Function Of Thylakoid Membranes Research Articles

Effect of light quality on the composition and function of thylakoid membranes in atriplex triangularis

Light quality was shown to exert well-coordinated regulatory effects on the composition and function of the thylakoid membranes as well as on the photosynthetic rates of intact leaves from Atriplex triangularis grown in continuous blue, white and red lights (50 μE · m −2 · s −1). The higher photosynthetic rates in plants grown in blue light, as compared to those in white and red lights, resulted from marked changes in both light-harvesting complexes and electron carriers. The concentrations of electron carriers such as atrazine binding sites, plastoquinone, cytochromes b and f and P-700 on a chlorophyll basis were markedly increased in Atriplex grown in blue light; and the apparent light-harvesting antenna unit sizes of Photosystems I and II were greatly reduced. Consequently, the electron transport capacities of Photosystems I and II were also increased as was the coupling factor CF 1 activity. Atriplex grown in red light had lower photosynthetic rates than those grown in blue or white light by incorporating changes in the composition and function of the thylakoids in a direction opposite to those caused by growth in blue light. When these regulatory effects of light quality were compared with those of light quantity [6,7], it is clear that Chla Chl b ratios, electron transport capacities of Photosystems I and II, concentrations of plastoquinone, atrazine binding sites, coupling factor CF 1 activity and the apparent antenna unit size of Photosystem II are more affected by light quantity, whereas light quality has a greater influence on the concentration of P-700, the apparent antenna unit size of Photosystem I and the overall photosynthetic rates of intact leaves.

Changes in composition and function of thylakoid membranes as a result of photosynthetic adaptation of chloroplasts from pea plants grown under different light conditions

The hypothesis that chloroplasts having different light-saturated rates of photosynthesis will have different proportions of the intrinsic thylakoid complexes engaged in light-harvesting and electron transport (Anderson, J.M. (1982) Mol. Cell. Biochem. 46, 161–172) has been tested. Peas were grown in light regimes which varied in light intensity, quality and time of irradiance, and ranged from sunlight through red to blue-enriched light of very low radiation. The electron-transport capacity at saturating light of Photosystem I and Photosystem II of chloroplasts isolated from light-adapted peas was 2-fold and 5–6-fold lower, respectively, in the lowest radiation compared to sunlight. There was a marked increase in the amount of total chlorophyll associated with the main chlorophyll a b- proteins (LHCP 1, LHCP 2 and LHCP 3) and a 2-fold decrease in the core reaction centre complex of Photosystem II (CP a) as the radiation decreased; the LHCP 1–3 CP a ratio changed from 3.5 to 9.0. The amount of chlorophyll associated with Photosystem I varied from 34% in sunlight to 27% in the lowest radiation, but the antenna size of Photosystem I was not markedly different; there was a 2-fold decrease in the amount of cytochrome f on a chlorophyll basis, which partly accounted for the decreased electron-transport capacity of Photosystem I. Since the increases or decreases in the levels of each of the components correlated with decreasing radiation, it is clear that the light-adaptation of both light-harvesting and electron-transport components is indeed closely co-ordinated.

Evidence for a close spatial location of the binding sites for CO 2 and for Photosystem II inhibitors

1. 1. CO 2-depletion of thylakoid membranes results in a decrease of binding affinity of the Photosystem II (PS II) inhibitor atrazine. The inhibitory efficiency of atrazine, expressed as I 50-concentration (50% inhibition) of 2,6-dichlorophenolindophenol reduction, is the same in CO 2-depleted as well as in control thylakoids. This shows that CO 2-depletion results in a complete inactivation of a part of the total number of electron transport chains. 2. 2. A major site of action of CO 2, which had previously been located between the two electron acceptor quinone molecule B (or R) and plastoquinone, appears to be in the same membrane protein which binds the Photosystem II inhibitor atrazine as suggested by the following observations: (a) CO 2-depletion results in a shift of the binding constant ( k b ) of [ 14C]atrazine to thylakoid mebranes indicating a decreased affinity of atrazine to the membrane; (b) trypsin treatment, which is known to modify the Photosystem II complex at the level of B, strongly diminishes CO 2 stimulation of electron transport reactions in CO 2-depleted membranes; and (c) thylakoids from atrazine-resistant plants, which contain a Photosystem II complex modified at the inhibitor binding site, show an altered CO 2-stimulation of electron flow. 3. 3. CO 2-depletion does not produce structural changes in enzyme complexes involved in Photosystem II function of thylakoid membranes, as shown by freeze-fracture studies using electron microscopy.

The influence of cations and methylamine on structure and function of thylakoid membranes from barley chloroplasts

The effects of cations on the structure and activity of isolated barley (Hordeum vulgare cv. Svalöfs Bonus) chloroplast lamellar systems were studied. Three separate effects of cations were recognizable. (1) Cations in high concentrations (in excess of 100–200 mM NaCl) are required to maintain granal stacks once the outer chloroplast envelope is broken. In 30 mM NaCl the majority of the lamellar systems exist as well-separated thylakoids, even in the presence of high concentrations of sucrose (330 and 660 mM). The lamellar systems photoreduce ferricyanide at coupled rates whether they are in the granal or non-granal configuration. (2) Cations are required for Hill reaction activity. This requirement becomes apparent following loss of thylakoid membrane integrity at very low cation concentrations. Maximum activation of the photoreduction of ferricyanide occurs at 30 mM NaCl, and divalent ions (Mg++, Ca++, Mn++) are 12 times more effective than monovalent ions (Na+, K+). (3) Cations are necessary to preserve the integrity of thylakoid membranes. Below about 8 mM NaCl, the thylakoids swell and this change is accompanied by progressive loss of Hill activity and the capacity to maintain a light-dependent proton gradient. Divalent cations are more than 200 times as effective as monovalent ions in preventing these changes. Hill reaction activity, but not proton pump activity, is regained by adding cations back to the swollen thylakoids (cation effect 2, above) and the new rate of Hill reaction activity is the same as in chloroplasts uncoupled with methylamine. The re-establishment of Hill reaction activity is accompanied by appression of the swollen thylakoids. The uncoupler methylamine causes stacking of thylakoids and collapsing of intrathylakoidal spaces.