The living world

What is living-?

The main characters of living world are

Not the defining properties

(1) Growth

(2) Reproduction

Defining Properties

(3) Metabolism

(4) Cellular organisation

(5) Consciousness

The character which has no exception is called as the defining property of life.

GROWTH

• The increase in mass or overall size of a tissue or organism or its parts is called growth.
• The increase in mass and increase in number are twin characters of growth.
• Growth is an irreversible permanent increase in the size of an organ or its parts or even of an individual cell.
• Growth is the result of metabolism when the synthetic reaction or anabolism is more than that of the destructive process or catabolism growth occurs.
• In the reverse situation (anabolism ↓, catabolism↑) there will be de-growth or negative growth.

Indeterminate Growth – unlimited growth

The growth which occurs continuously throughout their life span is indeterminate growth or unlimited growth. It occurs in plants and not in animals.

Determinate Growth – limited growth

Growth which occurs only up to a certain age is determinate growth or limited growth. It occurs in animals. However, cell division occurs in certain tissues to replace lost cells.

In majority of higher plants and animals, growth and reproduction are mutually exclusive events.

Because both living and non-living exhibit growth so it cannot be taken as defining property.

Growth from the inside (intrinsic growth) can be taken as defining property.

REPRODUCTION

• Production of new individual or progeny is called as reproduction.
• Reproduction in the case of the multicellular organism is the production of progeny, possessing feature more or less similar to those of parents.
• Reproduction in the case of unicellular organisms like bacteria, unicellular algae or amoeba is the increase in the number of cells. Means in unicellular organisms the growth and reproduction are synonyms or same.
• Reproduction is not found in any non-living object.
• Some living organisms which can /do not reproduce like mules, sterile human couples, worker bees.
• Hence, reproduction also cannot be taken as defining characteristics of living organisms.

Types of Reproduction

Asexual Reproduction – It is a type of reproduction in which gametic fusion or fertilization and meiosis are not involved.

There are some methods of asexual reproduction.

• By asexual spheres – In algae and fungi,
• By budding – In yeast and hydra.
• By fragmentation – In filamentous algae, fungi and the protonema of moss plant.
• True regeneration – Fragmented organisms regenerate the lost part of its body and become a new organism. E.g. Planaria.
• Regeneration is a process in which only lost part of the body is repaired or regained. E.g. star fish, lizard etc.

Sexual Reproduction – Reproduction in which gametes are formed by meiosis and fertilization also take place to form progeny is called sexual reproduction.

Metabolism

• The sum total of all the chemical reactions occurring in our body is metabolism.
• All living organisms, both unicellular and multicellular exhibit metabolism.
• No non-living object shows metabolism.
• Metabolism is a defining character of living organisms because it has no exceptions.
• All organisms are made of the small or big chemicals. These chemicals perform thousands of reactions and form some other chemical also in the bodies of living organisms.
• All plants, animals, fungi and microbes exhibit metabolism.

Cellular Organization

• A cell is the basic unit of life.
• All living organisms are composed of cells. Some are composed of a single cell and are called unicellular organisms while other are composed of many cells and are called multicellular organisms.
• Unicellular organisms are capable of independent existence and performing essential functions of life.
• Anything less than a complete structure of a cell does not ensure independent living. Hence “cell is a fundamental structural and functional unit of all living organisms.”
• Hence cellular organization is also a defining property of all living organisms.

Consciousness 

Ability to sense the surrounding environment and respond to these environmental stimuli is called as consciousness.

• Consciousness is the most obvious and technically complicated feature of all living organisms.
• We sense these physical, chemical or biological stimuli through our sense organs.
• Plants also sense and respond to external factors like light, water, temperature, other organisms, pollutants etc.
• All organisms from the prokaryotes to complex eukaryotes show consciousness to environmental clues.
• e.g. of consciousness seen in organisms
  • Plants perform flowering in a particular season (Photoperiodism), some animals perform budding in a particular season only and all organisms handle the chemicals entering their bodies etc.
• When human is concerned, a very high level of consciousness is found in us because of our very well developed nervous system and supreme level of skill of communication which is called as self-consciousness.
• Human is very fast to respond towards the external stimuli and even it can think or predict about possible changes of surroundings also can prepare accordingly to the surrounding situations. Further, a human can even change its surrounding situations up to a limit so this topmost or climax level of consciousness is regarded as self-consciousness which cannot be seen elsewhere.
• Self-consciousness is thought to be present only in human.

All the living phenomena are due to underlying interactions between different components of an individual or organ or tissue or cell.

Living organisms are self-replicating, evolving, and self-regulating interactive system capable of responding to external stimuli.

Adaptations and homeostasis are also very important characters of living.

LEVELS OF BIOLOGICAL ORGANIZATION

• Biological organization starts with sub-microscopic molecular level passes through the microscopic cellular level and microscopic and macroscopic organization level and ends in ecosystem and biosphere.
• The hierarchy of biological organization reveals that atoms are the lowest unit at the molecular level while the cells are the smallest unit at the microscopic level.
• Within the ecosystem, an individual forms the smallest unit. A larger unit that ecosystem is the landscape, which is a geographical unit with history.
• Organisms are the open system because they interact continuously with their environment.

Adaptation

• Adaptation to the environment is one of the most distinctive features of living beings.
• Adaptation may be defined as any characteristics of an organism, which makes the organism better suited to its environment.
• Adaptation may be structural physiological or behavioral in responses to the environment.

Types of Adaptation

Long-term adaptation – They are permanent inheritable changes in structure and function produced in an organism due to evolution for a better mode of living and adjustment with their environment.

1. Protrusion of ankle bone for standing erect.
2. Opposable thumb for holding or gripping.
3. The beaks of birds are of the different shape for their specific feeding habits.
4. The kangaroo uses its thick tail as a fifth limb to balance its body.
5. The desert plants are either leafless or have leaves modified to spines to check the loss of water, stem is water storing and has thick cuticle covered by layer or wax.
6. Female Anopheles requires mammalian proteins for egg formation so their mouth parts are adapted for sucking blood.

Short term adaptations – They are temporary changes that are produced in response to the specific condition and are meant for adjustment to unfavourable conditions.

1. Skin becomes dark on exposure to sunlight due to the accumulation of skin pigment melanin in the epidermis. It helps in protecting the underlying tissue from ultraviolet rays of the sun.
2. The dormant seeds suspend their activities and appear to be dead. In the favourable conditions, dormancy is broken and the seeds germinate.
3. A hibernating animal suspends all the activities to save energy. It keeps its metabolic rate very long. It survives on stored fats.
4. Plants show adaptive reactions e.g. phototropism.

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