The light-dependent stage of photosynthesis is the series of events that only occurs when light is present. It involves the use of energy from light to split water molecules and produce ATP and reduced NADP. There are two reaction pathways that occur as part of the light-dependent reaction, called non-cyclic photophosphorylation and cyclic photophosphorylation.

Non-cyclic photophosphorylation

» Chlorophyll molecules in PSI and PSII absorb light energy. The energy excites electrons, raising their energy level so that they leave the chlorophyll. The chlorophyll is said to be photoactivated.
» PSII contains an enzyme in an oxygen-evolving complex that splits water when activated by light. This reaction is called photolysis (‘splitting by light’).
» The water molecules are split into oxygen and hydrogen atoms.
» Each hydrogen atom then loses its electron to become a positively charged hydrogen ion (proton), H+.
» The electrons from the hydrogen are picked up by the chlorophyll in PSII, to
replace the electrons they lost.
» The oxygen atoms from the water join together to form oxygen molecules, which
diffuse out of the chloroplast and into the air around the leaf.
2H2O >> 4H^+ + 4e^- + O2 (in presence of light)
» The electrons emitted from PSII are picked up by electron carriers (electron transport chain) in the membranes of the thylakoids.
» Similarly, to oxidative phosphorylation in mitochondria, as the electrons move along the chain, they release energy.

– This energy is used to actively transport protons (hydrogen ions) across the thylakoid membrane into the space between the membranes.
– A high concentration of protons builds up in the thylakoid space.
– The protons are allowed to move back by facilitated diffusion into the stroma from the thylakoid space through ATP synthases, by chemiosmosis. This provides energy to cause ADP and inorganic phosphate to combine to make ATP. This is called photophosphorylation.

» At the end of the electron carrier chain the electrons are picked up by PSI to replace the electrons the chlorophyll in PSI had lost.
» The electrons from PSI are passed along a different chain of carriers to NADP.
» The NADP also picks up the protons (hydrogen ions) from the split water
molecules. The NADP becomes reduced NADP.

If you follow one electron from a water molecule, you can see how it:

• Is taken up by PSII
• Has its energy raised as the chlorophyll in PSII absorbs light energy
• Loses some of this energy as it passes along the electron carrier chain
• is taken up by PSI
• Has its energy raised again as the chlorophyll in PSI absorbs light energy
• becomes part of a reduced NADP molecule

At the end of this process, two new substances have been made. These are ATP and
reduced NADP. Both of these will now be used in the next stage of photosynthesis –
the light-independent stage.

Cyclic photophosphorylation

There is an alternative pathway for the electron that is emitted from PSI.
» The electron can simply be passed along the electron transport chain, then back
to PSI again.
» ATP is produced as it moves along the electron transport chain (photophosphorylation).
» However, no reduced NADP is produced. This is called cyclic
photophosphorylation.

Practice Questions.

Describe the structure of photosystems and explain how a photosystem functions in cyclic photophosphorylation.

• arranged in light harvesting, clusters/system ;
• primary pigments/chlorophyll a ;
• at reaction centre ;
• P700/P1, absorbs at 700(nm) ;
• P680/P11, absorbs at 680(nm) ;
• accessory pigments/chlorophyll b/carotenoids, surround, primary pigment/reaction centre/ chlorophyll a ;
• pass energy to, primary pigment/reaction centre/chlorophyll a ;
• P700 / PI, involved in cyclic photophosphorylation ;
• (light absorbed results in) electron excited ;
• emitted from, chlorophyll/photosystem ;
• flows along, chain of electron carriers/ETC ;
• ATP synthesis ;
• electron returns to, P700/P1 ;

Describe how non-cyclic photophosphorylation produces ATP and reduced NADP.

• photosystem I (PI) and photosystem II (PII) involved ;
• light harvesting clusters ;
• light absorbed by accessory pigments ;
• primary pigment is chlorophyll a ;
• energy passed to, primary pigment / chlorophyll a ;
• electrons, excited / raised to higher energy level ;
• (electrons) taken up by electron acceptor ;
• (electrons) pass down electron carrier chain (to produce ATP) ;
• PII has (water splitting) enzyme ;
• water split into protons, electrons and oxygen ;
• photolysis ;
• electrons from PII pass to PI / electrons from water pass to PII ;
• to replace those lost ; give either in relation to PI or PII
• protons and electrons combine with NADP (to produce reduced NADP) ;