SIGNIFICANCE OF PHOTOSYNTHESIS

  • Photosynthesis is the most significant, natural process during which light energy is trapped and converted into chemical form of energy.
  • Photosynthesis involves synthesis of food. The plants use this food as a source of energy and this food is made available for all heterotrophic organisms.
  • All living organisms, directly or indirectly depend on this process for energy. (Except chemoautotrophs.)
  • Photosynthesis releases O2, which is indispensable for life, maintains balance of O2 and CO2 in the atmosphere and helps to purify air.
  • Oxygen in the atmosphere may be used for ozone formation.
  • Photosynthesis being greater than respiration, biomass is built in plants and this provides feeding, hiding, nesting places for animals.
  • Various fossil fuels like coal, petroleum and natural gas are products of photosynthesis occurred in the geological past.
  • The plant products such as, timber, cotton, alkaloids, resins, gums, tannins, steroids, rubber, etc. are the products of photosynthesis, and are economically important.

 

FACTORS AFFECTING RATE OF PHOTOSYNTHESIS

Yield of crop plants depends on the rate of photosynthesis.

Photosynthesis is affected by:

  • Internal factors (number, size, age and orientation of leaves; amount of chlorophyll, rate of translocation of food etc.)
  • External i.e. environmental factors such as; availability of light, CO2 concentration, temperature and soil water.

External Factors

Light:

There are three aspects of light as a factor; light quality, intensity and duration.

Rate of photosynthesis is more in the red and blue region.

There is a linear relationship between light intensity and rate initially.

At higher light intensity further increase is not possible and at very high light intensity, beyond a point, it causes breakdown of chlorophyll and decrease in the rate.

CO2 concentration:

Carbon dioxide is the major limiting factor for photosynthesis.

The CO2 concentration is very low in the atmosphere between 0.03 and 0.04%.

Increase up to 0.05 % can cause an increase in the rate; beyond this it can become damaging over longer periods.

 

Temperature:

The dark reactions i.e. light independent reactions are controlled by enzymes and are more sensitive to temperature than light reaction.

The temperature optimum for photosynthesis depends on the habitat the plants are adapted to; tropical plants have a higher temperature optimum than temperate plants.

 

Water:

Though water is one of the raw materials for photosynthesis, the effect of water as a factor is more through its effect on the plant and not directly on the rate of photosynthesis.

Water stress causes closure of stomata; leading to low avail-ability of CO2. It brings about loss of turgidity in leaves and wilting.

 

Law of Limiting Factors

Though all the environmental factors simultaneously affect the rate of photosynthesis, usually one factor is the major cause or is the one that limits the rate.

Law of Limiting Factors- F.F. Blackmann, in 1905 proposed that at any given time rate of photosynthesis depends on factor which is the lowest.

For example,

if optimum light and CO2 is available and temperature is very low, then rate of photosynthesis will be controlled by temperature;

i.e. it will increase with the increase in temperature, and then there will be a steady state.

Further increase in rate is possible, only if there is increase in CO2 concentration.

Rate will increase with the increase in CO2 concentration, but again there will be a steady state and further increase is possible, if availability or intensity of light increases.

Thus at a time only one factor controls the rate and is called limiting factor.

This law of limiting factors is applicable to any biochemical reaction;

 

"If a chemical/biochemical process is affected by more than one factor, then the rate will be determined by the factor which is nearest to its minimal value."

It is the factor which directly affects the rate if its quantity is changed."