Compared to excessively wide range of temperatures existing in this universe, life is known to exist in very narrow range of temperature only.  This in itself indicates that temperature has significant effect on living organisms as a limiting factor.

Temperature regulates all chemical processes & many physical processes. It controls feeding, respiration, growth, reproduction, geographical distribution etc. of living organisms.

Range of temperatures can be classified as: (Average varies from species to species)

Minimum Temperature:

  • Minimum temperature at which an organism can live in active state, indefinitely.
  • It is the minimum temperature required for all metabolic processes necessary for life.
  • If an organism is subjected to temperatures below its minimum temperature, it enters into condition of inactiveness, sometimes called ‘Chill Coma’.
  • The lowest temperature at which survival is possible is called minimum survival temperature.

Optimum Temperature:

  • Temperature at which physiological activity in living organisms is maximum & reproduction is vigorous.
  • From minimum temperature as the temperature is increased, the physiological activity also increases & at optimum temperature it is maximum.

Maximum Temperature:

  • Maximum temperature at which an organism can live in active state, indefinitely.
  • If the temperature is raised above this maximum temperature, the living organism will enter state of inactivity called ‘Heat Coma’.
  • The maximum temperature at which survival of organism is possible is called maximum survival temperature.

Classification of living organisms based on temperature tolerance:

  1. Eurythermal: Living organisms which can tolerate large variations in temperature. Example- Wall Lizards, toads, human etc.
  2. Stenothermal: Living organisms which can tolerate only a small variation in temperature. Example- Fishes, Snails, Coral reefs etc.

Thermoregulation (Heat Regulation)

It is the ability to maintain optimum temperature range by an organism.

[In contrast- Thermoconforming is the ability of an organism to change its body temperature according to its environment temperature]

Classification of organisms based on thermal characteristics:

  1. Endothermic & Exothermic
  2. Homeothermic & Poikilothermic

1) Endothermic & Exothermic

Endothermic: (Warm Blooded)

  • Maintains body temperature at metabolically favorable degrees.
  • Use largely, heat generated from internal body processes for maintaining the body temperature.
  • Examples: Birds & Mammals.

Exothermic: (Cold Blooded)

  • Body temperature is dependent on external sources like sunlight, heated rock surface etc.
  • Internal body source for heat is relatively negligible as compared to body of the organism.
  • Example: Invertebrates, Fishes, Amphibians, Reptiles etc.

Heterothermic: Heterothermic organisms act both as endothermic & ectothermic at one or other time.

2) Homeothermic & Poikilothermic

(Based on how stable is an organism’s body temperature)


  1. Maintains a stable internal body temperature regardless to external influence.
  2. Homeotherms are not necessarily endotherms.
  3. Occurs in warm blooded organisms.


  1. Internal body temperature varies considerably.
  2. Terrestrial ectoderms are considered poikilotherms, besides other organisms.

Why all ‘Cold blooded organisms’ or ‘Ectoderms’ are not ‘Poikilotherms’?

Some ectoderms (body temperature dependent on external temperature) live in temperature constant environments up to the extent that they are able to maintain a constant internal temperature w.r.t. their external environment, which means that they have stable thermoregulation somewhere present in contrast to poikilotherms which have variable internal body temperature – when we are talking about body temperature stability.

Effects of temperature on plants & animals:

  • Effect on metabolism:
    • Most metabolic reactions are under control of enzymes which in turn are sensitive to changes in temperature.
    • Up to certain limit, the increase in temperature results in increased enzyme activity and hence increased metabolism.
    • Beyond a certain limit, varying enzyme to enzyme, increase in temperature brings about retardation in enzyme activity and hence dependent metabolism.
    • Example: Activity of enzyme liver arginase, acting on arginine, increases from 17⁰C to 48⁰C, beyond 48⁰C its activity is retarded.
  • Effect on reproduction:
    • Maturation of Gonads or sex cells takes place at particular temperature depending upon species.
    • Liberation of gametes also take place at particular temperature varying species to species.
    • Example: Calliphora sericite (Blow fly) – Number of eggs laid per female in one laying increases with increase in temperature upto 32.5⁰C, but with further increase number of eggs laid decreases.
    • Flowering in plants is also affected by temperature fluctuations, termed thermoperiodism.
    • Temperature also affects fecundity i.e. reproductive capacity in animals. Example: female Chrotogonus traychpterus becomes sexually mature at 30⁰C and 35⁰C as compared to 25⁰C.
  • Effect on Growth and Development:
    • Incubation period in blow fly, Calliphora sericate, decreases with temperature increase.
    • In Oyster, Ostraea virginica, length of body increases (from 1.5mm to 10.3mm) with increase in temperature (from 10⁰C to 20⁰C).
    • Corals do not flourish well in water temperature below 21⁰C.
    • Desiccation: Due to rapid transpiration and slow absorption during winters, plant tissues get dehydrated and injured.
    • Chilling Injury: Damage to plant parts when exposed to temperatures higher than freezing point. Tropical/Sub-tropical are more susceptible to chilling injury when exposed to low temperatures for some time.
    • Freezing Injury: damage to plant tissues when exposed to temperatures below zero degree Celsius due to freezing of intracellular water.
    • Cold Resistance: Ability of Perennial plants to tolerate low temperatures.
    • Heat Injury: Extremely high temperatures cause adverse effects on physiological processes like transpiration, respiration, metabolism etc. and final death of plants.
  • Effect on Crossing Over:
    • In Drosophila, certain characters show a greater percentage of crossing over above or below optimum temperatures. Development of organs like wings, eyes etc. can be influenced by fluctuating temperatures under which pupae/larvae are kept. Example: Wings tend to be longer at high temperatures and shorter at low temperatures.
  • Effect on Sex-ratio:
    • In Daphnids, under normal temperature, parthenogenetic eggs are laid which develop into females. With increase in temperature sexual eggs which after fertilization develops into either male or female.
    • There is significant increase in male with increase in temperature in Macrocyclops albida.

Division of World’s Vegetation based on temperature conditions:

  1. Megatherms: Plants living in high temperatures throughout year. Example: Equatorial & Tropical rainforest plants.
  2. Mesotherms: Plants living in alternating high & low temperatures. Example: Tropical Deciduous forest plants.
  3. Microtherms: Plants living in low temperatures. Example: Mixed coniferous plants.
  4. Hekistotherms: Plants living in extreme low temperatures. Example: Alpines.

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