Instant Notes: Plant Biology

produces NADPH and a proton gradient across the thylakoid membrane.

NADPH is used directly in the Calvin cycle(below). The proton gradient is

used to drive adenosine triphosphate (ATP)synthase, the enzyme that makes

ATP.

The second photosystem, PS-I, can work independently of PS-II. This occurs

when electrons are transferred from cytochrome b/fback to P 700 via plasto-

cyanin and re-energized by light. This process, termed cyclic electron flowis

illustrated in Fig. 2. As the electrons never reach NADP reductase, the system

only generates a proton gradient.

ATP is produced by ATP synthase(Fig. 3). This is a large protein complex

located in the thylakoid membrane. As the protons flow down the electrochem-

ical gradient from the thylakoid lumen into the stroma, the energy is used to

synthesize ATP by the phosphorylation of adenosine diphosphate (ADP) by the

ATP synthase complex. ATP synthesis driven by cyclic electron flow (PS-I only)

is termed cyclic photophosphorylation, while ATP synthesis driven by non-

cyclic electron flow (PS-I and PS-II) isnon-cyclic photophosphorylation.

The next stages of photosynthesis do not require light and are termed the

carbon-fixation reactions. They do require the ATP and NADPH generated by

the light reactions and result in the incorporation (fixation) of carbon into carbo-

hydrates. The carbon-fixation reactions in many plants occur in the stroma of

the chloroplast by the Calvin cycle.

The carbon-
fixation
reactions

140 Section J – Metabolism

Redox

potential

(V)

P700

Photosystem

I

P700*

Light

Fd

PC

Cytochrome

bf

complex H+ pump

+

Fig. 2. Cyclic phosphorylation in green plants.