Abiotic Factors & Responses to Abiotic Factors Notes | Study Biology Class 12 - NEET

NEET: Abiotic Factors & Responses to Abiotic Factors Notes | Study Biology Class 12 - NEET

The document Abiotic Factors & Responses to Abiotic Factors Notes | Study Biology Class 12 - NEET is a part of the NEET Course Biology Class 12.
All you need of NEET at this link: NEET


(A) Light

(B) Temperature

(C) Soil

(D) Other

It is a complex physical environmental factor. Light is measured by luxmeter or photometer. It is a electromagnetic spectrum.

Solar Constant – Solar radiation before entering the atmosphere carries energy at a constant rate i.e., 2 cal cm–2 min–1 known as the solar constant.

In solar radiation wavelength (l ) of light or visible spectrum is 0.4 to 0.7 mm (400–700 nm) it is also called photosynthetically active radiation (PAR).

The U. V. radiation distinguished in
U. V. – C → (0.100 to 0.280 μm)
U. V. – B → (0.280 to 0.320 μm)
U. V. – A → (0.320 to 0.400 μm)

Albido value –
The ability of a surface to reflect the incoming radiation is called albido value (AV) it is 80% for fresh snow, 20-30% for sand, 5-10% for the forest.

Abiotic Factors & Responses to Abiotic Factors Notes | Study Biology Class 12 - NEET

Effect of light –

Photokinesis – Regulation of speed of locomotion due to light is called photokinesis e.g. larvae of mussel crab move faster if light intensity increases.


Temperature is measured by the thermometer and under water by thermistor.

Effect of Temperature on plants –

Temperature causes dessication (injury due to dehydration), chilling injury (injury of plant due to cold climate) and freezing injury (injury to plant cell due to ice crystal formation in the intercellular spaces)

Effect of Temperature on animal –

Temperature affect the absolute size of an animal and its body parts.

1. Bergman rule –

Birds and mammals attain greater body size in cold region and lesser in warm region.

2. Allens rule –

The tail, ears, limbs, eyes, snout and hair of mammals are smaller in colder region and larger in warm region.

3. Jorden's rule –

Fishes in cold water possess more vertebrae than those living in warm water.

4. Gloger rule –
Warm blooded animals in hot and humid area (tropical region) are more darker in colour (heavily pigmented) than cold area.

5. Renschs rule –

Birds in cold region have narrow wings and in warm region have broader wings.


Type of migrationExamplesActivities
Long-distanceArctic tern

Nests close to north pole in summer. Flies from North (Arctic) to Antarctica in autumn and returns to North pole again each spring.

Short-distanceCaribou, Elk and whaleMigrates in search of food each winter to warmer place.
PeriodicLocust (Tiddi)Large population migrate in search of feeding grounds.

Note : Thermal Migration – Thermal migration has been seen in birds (siberian cranes, arctic tern), mammals (Bison, caribou), fishes (salmon) etc.

Acclimatisation –

Acclimatisation is a gradual physiological adjustment of the organism to the slowly changing new environmental condition. If there is a shift in some environmental factor beyond the tolerance range of an organism the latter can come to the resting stage or migrate.

Shelford law of tolerance –
Every organism has minimum and maximum limit of tolerance (ecological amplitude) with respect to environmental factors like temperature, sunlight or nutrient concentration in between those limits the central optimum range are found in which organisms are abundant this is known as optimum zone of tolerance.

Abiotic Factors & Responses to Abiotic Factors Notes | Study Biology Class 12 - NEET

On the basis of temperature vegetation is divided into four groups

Megatherms – The plants growing in high temperature throughout the year

e.g. Tropicalrainforest

Mesotherms – The plants growing in alternate high and low temperature

e.g. Deciduous tropical forest

Microtherms – Plants growing in low temperature

e.g. Coniferous forest

Hekistotherms – Plant growing in very low temperature

e.g. alpine vegetation

On the basis of temperature animals divided into two groups

(1) Eurythermous or Homeo thermal/endo thermal/warm blooded – Animals which are able to tolerate wide variation of temperature

e.g. birds, man, etc.

(2) Stenothermous or Poikilothermal/ecto thermal/cold blooded – Animals which are unable to tolerate wide variation in temperature

e.g. Arctic fishes Reptiles, Amphibians

Regulaters :

Organisms who are able to maintain homeostasis by physiological (some time behavioural also) means which ensures constant body temperature, (thermoregulation) constant osmotic concentration (osmoregulation) eg. all birds and mammals.

Abiotic Factors & Responses to Abiotic Factors Notes | Study Biology Class 12 - NEET

Conformers :
Organism who cannot maintain a constant internal environment means their body temperature or osmotic concentration change with external environment.

The response of an organism to a range or gradient of an environmental factor (temperature, light, nutrient).

Note : If the stressful external condition are localised or remain only for a short duration the organism has two
alternatives, tosurvive.

(i) Migration

(ii) Suspension


Thermal stratification occurs in deep water body because of difference in temperature of water at different

Mainly three layer or zone occurs in water below

1. Epilimnion – The top layer gains warmth.

2. Metalimnion/thermocline – Middle region steady decline in temperature or a gradual change in temperature.

3. Hypolimnion – Bottom which is not affected by temperature.

Abiotic Factors & Responses to Abiotic Factors Notes | Study Biology Class 12 - NEET

Difference in temperature of water at different depths results in thermal stratification in deep water bodies. During summer, temperature is higher on the surface water, whereas in lower layer temperature is low.

During winter in a temperature lake, water is at freezing temperature on the surface, whereas in the lower layer temperature is about 4°C. The surface water is cooled during autumn, and warmed in spring. This results in a free mixing of water in the whole water body, also known as autumn and spring turnover.

During spring and autumn due to turnover of water oxygen and nutrients are redistributed, resulting in a bloom of phytoplankton growth while during winter and summer, growth of phytoplankton is low due to low nutrients and oxygen availability.

Abiotic Factors & Responses to Abiotic Factors Notes | Study Biology Class 12 - NEET


All the ocean basins are roughly of the shape of a wash basin or an inverted hat.

Type I – On the basis of structure :

From the coast line a gradually sloping region extends for about 160 km into the sea. This zone is called continental shelf and has a depth of 8-200 meters. The angle of the slopes then abruptly steepens to form the continental slope. The continental slope levels off into a more or less horizontal ocean floor. The depth of the ocean floor is in the range of several thousand meters.

Type II – On the basis of availability of light :

The vertical zones of the ocean are determined by the availability of light or penetration of light for photosynthesis that is 

  • Photic or Euphotic zone – It is upper lighted zone up to a depth of about 200 meters.
  • Aphotic zone – Middle region where diffused light penetrate which is insufficient for photosynthesis, it extends up to the depth of 200-2000 meters.
  • Abyssal zone – It is deeper part where light do not reach below 2000 meter, is the area of perpetual darkenss.

Type III – On the basis of environment :

Three major environment may be recognised in the ocean basin.

  • The Littoral zone – Comprising the edge of the continental shelf.
  • The benthonic zone – Formed of sea floor along with the continental slope and the aphotic and abyssal zone.
  • The pelagic zone – Constituting the water of the ocean basin.


  • The shallow shore region of a marine area is called neritic zone.
  • The part have been cut off from river is called ox-bow-lake
  • Estuaries - The region where, river enter the ocean are known as estuaries.


Altitude → Height above the sea surface of any place.
Latitude → The distance of any place from the equator

On the basis of variation in mean temperature along latitude the main climatic regions are
(1) Tropical = 00 – 200 latitude
(2) Subtropical = 200 – 400 latitude
(3) Temperate = 400 – 600 latitude
(4) Arctic and antarctic = 600 – 800 latitude

Abiotic Factors & Responses to Abiotic Factors Notes | Study Biology Class 12 - NEET
Note :

  • The temperature and light values are maximum at the equator, decreases gradually towards the pole.Effect of altitude and latitude are almost same on temperature.
  • The types of vegetation from sea level to measuring altitudes are similar to increasing latitude (distance from equator)

On the basis of the soil condition there are seven ecological group of plant

  • Halophytes – Plants grow in saline soils
  • Psamophytes – Plants grow in sand
  • Lithophytes – Plants grow on rock surface
  • Chasmophytes – Plants grow in rock crevices
  • Chersophytes – Plant grow in waste land
  • Eremophytes – Plant grow in dry area
  • Cryophytes or Psychrophytes – Plant grow in low temperature or cold soil

Soil – Soil is the uppermost layer of earths crust formed by weathering of rocks. It is the mixture of living or non living materials.

Minerals 45% + Water 25% + Air 25% + Organic matter (living + non living) 5%

Soil formation is slow process 1 inch soil is formed in 500-1000 years

Pedogenesis – development of soil or soil formation

Pedology (Edaphology) – study of soil

Soil mineral matter –
As a result of weathering the mineral particles of different size are formed. The soil is divided into five types on the basis of size of soil particles.

Soil TypeSize of particles
Clayless than 0.002 mm
Silt0.002 – 0.02 mm
Fine sand0.02 – 0.20
Coarse sand0.20 – 2.0
Gravel or Grit2mm – 5mm
Coarse GravelAbove 5.00

Sandy Soil = 85% sand + 15% clay or silt or both

Loamy Soil = 70% sand + 30% clay or silt or both

Silt Soil = 90% silt + 10% sand

Note : Loam Soil is the best soil for growing of crops, it has high water holding capacity, high aeration and high root penetration.


The dead organic matter present in soil is called humus, which is formed by decomposition of plant and animal remains. Freshly fallen plant and animal material called detritus or litter, partially decomposed litter is called duff. Fully decomposed litter is called humus.

Litter → Duff → Humus

Decomposition (Formation of Humas) :

Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients and the process is called decomposition.Dead plant remains such as leaves, bark, flowers and dead remains of animals, including fecal matter, constitute detritus, which is the raw material for decomposition. The important steps in the process of decomposition are fragmentation, leaching, catabolism humification and mineralisation.

Detritivores (e.g.earthworm) break down detritus into smaller particles. This process is called fragmentation. By the process of leaching, water soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts. Bacterial and fungal enzymes degrade detritus into semple inorganic substances, This process is called as catabolism.

It is important to note that all the above steps in decomposition operate simultaneously on the detritus. 

Humification and mineralisation occur during decomposition in the soil. Humification leads to accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature it serves as a reservoir of nutrients. The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation.

Decomposition is largely an oxygen-requiring process. The rate of decomposition is controlled by chemical composition of detritus and climatic factors. In a particular climatic condition, decomposition rate is slower if detritusis rich in lignin and chitin, and quicker, if detritusis rich in nitrogen and water-soluble substances like sugars. Temperature and soil moisture are the most important climatic factors that regulate decomposition though their effects on the activities of soil microbes. Warm and moist enviroment favour decomposition wheres low temperature and an anaerobiosis inhibit decompositon resulting in build up of organic materials.

Two types of Humus –
(i) Mor (Coarse terxtured humus) – It is raw humus and is formed in acidic soil (PH - 3.8 – 4.0) in which decomposition of litter is slow because it has less number of decomposer organism.

(ii) Mull – This is completely decomposed litter i.e. humus because rate of decomposition is fast due to high PH of soil.


It is of three types

Holard – The total amount of water present in the land is called holard.

Hygroscopic water – A thin layer of water attached tightly to soil particles due to forces of cohesion or adhesion. This is non available to plant.

Gravitational water – The water that moves downwards due to gravity, it is non available to plant.

Capillary water – Inter particle spaces or porous spaces act as minute capillaries and some amount of  water is present in these capillaries due to surface tension, This is capillary water. Itis theonly formof soil water which is available to.plant.

Combined water – The water which is present as hydroxides of iron, aluminium silicon, etc. is called
combined water.

Some Terminology :
1. Field capacity – When soil holds all the water except gravitational water, it is known as field capacity. It is upper limit of water availability.

Field capacity = Holard – Gravitational water,
Field capacity = Capillary water + Hygroscopic water + Combined water

2. Water holding capacity (Storage capacity) –

W.H.C. = Holard – (Hygroscopic water + Combined water + Gravitational water)

3. Wilting point :The lower limit of water availability of a soil is called wilting point.

Note :
1. The water potential of soil at field capacity is (–0.01 Mpa) [Mpa = Mega pascal]

2. The water potential of soil at wilting point is (–1.5 Mpa)

3. Amount of water in soil measured by tensiometer.


Abiotic Factors & Responses to Abiotic Factors Notes | Study Biology Class 12 - NEET

Note : A Horizon + B Horizon = Solum

Type of soil on the basis of the mode of their formation –

(1) Residual soil – The soil in which soil formation i.e., weathering and pedogenesis, occurs at same place.

(2) Transported soil – Soil in which weathering process occurs at one place and pedogenesis occurs at another place.

(a) Colluvial soil → Itisbroughtbygravity.

(b) Alluvial soil → It is brought throughwater, it is highly fertilesoil.

(c) Glacial soil → Soil isbrought by ice.

(d) Eolian soil → This soil is brought through wind.



In the tropical area due to high temperature, high rainfall, litter is decomposed very rapidly in A layer. Due to mineralization of litter Al and Fe are liberated in the upper layer (A - layer) of soil, colour of this soil becomes redish-brown, this process is known as laterization.

In temperate area temperature is low and high humidity occurs. Humus and minerals contents dissolve and percolate with water and are leached from A layer to B layer. Due to loss of chemicals the colour of soil of A layer (horizon) turns to light ash colour. This process is known as podsolization and soil is known as podosols.


In tundra region due to low temperature and humid condition humus is formed in less quantity and moves slowly in B layer. So colour of B layer becomes blue – grey due to deposition of Fe salt. This process is known as gleization and soil is known as gleys.

Note :

  • Best pH of the soil for cultivation of plant is 5.5 – 6.5
  • Excess water produces salinity problem in soil.
  • Calcifuge plants → Those plants which can grow in little amount of calcium in soil
    (pH – 3.8 to 4.0) e.g. Rhododendron, Rumax etc.
  • Calcarious soil → Soil having excess of calcium carbonate.
  • Alkaline soil can be corrected by adding gypsum (CaSO4) and heavy irrigation whereas acidic soil can be corrected by adding lime Ca(OH)2
  • Availability of nutrients from the soil is related with pH of soil.

Note :

  • Black cotton – Soil of Maharashtra are commonly known as regur soil
  • Sierozen – Light coloured soil of arid region
  • Bog soil – Soil of marshes and swamps
  • Solnetz/Solansltz – Black alkaline soil
  • Peat soil – In which 90% humus in present.
  • Solonchak – White alkaline & Saline soil


1. Precipitation – It is a source of soil water. The hydrological cycle is the movement of water between earth, air and atmosphere.

Forms of precipitation –

(i) Drizzle – Minute drops of water floating in air
(ii) Rain – Large drops of water
(iii) Snow – Water in solid form
(iv) Sleet – Ice in minute granular from
(v) Hails – Large balls of ice

2. Humidity –
Atmospheric moisture in the form of invisible vapour is known as humidity. It is expressed in terms of relative humidity.

Relative humidity –
The amount of moisture in air as percentage of the amount which the air can hold at saturation at the existing temperature. Abiotic Factors & Responses to Abiotic Factors Notes | Study Biology Class 12 - NEET

Note :
R.H. is measured by Psychrometer.
Wind speed is measured with the help of anemometer.

3. Topography –
It includes the physical features of the earth like altitude, slope, exposure, mountain chains valleys plants. It affects distribution of organism by influencing the climatic factor like light, wind, rainfall etc.

The document Abiotic Factors & Responses to Abiotic Factors Notes | Study Biology Class 12 - NEET is a part of the NEET Course Biology Class 12.
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