Photosynthesis: Definition, Formula, Diagram, Steps & Importance

Photosynthesis is the process by which green plants convert light energy from the sun into chemical energy in the form of organic compounds, such as glucose. The overall productivity of agricultural crops are influenced by the rate of photosynthesis in plants. The process utilizes carbon dioxide and water in the presence of sunlight and chlorophyll to yield glucose and oxygen.

• Apart from plants, blue-green algae (cynobacteria) and algae are have the potential of performing this process. 
• Green plants contain an important pigment called chlorophyll, which helps in capturing sunlight and thus starts this process.
•  All animals (heterotrophs), parasites and saprophytes are directly or indirectly dependent upon plants for their food.

Photosynthesis Diagram

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What is Photosynthesis?

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The biological process in which sunlight, water and carbon dioxide are utilised to form oxygen and energy with the help of bacteria is called Photosynthesis. The bacteria involved in this process are called photosynthetic bacteria such as cyanobacteria. Green plant cells prepare their food through the process of photosynthesis in the presence of sunlight, chlorophyll, water and carbon dioxide (CO₂) thus producing glucose and oxygen (O₂) as by-products.

• The (photos: light; synthesis: building up) photosynthesis definition was coined by Barnes in 1898.
• The availability of food on the earth is dependent on the rate of photosynthesis in higher plants.  
• Photosynthesis play a significant role in maintaining oxygen balance in earth’s atmosphere. 

The photosynthesis definition is associated with the release of oxygen in the earth’s atmosphere which is essential for the survival for organism. Chemosynthetic bacteria are the only class of organism which can survive without photosynthesis. They use chemical energy energy from the oxidation of inorganic compounds. 

C₃ and C₄ Photosynthesis

C₃ and C₄ are the two types of photosynthesis. The naming is done on the basis of the stable product formed during the photosynthetic reaction. C₃ photosynthesis is the characteristic feature of most of the plants in which the first stable product is a three carbon compound called 3-phosphoglyceric acid. 

• C₄ photosynthesis is the characteristic feature of only vascular plants such as maize, corn, sorghum, millet, etc.
• During the C₄ photosynthesis process, a four carbon stable product is formed which is called oxaloacetic acid.

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Site for photosynthesis

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The chloroplast containing mesophyll tissue (present in leaves) serves as the main site for photosynthesis in higher plants. The major pigment that plays a significant role in the photosynthesis reaction is chlorophyll A.

Along with chlorophyll A, there are also some other photosynthetic pigments like blue, red and brown pigments which play a supporting role in the overall reaction taking place in the chloroplast. However, in some plants, stem, root, and calyx are also utilised as the site of photosynthesis and thus called photosynthetic organs such as: ​

1. Photosynthetic roots – Assimilatory roots of Tinospora sp., Roots of Trichosanthes sp., aerial roots of orchids.
2. Photosynthetic stem – Stem of Opuntia dilenii, Euphorbia quadrangularis, Lagenaria sp., etc. 
3. Photosynthetic stipule – Leafy stipule of pea, wild pea, etc.
4. Photosynthetic bract – Bracts of Adhatoda sp., Acalypha sp., Nymphaea sp.,
5. Photosynthetic sepals – Leafy sepals of Mussaenda sp. Dillenia sp., etc.
6. Photosynthetic petals – Green petals of Annona sp., Artabotrys sp. etc.

Photosynthetic Pigments

Photosynthetic pigments are the synthetic pigments products of plants formed as a result of the metabolic activities of cells and play an important role in the photosynthesis of plants. In higher plants, photosynthetic pigments are of two types: Chlorophyll (Green coloured) and Carotenoids (Yellowish orange coloured). However, in algae another pigment is found called Phycobilins.

Chlorophyll

It is the green-coloured pigment found in plants. This pigment absorbs the red and blue light of the sun and reflects back the green light, so this appears to be green in colour. Chlorophyll contains four pyrrole rings (attached to phytol tail) and one Mg²⁺ at the centre.

1. Chlorophyll a: Found in all photosynthesizing plants except bacteria.
2. Chlorophyll b: Found in higher plants, bryophytes, pteridophytes, green algae.
3. Chlorophyll c: Found in diatoms, brown algae, red algae.
4. Chlorophyll d: Found in some red algae.
5. Chlorophyll e: Exceptionally found in golden algae.

Carotenoids

Carotenoids are orange to the yellow coloured pigment that occurs in the membrane of thylakoid as an accessory pigment. It protects against excess light damage in plants. The carotenoid pigments play a role in photosynthesis by absorbing light and passing it to chlorophyll where it is used in the photosynthetic process.

Xanthophyll

1. Yellow in colour.
2. Xanthophyll is present more than carotene in normal leaves.
3. Most common xanthophyll is luteol or lutein.

Carotenes

1. Yellow in colour.
2. Soluble in fat (lipochromes).
3. β-carotene is most common in green plants.

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Photosynthesis Formula

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In the process of photosynthesis, solar energy is converted into light energy. Energy from the sun is absorbed, thus the photosynthesis reaction is called an endothermic reaction. The photosynthesis formula can be given as—

Photosynthesis Formula 

Factors affecting photosynthesis

The factors that affect photosynthesis process are as follows-

1. Intensity of light: Light intensity has a direct effect on the rate of photosynthesis. As the intensity of light increases, the rate of photosynthesis also increases and vice-versa. 
2. Concentration of CO₂: The rate of photosynthesis increases with an increase in the concentration of CO₂. However, a CO₂ concentration of approximately 400 ppm is optimum concentration for photosynthesis whereas the process is haulted once the concentration of CO₂ reaches 100ppm. 
3. Water content: The availability of water is an important factor that affect the rate of photosynthesis. The rate of photosynthesis is accelerated by higher content of water. However, plants in dry areas keep their stomata closed to retain the water stored in their body to carry out the process of photosynthesis.
4. Temperature: An optimum temperature between 25˚ – 35˚C is much more favourable for carrying out photosynthesis. 

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Photosynthesis Phases

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The photosynthesis process is further categorized into two types based on the utilisation of light energy: Light dependent and light independent photosynthetic reactions. The light dependent photosynthetic reaction is called light phase reaction whereas the light independent photosynthetic reaction is called dark phase reaction.

Light Phase Photosynthetic Reaction

This is the very initial phase of photosynthesis where light is captured by chlorophyll and this leads to the excitation of the chlorophyll molecule. This leads to photolysis of water, formation of ATP and NADPH as well as evolution of O₂.

• These reactions take place in the presence of light.
• It occurs in the grana of the chloroplast.
• The end products are ATP, NADPH (reduced NADP), water and liberation of oxygen.

Photosynthetic Steps: 

Activation of chlorophyll: When a photon particle strikes the chlorophyll molecule, it loses an electron and becomes activated.

Photolysis of water [photos: light; lysis: to break]: Water molecule breaks down into H⁺ and OH^- in the presence of sunlight in the activated chlorophyll. This is known as photolysis of water.

H₂O ⇌ H⁺ + OH^-

Evolution of oxygen: The OH^- thus formed by photolysis of water, donates e- to chlorophyll (chlorophyll returns to ground state) and becomes OH. Number of OH molecules combine together to form H₂O and O₂.

OH^- → OH + e^-

4OH → 2H₂O + O₂

Formation of NADPH: NADP⁺ receives electron from OH^- and H⁺ from H₂O and forms NADPH (reduced NADP).

NADP⁺ + e^- + H⁺ → NADPH

Photophosphorylation: In the activated chlorophyll, in presence of sunlight, ADP and Pi combine together to form ATP. This is called photophosphorylation.

ADP + P_i → ATP

Dark Phase Photosynthetic Reaction

After the event of light reaction photosynthetic phase, a special cyclical phase of reactions take place that occurs independent of light. NADPH and ATP produced in light phase are utilized to reduce CO2 in this phase to produce glucose. This phase of reactions is called dark phase or light-independent phase reactions or Calvin cycle.

• Light is not required for the dark-phase reactions, i.e. these reactions are light-independent.
• It occurs in the stroma of the chloroplast.
• Glucose is the end product.
• RuBisCO (Ribulose 1,5-bisphosphate Carboxylase Oxygenase) is the main participating enzyme.

Photosynthesis Steps: The photosynthesis cycle is proceeded in four major steps which are mentioned below: 

• Carbon fixation by addition of CO₂: Six molecules of CO₂ combines with 6 molecules of RuBP (Ribulose 1,5-bisphosphate) to form 12 molecules of 3-phosphoglycerate (PGA) in the presence of the enzyme RuBisCO.
• Reduction of PGA to PGAL: Twelve molecules of PGA thus formed is reduced to form 12 molecules of phosphoglyceraldehyde (PGAL) or Glyceraldehyde-3-phosphate.
• Re-synthesis of RuBP: 10 of the 12 molecules of PGAL is then converted to 6 molecules of RuBP. This step utilizes 6 molecules of ATP.
• Synthesis of glucose: Remaining 2 molecules of PGAL is converted to 1 molecule of glucose. The photo of photosynthesis cycle is given below: 

Photosynthesis Cycle

Significance of Calvin cycle

1. Carbon assimilation by intake of CO₂ takes place in the Calvin cycle.
2. Regular production of stable carbon compounds provides energy to the living world.
3. From PGAld/ Glyceraldehyde-3-phosphate, different organic compounds are produced.

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Importance of photosynthesis

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Photosynthesis is an important reaction as –

1. Transformation and storage of energy

The radiant energy of the sun is composed of small packets of highly energized solar particles called photons which is required during the light reaction and produces ATP (chemical energy). 

This ATP is utilised in dark reactions and glucose is formed and stored as potential chemical energy.

During respiration, food materials are oxidized and released in the form of heat energy.

2. Maintenance of O₂ and CO₂ in the atmosphere

Normal percentage of O₂ and CO₂ in atmosphere are 21% in and 0.03% respectively. Oxygen used during respiration and combustion is balanced by the liberation of O₂ during photosynthesis which in turn uses up CO₂ produced from respiration and combustion.

3. Food synthesis/ Storage of photosynthetic products

Photosynthesis is the only method of production of organic food in green plants. All heterotrophs are dependent upon green plants for food.

In different plants photosynthetic products are stored—

1. In root- carrot, radish, etc. 
2. In stem- potato, onion, ginger, etc. 
3. In leaf- spinach, cabbage, etc. 
4. In flower- cauliflower, etc. 
5. In fruits- Apple, guava, grapes, etc.
6. In seeds- paddy, wheat, coconut, etc.

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Things to remember

• Photosynthesis is the process by which plants convert light energy into chemical energy in the form of organic compounds.
• Chlorophyll is the primary pigment of photosynthesis process present in chloroplast of plant cells.
• Carotenoid is an accessory pigment which helps in protection of cells from high light intensity.
• The O₂ evolved during photosynthesis is generated from water (and not from CO₂).
• Light with wavelength of about 400-700nm is utilized in photosynthesis process.
• ATP or Adenosine tri-phosphate readily yields high energy on breaking of the bond between second and third phosphate groups and hence it is often called ‘Energy Currency of Cell’.
• The light phase photosynthetic reactions occur in grana of chloroplast.
• The dark phase photosynthetic reactions occur in stroma of chloroplast and yields glucose as an end product.
• RuBisCO is the key enzyme of Dark photosynthetic phase reactions.