A Guide to Photosynthesis

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The light-dependent reactions of photosynthesis are directly fuelled by light energy from the sun.

Recall that chloroplasts contain grana - stacks of thylakoids - suspended in a fluid called the stroma. The thylakoids contain the pigment chlorophyll.

The electron transport chain

When a photon - a small packet of light energy - of the right wavelength hits chlorophyll, it is absorbed by this large molecule, causing two electrons to be excited to a higher energy level. The excited electrons are then transferred from chlorophyll to an electron carrier, which is a protein.

A reaction in which electrons are transferred is a redox reaction. The substance which gains electrons is reduced, and the substance which loses them is oxidised (recall the OILRIG acronym: Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons). Therefore, chlorophyll has been oxidised and the electron carrier has been reduced.

Electrons are then transferred from the electron carrier to another electron carrier - another redox reaction - and then to another. Each of these reduced electron carriers is more stable than the last, so energy is released and used to convert ADP to ATP - see further down for details of how this happens.

Finally, the electrons are passed from the third electron carrier to a coenzyme called NADP (Nicotinamide Adenine Dinucleotide Phosphate). A coenzyme is simply a molecule that aids in an enzyme-controlled reaction. NADP aids by acting as an electron-carrying intermediate in redox reactions: in this case, NADP accepts the electrons originating from chlorophyll, and is thus reduced; in the later stages of photosynthesis, it then transfers these electrons to carbon dioxide.

The equation for the reduction of NADP is:

NADP+ + 2H+ + 4e- => NADPH + H+

The protons (H+) originate from the photolysis of water - see below.

Photolysis of water/regeneration of chlorophyll

As well as exciting chlorophyll, the photons from the sun have a second role in photosynthesis: splitting water.

The equation for the photolysis ("light-splitting") of water is:

2H2O => O2 + 2H+ + 2e-

The electrons reduce chlorophyll, replacing those that were lost in the electron transport chain and hence allowing the chlorophyll molecule to be excited again.

The protons:

take part in photophosphorylation (see below)

react with NADP+ to form NADPH + H+ (see above)

The mechanism of ATP production

The mechanism by which ATP is synthesised in photosynthesis (and also respiration) is called chemiosmosis.

(Apologies for poor diagram)

The energy released by the redox reactions in the electron transport chain is used to actively transport protons from the stroma into the thylakoids (across the thylakoid membrane).

This results in a large concentration and charge gradient between the stroma and the thylakoids.

A large amount of potential energy is stored in this gradient. With the gradient established, the protons will then diffuse back into the stroma through a transport protein called ATP synthase. True to its name, ATP synthase uses the stored potential energy of the concentration and charge gradient to convert ADP into ATP:

ADP + Pi => ATP

Since the energy for this phosphorylation originated from the photon, this is known as photophosphorylation.

Why so complicated?

The resultant equation of the light-dependent reactions is:

H2O + NADP+ + ADP + Pi => NADPH + H+ + O2 + ATP

So why bother with the intermediate stages? Why not just have the photon split water, and transfer the protons and electrons to NADP? Hess' Law tells us that the amount of energy released will be the same, so the energy would still be available for ATP production.

The answer is that evolution has favoured reaction pathways which release energy in several small, manageable bursts rather than reactions in which all the energy is released in one go. If a large amount of energy is released in one go, not only is it less efficient as less of this energy can be captured, but could also be damaging to the cell.

To Sum Up

A photon excites chlorophyll, causing an electron to be transferred from chlorophyll to an electron carrier.

The electron is passed through a series of electron carriers, with energy being released at each stage.

The energy released is used to pump protons from the stroma into the thylakoids, creating a concentration and charge gradient storing potential energy.

As the protons diffuse back into the stroma, ATP synthase uses the potential energy to convert ADP to ATP (photophosphorylation).

The electrons are passed to the final electron carrier, NADP.

A photon splits water (photolysis) to electrons (transferred to chlorophyll), protons (transferred to NADP), and oxygen.

The net products are oxygen gas, ATP, and NADPH + H+.

Light-dependent reactions