What is Life? Grade 7 Science

An Organism is: An individual living thing, made up of one or many cells, that is capable of growing and reproducing.

The Five Rules! Biologists often use five rules to classify something as living or nonliving: Living things respond to their surroundings. Living things grow and develop. Living things are able to reproduce. Living things use energy. Living things are made of smaller building blocks called cells.

1. Living things respond to their surroundings. All living things respond to a stimulus. A stimulus is a change in an organisms surroundings that causes the organism to react. Stimuli may include changes in temperature, light or sound. An organism reacts to a stimuli with a response---an action or change in behavior.

2. Living things grow and develop. Growth is the process of becoming larger. Adding more mass and more cells!!! Development is the process of change that occurs during an organisms life to produce a more complex organism.

2. Living things grow and develop.

2. Living things grow and develop.

3. Living things are able to reproduce. The process of making more of the same kind of an organism is called reproduction.

4. Living things use energy. All living things take materials from their surrounding such as food, water and gases and use these materials to get energy. Energy is needed to grow and repair injured parts.

5. Living things are made of smaller building blocks called cells. A cell is the smallest unit of a living thing. To see most cells, you need a microscope, a tool that uses lenses to magnify small objects.

Cells Organisms may be composed of only one cell or many cells. Unicellular, or single celled organisms include bacteria, one of the most numerous organisms on Earth. Multicellular organisms are composed of many cells. The cells of multicellular organisms are specialized to do certain tasks. Example: Muscle (movement) versus nerve(send messages from brain) cells.

Cells Muscle cell Nerve cell

Francesco Redi Before the 1600’s, many people thought that living things spontaneously generated (just appeared) from non-living things! Redi provided evidence that life (maggots) came from eggs not rotting meat.

The experiment

Are barnacles alive? Now, lets look at an organism to review the five characteristics of life.

1. Living things respond to their surroundings. Barnacles respond to their environment by closing their shells at low tide and opening them at high tide.

2. Living things grow and develop. Barnacles grow and develop. They begin life as free- swimming creatures. Once they find a good spot, they “glue” themselves to a rock and form a shell.

3. Living things are able to reproduce. Barnacles reproduce. After fertilization from a male barnacle, females hold the eggs in their shells until they hatch.

4. Living things use energy. By waving their legs, barnacles capture food. They use energy from the food to move their legs, open and close their shells, and carry out all of their life processes.

5. Living things are made of smaller building blocks called cells. If you examined the legs of a barnacle with a microscope you would see that they are made of individual cells.

Your turn! Pick something that is alive and use the five characteristics of life to explain and/or illustrate why it is alive!