Response to stimuli is an important characteristic of life. Anything that causes a living organism to react is called a Stimulus (plural is stimuli). Stimuli can be external or internal. 

For instance, if you feel like going to the bathroom, it is an internal stimulus that is controlled by the brain. If the sun comes up on a warm day, it is an external stimulus that can cause a snake to come out and busk. The ability of the organism to react is called ‘irritability’.

It helps the organism to stay in balance. Living organisms have some senses (sight, smell, touch, taste, etc.) that can help them to detect changes in their external environment, as well as their internal balance and respond to them.

Some organisms (such as herbivores) respond to stimuli much quicker than others (such as plants).

Homeostasis

Just like all the changes that occur outside of our bodies (external environment), there are also changes in our internal environments too. For instance, our bodies need to keep constant body temperature, glucose level, water level, and so on for the cells to function well. But conditions outside our bodies such as temperature, exercise, eating, and so on, often affect our internal balance. The term used to describe the ability of an organism to maintain balance in its internal environment is called ‘homeostasis’.

Homeostasis is achieved by a mechanism involving three components. The Receptor (or sensor), The Control Center (Processor), and the Effector. Here is an example:

The receptor:
Sensors on your skin can detect when the temperature outside increases.

The control center:
The brain receives the signal from the sensor and processes it (finds a solution).

The effector:
Sweat glands get to work, and blood flow increases to produce sweat, which cools the organism down. This way, the organism’s original balance is restored.

Living organisms also need to maintain a balance in the external environment too. For example, if a loud bang wakes you up in the night, the fright makes your heart beat faster, you breathe faster, and your heart pumps more blood to the brain. The result is that you suddenly become fully awake and attentive. This natural response helps you to stay safer.

 Adaptation is the process that helps an organism survive in its environment. A polar bear, for example, will struggle to survive in a hot climate because the temperature will be too much for it. The polar bear’s heavy fur is best suited for colder regions, so we can say that polar bears are adapted to colder regions.

In summary, for any organism to be considered alive, it must have all these characteristics.