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Nutrition in Plants

Nutrition in Plants Synopsis

 Synopsis

 

Nutrition

  • Nutrition is a process of intake as well as utilization of nutrients by an organism.
  • Nutrients are the substances required by our body for its growth, repair, work and maintenance.
              

Autotrophic Nutrition

  • It is the mode of nutrition in which organisms synthesise their own food from simple inorganic substances such as water and carbon dioxide.
  • Examples: Green plants and algae are autotrophs.


Heterotrophic Nutrition

  • It is the mode of nutrition of organisms which cannot synthesise their own food, but they are dependent on other organisms for food.
  • Examples: yeasts, fungi, bacteria, human beings, tiger, monkey, birds, lion, cow etc.
  • Heterotrophs are classified into different type basis the nature of food consumed and the mode of feeding.



 

Photosynthesis

  • Photosynthesis is a physiological process by which plant cells containing chlorophyll produce food in the form of carbohydrates using carbon dioxide, water and light energy. Oxygen is released as a by-product of this process.

Importance of Photosynthesis

  • It is the primary source of food for all living organisms on the Earth.
  • It adds oxygen to the atmosphere to compensate for the oxygen being used in the respiration of organisms and burning of fuels.
  • It influences the productivity of agricultural crops.

 

Chloroplast

  • Chloroplasts are the green plastids present in the green stems, sepals and leaves.
  • They are the site of photosynthesis.
  • Each chloroplast is bounded by a double membrane envelope.
  • Its matrix is filled with a proteinaceous, colloidal matrix called stroma and an elaborate system of membranous lamellae called thylakoids. 
  • The stroma contains enzymes, DNAs, RNAs and 70S ribosomes.
  • They occur as flattened sacs of two types—grana lamellae and stroma lamellae.
  • Thylakoid membranes possess photosynthetic pigments–chlorophyll a, chlorophyll b, carotenoids, cytochromes (b and f), ATP synthetase and the enzymes required in the light reactions
    of photosynthesis.
  • Thylakoids are the site of light for photosynthesis.

 

Pigments Involved in Photosynthesis

  • Chlorophylls are green photosynthetic pigments present in all photoautotrophs.
  • These pigments absorb light near both ends of the visible spectrum—the violet–blue and red light—and reflect green light because of which they appear green.
  • In higher plants, chlorophylls are of five different types:

  • Chlorophyll a is bluish green, chlorophyll b is yellow green.
  • Chlorophyll c is found in brown algae, diatoms and dinoflagellates.
  • Chlorophyll d is found in red algae.
  • Chlorophyll e is present in Xanthophyceae along with chlorophyll a.

 

Photosynthetic Units and Photosystems

  • Green plants have two types of photosynthetic units called photosystems or pigments systems I and II.
  • Each photosystem contains 250–400 pigment molecules.
  • The pigments are organised into two light harvesting complexes or antennae within photosystem I and photosystem II.
  • The pigments help to make photosynthesis more efficient by absorbing the different wavelengths of light.
  • The single chlorophyll a molecule forms the reaction centre.
  • The reaction centre is different in PS I and PS II.
  • In PS I reaction centre chlorophyll a has an absorption peak at 700 nm while in PS II an absorption peak is at 680.
  • Hence, PS I reaction centre is called P700 and PS II reaction centre is called P680

 

Electron Transport Chain

                                     

   

 

  

   

  

  • The whole scheme of the transfer of electrons, right from PS II uphill to the acceptor, down the electron transport chain to PS I, excitation of electrons, transfer to another acceptor and finally downhill to NADP+ causing it to be reduced to NADPH + H+ is called the Z scheme.

 

Photolysis of Water

  • The electrons that were removed from PS II during the electron transport chain are replaced by the photolysis or splitting of water.
  • During this process, water molecules split into H+ and OH- ions.
  • The OH- ions being unstable in nature combine to form water and molecular oxygen along with the release of electrons.

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Light Reaction 

  • The light reaction of photosynthesis occurs only in the presence of sunlight in the thylakoid membranes of grana and stroma lamellae.
  • Light reaction is also called the photochemical reaction or Hill reaction or photophosphorylation.

 

Absorption of Light

  • The various pigments of photosystem II (P680) and photosystem I (P700) absorb sunlight of different wavelengths.

 

Excitation of Reaction Centres

  • The reaction centres receive light energy from their respective photosystems, become activated and their electrons become excited.
  • The electrons are transferred to primary electron acceptor.
  • Because of the transfer of electrons, P680 and P700 become electron-deficient or highly oxidised.

 

Photosynthetic Electron Transport and Formation of Assimilatory Power

  • The electrons then move downhill along the electron transport chain from phaeophytin (primary electron acceptor) to plastoquinone (PQ), cytochrome complex (cyt b6 and cyt f) and plastocyanin to finally reach the reaction centre of PS I (P700).
  • During this transfer, electrons lose energy which is used to generate ATP.
  • The electrons then move downhill along the electron transport chain from phaeophytin to plastoquinone (PQ), cytochrome complex (cyt b6 and cyt f) and plastocyanin to finally reach the reaction centre of PS I (P700).
  • During this transfer, electrons lose energy which is used to generate ATP.
  • From here, the electrons move down to ferredoxin and then to ferredoxin-NADP oxidoreductase, where NADP is reduced to NADPH.
  • This causes reduction of CO2 to sugar during the dark reaction of photosynthesis and provides the necessary energy.

 

Photophosphorylation

  • The process of ATP formation in chloroplast in the presence of light energy is called photophosphorylation.
  • After losing their energy during transport, electrons can be used in the following ways:
  1. Electrons from photosystem II are used by photosystem I. This is called non-cyclic photophosphorylation.
  2. Electrons from photosystem II return to the same photosystem. This is called cyclic photophosphorylation.

 

Biosynthetic Phase/Dark Reaction 

  • The dark reaction is the second phase of photosynthesis which occurs in the stroma of chloroplasts.
  • The reaction is also known as the biosynthetic phase or Blackman’s reaction or carbon
    dioxide fixation.
  • It uses ATP and NADPH2 for the fixation and reduction of CO2 to form carbohydrates.

Types of CO2 Fixation


Calvin Cycle or C3 Cycle

  • Each Calvin cycle is completed in three phases—carboxylation, reduction and regeneration.