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 .

References in periodicals archive ?
The light reactions are the ones that researchers normally measure to estimate how much carbon photosynthesis captures, but the viral shunting means that estimate could be too high.
These data suggested that malate inhibited the light reactions of photosynthesis.
We will apply our new capabilities to one of the most important problems in structural biology, which is to elucidate the dynamics of light reactions, electron transfer and protein structure in photosynthesis.
The light reactions on the thylakoid membrane all happen in a microsecond.
The backup of this process sends the light reaction into its ground state discharge of the electrons.
Photosynthesis can be divided into two types of reactions: the light reactions and the carbon-fixation (or dark) reactions.
Then, NADPH and ATP made in the light reaction power the reduction of C[O.
The light reactions must have light energy to initiate them, but the dark reactions will go to completion without light.
The thylakoids support the enzymes associated with the light reactions.
Though we read a bit about New Light reactions to slavery, aside from the important issues surrounding disestablishment, Mulder does not pay particular attention to what might have differentiated these southern New Lights from those in New England or the Mid-Atlantic region.
Photosystem II of the photosynthetic light reactions contains two redox-active tyrosine residues ([Y.
Light reactions involve processes in which solar energy directly converts substances to chemical potential by charge separation.