Chloroplasts, ATP and the light-dependent reactions

What you’ll learn

• Link chloroplast structure to photophosphorylation.
• Compare cyclic and non-cyclic electron flow.
• Explain the roles of water, ATP and reduced NADP.

Understand the idea

A chloroplast has a double envelope surrounding the stroma, with internal thylakoid membranes arranged into grana. Photosystems, electron carriers and ATP synthase occupy these membranes; enzymes for carbon fixation occur in the stroma. A large membrane area accommodates many light-harvesting complexes, while a small thylakoid lumen allows a proton gradient to develop.

Light excites electrons in chlorophyll. In non-cyclic photophosphorylation, electrons travel through carriers from photosystem II towards photosystem I and ultimately reduce NADP. Electron transfer releases energy that helps accumulate protons in the thylakoid lumen. Protons return to the stroma through ATP synthase, driving ATP formation from ADP and inorganic phosphate. Photolysis supplies replacement electrons, protons and oxygen; the released oxygen originates from water.

Cyclic electron flow uses photosystem I: excited electrons return through carriers, generating ATP without net production of reduced NADP or oxygen. ATP transfers energy over short timescales when hydrolysed; reduced NADP supplies reducing power. These products connect the light reactions to the synthesis of organic molecules, in which chemical energy can be stored for longer.

Key formulae

$$2\mathrm{H_2O}\longrightarrow4\mathrm{H^+}+4e^-+\mathrm{O_2}$$

$$\mathrm{ADP}+\mathrm{P_i}\longrightarrow\mathrm{ATP}+\mathrm{H_2O}\quad\text{requires energy}$$

$$\mathrm{ATP}+\mathrm{H_2O}\longrightarrow\mathrm{ADP}+\mathrm{P_i}\quad\text{releases usable energy}$$

Worked example

An isolated thylakoid preparation makes ATP when illuminated but produces no oxygen or reduced NADP. Explain a possible electron pathway.

1. ATP production implies that energy has supported proton movement and ATP synthase activity.
2. The absence of oxygen means that net photolysis is not demonstrated; the absence of reduced NADP means that electrons are not being transferred to that final acceptor.
3. Cyclic flow around photosystem I returns electrons to the electron-transfer pathway and can support ATP production alone.

Answer: The observations are consistent with cyclic photophosphorylation, although additional evidence would be needed to exclude other explanations.