A revolutionary discovery published in Nature has pushed back the timeline of oxygenic photosynthesis, the process that transformed Earth's atmosphere and paved the way for complex life. Scientists have identified fossilised photosynthetic structures within 1.75-billion-year-old microfossils, offering direct evidence of this crucial biological innovation's early existence.
2. Key Findings
- These structures, known as thylakoids, are membrane-bound compartments within chloroplasts where sunlight fuels the conversion of water and carbon dioxide into glucose and, critically, oxygen.
- Their presence in these ancient microfossils, presumed to be cyanobacteria, suggests that oxygenic photosynthesis may have evolved before the transformative Great Oxidation Event around 2.4 billion years ago.
- The discovery doesn't definitively resolve the debate on whether photosynthesis predated or followed the Great Oxidation Event.
- However, it provides a crucial stepping stone in understanding the timeline and potential factors contributing to this pivotal event.
- Further analyses of older microfossils could shed light on this question and reveal whether the evolution of thylakoids played a role in Earth's oxygenation.
- The study emphasises the importance of ultrastructural analyses in paleobiology. Similar investigations on exceptionally preserved microfossils hold the potential to expand the fossil record of oxygenic photosynthetic organisms and reconstruct the early ecosystems in which complex life emerged.
3. Photosynthesis
Photosynthesis is a fundamental biological process that enables plants, algae, and some bacteria to convert light energy into chemical energy. This transformative process plays a crucial role in sustaining life on Earth by producing oxygen and providing a primary source of organic compounds. Here's a comprehensive overview of photosynthesis, encompassing its mechanisms, stages, and significance.
3.1. Mechanism of PhotosynthesisPhotosynthesis primarily occurs in the chloroplasts of plant cells, where specialized structures called chlorophyll molecules capture light energy. The overall chemical equation for photosynthesis is:
6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂
This equation represents the conversion of six molecules of carbon dioxide (CO₂) and six molecules of water (H₂O) using light energy into a molecule of glucose (C₆H₁₂O₆) and six molecules of oxygen (O₂).
3.2. Stages of Photosynthesis
Photosynthesis can be divided into two main stages: the light-dependent reactions (light reactions) and the light-independent reactions (Calvin Cycle or dark reactions).
a. Light-Dependent Reactions
- Light Absorption: Chlorophyll molecules in the thylakoid membranes absorb light energy.
- Water Splitting: Water molecules are split into oxygen, protons, and electrons. Oxygen is released as a byproduct.
- Electron Transport Chain: Electrons move through a series of proteins in the thylakoid membrane, generating ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate).
b. Light-Independent Reactions (Calvin Cycle)
- Carbon Fixation: Carbon dioxide is incorporated into a five-carbon sugar molecule (ribulose bisphosphate) with the help of an enzyme, RuBisCO.
- Reduction Phase: ATP and NADPH generated in the light-dependent reactions are used to convert the five-carbon molecule into a three-carbon sugar (glyceraldehyde-3-phosphate).
- Regeneration of RuBisCO: Some molecules continue in the cycle, while others regenerate the initial five-carbon molecule to ensure the continuous functioning of the Calvin Cycle.
- Oxygen Production: Photosynthesis is the primary source of atmospheric oxygen. During the light-dependent reactions, oxygen is released as a byproduct when water molecules are split.
- Organic Compound Synthesis: The process produces organic compounds, primarily glucose, which serves as an energy source for plants and other organisms in the food chain.
- Carbon Dioxide Utilization: Photosynthesis plays a crucial role in maintaining the balance of atmospheric carbon dioxide levels by fixing CO₂ into organic molecules.
- Energy Transfer: The stored energy in glucose is used by plants for various metabolic activities. Additionally, it serves as an energy source for herbivores and, subsequently, carnivores in the food chain.
3.4. Factors Affecting Photosynthesis
Several factors influence the rate of photosynthesis:
- Light Intensity: Higher light intensity generally increases the rate.
- Carbon Dioxide Concentration: Adequate CO₂ is essential for efficient photosynthesis.
- Temperature: Moderate temperatures are optimal; extreme temperatures can hinder the process.
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For Prelims: Photosynthesis, Oxygen, Carbon dioxide
For Mains:
1. Explain the mechanisms and stages of photosynthesis, highlighting the importance of light-dependent and light-independent reactions. (250 words)
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Previous Year Questions
1. Which one of the following is the process involved in photosynthesis? (UPSC 2014)
A. Potential energy is released to form free energy
B. Free energy is converted into potential energy and stored
C. Food is oxidized to release carbon dioxide and water
D. Oxygen is taken, and carbon dioxide and water vapour are given out
2. Who discovered oxygen? (Soldier GD 2021)
A. Macmillan B. Marconi C. Joseph Priestley D. None of these
3. Consider the following statements: (UPSC 2018)
1. The Earth's magnetic field has reversed every few hundred thousand years.
2. When the Earth was created more than 4000 million years ago, there was 54% oxygen and no carbon dioxide.
3. When living organisms originated, they modified the early atmosphere of the Earth.
Which of the statements given above is/are correct?
A. 1 only B. 2 and 3 only C. 1 and 3 only D. 1, 2 and 3
4. The increasing amount of carbon dioxide in the air is slowly raising the temperature of the atmosphere, because it absorbs (UPSC 2012)
A. the water vapour of the air and retains its heat
B. the ultraviolet part of the solar radiation
C. all the solar radiations
D. the infrared part of the solar radiation
5. Human activities in the recent past have caused the increased concentration of carbon dioxide in the atmosphere, but a lot of it does not remain in the lower atmosphere because of (UPSC 2011)
1. its escape into the outer stratosphere.
2. the photosynthesis by phytoplankton in the oceans.
3. the trapping of air in the polar ice caps.
Which of the statements given above is/are correct?
A. 1 and 2 B. 2 only C. 2 and 3 D. 3 only
Answers:1-B, 2-C, 3-C, 4-D, 5- B
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