Levels Of Biological Organization – Zooconcept. In

Levels of Biological Organization :

In the living world biological organisation starts with submicroscopic molecular level passes through microscopic cellular level and microscopic or macroscopic organismal level and ends in ecosystems and the biosphere.

In the biological hierarchies organisation reveals that atoms are the lowest unit at the molecular level while the cells are smallest unit at microscopic level. Atoms combine to form molecules, which undergo chemical reactions to form organelles. Several organelles are contained in the cell. A group of cells meant for a specific function constitutes a tissue. In the organisms above tissue level of organisation, many tissues become engaged to form a particular organ, such as liver, stomach, lung, testis, ovary, etc. for a specific function. Many organs together forms life process, such as digestive system, respiratory system (or respiration), reproductive system (or reproduction), photosynthesis etc.

Plants and animals together called organisms. An organism in turn contains many individuals Groups of individuals that sexually interbreed form a species. Individuals form population. A group of organisms belonging to the same species constitutes a population. Populations of different species living in the same area make up a biological community. Community interaction (biotic), integrated with the non-living (abiotic) features of the environment form an ecosystem. Within the ecosystem, an individual forms the smallest unit. A large unit than ecosystem is the landscape, which is a geographical unit with a history. It shapes the features of the land organisms inhabit in land.


Living organisms are selfmaintaining systems. They need a continuous supply of energy in order to sustain life. The main source of energy is sun (solar energy or radiant energy). Plants convert this radiant energy into chemical energy (Potential energy) in the form of food. The animals consume food (energy) produced by plants. They utilize this stored energy and transform it into different forms. This exchange of matter and energy within the organism is called metabolism. Thus metabolism refers to all the enzyme catalyzed chemical reactions taking place within an organism or its individual cells.

The chemical reactions (metabolic reactions) can be divided into two categories:

(a) Anabolism – Those reactions concerned with the synthesis of cell constituents and cell products from simple substances are called anabolism (constructive reactions). These are usually endergonic reactions (energy requiring) requiring an input of energy.

(b) Catabolism – Those reactions are concerned with breakdown of complex molecules to simpler ones by exergonic processes. This involves energy liberating oxidative reactions (destructive).

Anabolism and Catabolism are insolubly connected as there is continuous synthesis and breaking down of organic compounds. When anabolism becomes excess of catabolism, growth is effected in organisms. The final products of metabolism which cannot be transformed by the organism are excreted out.

Special metabolism includes changes undergone by a particular substance or group of substances in the body of the organism e.g., metabolism of carbohydrates, proteins and fat. Metabolic rate is usually expressed in calories per kg. body weight per hour. It may be determined either from the quantity of CO₂ eliminated or the quantity of O₂ consumed over a definite period of time.

The rate of oxygen consumption in a humming bird is 1000 ml/gm weight/ hour during rest, but it is 13000 ml/gm weight/hour during flight. Hence the metabolic ratio is 13 times higher during flight. Basal metabolic rate (BMR) is measured 12 – 14 hours after the last meal at an ambient temperature of 25°C. The normal BMR value for a healthy man is 40 kilo calorie per square meter of body surface per hour (40 Kcal/m²/hr) and same for a healthy woman is 37 Kcal.m³ BMR is highest in smaller animal like shrew (smallest rat wing 4 gms and on the contrary an elephant has very low Basal metabolic rate i.e. 67 ml of oxygen consume /gm body weight /hour. Therefore, elephant can stand under starvation for some days.

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