Biology is the division of science that deals with life and living. And, in order to learn biology, one should know everything about life. But have you ever wondered what exactly life is? How can we say that something or someone is living? Why do biologists consider some things to be living and others not? Which criteria can define life?
As simple and silly as these questions look, they are quite difficult to answer. Even with modern technologies and advanced scientific approaches, scientists are still wrestling to find an ultimate definition to describe life. However, from the beginning, biology has put forth a few characteristics that can help us understand life in detail. These criteria allow us to identify whether a thing is alive or not.
Today in this session, we are going to explore eight such properties that make a thing alive.
Content: Living Things
What are the characteristics of living things?
There are many characteristic features that can define a living thing. Here, we have mentioned 8 of them.
1. Cellular Organization
Cells are the most basic, fundamental and functional unit of any living entity present on this planet. No matter whether the organism is as big as a whale or elephant or as small as bacteria, all are composed of cells. However, the number and organization of cells may vary from one specie to other.
In multicellular organisms like plants and animals, similar kind of cells combines to form a tissue. The tissues connect to produce an organ, and organs combine to develop an organ system. Various organ systems work together to form an organism.
Non-living materials, on the other hand, have atoms as their elementary unit. Thus, the presence of cells also becomes one of the distinguishing characteristics between living and non-living things.
2. Movement
Movement is one of the most exclusive features of a living entity. All the species that are alive exhibit movement in one or the other way.
We can define movement as an activity by which the whole body of the organism or a specific part of its body changes its place.
Or in simple words, you can understand it as an ability of an organism or its body part to move.
For instance, the cow moving around the field, the dog playing with a ball, a cat jumping from your balcony to your neighbours or a rat swiftly running away with cheese. All these activities involve movement. Some animals are very fast when it comes to movement, while some are pathetically slow.
The cheetah is capable of running up to 70 mph, which makes him the fastest-moving organism. On the other side, animals like sloths and snails are the slowest movers.
You might have seen animals moving on their own, but you may be surprised to know that plants move too. Yes, you read it right! Plants do move. However, unlike animals, plants cannot walk or run; they can only move their particular body parts. And this movement is very slow in comparison to animals.
For instance, plants turn their leaves in the direction of the sun, open and close flower petals, extend the roots in the direction of water, and bend the stem as per the need.
This way, all living things tend to show movement in some way, and this makes movement an important characteristic for defining living.
3. Respiration
Respiration is another essential characteristic that is present only in living things.
Each and every living organism use a source of energy for its survival. In most cases, the energy source is food, and the organism needs to extract energy from that food material. This process occurs via cellular respiration.
We can define Respiration as a set of biochemical reactions that degrades or breakdown the organic nutrient molecules in stepwise oxidation with the release of energy for metabolism. In simpler words, you can understand it as a process of release of energy from the food.
The energy liberated after respiration is trapped in the form of the chemical compound ATP, i.e. Adenosine Triphosphate. This ATP acts as an energy reservoir for the body whenever needed.
4. Excretion
Animals require energy to perform various tasks, and for that, they intake food. The digestive system in the body extracts the nutrients and converts them into energy molecules for use. In this process, some of the material is produced as waste. And the body needs to get rid of this waste material as it can be harmful. Apart from that, there is also a respiratory waste product which is CO2 in most cases and needs to be expelled from the body.
For instance, the human body excretes out undigested food via the process of egestion. Similarly, our bodies expel excess water and minerals in the form of urine and sweat.
The plants also generate waste products or, you can say, by-products after photosynthesis which are eventually eliminated by them.
This process of getting rid of the waste product is termed Excretion. It is also among the important characteristics of any living entity on this planet. No non-living thing ever generates or excretes waste; therefore, excretion becomes one of the distinguishing characteristics between living and non-living.
5. Nutrition
Nutrition is the physiological and biochemical process by which the living organism attains those substances which are used for their growth, metabolism, and maintenance.
These substances are nothing but food. Since all living species need energy for survival, therefore nutrition becomes a crucial trait of any living entity. No matter if the organism is short or long, small or big, nutrition is common in all of them.
Some animals might not require to perform nutrition very often, such as snakes. Smaller snakes may eat twice a week, but as they get bigger and more mature, it preys on larger animals and thus has to eat once a week or two.
On the other side, animals like a hummingbird, blue whales, American pygmy, moles etc., eat very frequently.
For plants, the mode of nutrition is autotropic. That means they prepare their own food by using naturally available raw materials like water, CO2 & sunlight. For them, the process of nutrition occurs until the sun goes down.
Fungi, on the other side, are saprophytes. For them, nutrition is to absorb nutrients from dead and decaying organic matter.
6. Reproduction
Reproduction is an ability of a living organism to produce offspring of its own kind. It is a unique characteristic as no non-living thing can ever do it.
Through the process of reproduction, living things make more of their kinds, and they are known as progenies or, in easier terms, babies.
As per the definition, we can say that reproduction aims multiplication and perpetuation of the species. This helps maintain the continuity and existence of the species on this planet. Otherwise, living entities could have been extinct easily without the ability to reproduce.
All living things, including plants, animals, microbes, fungi, and protists, perform reproduction to continue and expand their germlines.
For animals and plants, the process of reproduction is lengthy, and thus, the birth of the progeny takes time. But for microorganisms, this process is very fast. For instance, bacteria have only 20 minutes of doubling time, which means they can produce their new generation in just 20 minutes.
7. Growth
We can simply define growth as an increase in size and weight. Anything which is alive tends to grow either in size or weight and become larger.
In technical terminology, we can say that growth is a permanent, stable and irreversible increase in the volume, shape, size and dry mass of an organism.
Be it any organism, including plants, animals, microbes or fungi, all of them exhibit growth during their life span. But the criteria for growth can vary from one organism to other.
For instance, if we talk about the growth of a dog then it refers to growing from a puppy to an adult dog. Similarly, the transformation of kitten into cat, cub into lion, calf into cow, eaglet into eagle, chicken into hen, lamb into sheep etc., all represent growth.
On the other side, a seed germinating into a small plant and then transforming into a huge tree depicts growth. Similarly, an increase in the girth of tree bark also represents growth in plants.
But for microbes like bacteria which is a single cell, growth means becoming a larger cell from a smaller one.
8. Sensitivity
Sensitivity is the ability of an organism to sense the changes occurring in the external environment and respond accordingly.
Homeostasis
All living entities are sensitive to different types of things. And in order to maintain optimal functioning, the organism’s body tries to maintain a constant internal environment within a predefined narrow range. This process of maintaining a sustainable body condition is known as homeostasis.
For example, when the external environment is dry and hot, the body maintains its temperature at 37° C by sweating. Similarly, when you feel cold, the body shivers to generate heat and maintain the inner body temperature.
Animals also respond actively to changes. They are sensitive towards climate changes, food and water availability etc. For instance, polar bears living the cold climates have developed a body structure that helps in withstanding extreme cold temperatures. They produce a thick layer of fat called blubber that acts as an insulator resisting the outer cold to hamper inner body parts.
Plants also show sensitivity and homeostasis. They certainly detect and respond to light intensity, temperature, water and so on.
Phototropism
The plants are sensitive the light, and the sun is the ultimate source of light for them. so in order to get proper sunlight for photosynthesis, the plants turn their leaves in the sun’s direction. This phenomenon is known as Phototropism.
Some plants are sensitive to touch, such as Mimosa pudica, also known as the ‘Shame plant.’
Living and Non-Living
So, guys, these were the eight main characteristics which can help you identify life and define living. Although life is difficult to understand, these features certainly support exploring it in detail.
We can say that anything that meets the above-mentioned eight criteria can be called living.
All living things have these characteristics in common. However, some non-living things might also have some of these features. For instance, a fan moves, rubber expands in size, and water beads grow. But you cannot consider them as living.
Here, the point to remember is that to be a living thing; it should possess all eight characters.
Are Viruses Living?
Most times, we refer virus as an organism, but it doesn’t actually count as a living thing. This is because viruses do not meet the above eight characteristics. For instance, they cannot respire, they don’t grow, and they don’t excrete in any form.
Thus, it is wrong to consider viruses as living entities.
A Word From Science Queue
As I told earlier that there are many characteristics of living, and biologists are eagerly searching for more. Remember that the earth is under a search operation where there are still millions of things hidden. And as time passes, new information keeps on adding to the biological database.
This is why we shouldn’t give a specific number to the characteristics of life like 5, 7 or 8, as this number will keep on changing and expanding with time.
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