How Plants Carry Out Photosynthesis And What Is The Mechanism Of Photosynthesis?


Photosynthesis is a process by which all green plants prepare their own food. In this resource I discussed about what raw materials are required for photosynthesis, how the plant is able to gather these materials and what is the mechanism of photosynthesis. In this resource, I also discussed about the final fate of the reactants and products of photosynthesis reaction.

Introduction


Green plants are the only organisms present in nature which can prepare their own food through a process called Photosynthesis. Due to this reason only the green plants are known as autotrophs (Auto- self, trophs- nutrition) and also they are the primary producers which can provide food to all living organisms directly or indirectly. Photosynthesis is a process in which with the help of chlorophyll pigment present in the green leaves, energy trapped from solar radiation by the chlorophyll pigment, water absorbed from the soil by the roots and carbon dioxide obtained from the atmosphere will help the green plants to prepare their own food. The term Photosynthesis was first coined by Barnes in 1898.

Photosynthesis is a chemical process in which so many steps of chemical reactions are involved. Every step of these chemical reactions require an enzyme catalyst. The enzyme catalysts which are required for these chemical reactions are made available from the place (chloroplast of mesophyll cells of leaves) where this process occur. Chemically speaking that photosynthesis is a oxidation-reduction (Redox) reaction. In photosynthesis process, water molecules get oxidized while carbon dioxide molecules get reduced and hence this reaction is known as Redox reaction. Photosynthesis is also an endergonic process as this process take place through intake of energy from solar radiation. Biologically speaking photosynthesis is an anabolic process. In anabolic process complex molecules are synthesized from simple molecules through chemical reactions. Thus in photosynthesis, from simple molecules like carbon dioxide and water, complex carbohydrate molecule starch is formed.

The overall chemical reaction of photosynthesis can be represented as follows:

6Co2 + 12 H2O------------> C6H12O6 + 6O2 + 6H2O (in the presence of Sunlight and chlorophyll)

What are the raw materials required for photosynthesis and where they are available?


Leaf is the chief organ of photosynthesis in a green plant. The other parts which are green in color also to some extent can carry out photosynthesis since they contain chlorophyll pigment in them. The raw materials which are required for photosynthesis process are chlorophyll pigment, Sunlight, Carbon dioxide, water and minerals.

1. Chlorophyll


Chlorophyll is a green colored pigment present in the chloroplasts of mesophyll cells of green leaves. Nine different varieties of chlorophylls are present in green leaves, of which chlorophyll-a and chlorophyll-b are the most abundant type of chlorophylls present in a leaf. Along with these two main pigments, some accessory pigments like xanthophyll and carotenes also associate with them. The molecules of main pigment as well as the molecules of accessory pigments together form a photosynthetic unit in the chloroplasts of mesophyll cells. 250 molecules of all pigments both main pigments as well as accessory pigments) together form a photosynthetic unit called quantasome. The main working center of quantasome is formed by chlorophyll-a and chlorophyll-b. Each pigment present in a photosynthetic unit has an ability to absorb energy from a particular wavelength of light. The different pigment molecules present in the photosynthetic unit transfer the energy they have collected to the main photosynthetic center (chl-a and chl-b) from which the photosynthesis proceeds. A chlorophyll molecule basically include porphyrin (pyrrole rings) as its head and a long phytol tail. A magnesium metal atom is present at the center of its head region.

Where the chlorophyll pigment is actually located in a leaf?


Location of Chloroplasts in a leaf tissue
Location of Chloroplasts in a leaf tissue (Courtesy: Wikipedia)

Chlorophyll pigment is present in cell organellels called chloroplasts. These chloroplasts are present in the mesophyll tissue of a leaf present in between the upper epidermis and lower epidermis of a leaf. This mesophyll tissue is found in the form of palisade parenchyma and spongy parenchyma. The photosynthetic machines chloroplasts are located in these mesophyll cells only. In each mesophyll cell of leaf, usually contain about 100 chloroplasts.

Structure of Chloroplast


Structure of Chloroplast
Structure of Chloroplst Courtesy: Wikipedia)

Chloroplasts are green pigment containing cell organellels situated in the mesophyll tissue of a leaf. Chloroplasts are usually oval, spherical or elliptical in shape. Each chloroplast is surrounded by a double membraned layer with an outer and inner layers. In side the chloroplast a colorless fluid matrix called stroma is present. In the fluid matrix flattened double membraned sacs are present called thylakoids. A stack or pile of thylakoids is known as granum. The tube like structures which interconnect grana are known as frets or grana lamellae. Inside the thylakoid membranes chlorophyll pigment is present like a paint like coating. In the grana region of chloroplasts light reactions of photosynthesis occur and dark reactions of photosynthesis occur in the stroma of chloroplast.

2. Sunlight


One other important source which is required for photosynthesis is Sunlight. The energy required for the process of photosynthesis is trapped by the green plants from solar radiation. A white light ray of Sun when passed through a prism is split into seven different colors (VIBGYOR). These seven colors we obtain from a ray of Sun is known as visible part of the light or visible spectrum. But in a ray of sunlight some other invisible wavelengths of light rays are also present is known as invisible part of the spectrum. This invisible part of the spectrum of light is not useful for photosynthesis.
Visible Spectrum of Light
Visible Part of the Spectrum (Courtesy: Wikipedia)

Each colored ray of visible spectrum has its own wave length. Different colors present in the visible spectrum have a wavelengths range from 330 nano meters to 750 nano meters. Red part of the visible spectrum has the highest wavelength and blue part of the visible spectrum has shorter wavelength. Shorter wave lengths have more energy associated with them but they have lesser penetration power. Longer wavelength of light will have less energy associated with them but have more penetration power. Visible Wave length of light is measured in nano meters. Of all the different colored rays of visible spectrum red and blue colors are the most useful for photosynthesis. The reason for this is the chlorophyll pigments present in grana can utilize these wavelengths light more the than other wavelengths of light. Green part of the visible spectrum is not useful for photosynthesis because no chlorophyll pigments present in leaf can utilize this wavelength of light. The energy trapped by chlorophyll will be utilized for breaking down of water molecules in the light reaction of photosynthesis. Each ray of Sunlight include discrete packets of small units of energy called photons and the quantity of energy present in them is called quanta.

3. Carbon dioxide


In the atmosphere carbon dioxide is present as a gas of about 0.03% by volume of air. Carbon dioxide is the main reactant, enters into the site of photosynthesis i.e. into the mesophyll cells where chloroplasts are present through a physical process diffusion. Opening of stomata, mostly present in the under surface of leaves will aid in diffusion of carbon dioxide from its higher concentration (atmosphere) to the lower concentration of the leaf.

4. Water and minerals


Water is one of the most important resource for photosynthesis and is supplied through the xylem tissue from the roots of the plant to the mesophyll tissue of leaves. Along with water, minerals are also are transported to the different parts of the plants. In photosynthesis water is the source of hydrogen which reduce carbon dioxide to form the carbohydrate glucose. The minerals taken along with water will work as co-factors for enzyme catalysts in the chemical reactions of photosynthesis.

Mechanism Of Photosynthesis


Photosynthesis is a many stepped chemical reaction. Each step of this process requires an enzyme catalyst. The chemical reactions of photosynthesis can be divisible into two parts, light reactions and dark reactions. The site for light reactions is the grana of chloroplasts and the dark reactions occur in the stroma of chloroplasts of mesophyll cells. The enzyme catalysts required for light reactions and dark reactions are available in the grana and stroma regions of the chloroplast.

Light Reaction


Light reaction occurs in the grana of chloroplast. Light reaction occurs only in the presence of light and thus it is a light dependent reaction. Scientists have proved that photosynthesis occur under natural light but also will be carried out in artificial source of light. Scientists also determined that the rate of photosynthesis is higher in intermittent light than in continuous supply of light.


1. When the leaf is exposed to bright Sunlight during day time, the chlorophyll molecules (Quantasome-unit of photosynthesis) present in the grana of chloroplast get excited after absorbing photons from the radiant light energy.

2. By using this light energy absorbed by chlorophyll, water molecules are the split into hydrogen and hydroxyl ions. This splitting of water molecules into hydrogen and hydroxyl ions by using the light energy in the grana of chloroplasts is known as Photolysis.

4H2O----------->4H+ + 4OH- (in the presence of light and chlorophyll)

3. In the next step, the hydroxyl ions produced through photolysis get oxidized to water and oxygen. The oxygen evolved during photosynthesis is purely through splitting of water only but not from carbon dioxide which was a false notion held by many scientists in the past. But in the later times scientists proved through many experiments that the oxygen released during photosynthesis is through the splitting water molecules in the light reaction of photosynthesis.

4OH- ---------->4(OH) + 4e- (Electrons)

4OH------------>2H2O + O2 (evolved)

4. In the next step, the hydrogen ions formed in the photolysis of water helps in reducing a chemical compound NADP (Nicotinamide adenine dinucleotide phosphate) present in the grana of chloroplast into NADPH by using electron energy.

NADP + e- + 2H+ ------------> NADPH

5. Another chemical compound present in grana, ADP (Adenosine diphosphate) combines with inorganic phosphate (iP) by using the electron energy e- formed in photolysis of water and thus an energy rich compound or molecule ATP (Adenosine triphosphate) is formed. Thus the formation of ATP molecules from ADP and iP by using light energy in the light reaction of photosynthesis is known as Photophosphorylation.

In light reaction of photosynthesis two important chemical compounds are formed, NADPH and ATP. These two compounds (NADPH and ATP) are very essential for the dark reactions of photosynthesis to start and thus these two compounds together known as assimilatory power or the driving force for the dark reaction.

Dark Reaction or Blackman's reaction or Biosynthetic phase or Calvin cycle


Dark reaction occurs in the stroma part of chloroplast. The chemical compounds and enzyme catalysts required for these reactions are present in the stroma region of chloroplast. The name Dark reaction got for these reactions is not because these reactions occur at dark or night time but because for these reactions to occur there is no need of light energy. Thus Dark reaction is also known as light independent reaction. The Calvin cycle or the mechanism of Dark reaction was first discovered by Melvin Calvin and Benson. In 1961, Calvin got Noble prize for this discovery.

1. The carbon dioxide which entered into the mesophyll cells and then into chloroplast is picked up by RUBP (Ribulose bisphosphate) and thus RUBP is a carbon dioxide acceptor.

2. The carbon dioxide is then combine with assimilatory power (NADPH and ATP) formed in light reaction. Carbon dioxide get reduced to glucose by using the hydrogen source from NADPH and the energy required from ATP molecules formed in light reaction. Immediate product of photosynthesis is glucose and is transported in fluid form through the phloem tissue to all parts of the plant. The overall Dark Reaction of photosynthesis can be represented as follows-

6 RUBP + 6 CO2 + 18 ATP + 12 NADPH + 12 H+ --------> 6 RUBP + C6H12O6 + 18 ADP + 18 Pi + 12 NADP + 6 H2O

3. The glucose formed in photosynthesis if not used immediately is changed into starch through a process called polymerization. Polymerization is a chemical process in which glucose molecules formed in photosynthesis are linked to each other to form long chains of glucose called starch. Starch is a polysaccharide and it is the reserve food of plants. Starch will be stored as starch granules by the leucoplasts in the plant cells.

Fate of various reactants and products of Photosynthesis


Carbon dioxide and water are the main reactants of photosynthesis. Carbon dioxide is the main reactant which get reduced to glucose by the hydrogen source provided through photolysis via NADPH. Water is the other reactant which is the source of hydrogen to reduce carbon dioxide to the immediate product glucose of photosynthesis. The oxygen and water molecules formed as products in photosynthesis are actually formed due the involvement of water in photosynthesis.

The chief product glucose formed in photosynthesis if the plant need immediately it will use it as a source of energy or otherwise it will be changed immediately into starch through a process polymerization. This reserve food will be stored in different parts like leaves, roots and stem. If the plant faces starved conditions, the reserve food will be changed into glucose and will be transported through phloem tissue in fluid form to the needy places of the plant. Through cellular respiration these glucose molecules evolve energy necessary for the various activities of the cells of the plant. Part of the oxygen formed as byproduct during photosynthesis is used by the plant itself for its respiration and the remaining it releases into the atmosphere. Oxygen released during photosynthesis will replenish and balance the oxygen content of atmosphere. This is the only process in nature which aids in maintaining and balancing oxygen concentration in atmosphere. Water is one other byproduct formed in photosynthesis is imbibed by the plant itself for its life activities and if present in excess will leave in the form water vapor through transpiration.



(Courtesy: YouTube)


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