Photosynthesis is a process to synthesize food by using light energy. The light energy source is sun that has light spectrum of infrared, red orange, yellow, green, blue, violet, and ultraviolet. The light which is used in photosynthesis is visible light spectrum (violet-red).
Photosynthesis is done by green plants, some bacteria, and some protists. In this process, sun energy is used to convert 6 water molecules and carbon dioxide molecules into 1 glucose molecule and 6 oxygen molecules. The equation of photosynthesis is
6CO2 + 6H2O —————— C6H12O6 + 6O2
Plants get water from ground in the form of ground water. The water enters the plants through root, and it is carried up to leaves through xylem. Meanwhile, carbon dioxide from the air enters the leaves through stomata.
1. Photosynthesis Pigment
Photosynthesis only occurs in the cells that have photosynthesis pigment, particularly chlorophyll. The chlorophyll (green pigment of leaf) can be found mostly in leaves. The leaves are composed of palisade tissue and spongy tissue containing chloroplast (plastid).
From the structure, chloroplast consists of double membrane covering a chamber filled with liquid called stroma. The stroma is the enzyme-rich liquid where glucose formation takes place in photosynthesis. Besides stroma, in the chloroplast there is also found thylakoid and grana. Thylacoid membrane is the part of chloroplast with disc-shaped structure. Chlorophyll and the others pigment, such as carotene and xanthophyll, usually attach to the thylakoid membrane. Some thylakoid membranes are often arranged in stack called grana (plural: granum).
All pigments attaching to thylakoid membrane are able to absorb light energy of sun and use it in photosynthesis. Chlorophyll a absorbs blue-purple and red lights. Chlorophyll a has green color because it reflects green lights. Chlorophyll b absorbs blue and orange light and reflects green-yellow light. While carotene that has red-yellow color absorbs green-blue light. When a pigment absorbs light, one out of the three things will occur. First, the absorbed energy is released as heat. Second, the absorbed energy is directly passes through with longer wave, which is called fluorescence. Third, the absorbed energy triggers a chemical reaction such as photosynthesis.
2. Photosynthesis Phase
Photosynthesis consists of two process, light reaction and dark reaction. Light reaction occurs in grana (thylakoid membrane) whereas dark reaction occurs in stroma of chloroplast.
a. Light Reaction
Light reaction is a process that depends on the presence of light. The light absorbed by chlorophyll a causes electron exited so that it reaches higher energy level. In a set of chemical reaction, the energy will be converted into ATP and NADPH, whereas the water will be splitted by releasing oxygen as the product of the reaction. Moreover, ATP and NADPH are used to form carbohydrate in dark reaction.
In thylakoid membrane, chlorophyll and other pigments unite to form a photosynthesis structure. In photosynthesis, chlorophyll a, chlorophyll b, and carotene function to capture light energy (photon). The energy is, then, released from one pigment molecule to others until reaching reaction center.
The reaction center consists of 1 chlorophyll a molecule and 1 primary electron acceptor molecule. The molecule of primary electron acceptor will capture the exited electron from chlorophyll molecule of reaction center and release it to electron transport chain.
There are two types of photosystem, photosystem I and photosystem II. Each phtosystem has specific characteristics. In photosystem I, the chlorophyll of reaction center is calledP700 because chlorophyll a can absorb light energy 0n 700 nm wavelength. Photosystem I consists of 12 different protein subunits. Chlorophyll a has 96 molecules consisting of 2 molecules as the reaction center, 4 additional molecules, and 90 molecules as antenna pigment. The antenna pigment is necessary to capture light.
In photosystem II, the reaction center of chlorophyll a is called P680 because chlorophyll a can absorbs light energy in 680 nm wavelength. Photosystem II consists of 20 protein subunits. Fifty or more chlorophyll a molecules consists of 2 chlorophyll molecules as the reaction center, 2 additional molecules, 2 pheophytin molecules (chlorophyll without Mg2+), and other chlorophyll a molecules that function as antenna pigment and 2 plastoquinone molecule.
In light reaction, there occur 2 kinds of electron flow, cyclic and noncyclic electron flow. It is called electron flow because the electron that has been charged flows to the origin reaction center, while the other is called noncyclic electron flow because the energized electron is transferred to NADP + instead of to reaction center. Cyclic electron flow only generates ATP while noncyclic electron flow produces ATP and NADPH. The ATP formation in these processes occurs through photophosphorilation. Photophosphorilation is an energy transformationprocess from exited electron into pyrophosphate derived from ADP molecules.
NADPH formed in photosystem II function to bring the energy required in dark reaction. Electron in photosystem II will replace the exited electron in P700 molecule in photosystem I. That is way there is electron flow from water to NADH. Next, NADPH and ATP will be used to form carbohydrate in dark reaction.
b. Dark Reaction (Calvin Cycle/C-3 Pathway)
Dark reaction is an independent light reaction. It occurs if ATP and NADPH produced by light reaction are available and these compounds are used later to form carbohydrate.
Dark reaction begins with carbon dioxide fixation. Six carbon dioxide molecules enter the cell through stomata and are captured by ribulosa biphosphate (RuBP). Then RuBP (5 carbon molecules) will be converted into one glucose molecule in stroma. Dark reaction is also known as Calvin cycle.
In the next phase, the glucose s converted into 2 phosphoglycerate molecules (PGA, 3 carbons chemical compounds). Each of PGA will accept phosphate from ATP and hydrogen and electron from NADPH to form phosphoglyceraldehyde (PGAL, 3 carbon molecules). So, out of 6 fixated carbon dioxide molecules it is produced 12 PGAL. The 2 of the 12 PGAL are used to form one glucose, whereas the others of 10 PGAL are converted into 6 RuBP molecules. All of the RuBP will recapture the carbon dioxide molecules.
Some plants do preparation towards Calvin cycle which is also known as C-4 pathway. It is called C-4 pathway because the product from carbon dioxide fixation is 4 carbon molecules. In this reaction, carbon dioxide fixation is done by phosphoenolpyruvate (PE, 3 carbon compounds). The fixation result forms oxaloacetate acid (OAA, 4 carbon molecules). The carbon dioxide fixation by PEP is catalyzes by PEP carboxylase which has stronger bind to carbon dioxide than to RuBP carboxylase.
Then, OAA is converted into malate acid, and in mesophyll, it diffuses to bundle sheath where pyruvic acid is formed by releasing carbon dioxide. Pyruvic acid, with the help of ATP, is broken down into PEP and will be reused on C-4 pathway in mesophyll. Meanwhile, carbon dioxide entering Calvin cycle forms glucose which is then carried throughout the body plants. The examples of C-4 plant are grass, corn, and sugar cane.