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The light-dependent reaction

In the light dependent reaction, light energy is transformed into chemical energy. This uses an electron transport chain.

Light is absorbed by special photo-pigments present in protein complexes called photosystems.

There are two types of photosystem present in the thylakoid membranes of plant cells: photosystem I and II.

The main photo pigment is chlorophyll. Chlorophyll absorbs red and blue light, which is used in the light dependent reaction, and reflects green light. This is why most plants appear green.

The chloroplasts are clearly visible in these plant cells.
The chloroplasts are clearly visible in these plant cells.

The light dependent reaction is part of photosynthesis. This reaction photosynthesis can be broken down further into two systems.

  1. The light dependent reaction starts in photosystem II: Light energy oxidises the chlorophyll, and causes it to lose electrons (photo-oxidation). Two electrons are removed per photon.

  2. The electrons are transferred to photosystem I where they reduce NADP$${^+}$$, (a slightly different molecule from NAD$${^+}$$, the electron carrier in respiration). NADPH then diffuses into the stroma and enters the Calvin cycle.

  3. The electrons lost from photosystem II need to be replenished. Light energy is used to split water molecules in order to release electrons (photolysis) and oxygen.

The net reaction is $$\ce{H_2O \rightarrow 2e^- + 2H^+ +\frac{1}{2}O^2+ +2H^+}$$

This reaction is also called noncyclic photophosphorylation because electrons have to be replenished by a separate process.

ATP, NADPH and O$${_2}$$ are generated by the reaction.

ATP is produced during the light dependent reaction in photosynthesis. The method is the same as in the oxidative phosphorylation stage of respiration.

As an electron passes through the chain of redox reactions, small amounts of energy are released at every step. The energy drives a proton pump, pumping hydrogen ions from the stroma into the thylakoid space.

The high concentration of hydrogen ions drives ATP production via ATP synthase, as in oxidative phosphorylation.

There are some differences between the photosynthetic process and respiration:

  • The electrons to start the chain are donated by H$${_2}$$O instead of NADH and FADH$$_2$$.
  • The final electron acceptor is NADP$${^+}$$ instead of O$${_2}$$.
  • Both cyclic (in respiration) and non-cyclic (in photosynthesis) phosphorylation produce ATP, but only non-cyclic photophosphorylation produces reducing agents (such as NADPH).

At times when the plant cell has sufficient NADPH, the light dependent reaction diverts the course of its electron to a cyclic flow. Electrons are only emitted from photosystem I.

The excited electrons are cycled continuously around photosystem I without reducing NADP$$^{+}$$. This process is called cyclic photophosphorylation.

Cyclic photophosphorylation allows the cell to control the amount of carbon compounds it makes whist still producing ATP. Oxygen is not produced during this process.

Chlorophyll is the pigment responsible for absorbing sunlight during photosynthesis. In common plants, it is contained in chloroplasts.

The chlorophyll used by common plants can best absorb blue and red light. In other words, chlorophyll can best capture energy from blue or red light.

By contrast, chlorophyll can only absorb very little green light. In other words, it reflects almost all green light. This is why plants appear green.

Chlorophyll in plants absorbs blue and red light but reflects green light. Only the green light reflected from plants can reach our eyes.
Chlorophyll in plants absorbs blue and red light but reflects green light. Only the green light reflected from plants can reach our eyes.

The spectrum (or range) of light that an object or substance can absorb is called its absorption spectrum. The absorption spectrum of chlorophyll indicates which colours chlorophyll can absorb.

Absorption spectrum of chlorophyll in green plants shows a peak for blue light and another peak for red light. This means these two colours are best absorbed.
Absorption spectrum of chlorophyll in green plants shows a peak for blue light and another peak for red light. This means these two colours are best absorbed.