How Is ATP Formed During Photosynthesis: Detailed Facts


In this article we will discuss about how is atp formed during photosynthesis.   

The process of photophosphorylation occurs within the thylakoid membranes of the chloroplasts of the plant cells where during photosynthesis, the hydrogen ions are generated by the Electron Transport System (ETS) which works on converting the ADP and the inorganic phosphate to ATP. This process converts the light energy into chemical energy to produce ATP. 

What is ATP?

The full form of ATP is Adenosine triphosphate, which is identified as the energy carrying molecules in every cell of the living beings. It is released from the breakdown of CO2 into the simple form of sugar to be used as the energy source. 

how is atp formed during photosynthesis
ATP from Wikipedia

What makes ATP in photosynthesis?

In producing ATP during the process of photosynthesis, there are two processes, one is cyclic and the other is non-cyclic. 

Cyclic Photophosphorylation:

This is the process which is utilised by the prokaryotes in order to generate immediate energy through the simple conversion of ADP into ATP. It involves only one photosystem, PSI (P700) and follows a few basic steps, which includes:

Step 1: Absorption of light in PSI (P700): the light is absorbed by the pigments within PSI in order to be passed on to reach the reaction centre. The electron is released to be passed on to ferredoxin, which is the primary acceptor which is then passed on to cytochrome b6f to be further passed on to plastocyanin.   

Step 2: ATP Synthesis: The electron with the higher energy level is transferred through the Electron Transport Chain (ETC), where it loses energy. The released energy helps to generate a gradient by pumping H+ ions across the membrane. The H+ ions pass via ATP synthase during their descent through the gradient, which causes the creation of ATP, and this process is known as chemiosmosis.

Read more on Do All Bacteria Do Photosynthesis: Why, What Type, How And Detailed Facts

Non-Cyclic Photophosphorylation

This is a process which involves two different photosystems, PS I (P700) and PS II (P680), where electrons get removed from the water molecule and transferred through both the photosystems to produce NADPH and ATP. The basic steps in the process includes: 

Step 1: Absorption of light in PSII (P680): the light is absorbed by the pigments within the thylakoid membrane and is passed on to reach the reaction centre, where the energy gets transferred to P680 in order to boost an electron to higher energy level. Next, the electron with higher energy level gets passed via the acceptor molecule which is then replaced by an electron from the water molecule. This splitting of the water molecule is termed as “photolysis” to release oxygen by the plants. 

Step 2: ATP Synthesis: the electron with higher level of energy gets transported through the Electron Transport Chain (ETC) to lose energy in the process. The released energy facilitates the pumping of H+ ions from the stroma towards the thylakoid membrane in order to build a gradient. During the passing down of the H+ ions through the gradient, the H+ ions pass through ATP synthase which drives the production of ATP and this process is termed as chemiosmosis. 

Step 3: Absorption of light in PSI (P700): the electrons from PSII arrive at the reaction centre of PSI. Further, when the light energy is passed on by the pigments within the reaction centre, the electrons within P700 are excited to form higher energy levels to be transferred via the acceptor molecule. The missing electrons within PSI are replaced by the electrons from PSII. 

Step 4: Formation of NADPH: These electrons with higher energy levels are then transferred through a short leg of ETC and at the end of the chain, the NADP+ is changed to NADPH. 

Difference between Cyclic and Non-cyclic Photophosphorylation 

Cyclic PhotophosphorylationNon-cyclic Photophosphorylation
Photosystem I (P700) is involved.Photosystem I (P700) and Photosystem II (P680)  are involved.
Active reaction centre: P700Active reaction centre: P680
Only ATP molecules are synthesised and not NADPH. Both ATP and NADPH molecules are synthesised. 
No Oxygen is produced as a by-productOxygen is formed as By-product. 
Photolysis of water molecules does not occur. Photolysis of water molecules is an important step. 
Travelling of electrons is in a cyclic pattern. Travelling of electrons is in a non-cyclic pattern. 

How much ATP is produced in photosynthesis

Cyclic photophosphorylation can occur under low light conditions or during any special conditions like accumulation of increased NADPH molecules within the Chloroplasts. It is common in prokaryotes in terms of synthesising immediate energy.

This is an important process in photosynthesis as it ensures the formation of chemical energy in the form of ATP only. Here the electrons get excited to enter only the photosystem I. This process generates the formation of 2 ATP molecules and no NADPH or Oxygen molecules.  

In non-cyclic photophosphorylation, the initiation of the electrons being reduced is done through the process of photo-oxidation of the water molecule. This is the significant process in photosynthesis as it ensures the formation of chemical energy in the form of NADPH and ATP and releases atmospheric oxygen for other living beings to breathe. It generates the formation of 1 ATP molecule and 2 NADPH molecules.  

Sayantani Misra

Hi, I am Sayantani Mishra, a science enthusiast trying to cope with the pace of scientific developments with a master’s degree in Biotechnology. Let's connect through LinkedIn-https://www.linkedin.com/mwlite/in/sayantani-misra-a54a83200

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