Definition of Photosynthesis

Photosynthesis is a biological process through which green plants, algae, and some bacteria convert light energy, usually from the sun, into chemical energy stored in glucose. This process involves the intake of carbon dioxide(CO₂) from the atmosphere and water (H₂O) from the soil, and it releases oxygen (O₂) as a byproduct.

Photosynthesis Process

What is Photosynthesis?

Photosynthesis, literally "synthesis using light," is the process by which green plants and other organisms use sunlight to synthesize foods with the help of chlorophyll. It involves converting CO₂ and water into glucose, a complex carbohydrate, and releasing O₂ as a byproduct. This process is crucial for life on Earth as it is the primary source of energy for nearly all organisms.

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Importance of Photosynthesis

Photosynthesis is vital as it is the main mechanism for energy supply in the living world. Sunlight is the ultimate source of energy, and through photosynthesis, green plants capture this energy to produce organic material. The energy stored in fossil fuels also originates from ancient photosynthetic processes. Chemosynthetic bacteria, in contrast, derive energy by oxidizing inorganic substances and do not depend on sunlight.

Historical Aspect

Photosynthesis first appeared in cyanobacteria. Ruben and Kamen (1941) demonstrated that the oxygen released during photosynthesis comes from water. Photosynthesis is essential for the existence of life on Earth, making it the most important physico-biochemical process.

Annual Carbon Fixation

Annually, approximately 75 x 10¹²kg of carbon (as CO₂ ) is fixed through photosynthesis, producing around 1700 million tonnes of dry matter. Oceans, primarily through algae, account for about 90% of this total photosynthesis. However, only about 1 - 5 % of the incident radiation, or 2 - 10% of Photosynthetically Active Radiation (PAR), is captured by photosynthetic processes under favorable conditions.

Photosynthesis Locations

Photosynthesis occurs in the chloroplasts of green plant cells. Chloroplasts are complex structures containing the photosynthetic apparatus. These organelles are typically discoid or oval in shape but can vary in different algae species. Chloroplasts contain several structures, including thylakoids and stroma, where both photochemical and enzymatic reactions take place during photosynthesis.

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Types of Photosynthetic Pigments

Photosynthesis requires various pigments for light absorption. The most important pigments are chlorophylls, carotenoids, and phycobilins.

Chlorophyll

Chlorophyll is the primary pigment found in all photosynthetic organisms. There are several types of chlorophyll:

• Chlorophyll a: Found in all oxygen-evolving green plants and algae, it absorbs blue and red wavelengths.
• Chlorophyll b: Found in higher plants and green algae, it absorbs blue and orange wavelengths.
• Chlorophyll c, d, and e: Found in specific algae groups.
• Bacteriochlorophyll: Found in photosynthetic bacteria.

Chlorophyll molecules have two parts: the porphyrin ring, which is hydrophilic, and the phytol tail, which is hydrophobic.

Carotenoids

Carotenoids are unsaturated polyhydrocarbons composed of eight isoprene units. They include carotenes and xanthophylls. Carotenes, such as β-carotene, are precursors to vitamin A. Xanthophylls, like lutein, are more abundant and responsible for the yellow coloration of leaves in autumn.

Phycobilins

Phycobilins are protein-linked pigments found in red and blue-green algae. They include phycoerythrin, phycocyanin, and allophycocyanin, which absorb various wavelengths of light.

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Mechanism of Photosynthesis

Photosynthesis is an oxidation-reduction process in which water is oxidized and CO₂  is reduced to form carbohydrates. This process can be divided into two main stages: the light-dependent reactions (light reactions) and the light-independent reactions (dark reactions).

Light-Dependent Reactions

Light-dependent reactions, or the Hill reaction, occur in the thylakoid membranes of the chloroplasts. These reactions involve the photolysis of water, the evolution of oxygen, and the synthesis of ATP and NADPH (assimilatory power). The breakdown of water in the presence of sunlight releases electrons, hydrogen ions, and oxygen. The electrons are used to reduce photosystem II, and the hydrogen ions help reduce NADP to NADPH.

Photolysis of Water

The photolysis of water, confirmed by Ruben, Hassid, and Kamen (1941), involves the breakdown of water molecules to release oxygen. This process requires light energy, an oxygen-evolving complex, and an electron carrier.

Light-Independent Reactions

Light-independent reactions, or the Calvin cycle, occur in the stroma of the chloroplasts. These reactions do not require light directly but use the ATP and NADPH produced during the light-dependent reactions to reduce CO₂  to carbohydrates.

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Types of Photosynthetic Reactions

Photosynthetic reactions can be cyclic or non-cyclic. In cyclic photophosphorylation, the same electron returns to the original donor, whereas in non-cyclic photophosphorylation, the electron is consumed in a chemical reaction and replaced by another electron donor.

Photo-Excitation of Molecules

Photo-excitation involves the absorption of photons by pigment molecules, causing electrons to become excited and move to higher energy levels. These excited electrons can either return to their ground state, releasing energy as fluorescence or phosphorescence, or participate in chemical reactions.

Conclusion

Photosynthesis is a critical anabolic process for life on Earth, converting light energy into chemical energy stored in organic compounds. It involves complex biochemical reactions within the chloroplasts, utilizing various pigments to capture light energy. This process not only sustains plant life but also supports the energy needs of nearly all living organisms.