Photosynthesis is a process where green plants and some other organisms make use of carbon dioxide and water in presence of sunlight, and then recombine them into glucose. Energy is stored in the bonds of the glucose molecule and can be used by plants by breaking these bonds by means of another process called cellular respiration. The released energy by breaking bonds of glucose molecule is used for the metabolic activities of cell.
Importance of photosynthesis:
- Photosynthesis is important to bot autotrophs and heterotrophs.
- Photosynthesis forms the primary source of energy in autotrophs (eg: plants, trees, algae etc. which can make their own food through photosynthesis) .
- Heterotrophs depend for their food, directly or indirectly on autotrophs (eg: plants, trees, algae etc.). Hence for heterotrophs also, photosynthesis is equally important.
- Photosynthesis in plants and green algae growing deep inside the oceans are the true oxygen generators of the planet. Just to remind, Oxygen is the byproduct of photosynthesis in plants, algae and some other organisms. Without photosynthesis there will no oxygen on earth and with lack of oxygen , no life can exist on this planet.
- The products of photosynthesis contribute to the carbon cycle occurring in the oceans, land, plants, and animals.
- Plants and photosynthesis are the basis of fossil fuels.
Artificial Photosynthesis :
- Artificial photosynthesis is a chemical process done in bioreactors etc to recreate the process of photosynthesis but alter its final outcome for benefit of human needs.
- During photosynthesis process in plants, oxygen is expelled at the end of the reaction. In artificial photosynthesis, scientists tinker with the process to release liquid hydrogen or any other fuel useful for human needs.
- Algae are promising source of biofuels. It is the only eukaryotic organism capable of oxygenic photosynthesis which are equipped with a hydrogen metabolism.
- Chlorella was the first alga for the study of photosynthesis by Warburg (1919) and later by Calvin and Benson in their work on carbon dioxide assimilation in plants.
- It has served as an model to study photosynthesis, due to its simple cell cycle, high growth rate, and having photosynthetic and metabolic pathways similar to higher plants.
- Chlorella also happens to the first microalgae considered for mass cultivation and produced commercially
- Algae also release oxygen as byproduct of photosynthesis. However in certain conditions (anaerobic conditions), it also produce hydrogen gas in small amounts in presence of light. Only 3-5% of photosynthesis leads to hydrogen gas production.
- Generically modified versions of algae can produce good viable source of Hydrogen gas.
- Importance of Hydrogen gas as fuel is that, it doesn’t release CO2 after combustion but only pure water. This aspect of hydrogen gas is useful for human needs as it doesn’t pollute the environment.
- Electrons and hydrogen ions produced from photolysis of water is redirected (in anaerobic conditions or when excessive solar energy is absorbed, some organisms release surplus electron) via photosynthetic electron transport chain to Hydrogenase enzyme to produce Hydrogen gas.
- Hydrogenase enzyme is very sensitive to the presence of oxygen. This the reason why production of hydrogen is limited in algal photosynthesis as oxygen is also produced along with hydrogen gas ( O2 inactivates hydrogenases).
- Oxygen inactivation of hydrogenases might also explain, why plants don’t release Hydrogen gas but only produce Hydrogen ions and oxygen after photolysis of water molecule during light reaction.
- Artificial photosynthesis can be harnessed to produce oxygen and Hydrogen gas by splitting of water. It can also be used to fix CO2 artificially, to yield carbohydrate molecules.
- Helps in generating solar fuel from sunlight that can be stored and used where sunlight is not available.
Summary of Photosynthesis :
- Photoautotrophs are organisms that are capable of surviving on CO2 as their principal carbon source and convert it in presence of water and sunlight to carbohydrates (food) and release oxygen gas as byproduct in atmosphere – This is termed as Photosynthesis.
- The first autotrophs were though have carried out only nonoxygenic photosynthesis – compounds such as H2S were oxidized as a source of electrons. Here Sulphur is released instead of oxygen as observed in oxygenic photosynthesis.
- The evolution of oxygenic photosynthesis in cyanobacteria – water is oxidized ( source of electrons ) and oxygen gas is released in atmosphere. With the increase in amounts of oxygen in atmosphere, organisms slowly evolved to new habitat and this laid the platform of aerobic respiration.
- Photosynthesis occurs in Chloroplast.
- Photosynthesis is divided into two phases : Light reaction – occurs in thylakoid membrane of chloroplasts and dark reaction / Calvin Cycle / C3 Cycle / Calvin–Benson–Bassham (CBB) cycle, reductive pentose phosphate pathway – Occurs in Stroma of Chloroplasts.
- The light reactions begin with the absorption of photons of light by photosynthetic pigments (chlorophyll a mostly) – an electron is pushed into outer orbit (excited) which can be transferred to an electron acceptor.
- The electron transport chain begins with excitation of electron from Photosystem II (P680), which then moves to pheophytin (primary electron acceptor) – P680 becomes oxidized and Pheophytin is reduced.
- Next, the electrons are transferred to plastoquinone, then to multiprotein complex cytochrome b6 f , later to plantacyanin and finally to Photosystem I (p700+ – Oxidized form after losing electron).
- As each photon is also absorbed simultaneously by P700, the electron is transferred to a primary acceptor, A0, and then through several iron-sulfur centers of the PSI reaction center to ferredoxin. Electrons are transferred from ferredoxin to NADP+ forming NADPH.
- The positively charged (P680), which first started the excitation of electron is neutralized ( or compensated) by an electron from a cluster containing four manganese ions and a calcium ion attached to Photosystem II, by splitting of 2 water molecules.
- This splitting of water produces O2 and delivers four H+( protons) to the thylakoid lumen, which contributes to the proton gradient important of ATP synthesis by a process called photophosphorylation.
- This Noncyclic electron flow results in the oxidation of H2O to O2, with the transfer of electrons to NADP+, forming NADPH is termed as Z scheme.
- During the light-independent reactions, the chemical energy stored in NADPH and ATP is used in the synthesis of carbohydrates from CO2.
- CO2 is fixed by RuBP carboxylase (Rubisco) to a five-carbon compound, RuBP, forming an unstable six-carbon intermediate, which splits into two molecules of 3-phosphoglyceric acid (3PGA).
- NADPH and ATP formed during light reactions are used to convert PGA molecules to glyceraldehyde phosphate (GAP).
- For every 6 molecules of CO2 fixed : 2 molecules of GAP will be used for starch or glucose formation – remaining 10 molecules of GAP can be used to regenerate RuBP for additional rounds of CO2 fixation.
- Rubisco can also catalyze a reaction in which O2, rather than CO2, is covalently joined to RuBP. This process, which is called photorespiration.
- C4 and CAM plants possess an additional CO2-fixing enzyme, called PEP carboxylase, that is able to operate at very low CO2 concentrations.
- In C4 plants, CO2 fixation takes places in mesophyll cells and C3 cycle takes place bundle sheath cells.
- In CAM pathway plants, CO2 fixation and C3 cycle takes place at two different times (Night and day respectively).
The final product of light reaction of photosynthesis are:
- O2 gas
- H+ ions
The products of Calvin cycle of photosynthesis are:
- Glucose (carbohydrates)
- H+ ions
The overall products of photosynthesis are:
- Glucose (carbohydrates)
- Sulfur (in photosynthetic sulfur bacteria)
Links to pervious blogs on Photosynthesis :
Part I of Photosynthesis : Introduction
Part VI – Differences between Light and Dark reactions or Calvin Cycle
Part V – Photorespiration, C4 Pathway and CAM metabolism pathway
Part VI – C3, C4 and CAM types of photosynthesis comparison
Part VII – factors affecting the rate of photosynthesis
Image Credit : Summary of Photosynthesis – Openstax Biology.