What makes something alive

Are electrons alive? How about viruses and bacteria? Even scientists disagree about what it means for something to be alive. What are these properties? The first is organization.

That means that living things are made of cells. To be alive, something must have specialized parts that all do their own job. Second, living things must have metabolism. This includes all the biochemical reactions that take place in something that is alive. The third property is homeostasis.

That means that living things regulate the conditions inside their bodies. Living things also experience growth. They reproduce in some way, although this can take many forms. Living things must give response to changes in their environment. Finally, the last property of life is evolution.

Living things must be able to change over time. Not all experts agree with these properties of life. Consider the mule. However, most people would consider the mule to be alive. Still, the properties of life provide a starting point for understanding what makes something alive.

There are still many questions to answer. For example, what about viruses? Experts disagree over whether they are living things. Others disagree, as viruses do have many of the other properties of life. How about life on other planets? Will it look like life on Earth?

Most likely not. That means those searching for alien life have to keep open minds about what makes something alive. Our understanding of this will probably continue to develop as humans explore deeper into the universe. What do you think? Would you classify viruses as living things? For example, a human grows from a baby into an adult and goes through developmental processes such as puberty.

Organisms grow and develop following specific instructions coded for by their genes DNA. In order to function properly, cells need to have appropriate conditions such as proper temperature, pH, and appropriate concentration of diverse chemicals.

These conditions may, however, change from one moment to the next. For example, an organism needs to regulate body temperature through a process known as thermoregulation. Organisms that live in cold climates, such as the polar bear Figure 4 , have body structures that help them withstand low temperatures and conserve body heat. Structures that aid in this type of insulation include fur, feathers, blubber, and fat. In hot climates, organisms have methods such as perspiration in humans or panting in dogs that help them to shed excess body heat.

Even the smallest organisms are complex and require multiple regulatory mechanisms to coordinate internal functions, respond to stimuli, and cope with environmental stresses. Two examples of internal functions regulated in an organism are nutrient transport and blood flow.

Organs groups of tissues working together perform specific functions, such as carrying oxygen throughout the body, removing wastes, delivering nutrients to every cell, and cooling the body. All organisms use a source of energy for their metabolic activities. Some organisms capture energy from the sun and convert it into chemical energy in food such as grass and bacteria that can perform photosynthesis ; others use chemical energy in molecules they take in as food such as the condor seen in Figure 5.

Unless otherwise noted, images on this page are licensed under CC-BY 4. This view of living may be appropriate at this age but has some limitations and can lead to the alternative conceptions above. For example movement in plants is not apparent to students and consequently they may not consider plants living.

Decisions about whether things are alive or non-living remain problematic as not all life processes stop at the same time. For example, human fingernails and hair continue to grow for weeks after death. To build up a concept of living things focus on the similarities shared with unlike living things. There is a need to maintain a focus on the big ideas such as groups of classification and allow students to make connections between individual examples and the big ideas.

Children tend to think fires are living because they consume wood, move, require air, reproduce sparks cause other fires and give off waste such as smoke. This is a complex idea and is better dealt with at higher levels where concepts can be unpacked in more sophisticated ways. Hence it is not necessary to try to change these conceptions at this stage, but to recognise that students may hold and maintain this view at some stage as learners. Students collect objects from the schoolyard on a scavenger hunt and classify them into groups of their choosing.

Students will articulate their classification system.