light reactions

Light reactionsclick for a larger image
Fig. 211 Light reactions . The main chemical pathways in the light reactions of photosynthesis.

light reactions

those processes of PHOTOSYNTHESIS requiring light energy in which ATP and NADPH are formed, to be incorporated later into the CALVIN CYCLE. Various constituents are necessary for the light reactions to occur, namely:
  1. CHLOROPLASTS (in higher green plants) within parenchyma cells, particularly the MESOPHYLL tissue of leaves.
  2. various pigments on the inner membranes of the chloroplasts responsible for trapping light energy Chlorophyll a is the primary pigment, which, with one or more types of accessory pigment such as chlorophyll b and various carotenoids surrounds a single molecule of specialized chlorophyll a (P680 and P700), forming a ‘photosystem’. Photosystem 1 (PS I) contains P700 and photosystem II contains P680 .
  3. two ELECTRON TRANSPORT SYSTEMS (see Fig. 145 ).
  4. water.

Two separate processes of PHOTOPHOSPHYLATION occur:

  1. cyclic photophosphorylation in which the various pigments in PS 1 collect light striking the chloroplast, passing the energy on to P70 0 which undergoes PHOTOACTIVATION. An electron with raised energy levels is accepted by ferredoxin and passed on to an ETS where ATP is produced as the energy level falls back to the starting point.
  2. noncyclic photophosphorylation. Here the initial source of electrons is water, which releases electrons by separation of charge (previously thought to be by means of PHOTOLYSIS) and these are passed on to PS II, and then on to plastoquinone at a higher level. As in cyclic photophosphorylation, ATP is produced via the ETS, with the electron dropping down to PS I. Then, however, light causes the energy level of the electron to be raised to a level high enough to be accepted by ferredoxin. At this stage a second ETS is entered, leading to the production of NADPH, with hydrogen coming from the separation of water into ions. Two photosystems are involved in the reduction of NADP because there is insufficient energy in light to energize the electrons from water straight to NADP: it requires two steps.

Thus, the products of these two types of light reactions are ATP, NADPH and oxygen. The first two products enter the DARK REACTIONS of photosynthesis, where they become involved in the CALVIN CYCLE and the synthesis of PGAL and eventually of GLUCOSE. See Fig. 211 .

Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005
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