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Presence of protoplasm is a characteristic feature of living organisms and it provides the site for all vital activities; hence Huxley (1863) called it the 'physical basis of life'.
Characteristics Of Living Organisms
There are certain characteristics exhibited by living organisms, which distinguish them from the non-living.
Growth
- All living organisms grow.
- Growth is defined as the irreversible increase in the mass and/or overall size (volume) of an organism or its parts.
It occurs due to the synthesis and deposition of two types of substances:
- Protoplasmic substances, which are the living substances like nucleus (nucleic acids), cytoplasm, etc.
- Apoplasmic substances, which are the non-living substances secreted by the living cells, e.g., matrix and fibres in the connective tissue.
Growth involves the following processes:
- Cell division/proliferation
- Cell enlargement
- Secretion of apoplasmic substances.
- Cell differentiation/maturation
Increase in mass and increase in number of individuals are the twin characteristics of growth.
In plants, growth is continuous and occurs throughout life due to cell divisions in meristems; in animals growth stops after a certain age, but cell divisions occur in certain cells to replace the wornout cells.
In unicellular organisms, cell divisions increase the number of cells in individuals. In increase in mass and size is considered growth, then non-living objects like boulders, mountains and sand mounds grow in size too; so growth cannot be taken as a defining property of living beings.
Form and Size
All living organisms have a definite form/shape and are described as morphous; each species is unique and can be distinguished from all other species.
They also have a characteristic size, e.g., whale is the largest aquatic animal while elephant is the largest terrestrial animal; Macrocystis, a brown alga is the largest among algae.
The non-living things without a characteristic form, are called amorphous.
Organisation
Biological organisation or organisation of living organisms begins at the submicroscopic levels (molecular or organelle level), passes through microscopic levels [cell(s), tissues, etc.] and reaches the macroscopic (organismic) level.
All living organisms are made up of cell(s) and its/their products, called apoplasmic substances.
In unicellular organisms, the single cell performs all the vital activities of life, while in multicellular organisms, these cells join together and become specialised (differentiation and histogenesis) for various functions, i.e., there is division of labour.
Metabolism
The sum total of all the chemical reactions occurring in a living cell/organism, is called metabolism.
It involves exchange of matter and energy.
The constant breakdown and synthesis of molecules/compounds is necessary for the cell to continue its life.
There are two forms of metabolism and they are described below -
Anabolism : It is the constructive part of metabolism, that includes the synthetic activities, e.g., photosynthesis, protein synthesis, etc.
Catabolism : It is the destructive part of the metabolism that includes the breakdown activities, e.g., digestion, respiration, etc.
Reproduction
Reproduction refers to the production of young ones (progeny) of the same kind by the living organisms.
Reproduction may be by asexual or sexual methods.
Following are examples of methods of asexual reproduction:
- Filamentous algae, fungi, protonema of mosses, etc., multiply by fragmentation.
- Planaria like organisms show regeneration of the whole individual from any small fragment of the body.
- Yeast, Hydra and sponges reproduce by budding.
- Fungi also reproduce asexually by spore formation.
- Unicellular organisms (Amoeba, certain bacteria, etc.) increase in number by binary fission.
Reproduction too, cannot be considered an all-inclusive defining property of living organisms, for the following reasons:
- In unicellular organisms, growth is synonymous with reproduction, i.e., increase in number of cells is considered as growth; we are not clear about the usage of these two terms, growth and reproduction, in unicellular organisms.
- There are many organisms which do not reproduce at all, e.g., mules, the sterile worker bees and even some human couples who are infertile.
Response to Stimuli
Responding to stimuli is a unique feature of all organisms, ie., they sense and respond to changes in the temperature, light/dark cycles, chemicals in their environment, photoperiods, etc.
- Plants respond to photoperiods for their flowering.
- Seasonal breeders respond to changes of seasons.
Thus all organisms are aware of their surroundings and this consciousness is a definite property of living beings..
In due course of time, they develop adaptations to suit the environmental changes and gradually evolve into new types of organisms (speciation).
Adaptation
An adaptation is defined as any structural, physiological or behavioural characteristic of an organism, that enhances its chances of survival and ability to reproduce in its environment.
Adaptations can be broadly classified into two categories as:
- Short-term adaptations and
- Long-term adaptations.
Short-term adaptations
- These develop due to a temporary or a brief change in the environment.
- The change (physical, chemical or biological) constitutes a stimulus.
- The stimulus is perceived by the organism and sent to the brain in the form of impulses.
- The suitable response takes the shape of a short-term adaptation.
These short-term adaptations are important for the survival of organisms, but are not involved in evolution.
Examples
1. Our skin becomes tanned (dark/black), when exposed to the sun for a long time; the skin pigment, melanin accumulates in the top layer of skin, absorbs the excess radiation and protects the underlying tissues.
2. Stem grows/bends towards light (positive phototropism), while roots grow downwards towards gravity (positive geotropism).
3. Hibernating animals maintain a very low rate of metabolism and suspend all activities, until the return of favourable environmental conditions.
Long-term adaptations
- These are the adaptations which develop over a long period of time and result in permanent change(s) in the structure and/or function of an organism.
- These responses occur within the framework of natural selection, i.e., they are heritable and have a role in evolution.
Examples
1.Aquatic plants have aerenchyma to give buoyancy for floating.
2.Giraffe has a long neck to feed on the leaves of tall plants.
3.Kangaroo uses its tail as the fifth limb.
4.Night-blooming flowers are generally scented and white, to attract pollinators.
5.Some orchid flowers e.g. Ophrys muscifera, resemble the shape and colour of certain species of bees/flies; this helps in their pollination.
Homeostasis
- The living state is a non-equilibrium steady state, maintained by self-regulated mechanisms.
- The capacity of living organisms to maintain a steady internal environment, is called homeostasis.
- It is necessary for the cellular chemical reactions to continue without interruption despite the changes occurring in the external environment.
- In multicellular organisms, all the cells except reproductive cells, contribute to the maintenance of homeostasis.
1.When you enter a dark room from bright light, you cannot see anything at first, but within a few seconds the eyes get adjusted to the dim light.
2.Human heart beats normally at the rate of 72 times per minute. When you run or climb stairs, the heart beats faster because of the need for more oxygen/blood supply to the tissues, but after sometime the normal heart rate is restored.
3.After vigorous exercise, the body temperature increases, but sweating occurs; when the sweat evaporates, there is a cooling effect and the temperature of the body is brought to normal.
Most of the homeostatic mechanisms in the organisms work through feedback systems and antagonism.
Feedback system
- It occurs at the level of enzymes, hormones, genes and even tissues.
- For example, if the product of an enzyme reaction accumulates in the cell/body, the enzyme reaction is inhibited by the product itself, until the product is used up/removed.
- Similarly, the level of hormones in the blood is maintained constant by interaction among hypothalamus, pituitary and the respective endocrine glands.
Antagonism
When two reactions or movements occur in opposite directions, they are said to exhibit antagonism.
For example, the hormone insulin decreases the level of glucose in the blood, by stimulating oxidation or glycogenesis by liver or synthesis of fats by adipose; but the hormone glucagon increases the level of glucose in blood by stimulating breakdown of glycogen.
Cellular steady state forms the basis of all steady states and also of adaptation and self perpetuation.
Death and its Significance
Death is a biological event, which occurs due to permanent breakdown in body functioning (all vital activities), usually occuring due to lack of oxygen supply to tissues.
- All living organisms are mortal and have a lifespan; but the single-celled or unicellular organisms are considered immortal, as the single cell divides into two daughter cells and there is no natural death for them.
- Death naturally keeps a check on the size of a population and prevents overcrowding and its consequences.
It is said that "Death Keeps No Calender".
Organisms Are Open Systems
In an open system, matter and energy are transferred/exchanged between the system and its surroundings.
The organisms also interact continuously with their environment for obtaining nutrients, oxygen, water, etc., and sending out the metabolic wastes.
A Shift In Approach To Study Organisms
In recent times, there is a shift in approach to study organisms from the traditional morphology, anatomy or cell biology to molecular level.
Studies at the molecular level, have yielded the following important points:
1. The basic/fundamental chemical organisation of all living beings is remarkably similar and complex.
2. Life is chemically based on the universal physico-chemical laws.
3. Living matter in the biosphere interacts with its surroundings.
The following are the characteristics of all living organisms at the molecular level:
1. As Linus Pauling has stated, 'Life results, due to a relationship of molecules and is not a property of any one molecule/compound'.
2. All living organisms are composed of carbon, hydrogen, nitrogen and oxygen; hence the molecular organisation is basically organic in nature, e.g., glucose is found only in living cells.
3. Both micromolecules (e.g., sodium chloride, water, glucose, etc.) and macromolecules (proteins, polysaccharides, nucleic acids, etc.) occur in living cells in nature.
4. These molecules have specific role(s) to play in the metabolism.
5. An increase in the variety of molecules in a cell provides scope for a wide range of chemical reactions in living cells.
Life As An Expression Of Energy
All activities of organisms, such as growth, movement, active transport, etc., need energy.
According to the first law of thermodynamics, energy can neither be created nor destroyed; it can only be transferred or transformed.
Organisms take in energy in the form of food and transform it to the form of chemical energy (ATP), they use; autotrophs take in energy in the form of light, but convert it into ATP during photophosphorylation.
The total amount of energy present in a biological system, is called enthalpy; it includes:
(i) usable/free energy, that can do work
(ii) unusable energy, that is lost to disorder.
▷ The cells derive their usable energy by breaking the bonds of high energy compounds.
▷ The energy needed to destabilise chemical bonds and to initiate a chemical reaction, is called activation energy.
▷ Reactions that occur without the supply of energy from outside source, are called spontaneous reactions.
▷ Energy yielding chemical reactions are called exergonic reactions.
▷ A chemical reaction which requires input of free energy from outside source, is called an endergonic reaction.
▷ An exergonic reaction normally has a first endergonic step, because the two molecules involved must come unusually closer and one or more bonds in them must break.
▷ Energy is transferred from high energy areas to low energy areas and this results in collision or disorder.
▷ Entropy is a measure of the disorder or randomness; entropy increases with every transfer and transformation of energy.
▷ The energy currency of living systems is adenosine triphosphate (ATP), which is hydrolysed in the presence of enzyme ATPase into ADP (adenosine diphosphate) and inorganic phosphate, to liberate the energy in its terminal bond.
ATP + ATPase → ADP + Inorgranic Phosphate + Energy