What is Living — Definition

Definition

When we look around, we see a vast array of things – some move, some don't; some grow, some don't. How do we decide if something is 'living' or 'non-living'? Biologists have identified a set of fundamental characteristics that, when present together, help us define life. Imagine a tiny seed, a towering tree, a buzzing bee, or even yourself – what do you all have in common that a rock or a cloud doesn't?

Firstly, all living organisms grow. This means they increase in size and number. For multicellular organisms like us, growth is an increase in mass and the number of cells. For single-celled organisms, it's primarily an increase in cell size and then division.

However, growth can also happen in non-living things, like mountains growing due to accumulation of snow or crystals growing. The key difference is that living organisms grow from *within* (intrinsic growth), while non-living things grow by *external accumulation*.

Secondly, living organisms reproduce. They create offspring similar to themselves, ensuring the continuation of their species. This can be sexual reproduction (involving two parents) or asexual reproduction (involving a single parent).

But wait, there are exceptions! Mules, sterile worker bees, and infertile human couples are living but cannot reproduce. So, while reproduction is a characteristic, it's not a *defining* one because not all living things reproduce, and some non-living things (like crystals) can appear to 'multiply' under certain conditions.

Thirdly, all living organisms exhibit metabolism. This is the sum total of all chemical reactions occurring inside their bodies. It involves breaking down substances (catabolism) and building up new ones (anabolism). From digesting food to synthesizing proteins, metabolism is constantly happening. A test tube reaction isn't considered metabolism because it's not happening within a living system. This makes metabolism a truly *defining* characteristic of life.

Fourthly, living organisms have a cellular organization. Every living organism, from the smallest bacterium to the largest whale, is made up of one or more cells. The cell is the fundamental unit of life. Non-living things do not have cells. This is another *defining* characteristic.

Finally, living organisms show consciousness. This means they can sense their surroundings and respond to environmental stimuli like light, water, temperature, other organisms, pollutants, etc. If you touch a hot stove, you immediately withdraw your hand – that's consciousness.

Even plants respond to light (phototropism) or touch (like the touch-me-not plant). The ability to sense and respond is universal among living things, making it a *defining* characteristic. Humans have a unique form of consciousness called self-consciousness, where we are aware of ourselves.

In summary, while growth and reproduction are important features, they have exceptions. Metabolism, cellular organization, and consciousness are the truly defining characteristics that collectively set living organisms apart from the non-living world.