Defining Features of Living Beings: Cellular Organisation, Metabolism, Consciousness, Homeostasis, Repair and Regeneration, Adaptation, Evolution and Death

Of all the planets present in the universe, what makes Earth different from all other planets? 

Look around and contemplate the answer. It's right there! 

The existence of life makes the Earth a unique planet. 

From there emerges the question - ‘What is life?’.

Life can be defined as a unique and complex interaction of different molecules that results in growth, reproduction, development, responsiveness and adaptation.

Out of all the characteristics of a living being, there are certain features which make a living being different from a non-living thing. These are considered as the defining characteristics of living beings. Let’s understand more about defining characteristics of living beings. 

Table of Contents

• Cellular organisation
• Metabolism
• Repair and regeneration
• Death
• Practice problems
• FAQs

Cellular Organisation

The smaller units and subunits that aggregate in a systematic matter to form an organism is referred to as the organisation. Living organisms are basically aggregates of small units, also called cells.

Specific cellular organisation can be seen only in living organisms. Different cell organelles normally interact with each other, so that they can function properly in a cell. The cells may be aggregated into tissues, certain tissues may be aggregated into organs and certain organs may be aggregated to form organ systems to achieve physiological functions.

Since cellular organisations can be observed only in living beings, cellular organisation can be called as a Defining characteristic of living things.

Metabolism

All the chemical reactions that take place in an organism sums up as metabolism.

None of the non-living entities undergo metabolism. Metabolism is exhibited by both the unicellular and multicellular organisms. Hence, metabolism is one of the defining characteristics of living organisms without exception.

Types of Metabolism

Metabolism involves two types of chemical reactions as follows:

Catabolism

The process of breaking down complex substances into simpler substances resulting in the release of energy is known as catabolism. Examples include breaking down proteins into amino acids. 

Anabolism

The process of formation of complex substances from simpler substances by utilising energy is known as anabolism. Examples include photosynthesis. 

Metabolic Reactions 

Metabolic reactions are not restricted to the living system. These can also be performed in an isolated manner in a test tube or laboratory and are considered neither living nor non-living. For example, by joining amino acids together we can make proteins in a test tube. 

Consciousness 

It is the ability of an organism or living being to respond to any stimuli. For example, folding of leaves in the touch-me-not plant or Mimosa pudica upon touching which serves as an external stimulus.

Types of Stimulus

Stimulus can be of three types as follows:

• Physical Stimulus - It is a stimulus that acts on the body from outside. Examples include the response of plants towards sunlight. 
• Chemical Stimulus - It is the stimulus exerted by any chemicals like gases, liquids or solids that is capable of creating a response. Examples include response to a bad odour by a human being. 
• Biological Stimulus - It is the stimulus created by a biological element. For example, the changes shown by macrophages when it comes in contact with a bacteria. 

Consciousness Exhibited by Living Organisms

All prokaryotes to the most complex eukaryotes exhibit consciousness by sensing and responding to environmental cues. Plants respond to various external factors like temperature, light, water, chemicals, pollutants, other organisms etc. 

A distinct example is photoperiod which regulates reproduction in seasonal breeders. 

Humans are the only living creatures who are self-conscious. In other words, they are aware of themselves.

Non-living things lack the ability to respond to external stimuli. On the other hand, all the unicellular and multicellular organisms respond to stimuli. Hence, consciousness is one of the defining features of living organisms.

Homeostasis

Homeostasis is defined as the maintenance of a suitable and constant environment internally in an organism for the functioning of different body organs. It is exhibited by each and every constituent cell of the body. Maintaining homeostasis is a characteristic feature of living organisms. The same is not exhibited by non-living organisms.

Repair and Regeneration

Repair and regeneration is the ability to repair, renew or regenerate the injured or damaged body parts. For example: Healing of wounds in animals.

Body repairing and regeneration such as wound healing is observed naturally in the living organisms.

Adaptation

In order to fight the worst conditions, organisms evolve to have certain biological features that help them to survive. These features are known as adaptations.

Types of Adaptations

Adaptations are of two types as follows:

Short Term Adaptations

Certain biological features that come into play only during stress conditions are known as short term adaptations. These disappear once the stress period gets over. For example, slowing down metabolism of most amphibians, reptiles and some mammals during hibernation.

Long Term Adaptations

Certain biological features that develop in response to stress conditions and remain throughout life are known as long term adaptations. These features pass on from generation to generation playing a vital role in evolution. For example, adaptation of claws of birds as per their perching habits like clinging and climbing in woodpecker, swimming in duck, running in ostrich, perching in pigeon, striking and catching in hawk.

Evolution

Evolution is the process of change in morphological, physiological or genetic characteristics that are passed on to the next generations as a result of natural selection. It was defined by Darwin. Non-living organisms do not undergo selection pressures and hence, do not exhibit this feature.

Death

The end or termination of an individual’s life is termed as death. Non-living things do not die. Hence, it is the feature of living organisms only.

Signs of Death

An organism shows clinical and biological signs of death as follows:

Clinical signs of death

Certain clinical signs that signify death are the following:

• Stoppage of pulse.
• Stop breathing.
• Stopping the beating of the heart.
• Dilation of pupils.
   

Biological signs of death

There are different biological signals that signify death. It include the following:

• The brain stops functioning.
• Blood starts to settle in the lower portion of the body
• The body temperature reduces.
• The duration of muscle contraction prolongs. 

Causes of Death

Causes of death can be natural or accidental. Natural death may be due to ageing or occurrence of a fatal disease. Death can also be caused due to accidents, natural disasters etc.

Practice problems of Features of Living Beings

Question 1. The tail of the lizard got separated from the body. After a few days, the tail of the lizard appeared again. Which characteristic of living organisms is referred to here? 

a. Reproduction
b. Development
c. Regeneration
d. None of the above

Solution: The tail of the lizard appeared again as it was able to regenerate its tail. Repair and regeneration is the ability of living organisms to repair, renew or regenerate the injured or damaged body parts. For example, the healing of wounds in animals.

Question 2. In order to cope with the stress conditions, metabolism of various animals is slowed down. This type of adaptation is termed as ________.

a. Short term adaptation
b. Long term adaptation
c. Permanent adaptation
d. None of the above

Solution: Slowed down metabolic rate is a type of short term adaptation. Certain biological features that come into play only during stress conditions are known as short term adaptations. These disappear once the stress period gets over. For example, slowing down metabolism of most amphibians, reptiles and some mammals during hibernation. Hence, the correct option is a.

Question 3. Which of the following cannot be considered as a defining characteristic of living things?

a. Evolution
b. Consciousness
c. Homeostasis
d. Growth

Solution: Growth is defined as the irreversible increase in mass, size, dry weight and number of an individual. Growth can be seen in both living and nonliving things. Non living things show extrinsic growth i.e. growth that happens due to deposits outside the body. For example: Growth of sand dunes, mountains, boulders, glaciers etc. Since growth can be seen in both living as well as non-living things and thus cannot be the defining property of living beings. Hence, the correct option is d.

Question 4. Arpit touched leaves of the plant which caused the folding of leaves. Identify the plant and the respective defining characteristics of living organisms shown by the plant.

a. Solanum tuberosum; homeostasis
b. Nepenthes alata; adaptation
c. Solanum tuberosum; consciousness
d. Mimosa pudica;consciousness

Solution: Folding of leaves upon touching is seen in the touch-me-not plant or Mimosa pudica. This particular ability of an organism to respond to any stimuli is defined as consciousness. Hence, the correct answer is option d.

FAQs of Features Of Living Beings

Question 1. What is metabolism?

Answer: All the chemical reactions that take place in an organism sums up as metabolism. None of the non-living entities undergo metabolism. Metabolism is exhibited by both the unicellular and multicellular organisms. Hence, metabolism is one of the defining characteristics of living organisms without exception.

Question 2. Is consciousness a defining characteristic of living organisms?

Answer: Ability of an organism to respond to any stimuli is defined as consciousness. For example, folding of leaves in the Chui-Mui plant or touch-me-not plant or Mimosa pudica upon touching which serves as an external stimulus. From all prokaryotes to the most complex eukaryotes exhibit consciousness by sensing and responding to environmental cues. Plants respond to external factors like temperature, water, light, pollutants, chemicals, other organisms etc. A distinct example is the photoperiod, which regulates the reproduction in seasonal breeders. Humans are the only living creatures who are self-conscious. Non-living things lack the ability to respond to external stimuli. On the other hand, all the unicellular and multicellular organisms respond to stimuli. Hence, consciousness is one of the defining features of living organisms.

Question 3. How is anabolism different from catabolism?

Answer: Catabolism is the process of breaking down complex substances into simpler substances resulting in the release of energy while anabolism is the process of formation of complex substances from simpler substances by utilising energy.

Question 4. What is homeostasis? How is it important for living organisms?

Answer: Homeostasis is defined as the maintenance of a suitable and constant environment internally in an organism for the functioning of different body organs. It is exhibited by each and every constituent cell of the body. Different living organisms live in different environmental conditions. For example, polar bears are adapted to live in extreme cold environments while camels are adapted to live in extreme hot environments. It becomes very important to maintain the internal temperature of the body according to the changing environment where homeostasis comes into play. Maintaining homeostasis is a characteristic feature of living organisms.

Other Related Topics

Other Related Topic Of Biology

• General characteristics of living
• features of living beings