Have you ever sat back and wondered what separates living things from none living things? Both living and non-living things are made up of elements bound together by atoms. How can we distinguish ourselves from say, a car, which shares many characteristics we do? What characteristics do we share with a car? Like a car, your body needs fuel to function and move around the environment. Does that mean that a car is a living entity? No, of course not. So how do we define living? Throughout their study of the natural world, biologists have come up with seven characteristics which distinguishes living from non-living things. The order of these characteristics does not matter. What is important is that, in order for something to be considered alive, all of these characteristics are met.
I. Nutrition
The first characteristic of living things is their ability to take in materials from their environment such as fruit, meat or plants. Living things use the proteins and carbohydrates found in the surrounding environment for energy. Their bodies break down the nutrients found in food.
II. Movement
The second characteristic of living things is, they need to be able to move. Now this characteristic does not say their movement needs to be noticeable to other living things, just that they move. For example, while a leopard's movement may be noticeable to us, the movement of a tree may not be.
III. Respiration
The third characteristic of living things is their ability to release energy from food substances. Cells within a living thing release the energy locked away in food, allowing living things to carry out the other characteristics on this list.
IV. Growth
The four characteristic of living things is they need to be able to grow. This is done through a complex process of cell division, creating new permanent cells, which can only be accomplished through breaking down of nutrients and proteins.
V. Reproduction
The fifth characteristic of living things is they need the ability to reproduce. A car may share some characteristics from this list, but it cannot reproduce like humans and animals can. Plants also reproduce.
VI. Excretion
The sixth characteristic of living things is they create waste from the energy they use. Cars create waste by burning oil and creating smoke, but they are not alive. Humans and animals produce waste in the form of feces and urine.
VII. Sensitivity
The last characteristic of all living things is their ability to sense and respond to stimuli in their environment. This includes things such as gravity, water, light, temperature, etc.
These seven characteristics are what biologists use to classify living things. As humanity's understanding of the natural world grows, these characteristics may very well be refined and added to. For now though, they serve us well in understanding the differences between living and non-living things.
Source: http://assets.cambridge.org/97805216/80547/excerpt/9780521680547_excerpt.pdf