Class 11 The Living World
Biology is the science of life forms and living processes. The living world comprises an amazing diversity of living organisms. Biology is the story of the evolution of living organisms on earth. All living organisms present, past and future, are linked to one another by the sharing of common genetic material, but to varying degree. Life is unique, complex cellular organisation of molecules. According to Lamarck (1809), nobody can have a life if its constituent parts are not cellular.
Life is unique, complex cellular organisation of molecules and the cells themselves that show various types of chemical reactions which lead to the availability of energy, growth, development, responsiveness, adaptation and reproduction.
Characteristics of Living Beings
♠ The various characteristics shown by living beings are as follows:
♦ Individuality: Each living being has a distinct individuality. It can not be broken into two or more parts.
♦ Cellular Structure: Each living being is a complex entity which is formed of one or more cells. The cells are made of protoplasm, popularly called living matter.
♦ Definite Shape and Size: Every living being has a definite shape (amorphous) and size.
♦ Organisation: A living being has an organisation, that is, the living being consists of several components which cooperate with one another for the well-being of the whole organism. Each level of organisation has its own properties which are not found in its constituents. Biomolecules are organised to form a cell organelle. Cell organelles are similarly organised to form cells. Cells are organised into tissue, tissue into organs, organs into organ systems and organ systems into an individual. Because of this organisation, living beings are also called organisms.
♦ Energy: Living beings constantly require energy not only to perform various activities of the body but also to overcome entropy or tendency to randomness. The source of energy is food. It is required by every cell of the body.
♦ Homoeostasis (Homoeostasis): An internal environment suitable for the functioning of body organs is present in every living being. It is quite different from the external environment. Changes in the external environment do not have much impact on the internal environment as the living beings have a self-regulated system to adjust and maintain the internal environment. The phenomenon is called homoeostasis (Gk. homo is -– alike, stasis – standing). Homoeostasis is also present in each cell of a multicellular organism. It shows homoeostasis or ability to maintain perfect internal environment through the self-regulated system. Homoeostasis brings about changes in the internal environment that bring about the adjustment to variations in the external environment.
♦ Protoplasm: All body functions and properties of life are actually due to protoplasm. Protoplasm is the physical basis of life.
♦ Metabolism: All organisms are made of a variety of chemicals. The sum total of all chemical reactions occurring in an organism due to specific interactions amongst different types of molecules within the interior of cells is called metabolism. (Gk. metabole-change). Metabolism involves the transformation of matter and energy within an organism and their exchange with the environment. All activities of an organism including growth, movements, development, responsiveness, reproduction, etc. are due to metabolism. No nonliving object shows metabolism. However, metabolic reactions are neither living nor non-living. The isolated in vitro metabolic reactions can, however, be called biological reactions or living reactions as they involve biochemicals.
Metabolism is of two kinds, catabolism and anabolism. Anabolism includes all the “building up” reactions, e.g., synthesis of organic compounds from CO2 and H2O during photosynthesis, the formation of starch from glucose, production of proteins from amino acids, the formation of lipids from fatty acids and alcohols. Energy is stored (as potential energy) in the process.
Catabolism (= katabolism) constitutes “breakdown reactions”. It is also known as destructive metabolism because it involves breaking of complex substances into simpler ones. The potential energy present in the complex substances is converted into kinetic energy. Respiration is an example of catabolism. It releases energy for performing different body activities.
♦ Growth: Growth is an irreversible increase in mass and cell number of an individual. A multicellular organism increases its mass by cell division. In plants, growth continues throughout life as they have meristematic areas where cell divisions occur regularly. In animals, growth occurs to a certain age after which cells divide only to replace worn out and lost cells. Unicellular organisms also grow by cell division. However, cell division is also a means of reproduction in them. In higher animals and plants, growth and reproduction are mutually exclusive. Living organisms show internal growth due to the addition of materials and formation of cells inside the body. Such a method is called intussusception (L-into-within, sus si pere-to receive).
Growth occurs when anabolism exceeds catabolism. Two types of substances– protoplasmic and apoplasmic are formed for growth. Protoplasmic substances are constituents of living matter. They bring about the increase in bulk of protoplasm. Apoplasmic substances are non-living substances, e.g., cell wall, matrix, etc. Formation of the two types of substances is followed by cell division and cell enlargement.
♦ Death: After a period of time each living being dies. The natural death occurs due to wear and tear.
♦ Life Cycle: Each individual passes through a definite life cycle of birth, growth, maturation, reproduction, ageing and death.
♦ Consciousness: It is awareness of the surroundings and responding to external stimuli. The external stimuli can be physical, chemical or biological. The stimuli are perceived by sense organs in higher animals, e.g., eyes, nose. Plants do not possess such sophisticated sense organs. However, they do respond to external factors such as light, water, temperature, pollutants, other organisms, etc. All organisms, from the prokaryotes to the most complex eukaryotes can sense and respond to environmental cues. Photoperiods influence reproduction in animals and plants which breed during a particular season. All organisms handle chemicals entering their body. All organisms, therefore, are ‘aware’ of their surroundings. A human being is the only organism who is aware of himself, i.e., has self-consciousness.
♦ Regeneration: Regeneration is the ability to form lost parts. It is limited to certain organs in higher animals. Trambley (1742) was the first to study regeneration (in Hydra). T. H. Morgan studied the mechanism of animal regeneration.
♦ Movements: Living beings show movement of their parts. Some are able to move from place to place. The phenomenon is called locomotion.
♦ Variations: They possess variations and have the ability to evolve with time. The genetic material undergoes mutations and reshuffling of genes. This causes variations. Variations are so abundant that no two individuals of the same race are exactly similar.
♦ Adaptation: (L. add-toward, apt-adjust). Useful inheritable variations or changes in form, function and behaviour which help an organism to adjust well and successfully in its environment are called adaptations. Adaptations allow the organisms to overcome seasonal and other changes in the environment. They are of two types, short term and long term.
♦ Reproduction: It is the formation of new individuals of the similar kind. Life arises from pre-existing life. Young ones grow and become mature. They develop the faculty to produce young ones of their own type. Some organisms reproduce asexually while other sexually. However, reproduction is not a must for all living beings, e.g., mule., worker honey bees, etc.
NCERT biology 11