Within the world there are millions of living things, but deciding what classifies an organism as living or dead is a task. Understanding the characteristics that classify living things is important. Characteristics that have been determined by scientists to decide if something is living or dead include reproduction, growth, adaptability and movement are some characteristics of living things.
Cellular Make-Up and Organization
Living things are composed of cells. These cells are organized into groups such as organelles, molecules and other multicellular classifications. Not only should cells be organized, but cells should also have the ability to reproduce themselves, showcase movement and show a response to certain stimuli in order for a scientist to consider those cells within an organism to be considered living.
Nutrition and Consumption
In order for something to be living it must consume and gain from nutrition. The process of nutrition involves an organism gaining energy from raw materials, food, and nutrients -- vitamins, fats, proteins and carbohydrates. Gaining this nutrition is done through consumption. Not every living organism intakes food like a human being. Consumption can be also through absorption of nutrients in the soil like a plant or tree would do in order to gain nutrition and energy.
Sensitivity and Response
Living organisms are sensitive to changes around them, things that touch them or even temperature. To be classified as a living organism something has to be sensitive to what is around it as well as respond to those things. Things that every living organism can be sensitive to are items such as light, temperature and even gravity. For example, a plant that is not receiving enough light will respond by slowly growing towards the source of light it needs in order to survive.
Living organisms grow and reproduce to make other living organisms. This can be in the form of growing new cells through asexual reproduction or producing other living organisms through sexual reproduction. These new living organisms that are reproduced will contain DNA similar to that of the cell it came from as well. When a plant reproduces, a male plant will create pollen that is released by their stamen, the pollen producing organ of plants, and then it is released in the air for the pistil, reproductive organ of female plants, to receive. This will in turn create seeds for the pistil to release into the wind and on the ground for further plants to form.
Growth and Adaptability
A living organism may stay the same size, but it can grow in other ways -- such as growing in number of molecules. Other organisms will physically change and adapt to the world around them as a means for survival or part of the growing process. Plants, for example, will grow from seeds to sprouts to full stems. They will also adapt to the world around them by adjusting to things like nutrients in the soil, pH of water and even lack of water in general through evolving their growth process.
A living organism will interact with another living organism -- whether it is the same type of organism, a threat or a neutral organism, there is some form of interaction between the two. For example, flowers interact with bees by releasing pollen into their male stamen in order for that pollen to be picked up and dispersed among female plants for reproduction. Plants like the Venus flytrap will interact with nature around it by enclosing itself over flies, lizards and other edible insects in the event one lands on the interior of its petals.
All living organisms have some form of respiration. This is the release of energy from the organism after food and nutrition has been broken down and their cells release energy in order to grow, adapt or even reproduce.
To classify an organism as “living” it must exhibit some form of movement. Though humans and animals obviously move, other items such as plants will still have their own form of movement -- even when it is hard to see with the human eye. For example, plants can move their buds or leaves toward sunlight or away from shaded areas in order to promote growth. These movements are slight and do not occur overnight whereas when humans or animals move it can be noticed instantly.
Living things will gain energy from the world around them. This can be in the form of plants gaining energy through sunlight by photosynthesis, the chemical reaction in which carbon dioxide is converted into organic compounds like sugar for energy, or humans gaining energy through the food they eat and developing adenosine triphosphate. Adenosine triphosphate or ATP is a coenzyme produced in humans and animals that gives an organism energy. Organisms can gain ATP through things like nutrition or photosynthesis.
All living things must be able to contain and maintain their bodies' internal states, known as homeostasis. This can involve regulating internal temperature, pH levels and even water. For example, a human body will sense changes around it such as temperature, and the nervous system will receive the sensory output from the skin, letting it know to use energy in order to create more heat within the body. This will also trigger sensory receptors in the brain in which a human can know to put on a sweater or take off a layer of clothing to adjust his body temperature as well.