Introduction to Photosynthesis

Photosynthesis is a physico-chemical process by which green plants, algae, and some bacteria use light energy to synthesize organic food from carbon dioxide and water. It is the foundation of all food chains and the only natural process responsible for the continuous supply of oxygen to the atmosphere.

Key idea: No photosynthesis = no food + no oxygen = no life on Earth.

Early Experiments on Photosynthesis

1. Joseph Priestley (1770) – Role of Air

  • Performed the bell jar experiment using a burning candle, a mouse, and a mint plant.
  • In a closed jar, both the candle and mouse died due to lack of fresh air.
  • When a mint plant was added, the candle kept burning and the mouse survived.

Key Conclusion:

  • Plants restore oxygen to the air that is used up by respiration and combustion.
  • He indirectly discovered oxygen (formally in 1774).

Memory hook: Plants repair polluted air.

2. Jan Ingenhousz (1779) – Role of Light and Green Parts

  • Repeated Priestley’s experiment in light and dark.
  • Found that plants purify air only in sunlight.
  • Using aquatic plants, he observed oxygen bubbles forming only around green parts in light.

Key Conclusions:

  • Sunlight is essential for photosynthesis.
  • Only green parts (chlorophyll-containing tissues) release oxygen.

Memory hook: Light + Green = Oxygen

3. Julius von Sachs (1854) – Food Formation

  • Demonstrated that photosynthesis produces glucose.
  • Showed that glucose is stored as starch in plants.
  • Proved that chlorophyll is located in chloroplasts.

Key Conclusions:

  • Starch is the first visible product of photosynthesis.
  • Chloroplast is the site of food synthesis.

Memory hook: Photosynthesis → Glucose → Starch

4. T.W. Engelmann (1843–1909) – Action Spectrum

  • Split white light into its spectrum using a prism.
  • Illuminated a green alga (Cladophora) placed with aerobic bacteria.
  • Bacteria gathered near blue and red regions of light.

Key Conclusion:

  • Photosynthesis is maximum in blue and red light.
  • This was the first action spectrum of photosynthesis.

📌 Memory hook: Blue + Red = Best photosynthesis

5. Cornelius van Niel (1897–1985) – Source of Oxygen

  • Studied photosynthesis in purple and green sulphur bacteria.
  • Proposed a general equation: 2H₂A + CO₂ → 2A + CH₂O + H₂O
  • In green plants: H₂A = H₂O → Oxygen released.
  • In bacteria: H₂A = H₂S → Sulphur released.

Key Conclusion:

  • Oxygen released in photosynthesis comes from water (H₂O), not CO₂.

Memory hook: Water splits → Oxygen comes out

Site of Photosynthesis: The Chloroplast

Photosynthesis takes place in the green parts of the plants, primarily the leaves, within specialized organelles called chloroplasts.

Chloroplast Structure:

  • Double Membrane: Enclosed by a double membrane (outer and inner).
  • Stroma: An aqueous, protein-rich fluid filling the interior. The dark reactions (Calvin cycle, or carbon fixation) occur here. It contains enzymes, small 70S ribosomes, and a circular DNA molecule.
  • Grana (singular: Granum): Membranous, flattened sacs called thylakoids are stacked on top of each other to form grana. The light-dependent reactions occur in the thylakoid membranes.
  • Stroma Lamellae: Thylakoids of different grana are connected by flat membranous tubules called stroma lamellae.

Division of Labour

There is a clear division of labour within the chloroplast:

  1. Light Reactions (in Grana/Thylakoids): The membrane systems are responsible for trapping light energy and for the synthesis of ATP and NADPH (the assimilatory power). This phase involves the splitting of water and the release of oxygen.
  2. Dark Reactions (in Stroma): The stroma is the site where enzymes use the energy from ATP and NADPH to reduce CO₂ and synthesize sugars (starch).

Memory Capsules – Quick Revision

Early Scientists & Contributions:

  • Priestley: Plants restore oxygen
  • Ingenhousz: Light + green parts needed
  • Sachs: Starch formation, chloroplast
  • Engelmann: Blue & red light best
  • Van Niel: O₂ comes from water

Chloroplast Rule:

  • Grana = Light reaction
  • Stroma = Dark reaction

One-line Formula: ➡️ Light reaction makes energy, dark reaction makes food.

💡 Questions and Answers

Q1. Where does photosynthesis occur in a plant?

A1. Photosynthesis occurs in all green parts of the plant, mainly in the leaves, because they contain chloroplasts with chlorophyll.

Q2. What did Priestley’s bell jar experiment prove?

A2. It proved that plants have the ability to purify air by releasing oxygen, which supports breathing and burning.

Q3. Why is sunlight essential for photosynthesis?

A3. Sunlight provides the energy required to split water and start the photosynthetic reactions. Without light, oxygen is not released.

Q4. What was the importance of Engelmann’s experiment?

A4. It showed that photosynthesis works best in blue and red light, proving these wavelengths are most effective.

Q5. How did van Niel prove the source of oxygen in photosynthesis?

A5. By comparing bacteria and green plants, he showed that oxygen always comes from the hydrogen donor, which is water in plants, not CO₂.

Q6. Explain the division of labour in a chloroplast.

A6.

  • Grana: Capture light and make ATP & NADPH.
  • Stroma: Uses this energy to convert CO₂ into carbohydrates.