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Light-Dependent Reactions (In the Thylakoid Membrane). By Thomas Yee, Dalton Wolfe, Mawadda Tarhuni.

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light dependent reactions in the thylakoid membrane

Light-Dependent Reactions (In the Thylakoid Membrane)

By Thomas Yee, Dalton Wolfe, MawaddaTarhuni

electron transport 1

The chlorophylls (a class of light-absorbing pigments used in photosynthesis) and carotenoids (a light absorbent compound which functions as an accessory pigment in photosynthesis) are grouped in clusters of a few hundred pigment molecules in the thylakoid membrane.

  • Each of these pigment molecules is referred to as a photosystem.
  • Two types of photosystems are known: photosystem 1 and photosystem 2
  • They are similar in terms of the kinds of pigment they contain, but they have different roles in the light reactions.

Electron Transport (1)

Examples of Carotenoids

electron transport 2
Electron Transport (2)
  • The light reactions begin when accessory pigment molecules in both photosystems absorb light
  • By absorbing light, those molecules acquire some of the energy that was carried by the light waves.
  • In each photosystem, the acquired energy is passed quickly to other pigment molecules until it reaches a specific pair of chlorophyll a molecules.
  • Light reactions in the thylakoid is a five step process.
light reactions step 1
Light Reactions Step 1
  • Light energy forces electrons to enter a higher energy level in the two chlorophyll a molecules of photosystem 2. These energized electrons are said to be “excited.”
light reactions step 2
Light Reactions Step 2
  • The excited electrons have enough energy to leave the chlorophyll a molecules. Because they lost electrons, the chlorophyll a molecules have undergone an oxidation reaction. Because an oxidation reaction must be accompanied by a reduction reaction, some substance must accept the electrons that the chlorophyll a molecules have lost. That substance is a molecule in the thylakoid membrane known as the primary electron acceptor.
light reactions step 3
Light Reactions Step 3
  • The primary electron acceptor then donates the electrons to the first of a series of molecules located in the thylakoid membrane. This series of molecules is called the electron transport chain, because it transfers electrons from one molecule to the next in series. As the electrons pass from molecule to molecule in the electron transport chain, they lose most of the energy that they acquired when they were excited. The energy they lose is harnessed to move protons in to the thylakoid.
light reactions step 4
Light Reactions Step 4
  • At the same time light is absorbed by photosystem 2, light is also absorbed by photosystem 1. Electrons move from a pair of chlorophyll a molecules in photosystem 1 to another primary electron acceptor. The electrons that are lost by these chlorophyll a molecules are replaced by the electrons that have passed through the electron transport chain from photosystem 2.
light reactions step 5
Light Reactions Step 5
  • The primary electron acceptor of photosystem 1 donates electrons to a different electron transport chain. This chain brings the electrons to the side of the thylakoid membrane that faces the stroma. There the electrons combine with a proton NADP+. NADP+ is an organic molecule that accepts electrons during redox reactions. This causes the NADP+ to become reduced to NADPH.
restoring photosystem 2
Restoring Photosystem 2
  • If electrons from photosystem 2 were not replaced, both electron transport chains would stop, and photosynthesis would not occur.
  • Water molecules provide the replacement electrons.
  • An enzyme inside the thylakoid splits water molecules into protons, electrons, and oxygen.

2H2O  4H+   +   4e-   +  O2

  • For every two molecules of water that are split, four electrons become available to replace those lost by chlorophyll molecules in photosystem 2. The protons that are produced are left inside the thylakoid, while the oxygen diffuses out of the chloroplast and can then leave the plant.
  • Oxygen can be regarded as a by-product of the light reactions. It’s not needed for photosynthesis but it is for cellular respiration.

Water Molecule

  • Kevin, Rogers. (2012) Energy [Photosynthesis], Retrieved November 4, 2012, from:
  • Tim Soderberg. (2012) Organic Chemistry [Hydrogenation/dehydrogenation reactions of carbonyls, imines, and alcohols], Retrieved November 4, 2012, from:
  • [Untitled photograph of chlorophyll a]. Retrieved November 4, 2012, from:
  • Dr. Todd Rosentiel. (2007) Mitochondria [Electron Transport Chain], Retrieved November 4, 2012, from:
  • [Untitled photograph of photosystem] Retrieved November 4, 2012, from:
  • Gary, Kaiser. (2001) Noncyclic Photophosphorylation, Retrieved November 4, 2012, from:
  • Otto, James Howard., and Albert Towle. Modern Biology. New York: Holt, Rinehart and Winston, 1973. Print.