Tag: metabolism, cell respiration, and photosynthesis

Photosynthetic bacteria posses only one type of photosystem and are mostly anaerobic and contains two type of pigment bacteriochlorophylls and bacterioviridin while cyanobacteria or blue-green algae posses two photosystems  PS I and PS II.

Photosystem I is one of the two membrane-bound photosystems of plants, algae and cyanobacteria that facilitate light-determined electron transport from water to NADPH. It utilizes absorbed light for electron transport from plastocyanin on the lumenal side to ferredoxin on the stromal side of the thylakoid membrane. In plants, this special integral membrane complex consists of more than 15 protein subunits, approximately 175 chlorophyll molecules, 2 phylloquinones and 3 Fe$ _4$S$ _4$ clusters. Whereas Photosystem II (of cyanobacteria and green plants) is as many as 35 chlorophyll a, 12 beta-carotene, two pheophytins, two plastoquinone, two heme, one bicarbonate, 20 lipid molecules and other ionic clusters. Thus Photosystem I contains more chlorophylls and accessory pigments.

DCMU or Dichlorophenyl dimethyl urea inhibits PS II in photosynthetic plants by blocking electron transfer from plastoquinone to cytochrome. DCMU binds to and blocks the site of plastoquinone, thus hinders the path and growth of plants. It is used as herbicide and algicide. It is also used in studying photosynthetic activity.

Being a light reaction, non-cyclic photophosphorylation occurs in the thylakoid membrane. First, a water molecule is broken down into 2H⁺ + 1/2 O₂ + 2e⁻ by a process called photolysis (or light-splitting). The two electrons from the water molecule are kept in photosystem II, while the 2H⁺and 1/2O₂ are left out for further use. Then a photon is absorbed by chlorophyll pigments surrounding the reaction core center of the photosystem. The light excites the electrons of each pigment, causing a chain reaction that eventually transfers energy to the core of photosystem II, exciting the two electrons that are transferred to the primary electron acceptor, pheophytin. The deficit of electrons is replenished by taking electrons from another molecule of water. 

A) PS I can funtion at wavelengths of 700 nm or less. So, it will be active at 550 nm. But its maximum activity is at 700 nm.
B) It will be active at 680 nm.
C) It will be active at 690 nm.

Photosystems refer to PSI and PSII. Photosystem II contains chlorophyll a, as well as up to 50% chlorophyll b. It probably evolved later as a supplement to Photo I. It is needed to capture enough energy to do the biosynthetic reactions of the dark reaction. Its reaction centre is a molecule called P680 which absorbs light maximally at 680 nm. Similarly, PSI or P700 absorbs light at 700nm. A reaction centre comprises several (>10 or >11) protein subunits, that provide a scaffold for a series of cofactors. The cofactors can be pigments (like chlorophyll, pheophytin, carotenoids), quinones, or iron-sulfur clusters and electron acceptors for transduction in the electron transport chain. 

In cyclic phosphorylation, oxygen is not evolved as the by-product where as, oxygen is evolved as a by-product during non-cyclic phosphorylation. Calvin cycle is the second stage of photosynthesis in which carbon atoms from carbon dioxide are combined, using the energy in ATP and NADPH, to make glucose.

The light reaction takes place in thylakoid discs. There, water is oxidized and oxygen is released. The hydrogen is accepted by $NADP$ and hence get reduced to $NADPH _2$.