Photosynthesis In Higher Plants Notes

Introduction

Photosynthesis is a vital process that occurs in the chloroplasts of green plants, algae, and some bacteria. It is the process by which these organisms convert light energy into chemical energy, specifically glucose, which is used to fuel their activities. Understanding photosynthesis is crucial for NEET aspirants as it forms a significant part of the Botany syllabus. This study note will break down the complex process of photosynthesis into manageable sections and explain each part in detail.

Overview of Photosynthesis

Photosynthesis can be broadly divided into two stages:

1. Light-dependent reactions (Light Reactions)
2. Light-independent reactions (Calvin Cycle or Dark Reactions)

Light-Dependent Reactions

These reactions occur in the thylakoid membranes of the chloroplasts and require light to produce ATP and NADPH. The key events include:

1. Absorption of Light:
   • Chlorophyll and other pigments absorb light energy.
   • This energy is used to excite electrons to a higher energy state.
2. Photolysis of Water:
   • Water molecules are split into oxygen, protons, and electrons.
   • The reaction can be represented as: $$2H_2O \rightarrow 4H^+ + 4e^- + O_2$$
3. Electron Transport Chain (ETC):
   • Excited electrons pass through a series of proteins embedded in the thylakoid membrane.
   • Energy released during this transfer is used to pump protons into the thylakoid lumen, creating a proton gradient.
4. Synthesis of ATP:
   • Protons flow back into the stroma through ATP synthase, driving the conversion of ADP and inorganic phosphate (Pi) into ATP.
5. Formation of NADPH:
   • Electrons reduce NADP^+ to NADPH, a high-energy molecule used in the Calvin Cycle.

Light-Independent Reactions (Calvin Cycle)

The Calvin Cycle occurs in the stroma of the chloroplasts and does not require light. It uses ATP and NADPH produced in the light-dependent reactions to convert CO_2 into glucose. The cycle includes three main phases:

1. Carbon Fixation:
   • CO_2 is fixed by the enzyme RuBisCO to ribulose-1,5-bisphosphate (RuBP), forming 3-phosphoglycerate (3-PGA).
2. Reduction Phase:
   • ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P).
3. Regeneration of RuBP:
   • Some G3P molecules are used to regenerate RuBP, enabling the cycle to continue.
   • The overall reaction can be summarized as: $$6CO_2 + 18ATP + 12NADPH + H_2O \rightarrow C_6H_{12}O_6 + 18ADP + 18Pi + 12NADP^+$$

Factors Affecting Photosynthesis

Several factors influence the rate of photosynthesis, including:

1. Light Intensity:
   • Higher light intensity increases the rate of photosynthesis to a certain point.
   • Beyond this point, the rate plateaus.
2. Carbon Dioxide Concentration:
   • Increased CO_2 concentration boosts the rate of photosynthesis until the enzyme RuBisCO becomes saturated.
3. Temperature:
   • Photosynthesis has an optimum temperature range.
   • Extremely high or low temperatures can denature enzymes, reducing the rate.
4. Water Availability:
   • Water stress can close stomata, reducing CO_2 uptake and thus photosynthesis.

Types of Photosynthesis

Plants have evolved different pathways to optimize photosynthesis under varying environmental conditions:

C3 Photosynthesis

• The most common form, occurring in most plants.
• Involves the direct fixation of CO_2 by RuBisCO to form 3-PGA.

C4 Photosynthesis

• Adapted to high light intensities, high temperatures, and low CO_2 conditions.
• CO_2 is first fixed into a four-carbon compound in mesophyll cells, which is then transported to bundle-sheath cells where the Calvin Cycle occurs.

CAM Photosynthesis

• Found in plants adapted to arid conditions.
• Stomata open at night to fix CO_2 into organic acids, which are then used during the day for photosynthesis.

Conclusion

Photosynthesis is a complex but fascinating process that is essential for life on Earth. Understanding its intricacies, from light-dependent reactions to the Calvin Cycle, is crucial for NEET aspirants. Remember to consider the factors affecting photosynthesis and the different pathways plants use to optimize this process.

Diagram showing the light-dependent reactions in the thylakoid membrane and the Calvin Cycle in the stroma of a chloroplast.

By breaking down each component and understanding the nuances, you will be well-prepared for any questions on this topic in the NEET exam.