To study biological processes, we must first have a good understanding of what life and living beings are. What is life "made of?" How can we define what it means that something is "alive?" This topic will give you the answers to those questions.
What the living being is
First things first, and if you want to understand the differences between plants and fungi or between an octopus and a seagull you have to be sure of what life is and what a living being is.
First, all living things are composed of one or more cells, and their cellular structure is important. A cell is a discrete unit of life. An organism can be single-celled (for example, bacteria or amoeba) or multicellular (this is you, the birds and trees outside the window, mushrooms in the forest nearby, and your cat lying on your laptop trying to explain a mistake in your code).
The cell is surrounded by a cell membrane that acts as a border and a filter. Through it the cell consumes nutrients, removes waste, and communicates with the environment. The cell membrane allows the second property of a living thing to be fulfilled — growth and metabolism.
The cell grows and develops on its own and does not need a host organism, so the third property of living is the ability to exist independently.
The fourth property of living things is reproducibility. In different organisms, reproduction can look very different: bacterial division, live birth of adorable kittens, or the fluff from a dandelion are all means of reproduction.
From the reproductive process come two more properties — variability and heredity. Heredity allows the daughter organism to inherit the genetic material of the mother organism. Variability allows "children" to inherit changes from parent organisms — this helps the population survive due to the spread of beneficial genetic features.
The seventh property of living is irritability, meaning that a cell or an organism must constantly respond to environmental changes. Due to irritability, the physiological parameters of the body change. This is pulling the hand away from a hot surface and flower rosettes turning towards the sun – responding to their environment is one of the main features of living beings!
These are the general properties of all living things. Why did we consider them? In addition to the "living" kingdoms, there is one "not quite alive" — this is the kingdom of viruses. Viruses can reproduce, but they are not able to do it on their own (they need a host cell), and they are not able to metabolize or grow. Viruses do not have a cellular structure. They consist of their genetic material (nucleic acid) and the capsid that surrounds it. A capsid is a shell made up of proteins and sometimes lipids. It is still unknown exactly where viruses came from, although there are several well-described theories. According to one of them, viruses are the remains of "junk" nucleic acid that "escaped" from living organisms.
Conclusion
We have studied the main properties of living things — this is important, because a precise definition of life allows us to study both the biology of living things as well as define biological entities that are not alive — like viruses. Life is diverse, but it is formed according to a singular "canon": All living organisms are made up of cells, they grow and metabolize, they reproduce and pass on their genes to daughter organisms, they have variability among individuals, they live independently, and they respond to environmental changes.