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Transformation of light energy into chemical energy (ATP) and reducing power (NADPH)

The synthesis of ATP or phosphorylation of ADP occurs thanks to the energy provided by photons of light, which is why this process is called photophosphorylation.

The photophosphorylation occurs when, at the excited by the action of light, a flow of electrons from the photosystems are conducted through the different electronic acceptors to NADPH, while a proton gradient generated whose energy is used to synthesize ATP.

When electrons are falling from higher to lower energy levels along the transport chain, the released energy is used to pump protons (H+) from the stroma into the interior space of the thylakoid. Thus a proton gradient is formed between each side of the thylakoid membrane. Inside the thylakoid there is an acidic pH (pH = 5), and outside, basic (pH = 8).

Protons cannot directly cross the thylakoid membrane, so they return to the stroma, down the gradient, through proton translocating ATPases. When they pass, the gradient disappears and they have generated energy that is used to phosphorylate ADP and synthesize ATP.

Two types of photosynthesis are distinguished, depending on whether the PSI and PSII participate jointly or if only the PS I does it:

Difference between photophosphorylation and oxidative phosphorylation.

The photophosphorylation is associated with an anabolic process that occurs in the chloroplasts of plant cells, and releases oxygen.

The oxidative phosphorylation is associated with a catabolic process that occurs in the mitochondria of the animal and plant cells, and requires oxygen as the terminal electron acceptor.

Main processes of the photochemical or light phase or dependent on light Capture of light by photosystems
Photolysis of water
Electronic transport
ATP synthesis
Synthesis of NADPH


Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Andalusia, June 2018, option A, question 1.

a) Indicate the phases of photosynthesis [0,2] and the basic processes that take place in each of them [1].

b) Describe the photophosphorylation [0.6] and its location within the corresponding cell organelle [0.2].

Aragon. June 2003, option A. Question 3.-

Please answer the following questions precisely and briefly (2 marks):

a) How is light energy transformed into chemical energy? What role does water play in these processes? (1 point)


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