Describe the Mechanism and the Factors affecting Photosynthesis.

All metabolism begins with the chemical raw materials that are collectively referred to as food . Some organisms are capable of synthesising organic materials from inert inorganic substances . Such organisms are known as autotrophs . Whereas , others depend upon autotrophs directly or indirectly for the purpose of food and known heterotrophs . The autotrophic organisms are further categorized as 
a) photoautotrophs and 
b) chemoautotrophs . 
PHOTOSYNTHESIS ICSE Class 10th Biology Chapter 5

Photoautotrophs are the green coloured plants and some algae . They synthesise living matter ( organic compounds ) from the inert inorganic materials in the presence of chlorophyll and solar energy . Whereas , chemoautotrophs utilize energy released by their chemical processes , for the purpose of synthesising their food , e.g. sulphur bacteria .

 On the other hand , heterotrophs depend upon the autotrophs directly or indirectly for the purpose of their food . facile Photosynthesis is a physiological process by which green plants in the presence of sunlight and chlorophyll prepare food in the form of carbohydrates using carbon dioxide and water . 

RAW MATERIALS REQUIRED FOR PHOTOSYNTHESIS 

Four important materials are required for the process of photosynthesis . These are :

  1. Carbon dioxide ( CO2 ) , 
  2. Water ( H20 ) , 
  3. Radiant ( Solar ) energy , and 
  4. Chloroplasts . 

(1) Carbon dioxide ( CO ) : 


It is the raw material for the formation of glucose . Plants obtain CO2 from the atmosphere through the stomata . The stomata open or close depending upon the need of CO2 by the plants .

(2) Water ( H20 ) : 

It is the second raw material for the process of photosynthesis . Plants get water from the soil by absorption.

(3) Radiant energy ( Solar energy ) : 

The Tradiant energy is trapped from the solar energy ( sun ) by the green coloured pigment chlorophyll - present in the chloroplast .

(4) Chloroplasts : 

These are green coloured plastids found in the green coloured part of plants . They are very abundant in the leaves ( normally 40-50 in each cell ) . In the leaves they are present mainly in mesophyll cells located between upper and lower epidermis ( i.e. , in palisade cells and spongy cells ) . There may be 50,00,000 chloroplasts per square mm of leaf surface . Chloroplasts are minute , microscopic cell organelles of plant cells . Each chloroplast can be differentiated into three parts i.e.
a) membranes
b) stroma and
c) thylakoids.

( a ) Membranes : The chloroplast is covered by a double membraned envelope . Each membrane is  lipo proteinaceous in chemical nature and selectively permeable . In - between the two membranes is present a narrow space called periplastidial space .

( b ) Stroma : The chloroplast is filled with a colourless ground substance or living matrix called stroma .

( c ) Thylakoids : In the stroma are present flattened membranous sac - like structures called thylakoids . The thylakoids are present like the piles of coins . These piles of thylakoids are called grana . The grana are connected by stromatal or fret lamellae .

Chlorophyll : 

The vital pigment , is found in walls of thylakoids . The chlorophyll is a highly complex organic substance . It is made of carbon , hydrogen , oxygen , nitrogen and magnesium . There are five types of chlorophyll molecules out of which commonly occurring and most abundant are chlorophyll - a and chlorophyll - b .

The chlorophyll molecules are associated with the membranes of thylakoids . About 250 molecules of chlorophyll form one quantasome . It is capable of trapping one quantum of energy and converting into chemical energy . This process is called photochemical act .

In addition to chlorophyll molecules , the chloroplasts also contain accessory pigments carotenes and xanthophylls . These pigments help in the transference of light energy to chlorophyll - a . Chlorophyll is highly sensitive to light . Too much light may destroy it .

Mechanism of opening and closing of Stoma :

The opening and closing of stoma is regulated by turgidity of guard cells . The stoma is open when guard cells are turgid . On losing turgidity in guard cells the stoma gets closed .

According to sugar concentration theory ( Old one ) : 

During daytime the guard cells start photosynthesis . The sugar produced during this process increases the osmotic pressure . Due to increased osmotic pressure water enters in the guard cells because of endosmosis , from the adjoining cells . Hence , the guard cells become turgid and bulge outwards due to their thin outer wall thus widening the stomatal opening lying between them . 

According to potassium ion concentration theory ( New one ) : 

The opening and closing of stoma depend on the generation of potassium ion gradient . During day , photosynthesis occurs in chloroplast of guard cell which leads to the production of ATP in the light reaction phase . ATP pumps the potassium ions of adjoining cells into guard cells .

Increased potassium ion concentration in guard cells makes them hypertonic , hence , water from adjoining cells rushes inside and the cells become turgid . The guard cells move outwards to open out the stomatal pore . At night , potassium ions leak out thus reducing the turgor of the guard cells and the stomatal pore closes .

MECHANISM OF PHOTOSYNTHESIS

Mesophyll cells ( palisade and spongy cells ) in the leaves are the main centre of photosynthesis . During daytime the chlorophyll molecules trap solar energy . This trapped energy is utilized for the synthesis of organic compounds ( glucose ) using CO2 and H20 as inorganic ingredients .

Oxygen gas is evolved as a by - product of this process . The overall reaction that takes place in photosynthesis can be expressed as follows :
6CO2 + 12H20 → C6H12O6 + 602 + 6H20

The 6 molecules of water liberated in the equation above are those that are reformec during a chain of reactions and not out of the original ones .

FORMATION OF STARCH AND OTHER ORGANIC COMPOUNDS

The process by which several molecules of glucose are transformed to produce one molecule of starch is called polymerisation . The starch is insoluble in water . It is used for the storage of food in the plant cells . In some plants glucose is converted into sucrose or other sugars .

FATE OF END - PRODUCTS OF PHOTOSYNTHESIS

Photosynthesis results in the formation of three types of end - products :

  1. Glucose ( C6H1206 ) - a simple sugar 
  2. Water 
  3. Oxygen . 
Some of the glucose produced during photosynthesis is oxidized immediately for the release of energy , required for the metabolism of cells . The rest of the glucose is converted into insoluble starch . It is stored in the cells as reserve food material . It is also transported to non - photosynthetic cells in the form of sucrose .

Water produced during photosynthesis is reutilized for the continuity of this process .Oxygen produced during photosynthesis is partially utilized by the plant cells for oxidation . Rest of the O2 is released into the atmosphere through the leaf surfaces , specially through stomata . The oxygen released helps to maintain the air composition constant .

This 02 is used by the animals and plants for the purpose of oxidation of glucose during respiration . The process of respiration releases CO2 which is utilized by the plants in photosynthesis . It means respiration and photosynthesis are antagonistic to each other and help to maintain O2 and CO2 concentration in the air constant .

PRICE OF PHOTOSYNTHESIS 

Stomata are primarily meant for the diffusion of CO2 which is to be used for photosynthesis . But when CO2 diffuses in at the same time , a large amount of H2O is lost due to transpiration . So , we say that the plants have to pay the price of photosynthesis in the form of transpiration .

Fate of carbohydrates formed in photosynthesis 

Carbohydrates formed in photosynthesis are used up in three main ways :

  1. Respiration : Most cells receive sugar through phloem . This is broken down during respiration to liberate energy the cells require . 
  2. Food storage : Some of the sugar may be stored for future use either in organs , such as bulbs , tubers , corms or rhizomes or for the next generation in seeds or fruits . Usually the sugar is converted into starch for storage but quite often it is stored as sucrose as in sugarcane . In fact , many of our vegetables are really plant food storage organs . For example , potato tubers are storage stems , carrot , turnip and beetroots are storage roots . In many cases , carbohydrates are converted to oils for storage e.g. , mustard oil and groundnut oil . 
  3. Growth : Sugar is also transported to actively growing parts of the plant where these provide energy and are used in the formation of cellulose , fats and proteins . 

TRANSPORTATION ( TRANSLOCATION ) OF ORGANIC COMPOUNDS IN THE PLANTS

Primarily , the process of photosynthesis i.e. , formation of glucose and other organic compounds takes place in the cells of leaves . But these organic compounds are needed by all the cells of a plant . Formation of glucose in the leaf cells is continuous and very rapid . It cannot be transported to other cells with the same rapidity . 

That is why , most of the glucose is converted into insoluble starch for the purpose of temporary storage. At night the stored starch is reconverted into soluble sucrose . The soluble sucrose in the form of solution is transported ( translocated ) to all the cells of the plant . 

Sucrose passes through the veins of leaves down through the phloem of the stem and roots . The sucrose is converted into glucose in different cells . It is partially oxidized in respiration to release energy . Rest of glucose is reconverted into starch for the purpose of storage . 

ADAPTATIONS IN LEAF FOR PHOTOSYNTHESIS 

  1. Large surface area : The surface area of the leaves is very large . It helps in the absorption of maximum light energy from the sunlight .
  2. Leaf arrangement : The leaves are arranged on the stem in such a way ( right angle to light source ) so that maximum surface area of leaves is exposed to sunlight . 
  3. Chloroplasts containing chlorophylls are concentrated in the upper layers of the leaf cells , near the cell walls . It helps in trapping maximum radiant energy from the sunlight quickly . 
  4. Numerous stomata in the leaves allow quick exchange of O , and CO2 through them . 
  5. Thinness of leaves further enhances the rapid transport of materials between adjoining cells . 
  6. Network of veins in the leaves helps in the rapid transport to and from the mesophyll cells of leaves . 

FACTORS AFFECTING PHOTOSYNTHESIS 

I. External Factors : 

  1. Light : Radiant energy of light is an important factor in three aspects i.e. , intensity , quality and duration . About 0.2-1 % of the total solar radiations is utilized in photosynthesis . Within limits , increase in light intensity increases the rate of photosynthesis . At very high light intensity , the photosynthetic apparatus may get damaged . This phenomenon is called solarization .Photosynthesis is greatly influenced by light quality . Blue and red regions of the visible spectrum are most effective and green the least .The process of photosynthesis is also directly proportional to the duration of light . 
  2. Carbon dioxide : If light intensity is good and the temperature is optimum , then increase in CO2 concentration increases the rate of photosynthesis . 
  3. Temperature : Temperature at which the rate of photosynthesis is maximum is called optimum temperature . It is 35 ° C . The rate of photosynthesis goes on increasing from lower temperature to 35 ° C . It again starts decreasing above 35 ° C and stops above 40 ° C . A rise of 10 ° C below the optimum temperature doubles the rate photosynthesis , provided the light intensity is high . It is called Q10 law .
  4. Water : Under normal conditions water has no direct bearing on the rate of photosynthesis . But , if the water supply is poor , then a decrease in photosynthesis has been observed . 

II . Internal Factors : 


  1. Chlorophyll : Deficiency of magnesium causes loss of chlorophyll , hence , leaves are not able to absorb sunlight ( Solar energy ) . 
  2. Leaf structure : The size of leaf , distribution of stomata and thickness of cuticle influences the amount of CO2 and light entering the leaf , hence , affects the rate of photosynthesis .

SIGNIFICANCE OF PHOTOSYNTHESIS 

Photosynthesis is the most important and basic process which sustains life on this earth . It has manifold significance , such as :

  • It synthesises food from inorganic substances . This food becomes the ultimate source of energy and life for all the living organisms . This helps in maintaining food chain amongst the producers , herbivores and carnivores . The food chain may have a number of stages but the starting point is always a plant ( producer ) . The non - green plants like fungi and bacteria obtain their food from decaying organic matter in their environment . Hence , it is very rightly said that " All flesh is grass " . 
  • It is the only known method which releases oxygen in the atmosphere . It keeps the O2 concentration constant . The consumption of O2 by living organisms during respiration is compensated by photosynthesis . 
  • It also helps to keep the CO2 concentration in the atmosphere constant . The CO2 being released due to the respiration of living organisms is being incorporated in glucose during photosynthesis . 
  • Coal , petroleum and natural gas are fossil fuels . These have been produced by the application of heat and compression on the past plants . 
  • All useful plant products , such as , timber , rubber , resins , drugs , oils , fibres , etc. are derived from the process of photosynthesis .

CARBON CYCLE

Carbon is the building material of all the organic compounds found in the living cells , e.g. carbohydrates , lipids , proteins and nucleic acids . In the biosphere , there are 4 sources of carbon :

  1. Carbon dioxide present in the air . 
  2. Carbon dioxide present in air gets dissolved in water easily . 
  3. Carbonates of the earth's crust derived from rocks . These give rise to carbon dioxide by the chemical changes . 
  4. The fuels , such as , coal and petroleum products , produce carbon dioxide after burning.

The carbon cycle can be defined as a series of chemical reactions in which carbon as an element from carbon dioxide is removed from the atmosphere , used up by living organisms in their body processes and is again returned to the atmosphere .

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