12. Photosynthesis topic for XI std

 

12.PHOTOSYNTHESIS

INTRODUCTION

·       All living organisms require energy to carry out their different life activities. This energy directly or indirectly comes from the sun.

·       The green plants are unique in the respect that they synthesize food in the presence of sunlight from simple substances like carbon dioxide and water.

"Photosynthesis is a Physicochemical or photo-biochemical process (anabolic & endergonic) in which organic compounds (carbohydrates) are synthesised from the inorganic raw material (H₂O & CO₂) in presence of light & pigments. O₂is evolved as a byproduct".

Light energy is conserved into chemical energy by photosynthesis.

·      90% of total photosynthesis is carried out by aquatic plants.

·      First true and oxygenic photosynthesis started in cyanobacteria (Blue - green algae).

·      In the Cuscuta (parasitic) and fungi (achlorophyllous), photosynthesis is absent.

·      Euglena is a photosynthetic organism and is the link between animals and plants. Though, they are photosynthetic in the presence of sunlight, when deprived of sunlight they behave like heterotrophs by predating on other smaller organisms.

·      Roots of Tinospora and Trapa are assimilatory or photosynthetic.

·      Atmosphere contains only about 0.03percent carbon dioxide by volume. This small percentage represents 2200 billion tons of CO₂in the atmosphere. The oceans contain over 50 times by amount of atmospheric CO₂ in the form of dissolved gas or carbonates. From these two sources, about 70 billion tons of carbon is fixed by the green plants annually.

Chloroplasts / site of photosynthesis

Chloroplasts are green plastids. These are the special protoplasmic organelles present only in the green cells of plants. Chloroplasts act as photosynthetic apparatus.

The entire process of photosynthesis is completed in each chloroplast. Hence these are the site of photosynthetic reactions.

i)           It is the structural and functional unit of photosynthesis which is studied by Willstatter et al.

ii)         It is bounded by double lipoprotenous membranes called peristomium or chloroplast envelope (40 -60 A°)

iii)       The outer membrane is thin and permeable to no. of solute and metabolites

iv)       The inner membrane is thick and selectively permeable.

v)         In between these two membranes a space is present known as periplastidal space (100 -200 A°)

vi)       Inside the peristomium a space or cavity is present which is filled by coloureless hydrophilic matrix called as stroma.

vii)      The matrix having no. of granules, enzymes, DNA, RNA, 70’S Ribosome.

viii)    The DNA of Chloroplasts is called chloroplast DNA or ctDNA or Plastidome.

ix)        Here, stroma is the site for the dark reaction.

x)         Inside the stroma 40 to 100 grana are present

xi)        Each granum is unit of plates which are arranged one upon another and such grana are connected with each other by thin lamellae called as intergrana lamellae or stroma lamellae or fret channels having the enzyme carboxylase useful for CO2 fixation.

xii)      Each disc like structure of grana separately called as Thylakoids. In prokaryotes like cyanobacteria, purple bacteria, etc., thylakoids are present but they lie naked in the cytoplasm

xiii)    According to the quantasomal theory the thylakoid having a membrane called as thylakoid membrane or fret membrane.

xiv)    The thylakoid membrane is made up of ultra-microscopic hemispherical units called quantasome.

xv)      The term quantasome is given by Park and Biggins.

xvi)    Each quantasome internally having 200 – 250 pigment systems like –

                Chlorophyll

                Carotenoids

                Phycobilins

•  Chlorophylls are tetrapyrrolic magnesium (Mg) porphyrin compounds.

·   Chemically chlorophyll molecule consists of two parts head of tetrapyrrol the Porphyrin ring and a long hydrocarbon tail called phytol attached to the porphyrin group.

•  Chlorophyll molecule has a Mg-porphyrin head and an alcoholic phytol tail. Head is hydrophilic and phytol tail is hydrophobic in nature.

 

     PHOTOSYNTHETIC PIGMENTS

         Chlorophyll a – C55H72O5N4Mg (CH3 group at IIIrd C of IInd pyrrole ring)

Chlorophyll b – C₅₅H₇₀O₆N₄Mg (CHO group at III^rd C of II^nd pyrrole)

Chlorophyll c – C₅₅H₃₂O₅N₄Mg,

Chlorophyll d – C₅₄H₇₀O₆N₄Mg,

Carotenes – C₄₀H₅₆

Xanthophylls – C₄₀H₅₆O₂.

·         Chlorophyll-a and carotenes are universal pigments, which are found in all O₂ liberating cells.

·         Chlorophylls are soluble only in organic solvents like ketones, ethers etc.

·         Stroma lamellae/stroma thylakoids lack PS II and enzyme NADP reductase.

·         By paper chromatography/chromatogram, produces different pigment colours which appear as:

·      Chlorophyll a - blue-green.

·      Chlorophyll b - yellow green.

·      Xanthophyll - yellow

·     Carotenoids - yellow to yellow – orange

 

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