Characteristics and Classification of Living Organisms

Living Things Characteristics and Classification: Life Cycle

Our planet is populated by a great variety of living organisms. How do we recognize them? In this article we will explain what living things are, what the characteristics and classification of living organisms are, and what are their vital functions.

What is a Living Organism?

Living things are organisms made up of cells that have the ability to carry out the basic functions of life. There are characteristics common to living things that differentiate them from those that do not have life such as a rock (which is inert).

Characteristics of Living Things for Children
Characteristics of Living Things for Children

Characteristics of Living Things for Children

Characteristics of Life

1. Organization

Living things, as the cell theory expresses, are made up of cells. Although living things appear very different from each other, they are all made up of cells. The cell is the structural unit of all organisms. Organisms can be made up of different numbers of cells of different types. Some, like bacteria, are made up of a single cell, they are unicellular, while other more complex organisms are multicellular, such as humans and animals.

Levels of Organization of Living Things for Kids

Multicellular organisms are not only a set of cells, but also have a high degree of organization and complexity. Similar cells group together to form tissues. Organs are related and give rise to organ systems. An organism has different levels of organization: cellular organization, tissue organization, organ organization, and organ systems organization.

Read also: The Cell: Parts and Functions

Levels of Organization of Living Things for Kids
Levels of Organization of Living Things

2. Growth

All living things grow. Growth is the increase in the size of cells or the increase in the number of cells in a living being. The growth process allows living beings to progressively increase their size due to the incorporation of nutrients until they reach the characteristic limits of their species. Single-celled organisms also grow. This growth process can last a lifetime, as in trees, or be limited to a stage and up to a certain height as in most animals.

3. Metabolism

Organisms need materials and energy to maintain their high degree of complexity and organization, to grow and reproduce. Metabolism is a set of processes that allows living beings to transform and take advantage of nutrients to obtain energy and develop their functions. It is the sum of all the chemical reactions that allow the cell to grow, preserve and repair.

  • The metabolism is anabolic (anabolism) when it allows to transform simple substances into complex ones. This results in energy storage, production of new cellular materials, and growth.
  • The metabolism is catabolic (catabolism) when there is a breakdown of complex substances with the release of energy. They are degraded or reduced to their simplest forms for the body to assimilate and transform into energy.

4. Homeostasis

Organisms, to preserve life and function properly, must maintain the constancy of the internal environment of their body. This process is called homeostasis. This term comes from the Greek meaning “to remain unchanged”. Homeostasis is the capacity by which an organism maintains its interior in balance, regardless of internal and external variants. Living beings perceive stimuli that come from outside and inside, and they respond. Among the conditions that must be regulated are: Body temperature, pH, water content. For example, when living beings are dehydrated, the sensation of thirst appears. When it is cold, the body begins to shiver, muscles contract to generate heat and maintain body temperature.

5. Irritability

Through irritability, living organisms respond to stimuli. Unicellular organisms respond to the stimulus with their entire being because it is a single cell. While multicellular organisms have specialized cells to detect different stimuli such as physical and chemical changes in the environment, whether internal or external. Examples of stimuli are: Light, pressure, temperature, water availability, surrounding air, soil composition.

6. Reproduction

Every cell comes from another cell. They reproduce, because they can give rise to other similar living beings. They replace those that are disappearing. There are two main types of reproduction: sexual and asexual reproduction. The genetic makeup of a species changes over many life spans.

7. Adaptation

Species evolve and adapt. Adaptation is the ability of living beings that allows them to accommodate environmental factors. This manifests through changes either in their structures or in their behaviors (sizes, colors) and is progressive.

Characteristics and Classification of Living Organisms

Summary: What is Life?

Characteristics of Living Things
Characteristics of Living Things

Characteristics of Living Things

  • Living things are made up of cells. Unicellular, multicellular.
  • Living things are organized. Organisms have structures and parts with special functions.
  • Reproduction: Organisms produce offspring. Sexual and asexual reproduction.
  • Living things grow and develop.
  • They Respond and adjust to their environment. They have the ability to maintain stable internal conditions. Maintain Homeostasis (homo-: same, -stasis: state).
  • Metabolism: Obtain and use energy eating or using sunlight.
  • Adapt and evolve: The evolution of species occurs as a result of successive adaptations.
  • Irritability: Respond to stimuli.

Read also: What is a Cell?

Life Cycle of Living Things

What is the life cycle? It is the vital process of an organism from its birth to its death. Living beings: They are born, grow, reproduce and die. The life cycle of living things varies according to the species. Some beings, like some insects, live only one day. On the other hand, humans or some animals and plants live for many years.

Life Cycle of Living Things for kids
Life Cycle of Living Things
  • They are Born. Living things are born from other living things.
  • They grow and develop. They are nourished, “nutrition” means that they eat, consume food (or manufacture it), breathe and eliminate the waste they produce.
  • They reproduce. Reproduction gives rise to other similar living beings.
  • They die: Every living being, when its life cycle ends, dies.

Characteristics and Classification of Living Organisms

The 3 Vital Functions of Living Things

There are key functions necessary for life. Vital functions are those processes that all living beings carry out to stay alive. To be categorized as a living thing, an organism must be able to do all of these. They are common to all without whom they would die. Even single-celled living things have vital functions. All organisms use resources from the environment and exchange information. They detect the changes that occur and respond because they are open systems.

1. Nutrition

It is what allows the body to transform food into usable nutrients, carry them to the proper place and dispose the waste that will not be used. The systems involved in nutrition are:

  • Digestive. The function of the digestive system is to transform food into small substances called nutrients so they can enter the cells.
  • Circulatory System. It is responsible for transporting nutrients, oxygen and waste for its elimination. It distributes blood throughout the body.
  • Respiratory System. Cells need oxygen to keep the body working. When oxygen combines with nutrients, they provide the energy we need to run, walk, think, sleep. When cells do their work, they remove carbon dioxide.
  • Excretory System: The body generates waste that must be expelled. The excretory system is in charge of eliminating them.

2. Interaction

Through this function, living beings detect what is happening around them (stimuli) and react appropriately. It allows them to get food and flee in cases of danger and thus guarantee the maintenance of their homeostasis and preserve life.

Examples of stimuli are: Change in temperature, pain, amount of water, light, sound, pH.

3. Reproduction

Reproduction is a necessary function for the conservation of the species. It is a process by which organisms produce their offspring. It allows organisms to give rise to new life. There are two main types of reproduction: sexual and asexual reproduction.

  • Sexual Reproduction: To form a new being, individuals of both male and female sex participate. Sexual reproduction requires two parents.
  • Asexual reproduction. Only a single individual is involved. It is the one used by less complex or unicellular organisms. Asexual reproduction requires only one parent.

Classification of Living Organisms

Living Things Classification for Kids
Living Things Classification for Kids

Living Things Classification and History

How did Aristotle classify living things? Aristotle was a philosopher of Ancient Greece who classified living beings into two large groups:

  • They move (Animals)
  • They do not move (Plants).

Aristotle developed the first system of classification of animals too. Aristotle’s two major groups of animals are dependent on whether or not they have “red blood.” Animals with red blood correspond to vertebrates, and animals without red blood correspond to invertebrates.

Another Classification: According to its Size

  • Large dimensions: such as humans, animals, plants, and others.
  • Small: Insects and microorganisms that cannot be seen.

In this way we can divide them into two groups:

  • They can be seen with the naked eye: Animals you can see without assistance (ants, dogs, elephants, etc).
  • Cannot be seen: A microscope is required to see them (microorganisms, bacteria).

Living Things Classification: 5 Kingdoms

They are classified into large groups called Kingdoms. All individuals belonging to the same Kingdom have the same basic characteristics. The most widely used classification groups living beings into 5 five Kingdoms:

  1. Animal Kingdom
  2. Plant Kingdom
  3. Fungi Kingdom
  4. Protista Kingdom
  5. Monera Kingdom
Living Things Classification: The 5 Kingdoms for kids
Living Things Classification: The 5 Kingdoms

1. Animals

  • Most can be seen with the naked eye.
  • They feed on other living things (or parts of them). They are heterotrophic beings.
  • At some point in their lives they move.
  • At a certain point, they reach their largest possible size. They do not grow indefinitely.

The animal kingdom has a first classification in:

  • Vertebrates: They have an internal skeleton with a backbone. They use the locomotor apparatus to move. They are classified as: Mammals, Birds, Fish, Amphibians, Reptiles.
  • Invertebrates: They do not have an internal skeleton although they may have some hard parts (for example the shell of a snail). They are classified as: Arthropods, Echinoderms, Worms, Mollusks, Coelenterates, Porifera.

Read also: Vertebrates and Invertebrate Animals

2. Plants

  • They are seen with the naked eye and are multicellular.
  • They make their own food. They are autotrophic beings.
  • They do not move although they can perform some movement (such as orienting their stems or leaves towards the sunlight).
  • They grow throughout life.

3. Fungi

  • The vast majority can be seen with the naked eye.
  • They feed on other living things. They do not make their own food. Most break down food in order to absorb its nutrients. (same as animals) Examples: yeasts, molds and mushrooms.
  • They do not move from one place to another, they live in fixed places (just like plants).
  • They can be considered an intermediate between plants and animals. They are found in soil, water, decomposing material, or on human skin. Molds grow on food and fruit. This kingdom is made up of eukaryotic organisms (their cells have a nucleus).


This group includes the Kingdom Protista and the Kingdom Monera.

  • They are not visible to the naked eye.
  • Many make their own food or eat other living things.
  • They have different colors, sizes, shapes.
  • They include: microscopic fungi, bacteria and protists.

4. Kingdom Protista

It is formed by unicellular organisms (a single cell with a nucleus). They are divided into: Protozoa and unicellular algae. They live in an aquatic or very humid environment. Some multicellular algae are also considered protists because they do not have tissues or organs.

5. Monera Kingdom

They are the smallest beings, they are made up of a single cell, therefore they have a very simple structure. They are the oldest beings on the planet. In this group we find bacteria that live in all environments and blue-green algae called cyanobacteria. They produce their own food and live-in aquatic environments. Cyanobacteria were very important when the atmosphere was generated.

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