Question

How do light, reaction centers, antennae complexes, and bacteriochlorophyll together cause an electron to have a very low E (i.e. a negative number with a large absolute value) in photophosphorylation?

Answer 1

Ans. -

The light dependent reactions use light energy to make two molecules needed for the next stage of photosynthesis: the energy storage molecule ATP and the reduced electron carrier NADPH. In plants, the light reactions take place in the thylakoid membranes of organelles called chloroplasts. There are two types of photosystems: photosystem I (PSI) and photosystem II (PSII).

Both photosystems contain many pigments that help collect light energy, as well as a special pair of chlorophyll molecules found at the core (reaction center) of the photosystem. The special pair of photosystem I is called P700 , while the special pair of photosystem II is called P680.

In a process called non cyclic photophosphorylation, electrons are removed from water and passed through PSII and PSI before ending up in NADPH. This process requires light to be absorbed twice, once in each photosystem, and it makes ATP . In fact, it's called photophosphorylation because it involves using light energy to make ATP from ADP.

Light absorption in PS I - When light is absorbed by one of the many pigments in photosystem II, energy is passed inward from pigment to pigment until it reaches the reaction center. There, energy is transferred to P680, boosting an electron to a high energy level.

ATP synthesis- The high-energy electron travels down an electron transport chain, losing energy as it goes. Some of the released energy drives pumping of start superscript, plus, end superscript ions from the stroma into the thylakoid interior, building a gradient. As \text H+, H, end text, start superscript, plus, end superscript ions flow down their gradient and into the stroma, they pass through ATP synthase, driving ATP production in a process known as chemiosmosis.

Light absorptionin PSII- The electron arrives at photosystem I and joins the P700 special pair of chlorophylls in the reaction center. When light energy is absorbed by pigments and passed inward to the reaction center, the electron in P700 is boosted to a very high energy level and transferred to an acceptor molecule. The special pair's missing electron is replaced by a new electron from PSII.

NADPH formation- The high-energy electron travels down a short second leg of the electron transport chain. At the end of the chain, the electron is passed to NADP+2start superscript, plus, end superscript to make NADPH.

The net effect of these steps is to convert light energy into chemical energy in the form of ATP and NADPH. The ATP and NADPH from the light dependent reactions are used to make sugars in the next stage of photosynthesis, the Calvin cycle. In another form of the light reactions, called cyclic photophosphorylation electrons follow a different, circular path and only ATP is produced.