The Living World

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Biology is the science devoted to the study of living organisms. It studies their structure, functioning, diversity, and relationships.

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Living organisms are self-replicating, evolving and self-regulating interactive systems capable of responding to external stimuli. This means they can maintain internal balance, interact with the envi…

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Metabolism, nutrition, respiration, irritability or consciousness, and adaptation are defining properties of all living beings. These are considered the core signs of life in this chapter.

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Growth is not defining because non-living objects can also increase in size, though not from within. Movement is not defining because non-living objects can move due to external force. Reproduction is…

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Metabolism is the sum total of all chemical reactions occurring in the body. Living organisms constantly make and change biomolecules through metabolic reactions. No non-living object exhibits metabol…

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Irritability is the ability to respond to changes in the internal and external environment. In humans, this includes self-consciousness. Response to stimuli is seen in organisms such as Mimosa pudica,…

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Adaptation is the tendency of living organisms to bring about changes in structure, function, or behaviour that improve survival in a particular environment. It helps organisms adjust to their surroun…

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In unicellular organisms such as bacteria, unicellular algae, and Amoeba, reproduction is synonymous with growth because an increase in number of cells is the main way the organism multiplies.