Characteristics of Life
Characteristics of Life
Introduction: All living things share a certain traits. If something is missing even one of these, then it automatically is not alive.
The first characteristic all living things share is that they are made up of one or more cells. A cell is the smallest unit that can support life. Bacteria and many protists are only a single cell, while a human and an elephant are made of trillions of cells. Each cell has its own metabolism that creates energy and rids itself of wastes. In higher organisms, they work together in structures called tissues, organs, and organ systems.
Secondly, all living things use energy. This allows them to carry out their individual processes, such are breaking down food or eliminating waste products. They also use this energy for building things, such as glucose in plant cells.
Living things are able to reproduce. This means they produce similar organisms to themselves. Reproduction can happen asexually, where the organism produces more of itself without the transfer of genetic material (such as binary fission in bacteria or budding in sponges), or sexually, where genetic material is transferred from one organism to another, usually via sperm. Most organisms reproduce in this fashion.
One reproduction has occurred, living things need to grow. They do this by dividing each cell over and over again. Additionally, each of those cells need to increase in size before it can split. In sexual reproduction, a single cell (the egg) divides thousands of time in order to form the new individual.
If you poke a dog with a stick, it will bark, growl, or at the very least, move. Living things respond to stimuli. Stimuli may be an increase in environmental temperature, a shortage of food supply, or the production of chemicals to keep insects from eating leaves.
The response to stimuli is not to be confused with adapting to the environment. Living things adapt when environmental conditions change. This is not an immediate reaction, like running away, but rather takes a lot of time. Evolution is the result of random mutations in DNA that produce favorable traits in an organism. If the environment has change such that the trait is useful, that organism has an advantage over one that does not have the trait. When that organism reproduces, it will pass along the favorable trait to its offspring. Over time, the entire population will have that trait.
Finally, all living things have a universal genetic code. This means that they all have DNA inside of them that produces their traits. This DNA is the same regardless of into which organism one looks. A mushroom's DNA will have the same structure as that of a tiger shark. It is the sequence of the nitrogen bases in the DNA that makes these creates, and all creatures unique. However, every one of them still has the same four nitrogen bases.
There are some scientists who classify viruses as living things. After a virus is able to reproduce (that is how it infects a cell) and evolve (this is why you need to get a flu shot every year). They may also have DNA or RNA and respond to stimuli (many viruses are temperature sensitive). However, since they do not use energy nor are they made of cells, they cannot be classified as living things. The diagram below shows the anatomy of a virus. The main "head" of the virus is called a capsid. This is where it stores its genetic material.
Directions for this Lesson: Answer the practice questions and then watch the video to learn more about what makes something alive.