Tag: photosystems

Photosystems are functional and structural units of protein complexes involved in photosynthesis, that together carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. There are 2 kinds of photosystems - photosystem I and II. In photosystem I, the reaction center is P-${ _7}$${ _0}$${ _0}$. In photosystem II, the reaction centers are P-${ _6}$${ _8}$${ _3}$. Thus, the correct answer is option A. 

P$ _{680}$ is the primary donor present in photosystem II. Structurally,  it's chlorophyll pigment dimer is present at the center of photosystem II. 680 suggests that the absorption is maximum at 680nm in red light. P$ _{680}$ or primary donor receives energy either by absorbing light or by excitation of electrons present in nearby chlorophyll. The excited electrons get captured by electron acceptor present in photosystem II, which is pheophytin and oxidized P$ _{680}$ is then reduced by electron generated from water during oxygenic photosynthesis.

Ferredoxin is the iron-containing, soluble compound in chloroplasts that helps in electron transportation and is the constituent of PS I which asses electrons to reductase complex that helps in the reduction of NADP$^+$ to NADPH which is a strong reducing agent.

Cyclic photophosphorylation is carried out by PS I only. This process takes place in stroma lamellae membrane. An external source of electrons is not required. Photolysis of water does not take place. There is no evolution of oxygen takes place because it is not connected with photolysis of water. Cyclic photophosphorylation produces ATP only. It does not involve the formation of NADPH. It operates under low light intensity, anaerobic conditions or when $CO _{2}$ availability is low. When only PS I is functional, the electron is circulated within the photosystem and the phosphorylation occurs, due to the cyclic flow of electrons. The membrane and lamella of the grana have both PS I and PS II, the stroma lamella membrane lack PS II as well as NADP reductase enzyme. The excited electron does not pass on to $NADP^{+}$ and is cycled back to the PS I complex through the electron transport chain.

The process of photosynthesis in plants involves a series of steps and reactions that use solar energy, water, and carbon dioxide to produce organic compound. One of the first steps in this complex process depends on chlorophyll and other pigment molecules. Chlorophyll is the green pigment molecule that make plants appear green. In photosynthetic plant cells, chlorophyll molecules are embedded in stacked membranes (thylakoids) contained in special membrane-bound organelles called chloroplast. The chlorophyll molecules are arranged in discrete units called photosystems. Each one of it contains hundreds of pigment molecules arranged into an "antenna complex" surrounding a reaction center.

• When Ttrr x Rrtt is performed, 4 types of phenotypes are obtained in equal numbers. (1:1:1:1)
  
• The gametes for Ttrr are Tr and tr and for Rrtt are Rt and rt.
• The possible genotypes are TtRr, Ttrr, ttRr, ttrr which are all different phenotypically.
• So. the correct answer is '1:1:1:1'.

Since carotene (alpha and beta), chl A, chl B are all found in both photosystem, the one which is only present in PS II is xanthophyll.