’t for the transportation of materials, bad and good, we wouldn’t be able to get oxygen and nutrients to all of our cells. This is another characteristic of all living things: the ability to move materials from one place to another. In humans and many other animals, we accomplish this by having blood that reaches every single cell in the body. In fact, our blood makes a full trip around our body about every 45 seconds. Single-celled organisms don’t have to transport molecules very far, but they still move molecules around their cells so that nutrients can get to parts of their cell that can make use of these nutrients.

A twig-like insect

In addition to moving nutrients around, all living things have to move energy around. Our stomachs digest our food and move that food to the small intestine, where most of the nutrients are passed directly to the blood. There is a tremendous amount of energy inside of these nutrients that helps our muscles move, our nerves send signals and our organs to expand and contract. If it weren’t for the fact that this energy could get to our muscles, we wouldn’t be able to move at all! All organisms have this same problem: Food comes in one place, but must be used in another place.

One of the smallest known snakes in the world

The final characteristic of life is the most complicated: Homeostasis. Put simply, homeostasis is the ability of an organism to maintain a balance between what’s going on outside and what’s going on inside itself. As humans, we keep our bodies at a pretty constant 98.6 degrees Fahrenheit (except when we’re sick and trying to create an unfavorable environment for bacteria or viruses). We maintain 98.6 F by sweating off heat when it’s hot outside of our bodies and by shivering and shaking to produce more heat when it’s cold. A dog does not have the ability to sweat, so it releases heat by panting; a reptile can’t produce its own heat, so reptiles need to sit in the sun or on warmed earth in order to digest their food. All organisms need to maintain a balance of more than just heat. Just think what would happen if you couldn’t keep all of the water inside of your body from evaporating! What would happen to a tree that couldn’t absorb water from its environment? All living things need to be in balance with their environment.

If we ever find an alien, there will certainly be a lot of questions to answer. For biologists, they will be quick to ask how it is that the alien accomplishes these five tasks that form the basis of all living things.

1. Identify the five characteristics of life and define each of them.
2. Describe homeostasis in your own words.
3. What are two substances that your body makes?
Put it together
4. Choose any organism. Predict the effects of this organism not being able to get rid of wastes.
5. Name something that is involved in human homeostasis that is not mentioned in the passage.
Think about it
6. Imagine that tomorrow, a new life form on a distant planet in the Milky Way is found. a) Name it. b) Describe it in one sentence. c) Draw a quick sketch of it. d) For each characteristic of life, how can the organism be considered alive?
7. Complete: After a ________ has been well-tested and confirmed by other scientists, it can become a ________ .
8. Summarize the experiment performed in the passage using the Describing Wheel.
Heart Rate Lab

by Dr. Ingrid Waldron

For this activity, after you learn how to measure heart rate accurately, your group will design an experiment to test how a stimulus or activity affects heart rate. During the next laboratory period, you will carry out your experiment, analyze your data, and prepare a poster describing your experiment.

Heart Diagram

  1. Why do you need to have a heart? Why do you need to have blood circulate to all the parts of your body?
  2. How does your heart pump blood? What is a heart beat?
  3. Does your heart always beat at the same rate?
  4. List 5 activities or stimuli that you think may increase a person’s heart rate. An activity is something a person does, and a stimulus is an input from the environment around a person.
  5. Why would it be useful for the heart to beat faster during these activities or in response to these stimuli?
  6. Are there any activities or stimuli that you think may decrease a person’s heart rate?

Each time the heart beats, blood is pumped into the arteries. As the blood surges into the arteries during a heart beat, each artery stretches and bulges. This brief bulge of the artery is called a pulse. You will be measuring heart rate by counting the number of pulses in the artery in the wrist in a 30 second interval. To measure heart rate, count the number of pulses in 30 seconds. Multiply that number by 2, and you will have the number of heart beats per minute.

  1. Choose one person in your group to be the subject, one person to measure the pulse count in the left arm, and one person to measure the pulse count in the right arm. The fourth person in the group will use the stop watch to time a 30 second interval, and will indicate when the count of beats should begin and end.
  2. Both people who are measuring pulse count should write down the number of beats for the 30 second interval before saying the number out loud. What is the pulse count for 30 seconds?
Group Member 30 second pulse count Beats per minute(Equal to 30 second pulse count times 2)
Trial 1 Trial 2 Trial 3
  1. Compare the results found by the two different people who were measuring pulse counts. Did you both count about the same number of pulses in the 30 second interval? If you got different results, why?
  2. Try to improve your technique, and repeat step 8 until both people who are measuring pulse counts get the same number of pulses in the 30 second interval (or within 1 or 2 of the same number). Once you have accurate readings, use the final, accurate set of measurements to calculate the heart rate for this subject (beats per minute). What is the heart rate?
  3. After this, you should switch roles. The people who were measuring pulse counts should now be the subject and the timer, and the people who were the subject and the timer should now measure pulse counts. Repeat steps 8 and 9 until the heart rate measurements are accurate.

Discuss how you could test your ideas concerning activities or stimuli which may increase or decrease heart rate. Choose a hypothesis that your group would like to test in your next lab class.

  1. Write your hypothesis down.

Plan your experimental procedure. You should keep everything in your experiment the same except for the one thing that you are testing for. When you write down your procedures, remember that the heart rate can be affected by even minor physical activity like changing seats, so you need to keep even this activity the same in order to test the effect of your stimulus or activity. Plan to have each person in the group be a subject in the experiment, in order to see whether different people have the same heart rate response to your stimulus or activity.

  1. Write down the procedure for your experiment. Include:
    1. What you plan to do to your subjects
    2. What the activity is
    3. When you will measure heart rate
    4. How often you will measure heart rate (e.g., 2 or 3 times before the activity, during the activity, after the activity)
  2. Make a data sheet to collect the data during your experiment next time. The data sheet should include places to record the:
    1. Names of each student in the group
    2. Resting heart rates for each subject before the stimulus or activity
    3. The heart rates during and/or after the stimulus or activity
    4. Anything you notice which might affect the results (e.g., other things which may be happening in the room during your experiment or changes in each subject’s mood during the experiment)
  3. Review your experimental plan from last time, and carry out the experiment for each subject in your group. Record your data in your data sheets.
  4. For each subject, calculate the difference between the resting heart rate and the heart rate during or after the stimulus or activity. Make a table which shows these change in heart rate values. Calculate the average change in heart rate for all subjects in the experiment, and record this average in the table.
  5. Do your results support your hypothesis? What conclusions can you draw from your experiment?