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Understanding the Role of Light in Photosynthesis

Updated on February 25, 2014

What About Photosynthesis?

The photosynthesis is the process used by plants, algae, and many species of bacteria to convert carbon dioxide to organic compounds using energy from the light.

The overall reaction of this process can be translated as: six molecules of water plus six molecules of carbon dioxide in the presence of light produce one molecule of sugar plus six molecules of oxygen:

6 H2O + 6 CO2 → C6H12O6 + 6 O2

The reaction starts when the energy from the light is absorbed by the green pigment called chlorophyll.

Photosynthesis changes the energy from the sun into chemical energy and splits water to liberate O2 and fixes CO2 into sugar
Photosynthesis changes the energy from the sun into chemical energy and splits water to liberate O2 and fixes CO2 into sugar | Source

The photosynthesis is a two stages process:

  • The first process is the Light Dependent Reaction (sometimes called the Light Reaction). In this process, one molecule of chlorophyll absorbs one photon and loses one electron. These electrons are shuttled through an electron transport chain, producing ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). Water is split in the process, releasing oxygen as a by-product of the reaction:
    2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2
  • The second process is the Light Independent Reaction (sometimes called the Dark Reaction). In this process, the carbon dioxide is captured from the atmosphere and is transformed in a three-carbon sugar in what is called the Calvin cycle:
    3 CO2 + 9 ATP + 6 NADPH + 6 H+ → G3P + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O

    First, the CO2 combines with a 5-carbon sugar (RuBP) to create two molecules of a three-carbon compound, glycerate 3-phosphate (GP). Then, the resulted GP, in the presence of ATP and NADPH produced in the Light Reaction stages, is reduced to glyceraldehyde 3-phosphate (G3P). Most of the G3P produced (5 out of 6 molecules) is used to regenerate RuBP so the process can continue. The remaining 1 out of 6 molecules of the resulted G3P often condense to form hexose phosphates (C6H12O6), which ultimately yield sucrose, starch and cellulose. The sugars produced during carbon metabolism yield carbon skeletons that can be used for other metabolic reactions like the production of amino acids and lipids.

The Light

As a general definition, the light is the visible portion of the electromagnetic radiation. It's wavelength starts around 400 nm, and ends near 700 nm. Special cells (inside the retina), sensitive to the electromagnetic radiation in this range, allow conscious perception of light and vision, including color differentiation; the human eye can distinguish around 10 million colors, and each color is associated with a small portion of the spectrum:

Electromagnetic spectrum with light highlighted
Electromagnetic spectrum with light highlighted | Source

The light is composed of many photons. A photon is an elementary particle that is best explained by quantum mechanics, and they exhibit wave–particle duality. They are the basic unit of light and all other forms of electromagnetic radiation.

How the plants see the light

As we mentioned in the beginning of this article, in the Light Dependent Reaction that takes part inside the plants, one molecule of chlorophyll absorbs one photon. But the chlorophyll is only sensitive to a small section of the light spectrum: blue and red:

Absorption spectrum of several plant pigments (left) and action spectrum of elodea (right), a common aquarium plant used in lab experiments about photosynthesis
Absorption spectrum of several plant pigments (left) and action spectrum of elodea (right), a common aquarium plant used in lab experiments about photosynthesis

We see the plants as green just because the blue and red component of the light is absorbed by chlorophyll.

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      shakeela nazeer 4 years ago

      your notes are good and easy wording i like its notes

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