We need some criteria to distinguish life from non-life in our study of biology and the natural world. The seven characteristics of life are the essential criteria all life shares, from the smallest bacteria to the most giant whales. It usually is a straightforward task, yet some substances between life and non-life are hard to judge, such as viruses and prions.
1. Cells make up the basic units of life
They are very similar in both a unicellular or multicellular organism as fundamental units. Bacteria and protists are examples of single-cell and, therefore, called unicellular organisms. Plants and animals may consist of many cells and are sometimes called multicellular organisms. Cells make their way to tissues, organs, organ systems, and the complete multicellular organism. Cells coordinate and cooperate with each other to maintain all the life processes.
2. Metabolism
Organisms change energy from their surroundings to forms that they can use to grow, reproduce, and establish other functions and activities required. Metabolism represents the whole chemical process in the organism. It is divided into two classes. Synthesis is the process of ingesting nutrients from the environment and converted into organics. Decomposition refers to the breakdown of substances in the organism to obtain the energy for life activities. An example is respiration. An organism grows if there is more synthesis than decomposition. An organism shows wasting if the synthesis is less than decomposition.
3. Response to stimuli
Living organisms can respond to environmental stimuli. It provides the ability to respond to change, locate food, avoid predators, and interact in many ways with the environment.
Examples
Unconditioned reflexes are characteristics considered innate properties of life. If you drop saline and sugar solutions on opposite sides of a slide, a paramecium will avoid the salt water and move toward the sugar solution. An unconditioned reflex is the time when your hand accidentally touches a hot stove and pulls back immediately before feeling pain.
The conditioned reflex stands for much more complex and higher-level neural activity. By training, it temporarily links two completely independent activities together. If one rang a bell every time before feeding the dog, and it was trained many times, the dog will start to salivate upon hearing the bell. If the training is not often practiced, the formed conditioned reflex will gradually become weak until it disappears.
4. Reproduction
Organisms duplicate individuals similar or identical to themselves to help their species survive. Reproduction may be either sexual or asexual. Most single-celled organisms reproduce asexually, while most multicellular organisms undergo sexual reproduction. For example, many bacteria and protists divide into two identical offspring if you overlook genetic mutations. Vertebrates produce sperm or eggs that contain half of the genetic material in the parent's body. When combined, the age of sperm and eggs produces another individual that has traits from both its father and mother.
5. Growth
Organisms may be able to ingest and synthesize various organic materials and make their bodies grow. For example, single-celled organisms duplicate their DNA and increase their volume size before cell division. The multicellular organism not only increases cell volume but also cell number. Some cells may differentiate into cells with entirely different functions.
6. Homeostasis
The conditions required for metabolism in an organism, for example, temperature, pH, ion concentration, etc. are limited to a very narrow range. Organisms maintain stable internal conditions with several regulatory mechanisms even when the external environment changes drastically. For example, human body temperature does not deviate much over a day. Human blood has a variety of inorganic salts which buffer the pH to around 7.35 – 7.45.
7. Evolution and adaptation to the environment
An organism reproduces to progenies with mutations or gene recombination to obtain new traits. Some characteristics are better adapted to the environment, so organisms with those characteristics can easily out-compete their competitors, and their offspring will inherit those genes. Hence, it is this process of natural selection through which evolution occurs, so only those remaining organisms living on this planet are chosen and adapted to the environment. Otherwise, they would have been selected out. From simple to complex, aquatic to terrestrial, this is a universal law of biological evolution.