These herbicides all inhibit photosynthesis in susceptible plants.

The photosynthetic pigments in the thylakoid membrane are arranged into photosystems that work to increase the efficiency of light capture. Each photosystem contains 250-400 pigment molecules (chlorophyll and accessory pigments) but only one pair of Chlorophylla molecule in each is situated in the reaction center. Ultimately the absorbed photon energy will be passed as an e- from pigment to pigment until it reaches one of the two types of reaction centers, PSI or PSII.

Photosynthesis occurs in the chloroplasts of the plant cells.

1. Draw and label a diagram showing the structure of a chloroplast as seen in electron micrographs.

Photosystem I (PSI), Photosystem II (PSII), and ATP Synthase.

Photosynthetic Inhibitors control many broadleaf and some grass weeds. In general, these herbicides inhibit photosynthesis by binding to D1 proteins of the photosystem II complex in chloroplast thylakoid membranes. Herbicide binding at this protein blocks electron transport and stops CO2 fixation and production of energy needed for plant growth. The death of plants, however, does not occur primarily from photosynthates depletion but rather from an indirect effect on other processes. Blocking electron transport in PSII systems promotes the formation of highly reactive molecules that initiate a chain of reactions causing lipid and protein membrane destruction that results in membrane leakage allowing cells and cell organelles to dry and rapidly disintegrate. In addition, some PSII Inhibitors affect other plant processes such as carotenoid biosynthesis and synthesis of anthocyanin, RNA, and proteins.

PSI and PSII absorb light through pigments (explained below).

ATP synthase (ATPase)





3--Bacteria that carry out oxygenic photosynthesis have 2 photosystems(PSI and PSII) with 2 reaction centers (RCI and RCII).

Electrons flow from H2O -- PSII -- PSI -- NADP+ generating O2, ATP and NADPH

The new structures of PSI and PSII from ..

Determination of efficient (photosynthetically speaking) light sources is important for several reasons. Economy in the costs of maintaining a coral reef aquarium certainly is a concern, but the health of zooxanthellae and hence their animal host should be of primary importance. Aesthetic concerns, such as the promotion of coral coloration through expression of fluorescent proteins and non-fluorescent chromoproteins, are of interest to many.

Difference between Photosystem I and Photosystem II ..

In contrast, at room temperature, energy transfer processesbetween the various photosynthetic complexes result in very similarfluorescence emissions, which mainly consist of fluorescence photonsemitted by PSII hindering a discrimination based on spectral ROIs(regions of interest).

Details of Photosynthesis in Plants

In plants and algae a two-stage photocenter is used. First photosystem II absorbs a photon and passes and electron to photosystem I, driving the proton pump to generate ATP. When a second photon is adsorbed by photosystem I and eventually passed along to nicotine adenine dinucleotide phosphate (NADP+) to generate NADPH (a two electron transfer). This two stage process is irreversible and non-cyclic and the two stages are needed to provide charge separation while gaining sufficient energy to drive the reduction of ferredoxin. Ultimately electron replacement comes from water, thereby producing oxygen.

PSI and PSII contain chlorophylls and ..

A pair of P680 molecules at the center energize a single light electron (photon) until it's released into an electron transport chain; this is repeated for all the light electrons.
Calvin Cycle
Second stage of photosynthesis, takes place in the stroma.

Non-cyclic photophosphorylation involves PSI and PSII.

These measurements have, up to now, mostly been carried out at lowtemperatures (77 K), as these conditions enable the differentiationbetween the fluorescence of Photosystem I (PSI) and Photosystem II(PSII).