Light Dependent Reaction of Photosynthesis

The chemical equation for photosynthesis is:
6CO2 + 6H2O -> C6H12O6 + 6O2

The equation for aerobic respiration is:
C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy

Photosynthesis creates glucose, while respiration breaks down glucose to release energy. Respiration uses up oxygen, while photosynthesis produces it. The point where respiration and photosynthesis occur at the same rate is called the compensation point.

Photosynthesis takes place in the chloroplasts. Chloroplast contains photosynthetic pigments, like chlorophyll and carotene, which absorb light for photosynthesis and are found in the thylakoid membrane. These pigments are attached to proteins, to form a photosystem. There are two photosystems, PS1 which absorbs light best at a wavelength of 700nm, and PS2 which absorbs light best at a wavelength of 680nm.

Coenzymes are molecules that aid the function of an enzyme. A coenzyme involved in photosynthesis is NADP, which transfers H+ ions (protons) from one molecule to another, reducing and oxidising molecules.

The light-dependent reaction takes place in the thylakoid membranes. There are two types of photophosphorylation, cyclic and non-cyclic.

Non-cyclic photophosphorylation produces ATP, NADPH and O2. You need to know that photosystem one and two are linked by electron carriers, which are proteins that carry electrons. The photosystems and the electron carriers form an electron transport chain, which is a chain of proteins where excited electrons flow.

The steps to non-cyclic photophosphorylation are:

1) Light energy excites electrons in chlorophyll. Light energy is absorbed by PS2, which excites electrons to a higher energy level. These high energy electrons are released from PS2 and move down the electron transport chain to PS1.

2) The excited electrons that are released from the PS2 need to be replaced, therefore light energy splits water into H+, 1/2O2, and electrons in the equation H2O  ->  H+ + 1/2O2 (photolysis).

3) Energy released from excited electrons is used to make ATP via a series of redox reactions down the electron transport chain. The excited electrons lose energy as they move down the electron transport chain. This energy is used to actively transport protons into the thylakoid membrane, creating a proton gradient between the stroma and the thylakoid. 

4) Protons move down their concentration gradient into the stroma through the enzyme ATP synthase in the membrane by facilitated diffusion. The energy from this movement is used to phosphorylate ADP to ATP.

5) The NADPH is produced when PS1 absorbs light energy, and moves the already excited electrons to an even higher energy level. A very excited electron, and a proton from the stroma, is sent to a NADP coenzyme reducing it to NADPH.


Cyclic photophosphorylation only occurs in PS1 and only produces ATP. It’s called cyclic because instead of the electrons being used to reduce NADP, they are instead moved back to PS1 via electron carriers. 

photophosphorylation, beginner biology

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