What is life? This is what scientists have been asking themselves for years. Now, to determine if something is alive or not, characteristics of life have been developed. For something to be considered alive, that something must have all the characteristics, not just one or two. If it does have all the characteristics, it is called an organism.
There are many different versions of these characteristics available, which doesn't make sense. I compiled a list by comparing the many lists I saw. This is what I believe to be an ultimate list from several lists.
1. Organisation: This characteristic doesn't mean that living things should be organised in their daily life but chemically. They have to be composed of atoms, which form molecules, which form cells. Similar cells put together make a tissue, and two or more tissues make organs. Organs then make a system. A living thing doesn't have to have a system, but at least one cell. These are called unicellular organisms (e.g. Amoeba). If the living thing has more than one cell, it is called a multicellular organism (e.g. Cat, Human).
Example: Muscle cells->Muscle Tissue->Heart (an organ made up of muscle tissue)->Circulatory System (Lungs and Heart and Blood Vessels working together)
2. Growth: All living things should be able to gather resources to use them to make their cells bigger, making their entire system enlarge.
3. Reproduction: All living things should be able to create another of its species. If it couldn't, species would not be able to survive because no young would be born to keep the species going. There are two kinds of reproduction: asexual and sexual. Asexual involves only one parent, for example, bacteria dividing their cells in two, creating two new beings. Sexual involves two parents.
4. Movement: All things should be able to move in some way. It is noticeable that a human, dog, centipede, can all move, but then what about plants? Plants move, only they move so slowly it is only noticeable after a certain number of time. For example, sunflowers move into the direction of the sun, flowering plants open their petals. The roots move down into the earth. Although it is not obvious, like a running animal, plants do move.
5. Response: Response is when an organism has the ability to respond to changes in its environment and to things happening in their bodies. Organisms respond to light, sound, and touch. Again, plants do not seem to be responding much, when you pull of one of their leaves, they don't jump back from shock and pain. But plants do respond. They have no nervous system, so they won't respond to touch, but they grow toward the light. If the sun changes place, they adapt themselves so that they end up back towards the sun. Another example would be that if a seed that is growing finds itself the wrong way, with its roots in the air, then the plant will respond to gravity and shift so that it's roots are back down in the earth.
6. Respiration: Respiration is how organisms get and use their food. There are two types of organisms for this characteristic: Autotrophs and Heterotrophs. Autotrophs are organisms that create their own food, i.e. plants. Heterotrophs are organisms that get food by eating other organisms, i.e. animals.
7. Metabolism: This is how an organism uses it's food, which food it requires, and how much of it it requires, and also what is poisonous to the organism. It is also the ability to create energy, and to divide it for separating it into different parts of the organism. A living thing must be able to metabolize.
All these characteristics must be present in something for it to be an organism, otherwise it is not alive. Is a cellphone alive? It reacts with it's environment (when you press buttons), it moves when it vibrates, but does it reproduce or break down its food to make energy?