• All living beings, including humans, rely on plants for their food. 
  • Green plants synthesize their own food through a process called photosynthesis. 
  • Photosynthesis is crucial as it not only provides food but also releases oxygen into the atmosphere. 
  • Photosynthesis is a physico-chemical process carried out by green plants. 
  • It utilizes light energy to produce organic compounds, serving as the primary source of food for all living organisms. 
  • This process is essential for sustaining life on earth as it harnesses sunlight energy. 
  • Green plants, being autotrophs, produce their own food through photosynthesis. 
  • Heterotrophs, including animals and humans, rely on plants for their nutritional needs. 
  • Photosynthesis is vital for two main reasons:

- It is the primary source of food production on earth, supporting the entire food chain.

- It contributes to the release of oxygen into the atmosphere, crucial for aerobic respiration and sustaining life. 

  • The absence of photosynthesis would lead to a scarcity of food and oxygen, posing significant challenges for all living organisms. 
  • Life on earth depends fundamentally on the continuous process of photosynthesis. 
  • Photosynthesis involves complex reactions within specialized cellular structures. 
  • The detailed understanding of the photosynthetic machinery and its reactions is essential for comprehending its significance in sustaining life.


Early photosynthetic experiments


Joseph Priestley's Discoveries (1733-1804)

  • In 1770, Joseph Priestley conducted experiments revealing the importance of air in the growth of green plants. 
  • Notably, Priestley discovered oxygen in 1774. 
  • Observations: A candle burning in a closed space (bell jar) would extinguish, and a mouse would suffocate, indicating a detrimental effect on air. 
  • However, when a mint plant was placed in the same bell jar, the mouse remained alive, and the candle continued burning. 
  • Hypothesis: Plants replenish the air by restoring what breathing animals and burning candles remove. 




Jan Ingenhousz's Contribution (1730-1799)

  • Using a similar setup, Jan Ingenhousz demonstrated the essential role of sunlight in purifying air fouled by burning candles or breathing animals. 
  • Experiment: Placing the setup once in darkness and once in sunlight. 
  • Observation: In sunlight, small bubbles formed around the green parts of the plant, identified later as oxygen. 
  • Conclusion: Only the green parts of plants release oxygen, and sunlight is crucial for this process.


Julius von Sachs' Discoveries (1854)

  • Sachs provided evidence for glucose production during plant growth, with glucose typically stored as starch. 
  • He identified chlorophyll, the green substance in plants, as being located within special bodies called chloroplasts. 
  • Sachs's studies revealed that glucose is synthesized in the green parts of plants and stored as starch.


T.W Engelmann's Experiment (1843-1909)


  • Engelmann conducted experiments using a prism to split light into its spectral components. 
  • He illuminated a green alga, Cladophora, in a suspension of aerobic bacteria to detect sites of oxygen evolution. 
  • Observations: Bacteria accumulated mainly in the regions of blue and red light, indicating their involvement in photosynthesis. 
  • Result: The experiment provided the first action spectrum of photosynthesis, resembling the absorption spectra of chlorophyll a and b. 
  • By the mid-nineteenth century, it was understood that plants utilize light energy to convert carbon dioxide (CO2) and water into carbohydrates. 
  • Empirical equation representing photosynthesis for oxygen-evolving organisms: CO2 + H2O --(light)--> CH2O + O2, where CH2O represents a carbohydrate like glucose.





Contribution of Cornelius van Niel (1897-1985)

  • Van Niel's studies of purple and green bacteria demonstrated that photosynthesis is a light-dependent reaction. 
  • Equation: 2H2A + CO2 --(light)--> 2A + CH2O + H2O, where H2O serves as the hydrogen donor and is oxidized to O2 in green plants. 
  • In some organisms like purple and green sulphur bacteria, H2S is the hydrogen donor, leading to the production of sulphur or sulphate instead of O2.


Explanation of the Equation

  • The overall process of photosynthesis involves multiple steps, not just a single reaction. 
  • Equation: 6CO2 + 12H2O --(light)--> C6H12O6 + 6H2O + 6O2, where C6H12O6 represents glucose and O2 is released from water. 
  • Explanation: Twelve molecules of water are used because water serves as both the hydrogen donor and a source of oxygen during the process of photosynthesis.


Chloroplast structure

  • Besides green leaves, photosynthesis can also occur in other green parts of plants. 
  • Mesophyll cells in leaves contain numerous chloroplasts, where photosynthesis primarily takes place. 
  • Apart from leaves, photosynthesis may occur in:

- Stems: Green stems contain chlorophyll and can perform photosynthesis.

- Green Fruits: Unripe fruits with green pigment can undergo photosynthesis.

- Young Shoots: Green portions of young shoots can carry out photosynthesis. 

  • Chloroplasts align themselves along the walls of mesophyll cells to receive optimal light. 
  • Flat surfaces of chloroplasts are parallel to the walls when light intensity is low. 
  • Chloroplasts are perpendicular to incident light when light intensity is high, ensuring maximum light absorption. 


  • Chloroplast Structure

1. Membranous System:

Membrane system traps light energy and synthesizes ATP and NADPH during light reactions (photochemical reactions).

- Grana: Stacks of thylakoid discs where light energy is captured.

- Stroma Lamellae: Interconnecting membranes that link grana. Enzymatic reactions occur here, synthesizing sugars from CO2 using ATP and NADPH produced in light reactions. Sugar formed in stroma is stored as starch. 

2. Matrix Stroma: Fluid-filled space surrounding grana and stroma lamellae.



  • Light and Dark Reactions
  • Light Reactions:

- Directly light-driven reactions occurring in the membrane system.

- Produce ATP and NADPH as energy carriers. 

  • Dark Reactions (Carbon Reactions):

- Indirectly light-driven reactions occurring in the stroma.

- Utilize ATP and NADPH from light reactions to synthesize sugars from CO2.

- Despite the name, dark reactions can occur in the presence of light, as they depend on products of light reactions.