Types of Photosynthesis: Light-dependent, Light-independent, C3, C4

How Many Types of Photosynthesis Are There?

From the meats and dairy products in grocery stores to breads, cereals and pastas, most foods that people consume link back to photosynthesis. Light energy enters a plant through pigment molecules called chlorophyll, which then releases the energy by splitting water molecules.

Photosynthesis takes place in organelles called chloroplasts, found in cells of plants and algae. Chloroplasts contain stacked, disc-shaped structures called thylakoids.

1. Light-dependent

The light-dependent reactions (also called photosynthesis-reactions) use sunlight to convert water molecules into ATP and NADPH, which are energy-storing molecules. The products of these reactions are used by the light-independent reactions to “fix” carbon dioxide and assemble sugar molecules.

In the light-dependent reaction, one molecule of the pigment chlorophyll absorbs a photon and loses an electron. This electron passes to a special enzyme in the photosystem known as cytochrome b6f. The cytochrome b6f then moves the electron to the center of the photosystem where it is joined with another chlorophyll molecule and passed through an electron transport chain that eventually leads to reduced NADP and ATP.

This is the first step in photosynthesis. It also produces the oxygen that is present in Earth’s atmosphere. The process takes place in the thylakoid membranes of the chloroplasts. The thylakoid membranes contain the light-sensitive pigments chlorophyll and other proteins. Both prokaryotic and eukaryotic organisms carry out this process, including cyanobacteria (diatoms, dinoflagellates) and green algae and plants, such as vascular plants, bryophytes, pteridophytes, gymnosperms, and angiosperms.

2. Light-independent

The light-independent reactions of photosynthesis use the chemical energy harvested by the light-dependent reactions to assemble carbon dioxide into organic molecules, such as glucose. These reactions take place in the stroma, a colourless fluid that surrounds the thylakoid discs of chloroplasts. They are collectively known as the Calvin cycle.

In the light-independent reactions, the ATP and NADPH produced by the thylakoid membranes are used to reduce and phosphorylate atmospheric carbon dioxide into a carbohydrate molecule called 3-phosphoglycerate (3PG) or G3P. 3PG can then be incorporated into an organic molecule, such as glucose.

The reversible reaction that turns water into glucose is called the light-independent reaction of photosynthesis. It is also the final step of photosynthesis that produces oxygen, and it happens in the stroma of chloroplasts in the dark (without sunlight). The reversible reaction in this stage is called the Calvin cycle. It combines six molecules of carbon dioxide and 12 molecules of water to produce one molecule of glucose.

3. C3

This is the process that most plants use to make food from carbon dioxide and water. It’s also why our planet is blanketed with an oxygen-rich atmosphere. During photosynthesis, plants, algae and cyanobacteria convert sunlight into energy that they then store as organic molecules such as carbohydrates.

The C3 photosynthesis pathway is the dominant process used by most land plants, including cereals, rice, cotton and potatoes. It uses a process called the Calvin cycle to remove carbon from atmospheric CO2 and turn it into organic molecules.

C3 plants can benefit from increasing levels of carbon dioxide in the air resulting from the climate crisis, but this may be offset by increased temperature that could cause stomatal stress. Research is underway to improve C3 photosynthesis in order to ensure that crops can continue producing food. One approach is to introduce C4 traits into C3 plants. Glycine transport loops in bundle sheath cells can reduce photorespiratory CO2 loss and increase C3 photosynthesis at a given temperature by compartmentalizing glycine decarboxylation in the bundle sheath cells.

4. C4

In C4 photosynthesis, light energy is used to split water in a process called photosynthesis, producing O2 and electrons. These are used to produce the ATP and NADPH required for carbon fixation. This produces a four-carbon intermediate compound in the chloroplast of a thin-walled mesophyll cell, which is then pumped to a thick-walled bundle sheath cell where it is converted to CO2.

The CO2 diffuses out of the bundle sheath cells, and the ATP energy used to pump it back to the mesophyll cell for conversion to PEP makes this type of photosynthesis less efficient than C3. In addition, it requires low CO2 concentrations. Consequently, C4 plants can thrive in arid regions and habitats with high temperatures, low water availability, and saline soils. The evolution of C4 photosynthesis is thought to have been driven by a combination of conditions. The appearance of C4 plants in the evolutionary record coincides with periods of global aridification and declining atmospheric CO2. Gene duplication, neofunctionalization and carbon conservation selection are leading theories to explain its origin.

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