Photosynthesis is the process that converts solar energy into chemical energy that is used by biological systems.
It is composed of two stages: Light Dependent Reaction (Photosystems I and II) and Light Independent Reaction (Calvin Cycle).
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Light Dependent Reaction
Steps of Photosystem II and I
STEP 1.
As pigment in P II absorbs a photon, one of its electrons is elevated to a higher energy state where it has more potential energy. The potential energy is transferred from pigment to pigment within the harvesting complex, exciting each complex as it continues in a wave like fashion until it reaches the P680 reaction center complex and the pair of chlorophyll α.
STEP 2.
The pair of Chlorophyll α are special because their location and other molecules surrounding them allow them to use the energy from the harvesting complex to not only boost an electron to a higher energy state, but to literally transfer it to the primary electron acceptor. The primary acceptor captures the electron, leaving P680 as P680+, and essentially with an electron ‘hole’ in it.
STEP 3.
At the same time, an enzyme catalyses the splitting of water into H +, electrons and oxygen. The electrons are supplied one by one to the P680+, filling the electron hole and giving it another electron to shoot up to the primary acceptor the next time it gets excited. The H+ are released into the thylakoid space, and the oxygen atom will immediately combine with another to make O2.
STEP 4.
Each electron that is captured by the primary acceptor of photosystem II is passes to reaction center complex of photosystem I via an electron transport chain. The exergonic fall of electrons to a lower energy via the chai provides energy for the synthesis of ATP. The H+ ions are pumped into the thylakoid space, contributing to a proton gradient that is used in chemiosmosis.
STEP 5.
In photosystem I, light is also absorbed by pigments in its harvesting complex, and in the same way that occurs in photosystem II, the pair of chlorophyll P700 release an electron to its primary acceptor. The electron hole created is filled by the electron that is carried from photosystem II, so that the process can continue to occur in a cycle. The electrons are passed through a series of redox reactions from the primary acceptor of P.S I down a second electron train and through a protein called Ferredoxin (Fd). This chain however, does not generate ATP
STEP 6.
The enzyme NADP+ reductase catalyses the transfer of electrons from Fd to NADP+. Two electrons are required, and one H+ from the stroma is also taken to form NADPH. The NADPH will be the energy carrier moving into the Calvin cycle
Light Independent Reaction
Calvin Cycle
The Calvin cycle occurs in the chloroplast stroma, the region between the thylakoid membrane and the organelle’s inner membrane, just after completing the light reaction of photosynthesis. The light reaction helps the Calvin cycle by providing ATP, which is its energy source, and NADPH for reducing ability.
image credits to: ScienceFacts.net
STEP 1: CARBON fIXATION
STEP 1: CARBON fIXATION
Stage 1: Carbon Fixation
It starts when carbon in the form of carbon dioxide enters through minute pores in the leaves called stomata, where they diffuse into the stroma of the chloroplast. Next, they combine with a five-carbon molecule ribulose-1,5-bisphosphate (RuBP) to form an unstable six-carbon intermediate that breaks down to form two 3-phosphoglyceric acid (3-PGA) molecules. This step of the Calvin cycle is catalyzed by the enzyme RuBP carboxylase/oxygenase, also known as rubisco.
STEP 2: REDUCTION
STEP 2: REDUCTION
Stage 2: Reduction
Using the energy from ATP, the three-carbon compound, 3-PGA molecules, produced in the carbon fixation stage, are converted into a three-carbon sugar glyceraldehyde-3-phosphate (G3P). This step involves the enzyme glyceraldehyde 3-phosphate dehydrogenase, in which NADPH from light reaction acts as the electron donor.
STEP 3: REGENERATION
STEP 3: REGENERATION
Stage 3: Regeneration
This is the final stage of the Calvin cycle that starts with G3P, the end product of the entire pathway. Some G3P is utilized in making glucose, while others are recycled to continue the cycle by combining with a carbon acceptor that turns into RuBP. The energy required to regenerate G3P is derived from ATP.
Purpose of the Calvin Cycle
Primary Functions
Making the unavailable form of carbon present in the atmosphere as carbon dioxide into the useable form of glucose that acts as the source of food and energy in plants. All other organisms indirectly depend on plants for their food and survival.
Forming the structural buildup in plants by preparing three-carbon sugar, that is utilized to make other sugars such as glucose, cellulose, and starch.
The carbon backbones formed in the Calvin cycle make nucleic acids, lipids, protein, and all other building blocks of cells in plants and animals.
Other Functions
Maintaining the level of carbon dioxide, a greenhouse gas, in the atmosphere and thus keeping the earth’s temperature under control.
Providing the source of energy for cellular respiration in plants.
Helping to continue the carbon cycle in nature.
Producing biofuels or carbon-neutral fuels that are environment-friendly.
2. Photosynthesis.pdfLight Reaction Photosynthesis
Calvin Cycle
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