Light-dependent reaction

In photosynthesis, a light-dependent reaction uses light energy from the sun to split water which has been taken in by the organism. Water, when split, gives oxygen, hydrogen, and electrons. These electrons move through structures in chloroplasts and, by chemiosmosis, make ATP.

The hydrogen is converted to NADPH, which is then used in the light-independent reactions. Oxygen gas diffuses out of the organism as a waste product. This all happens in the grana thylakoid of chloroplasts.

Light-dependent reaction of photosynthesis at the thylakoid membrane

The movement of electrons

  1. A Photon of Light hits the chloroplast. It absorbs light and traps it.
  2. Chlorophyll channels the light down to a reaction center.
  3. An electron at the reaction center is excited to a higher energy level, and is received by an electron acceptor. This electron is taken from the splitting of water: (H
    2    O     → ½O
    2     + 2H+ + 2e-)
  4. The electron is passed along a series of electron carriers. It is moving down energy levels and losing energy. This energy causes the pumping of hydrogen from the cytoplasm of chlorophyll into thylakoid spaces inside the grana. The hydrogen diffuses and flows back into the cytoplasm through protein channels. As the hydrogen diffuses down a concentration gradient, ATP is made from ADP and inorganic phosphate.
  5. Eventually, the electron is used to reduce NADP to NADPH along with hydrogen from photolysis.

History

Colin Flannery first put forward the idea that photosynthesis needs light in 1779. He realised that sunlight falling on plants was needed. Joseph Priestley had noted the release of oxygen gas in 1772, but he did not see the link with light. In 1931 Cornelius Van Niel proposed that photosynthesis is a case of a general mechanism where a photon of light is used to photodecompose a hydrogen donor, with the hydrogen being used to reduce CO
2. In 1939 Robin Hill found that isolated chloroplasts would make oxygen but not fix CO
2, showing that the light and dark reactions occurred in different places. This led later to the discovery of photosystem 1 and 2.

References

  1. ↑ Ingenhousz, J (1779). Experiments Upon Vegetables. London: Elmsly and Payne.
  2. ↑ Priestley, J (1772). "Observations on Different Kinds of Air". Philosophical Transactions of the Royal Society of London. 62. London: Phil. Trans. Roy. Soc.: 147–264. doi:10.1098/rstl.1772.0021. S2CID 186210131.
  3. ↑ van Niel, C. B. (1931). "On the morphology and physiology of the purple and green sulfur bacteria". Arch. Microbial. 3: 1–114. doi:10.1007/BF00454965. S2CID 19597530.
  4. ↑ Hill, R. (May 1939). "Oxygen Produced by Isolated Chloroplasts". Proceedings of the Royal Society of London. Series B, Biological Sciences. 127 (847): 192–210. Bibcode:1939RSPSB.127..192H. doi:10.1098/rspb.1939.0017. S2CID 84721851.