1. Is it alive?
What does it mean to be alive? Is a cactus alive? Is a seed alive? Is the air we breathe alive? What are the necessary characteristics? To hook students into ...
What does it mean to be alive? Is a cactus alive? Is a seed alive? Is the air we breathe alive? What are the necessary characteristics? To hook students into the question, they are introduced to “glue monsters” (sometimes known as “scooting glue”) and the class discusses whether the “monsters” are alive or not. Next, students are given cards with the names of various objects and asked to sort them into categories: alive, once was alive, never alive, and not sure. Finally, students create a list defining the characteristics of life – a set of characteristics that all living things share. The list is initially developed in pairs, then in larger groups of 4, and ultimately as a whole class. The final list is turned into a poster that can be referenced and modified throughout the remainder of the unit as students learn more about what it takes to be alive.
Can begin to discuss the necessary characteristics of life.
Can begin to categorize objects as alive or not alive.
Can recognize that movement does not necessarily mean something is alive.
1. Is it alive? - Logistics
10-15 min glue monsters demo
30-40 min alive or not alive card sorting and discussion
30-45 min create characteristics of life lists
The card sorting activity and initial creation of a list of the characteristics of life is done in pairs. These pairs will eventually merge in groups of 4 to compare, discuss, and revise their lists. The remainder of the discussion takes place as an entire class.
For the “glue monsters” demo, the teacher needs:
- 1 overhead projector
- 1 tube of Duco® cement glue (according to Flinn Scientific, this is the only brand of glue that works reliably)
- 1 clear Petri dish
- ground pepper or pencil shavings
For the card sorting activity, each pair of students needs:
- copy of the sorting cards on cardstock paper, cut out and placed in an envelope
For developing a characteristics of life list, each class needs:
- butcher paper or flip chart paper
1. Is it alive? - Background
Many middle school students believe that the defining characteristic of living things is that they move. When they see the “glue monsters” wiggle in the Petri dish, most will immediately assume they are alive. What is going on? Duco® cement is polymer mixed in a water soluble solvent. When the cement is exposed to air as it drops into the dish, a thin, solid polymer skin quickly forms around the liquid, solvent-polymer mixture. When the bead of cement is immersed in water, the solvent diffuses through the skin, causing the bead to shrink and the skin to rupture on one side of the bead. The solvent squirts out of the hole and the surface tension of the water on that side of the bead suddenly falls. Since the surface tension is now uneven, the bead will move away from the hole, towards the area with greater surface tension. The hole quickly repairs itself but the skin then bursts in another location. Thus, the bead appears to wiggle and twist as the surface tension changes depending on where the skin bursts.
Another way to demonstrate surface tension propulsion is to place a paper boat in a tub of water. Take a toothpick dipped in concentrated dish soap (which will lower the surface tension of water) and touch it to the water near the back of the boat. The boat with rush away from the toothpick.
As to the card sorting activity, students will struggle over many of the items. Do not expect students to correctly categorize items, even after a group discussion. The goal is to get students thinking and debating about the characteristics all living things share, not to get the “right” answer. Their classifications will also give you a good sense of their current state of understanding and sophistication. Keep a list of the items students disagree on or misclassified. Revisit these items at the end of the unit when students have mastered the major concepts. Keep in mind that there are some items that even scientists disagree on, such as viruses and prions. Some of the items (dirt and air) are mixtures in which some parts are alive and some are not. In addition, how you classify a part of a multicellular organism (like a single leaf, blood, or pollen) depends on your point of view. These ambiguous items provide opportunities to discuss the characteristics of life with your students.
Defining the characteristics of life is difficult and not completely clear cut. Although you will find different lists at different sources, most scientists agree that the following characteristics are shared by all living things:
- Living things are made of cells. The basic unit of life is the cell. All living things are composed of one or more cells.
- Living things grow larger over time.
- Living things reproduce. Each individual, given the right circumstances, has the potential to produce a new individual that resembles its parent.
- Living things respond to the environment. Sometimes this response takes the form of motion such as an animal running away from danger or a plant orienting towards the sun. Sometimes this response is more subtle such as closing certain membrane channels in response to changing salt concentrations.
- Living things metabolize, that is, they take in raw materials and convert them into energy and wastes. For instance, animals convert glucose and oxygen into energy and waste products (water and carbon dioxide) through the process of respiration.
- Living things evolve. Over many generations the traits of the species will change by natural selection to better fit the current environmental conditions. In other words, living things adapt to their environment. A basic assumption of evolution is heredity, the passing of traits from parent to offspring through genes. Thus it is also accurate to say that all living things inherit traits from their parents through some type of genetic material.
In addition to these 6, some lists included 2 additional characterisitics:
- Living things maintain homeostasis. That means that living things can maintain their internal environment relative to changes in the external environment. A good example is how our bodies maintain a constant body temperature – sweating to cool our bodies down and shivering to warm our bodies up when necessary.
- Living things are made of organic molecules. Organic molecules include proteins, lipids, carbohydrates (starches and sugars), and nucleic acids (like DNA and RNA).
The goal of these activities is not to force students to memorize the list above. Many are new, difficult concepts (like cells, metabolism, and organic molecules) that will develop over the course of the unit. Students should experience the process of creating the list themselves and revising it periodically as they learn new things. For instance, in the preliminary list, the items: “need nutrients”, “make wastes” and “need energy” may appear separately. After students learn about photosynthesis and respiration as metabolic processes, these 3 items can be combined under the umbrella of “living things metabolize”.
1. Is it alive? - Getting Ready
Glue monsters demo:
- Place a Petri dish half full of water on the overhead projector.
- Set aside a small dish of pencil shavings or black pepper.
- Try the demo yourself first, rehearsing the “release” of the “monsters” into the dish and their “feeding” with pepper or pencil shavings.
- Optional: wrap the tube of glue in paper or keep it in a brown paper bag so that students cannot tell what the “monsters” really are.
Alive or not alive card sorting:
- Copy sorting cards onto cardstock paper.
- Cut the cards apart.
- Place cards in an envelope.
- Optional: include some extra blank cards for students to add their own items to the set.
Characteristics of life list:
- Set out butcher paper or flip chart paper and markers.
1. Is it alive? - Lesson Plan
Glue monsters demo:
- Fill a Petri dish half full with water.
- Place the Petri dish on an overhead projector.
- Fabricate some story about small, blob-like monsters that live in the local watershed. Add a drop of Duco® glue to the water in the dish.
- To “feed” the “monster”, sprinkle a small amount of the pencil shavings or pepper near the “monster”. It should move towards the shavings or pepper and “eat” them.
- Add additional drops of glue and watch “monsters” interacting with one another.
- When the “monsters” slow down, turn off the projector and ask the students for their observations. What did they see? What happened when “food” was added? How did the monsters interact with each other? Most importantly, were they alive and how could you tell?
- After students reported their initial observations, do the demonstration again, this time very obviously showing students that it was a trick – the “monsters” were just drops of glue in water and the “food” was pencil shavings. Briefly explain the chemistry behind the demonstration.
- Discuss what characteristics they used to initially decide that the “monsters” were alive. Students will generally point out how they “moved on their own”. Ask students whether all living things “move on their own” and look for counterexamples like plants.
Alive or not alive card sorting:
- Describe the card sorting activity to the students. Working with a partner, they should divide the items on the cards into 4 categories: alive, never alive, once was alive, and not sure.
- Divide the students into pairs, distribute the sorting cards, and allow students to get started.
- Circulate around the room helping students that have questions. If an item is unfamiliar, describe what the item is without necessarily giving away whether it is alive or not.
- If groups finish early, you may ask them to come up with their own cards and add them to the set.
- When most groups are done, pull the class together to discuss their conclusions. Begin the discussion by creating a master list on the board. Have students name the items within each category that were easy to classify. Then discuss the more difficult items one by one. Don’t tell students the “right” answer. Allow them to make mistakes. In your discussion, repeatedly ask students to explain:
- what criteria they used to make their decisions
- what all the living things have in common
- You may wish to keep a list of those items that were miscategorized and those that had disagreements. Revisit this activity at the end of the unit as a way to review.
Characteristics of life list:
- Give each pair of students the task of creating a list of characteristics that they believe all living things have in common. Students should make sure that each criteria applies to all the living things from the card sorting activity. It is important that both students agree on every item. Both students should make their own copy of the list on a separate sheet of paper. Allow students around 15 minutes to discuss the problem and come up with their list.
- When each pair has a list, rearrange students into groups of 4. The original pairs should be split apart into different groups. In these new groups, the goal is to come up with a consensus list, one that each person in the group agrees with. Do not allow voting. Allow students around 10 minutes to come to consensus.
- Have each group read their list to the whole class. As they read out their lists, write down the characteristics on the board putting tally marks next to those reported by multiple groups.
- Moderate a discussion to come up with a consensus list for the whole class. Have different groups explain the reasoning behind criteria that there is disagreement about. When everyone (including you as the teacher) is comfortable with the consensus list, create a class poster with all the characteristics of life listed. This poster will be revisited and revised over the course of the next few weeks as students become more sophisticated and learn new concepts.
1. Is it alive? - Assessment
Play the game “5 Alive”. On a piece of paper, the person who is “it” should write the name of any item that they know for sure is alive or not. The rest of the group gets to ask 5 yes or no questions to figure out if the mystery item is alive.
- Many of the other activities in this box extend upon this first lesson.
- The Life Trap activity demonstrates that living things can be microscopic, grow and reproduce.
- The Testing for Life activity introduces the idea that all living things are made of the same organic molecules and has students test for proteins, starches and sugars.
- The Seeing Cells activity introduces students to the idea that all living things are made of cells.
- The Cell Energy activity brings up the concept of cell metabolism.
- The Life on Mars project asks students to design 3 experiments to determine whether there is anything living in a sample of “Martian soil”.
- A great resource for additional lessons on the characteristics of life is the Life in the Universe curriculum, published by the SETI Institute.
- Another resource is the Searching for Life curriculum from NASA.
1. Is it alive? - Sources and Standards
I discovered the “glue monster” or “scooting glue” demo from Flinn Scientific (click on “Glue Monsters” to download the pdf file). Kitchen chemistry also provides a write up for the same activity with a better description of the chemistry behind the demo. For a quicktime movie of a paper boat “fleeing” from a dish soap coated toothpick, see the University of Iowa Physics and Astronomy Lecture Demonstrations.
To learn more about the characteristics of life, see the following sites:
- The Open Door Website
- Astrobiology: The Living Universe
1. All living organisms are composed of cells, from just one to many trillions, whose details usually are visible only through a microscope.
Structure and Function in Living Systems
5. The anatomy and physiology of plants and animals illustrate the complementary nature of structure and function. As a basis for understanding this concept:
a. Students know plants and animals have levels of organization for structure and function, including cells, tissues, organs, organ systems, and the whole organism.