Living Things: Definition, Types, Characteristics, Classification

Living Things Overview 

Any organism or living form that demonstrates a trait associated with life or being alive is referred to as a "living thing." Cells, which are tiny building blocks, make up all living things. They are capable of movement and have a metabolism. Living things have senses, eat, breathe, grow, and move. Even their young can be reproduced by them. They possess the senses of taste, smell, sight, and touch. All living things eventually pass away as they get older because their organs and senses stop working. Plants are an exception to this rule because they do not walk like humans or other living things do. Living things are the only organisms that exhibit movement. 

Living Things Definition

An organism that needs food to grow, air to breathe, and water to survive is said to be a living being. A living thing has the ability to sense, move, and reproduce. There are both living things and non-living things on Earth. Trees, birds, and other living things are examples. For the trees to survive, they need to breathe air and have access to nutrients, water, and sunlight. The birds can fly, they eat to gain weight, and they develop their eggs into young by hatching them. They are able to see, smell, taste, hear, and feel. 

What is Living?

The cells that make up living things are capable of performing a variety of cellular functions. Below are some of the characteristics that best define what a living thing is:

1. Growth

Typically, the mass and number of individual organisms increase to indicate growth. Humans continue to age visibly after a certain point, but as long as cells divide, new cells continue to replace the old ones in the body. Because of the buildup of particles, some inanimate objects, like mountains, also appear to expand in size. Cell division leads to the growth of living things. Therefore, growth is not a defining quality.

2. Reproduction

It is the process by which an organism creates a person who is similar to itself. Asexual and sexual reproduction are the two main types of reproduction. Like humans, the majority of mammals reproduce sexually. However, when thinking about unicellular organisms like bacteria, growth and reproduction are unclear because they both result from cell division. Therefore, reproduction once again cannot be regarded as a characteristic that best describes living.

3. Metabolism

It includes every chemical process taking place inside a living cell or body. As a result, it is a typical trait of living things. Non-living things do not show metabolism.

4. Cellular Organization

Cells are the basic building blocks of organisms. They also adhere to the organization of cells, tissues, and organs, which in turn combine to form the organ system. An organism is made up of various organ systems. This kind of cellular organization is absent in nonliving beings. Muscles, tissues, and nerve cells are a few examples. 

5. Consciousness

It is a characteristic that determines how an organism responds to an outside stimulus or environmental change. It is a characteristic of all living things. They have the capacity to detect any environmental change.

Read more about Terrestrial Ecosystem, Entomophily or Insert Pollination and Gemmules.

Characteristics of Living Things 

Organisms considered to be living exhibit the traits of life. The following distinguishes living things from non-living things-

1. An organized structure- Living things are structured systems. It could be single-celled, like a bacterial cell, or multicellular, like the cells found in animals and plants. The basic biological building block of an organism is a cell. The cell performs various cellular processes in an organized, systematic way. Protoplasm is what makes up a cell, and a plasma membrane surrounds it. Organelles are cytoplasmic structures that are suspended in the cell's cytosol and have distinct roles and functions.
2. Energy-requiring- To survive, living things need energy. Energy is crucial because it powers a cell's numerous metabolic processes. Through photosynthesis, where light energy is transformed into chemical energy, organisms can create energy. Another method is through cellular respiration, in which energy is obtained biochemically from an organic substance (such as glucose) and then stored in an ATP-like biomolecule for later use.
3. Reproductive capacity- Reproduction is a function of life. Living things have two ways of making copies of themselves: sexual reproduction and asexual reproduction. In sexual reproduction, the two parents' male and female sex cells combine to form a zygote, which will eventually grow into a being of its own kind. Contrarily, asexual reproduction is a form of reproduction that doesn't use sperm or gametes for fertilization. The offspring comes from only one parent. Examples include parthenogenesis, sporogenesis, binary fission, budding, vegetative propagation, apomixis, nucellar embryony, and vegetative propagation.
4. Growth- Living things grow. Growth at the cellular level can refer to both an increase in quantity and size. The increase in the number of cells is a result of cell division. Animal stem cells and plant meristematic cells both divide to produce new cells. The rise in cytoplasmic mass is frequently cited as the cause of the increase in cell size. In the cell cycle, the cell goes through a number of phases. The new cell created during mitosis typically goes through interphase. It is the phase in the cell cycle wherein the cell grows in size. The cell may replicate its DNA to get ready for the subsequent cell division if it has not fully differentiated. Plants gain volume in new cells by ingesting and storing water in vacuoles. Some plant cells develop a secondary cell wall between the primary cell wall and the plasma membrane. At the tissue level, growth in vascular plants is of two types: primary and secondary. While secondary growth is related to lateral growth brought on by the formation of secondary xylem from the vascular cambium, primary growth involves vertical growth as primary xylem forms from the procambium. 
5. Metabolism- An organism metabolizes. The term "metabolism" describes the various actions that keep a cell or an organism in a living state. Some of the homeostatic processes that are examples include cell growth, respiration, reproduction, response to stimuli, sustenance, biomolecular synthesis, waste elimination, and other processes. Catabolism and anabolism are the two types of metabolism. Living things engage in catabolism, which involves degradative chemical reactions that result in the disintegration of large molecules into smaller ones, releasing energy in the process. Anabolism is a chemical process where molecules are constructed from smaller building blocks.
6. Responsive to stimuli- Living things react to stimuli and change with their surroundings. It is capable of detecting environmental changes, particularly when cells acting as receptors are present. The five basic senses that humans have are sight, hearing, smell, touch, and taste. Other senses include thermoception, kinesthetic (which detects the positions of body parts), vestibular (which detects body movement, direction, and acceleration), internal (interoception), and so on. It can not only notice changes in its environment but also adjust to them.
7. Movement- Every living thing moves. A living thing can react appropriately to stimuli because it is able to detect them in its environment. Animals move, for instance, to forage, avoid predators, and look for a potential mate. Animals can move around whenever they want, but plants only have a limited range of movement known as nastic movement (e.g., thigmonasty, nyctinasty).
8. Death- Death is a natural occurrence for all living things. A living thing has life, and life eventually comes to an end. Biological aging is referred to as "senescence." It is the process by which living things gradually lose their health over time. Gradually, the organism's capacity to function and respond to stressors declines. It consequently becomes more susceptible to illnesses and dysfunction. The cell no longer divides at the cellular level, though it may still be metabolically active. Telomere shortening, which causes DNA damage, is one of the organic causes of cellular senescence. On the other hand, some living things are thought to be immortal because they appear to escape death. Examples include the regenerating flatworms, the age-reversing jellyfish Turritopsis doohmii, and the seemingly indestructible tardigrades.
9. Non-cellular life- Are viruses considered living things? This question has sparked major debate among biologists for so long. Since they seem to be alive when they are inside their host, some people would classify viruses as living things. They have genetic material, reproduce, and change through natural selection. Others, however, do not perceive them as living creatures because, when removed from their host, they essentially cease to exist. Viruses cannot reproduce independently. They rely on the machinery of the host cell to do this. Consequently, viruses are neither wholly alive nor dead. Viruses appear to be inanimate and inactive when they are outside of their host. Once inside their host, they became active and were able to replicate and use the host cell's structures. Another group that appears to be non-cellular life are viroids. They are circular, single-stranded, short strands of RNA that are pathogenic and infectious. 
10. Research- The term "biology" (also known as "biological science") refers to the study of living things. A biologist is a specialist in this area. Morphology, anatomy, cytology, histology, physiology, ecology, evolution, taxonomy, and pathology are a few topics of biological study.

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Classification of Living Things

Animals and plants are the two broad categories into which living things are divided. They are both eukaryotic. They are differentiated according to their traits, mode of nutrition, reproductive process, etc. Plants are non-motile, photosynthetic, and have a cell wall, while animals are heterotrophic, motile, and lack a cell wall. Being prokaryotes, bacteria are neither plants nor animals. They are single-celled organisms, bacteria, and archaea. They lack a nucleus despite having cell walls made of various materials.

1. Kingdoms

The kingdom is a living thing's initial level of classification. There are five kingdoms, and the animals are divided into each kingdom according to how they obtain food, what kind of cells they have, how many cells they have, etc.

2. Phylum

The organism with the greatest physical similarity within a kingdom is placed under the phylum, which is the next level of classification for living things after the kingdom. The physical resemblances among the organisms in a phylum are based on their shared ancestry.

3. Classes

The phylum's organisms are further broken down into classes. The organisms in classes share more traits than a phylum as a whole. Given that they consume their mother's milk as infants, humans are considered to be mammals.

4. Order

Orders are a further division of a class. Using taxonomy, the organisms are divided into orders. This taxonomy is a list of traits used to categorize organisms into different orders and identify which organisms belong to which order.

5. Families

Orders are further divided into families. Families of organisms share more characteristics than organisms at any other level of classification. The organisms of a family are related to each other. The Hominidae family includes humans.

6. Genus

Based on the organism's generic name, the genus is a classification of the organism. Between animals and plants, there are many different genera. The first part of the organism's two-part name is completed by the genus.

7. Species

At this species level, classification is done in the most precise manner. The ability to reproduce with another organism of the same species is the basis for classifying an organism as a species. The species of an organism determines the second part of its two-part name.

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Properties of Five Kingdoms 

The properties of the Five Kingdoms are-

1. Monera

• Prokaryotic cells make up the organism.
• Despite being noncellulosic, they have a cell wall.
• They are organisms with only one cell.
• They eat in an autotrophic and heterotrophic manner.

2. Protista

• The cells are eukaryotic in nature.
• They have a nuclear membrane and a cell wall.
• They are a single-celled organism.
• They eat in an autotrophic manner.

3. Fungi

• They are eukaryotic cells.
• They have a nuclear membrane and a cell wall.
• They are organisms with multiple cells.
• Heterotrophic feeding is their mode of nutrition.
• They are unable to grow their own food.

4. Plantae

• The cells are eukaryotic in nature.
• They possess a non-cellulosic cell wall.
• There is a nuclear membrane.
• They (the tissue or organ) are multicellular organisms.
• Autotrophic nutrition is the mode of life.

Read more about the Scientific Names of Animals and Plants.

5. Animalia

• The cells are eukaryotic in nature.
• Their cell walls are absent.
• Their nuclear membrane is present.
• They (the tissue, organ, and organ system) are multicellular organisms.
• Due to their heterotrophic nature, they are unable to produce their own food.

Read more about the Difference Between Endosmosis and Exosmosis.

Difference between living and Non- Living things

The table below provides a comparison of the differences between living things and non-living things-

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Things to Remember

1. Living things are any organisms that can move, eat, reproduce, breathe, and have senses.
2. Living things can have a single cell or many.
3. Living things are divided into seven categories: Kingdoms, Phylum, Classes, Order, Families, Genus, and Species.
4. Animalia, Plantae, Protista, Fungi, and Monera are the five kingdom classifications.