Introduction
Our world is full of an astonishing variety of organisms — from giant trees to tiny bacteria. What makes something "living"? All living organisms share certain fundamental characteristics that distinguish them from non-living things. In this chapter we explore these characteristics.
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Key Concepts and Definitions
Living organisms are entities that carry out all the basic life processes: nutrition, respiration, excretion, growth, reproduction, movement, response to stimuli, and lifespan.
Non-living things do not carry out these processes on their own (e.g., a rock, a pen, a table).
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Characteristics of Living Organisms
1. Nutrition
All living things need food for energy and growth. Plants make their own food through photosynthesis (autotrophs). Animals eat plants or other animals (heterotrophs).
2. Respiration
Living things break down food (glucose) to release energy. This process uses oxygen in most organisms and releases carbon dioxide.
Formula: Glucose + Oxygen → Carbon dioxide + Water + Energy
3. Growth
All living organisms grow — they increase in size, mass, and complexity. Plants continue growing throughout their lives; most animals stop at a certain size.
4. Reproduction
Living organisms produce offspring of their own kind. This ensures continuation of the species. Can be sexual (two parents) or asexual (one parent — e.g., budding in Hydra, spores in fungi).
5. Movement
All living things show movement. Animals move from place to place (locomotion). Plants move parts of themselves — leaves turn toward light, roots grow downward (but plants do not move from place to place).
6. Response to Stimuli
Living organisms respond to changes in their environment (stimuli). A plant bends toward light (phototropism). A touch-me-not (Mimosa) plant folds its leaves when touched. Animals respond to sounds, pain, light, etc.
7. Excretion
Living organisms produce waste and remove it. Humans excrete urine and sweat; plants release oxygen (waste of photosynthesis) and water vapour through their leaves.
8. Lifespan
Every living thing has a fixed lifespan — it is born, grows, reproduces, and dies.
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Worked Examples
Is a car living or non-living?
- A car moves, uses fuel (energy), and produces waste gas.
- However, it does NOT grow, reproduce, or respond to stimuli on its own.
- Conclusion: A car is non-living. It needs a human driver; it cannot do these things independently.
Photosynthesis as nutrition
- A mango tree absorbs water (from roots) + carbon dioxide (from air) + sunlight.
- It produces glucose (food) + oxygen.
- This is the nutritional process of an autotroph.
Why seeds can remain dormant
- Seeds are alive but show no visible signs of life (no growth, no movement) when dry.
- When given water and warmth, they germinate.
- This shows that life processes can slow dramatically but the organism is still living.
Movement in plants — Mimosa pudica (touch-me-not)
- Touch the leaves of Mimosa pudica.
- They fold inward quickly (within seconds).
- This is a response to stimulus (touch). Plants can move and respond, just more slowly than most animals.
Comparing a living tree and a wooden table
- Both the tree and the table come from wood.
- The living tree: grows, makes food, reproduces, responds to environment.
- The wooden table: none of these. It is dead (once-living) material — now non-living.
Reproduction ensures species survival
- A pair of frogs lays hundreds of eggs.
- Most eggs are eaten or die; a few survive to become adult frogs.
- Without reproduction, the frog species would die out in one generation.
Excretion in plants vs animals
- Animals: kidneys filter blood; waste leaves as urine.
- Plants: release oxygen during photosynthesis and carbon dioxide during respiration — both are waste products of their respective processes. Water vapour is also released through stomata (transpiration).
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Key Formula
Photosynthesis: Carbon dioxide + Water + Sunlight → Glucose + Oxygen
Respiration: Glucose + Oxygen → Carbon dioxide + Water + Energy
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Common mistakes
- Thinking fire is living — fire moves, "grows," and produces waste (smoke), but it cannot reproduce or respond to stimuli independently. It is not living.
- Thinking movement means locomotion — plants move their parts; movement does not require travelling from place to place.
- Confusing respiration (breaking down food for energy, done by ALL living things) with breathing (physical intake of air, done only by animals with lungs or gills).
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Summary
All living organisms share seven characteristics: nutrition, respiration, growth, reproduction, movement, response to stimuli, excretion, and have a lifespan. Plants and animals share all these traits, though in different ways. Non-living things (rocks, cars, fire) may show one or two traits but not all — only living organisms show all characteristics.