From my experience to teach photosynthesis, I like to use the diagrams in the Biology: A global Approach written by Campbell N. A. with his friends. The numbering used with the diagrams make the explanation of the steps of the process easier to be understood.
The above video shows the cells of Elodea sp. showing the movement of the chloroplasts in the cytoplasm.
A structure of a chloroplast
A photosystem
(From Campbell, N.A. et al. (2018). Biology: A global approach)
(From Campbell, N.A. et al. (2018). Biology: A global approach)
LIGHT DEPENDENT REACTION
Light dependent reaction occurs in the thylakoid membrane. Light dependent reaction converts the light energy to chemical energy in the form of ATP and NADPH. Light dependent reaction involves noncyclic photophosphorylation and cyclic photophosphorylation.
Noncyclic photophosphorylation
(From Campbell, N.A. et al. (2018). Biology: A global approach)
(From Campbell, N.A. et al. (2018). Biology: A global approach)
In noncyclic photophosphorylation,
1.
- The light energy strikes one of the antenna pigment molecules in a light-harvesting complex of PS II.
- One of the electrons in the pigment molecule is boosted to a higher energy level.
- As the electron falls back to its ground state, an electron in a nearby pigment molecule is simultaneously excited.
- The light energy is relayed from one pigment molecule to other pigment molecules until it reaches the P680 in PS II.
- Then the electron in the P680 is photoexcited.
2.
- Then the photoexcited electron is transferred from P680 to the primary electron acceptor of PS II, creating an electron-hole in P680.
- Then the photoexcited electron is transferred from P680 to the primary electron acceptor of PS II, creating an electron-hole in P680.
- The P680 now becomes P680+.
*P680+ is the strongest biological oxidizing agent.
3.
- An enzyme split a water molecule into 2 electrons, 2 hydrogen ions (H+) and an oxygen atom.
- The electrons from the splitting of water, replace one by one the missing electron in P680 which was transferred to the primary electron acceptor.
- The hydrogen ions (H+) are released into the thylakoid space and create a proton gradient across the thylakoid membrane.
- The oxygen atom combines with an oxygen atom from the splitting of another water molecule to form an oxygen molecule, O2.
*P680+ is the strongest biological oxidizing agent.
3.
- An enzyme split a water molecule into 2 electrons, 2 hydrogen ions (H+) and an oxygen atom.
- The electrons from the splitting of water, replace one by one the missing electron in P680 which was transferred to the primary electron acceptor.
- The hydrogen ions (H+) are released into the thylakoid space and create a proton gradient across the thylakoid membrane.
- The oxygen atom combines with an oxygen atom from the splitting of another water molecule to form an oxygen molecule, O2.
4.
- Then each photoexcited electron is passed from the primary electron acceptor of PS II to the first electron transport chain until it reaches the P700, starting from an electron carrier called plastoquinone (Pq) to a cytochrome complex, then to a protein called plastocyanin (Pc).
- As the electrons flow down the electron transport chain, the energy released used to pump protons (H+) into the thylakoid space.
5.
5.
The potential energy stored in the proton gradient is used to make ATP from a process called chemiosmosis.
6.
- At the same time, a photon of light strikes the pigment molecules in PS I.
- The light energy released is relayed from one pigment molecule to other pigment molecules until it reaches the P700 in PS I.
6.
- At the same time, a photon of light strikes the pigment molecules in PS I.
- The light energy released is relayed from one pigment molecule to other pigment molecules until it reaches the P700 in PS I.
- Then the electron in the P700 is photoexcited.
- Then the photoexcited electron is transferred from P700 to the primary electron acceptor of PSI, creating an electron-hole in P700. - The P700 now becomes P700+.
- The electrons from the first electron transport chain replace one by one the missing electron in P700.
7.
- Each photoexcited electron from P700 is passed from the primary electron acceptor of PS I down a second electron transport chain through a protein called ferredoxin (Fd).
8.
- The enzyme NADP+ reductase catalyzes the transfer of electrons from Fd to NADP+ to form NADPH.
(From Campbell, N.A. et al. (2018). Biology: A global approach)
In cyclic photophosphorylation,
- The photoexcited electron from PS I is occasionally shunted back from ferredoxin (Fd) along the first electron transport chain which from cytochrome complex then to plastocyanin (Pc).
- Then the electron is transferred to P700 to stabilize it.
- After that the electron in P700 is photoexcited and transferred to the primary electron acceptor, creating a hole in P700.
- After that the electron in P700 is photoexcited and transferred to the primary electron acceptor, creating a hole in P700.
- Then the electron is transferred back to ferredoxin (Fd).
- This cyclic flow generates ATP only through chemiosmosis.
- This cyclic flow generates ATP only through chemiosmosis.
- There is no production of NADPH and no releasing of oxygen in this cyclic flow.
LIGHT INDEPENDENT REACTION
Calvin Cycle
(From Campbell, N.A. et al. (2018). Biology: A global approach)
Calvin cycle is an anabolic process which build carbohydrates from smaller molecules and consumes energy. Carbon enters the Calvin cycle in the form of carbon dioxide and leaves the cycle as a three-carbon sugar called glyceraldehide 3- phosphate (G3P). To produce nett synthesis of one molecule G3P, the cycle takes three times which fix one molecule of carbon dioxide per turn of the cycle.
3 phases:
(From Campbell, N.A. et al. (2018). Biology: A global approach)
Calvin cycle is an anabolic process which build carbohydrates from smaller molecules and consumes energy. Carbon enters the Calvin cycle in the form of carbon dioxide and leaves the cycle as a three-carbon sugar called glyceraldehide 3- phosphate (G3P). To produce nett synthesis of one molecule G3P, the cycle takes three times which fix one molecule of carbon dioxide per turn of the cycle.
3 phases:
1. Carbon fixation
- Each molecule of carbon dioxide is attached to a five-carbon sugar called ribulose bisphosphate (RuBP).
- This reaction is catalyzed by RuBP carboxylase-oxygenase or rubisco.
- The product is a six-carbon intermediate that is short-lived because it is unstable which immediately splits into two molecules of 3-phosphoglycerate for each molecule carbon dioxide fixed.
* Rubisco is the most abundant protein on Earth.
- This reaction is catalyzed by RuBP carboxylase-oxygenase or rubisco.
- The product is a six-carbon intermediate that is short-lived because it is unstable which immediately splits into two molecules of 3-phosphoglycerate for each molecule carbon dioxide fixed.
* Rubisco is the most abundant protein on Earth.
2. Reduction
- Each molecule of 3-phosphoglycerate receives an additional phosphate group from ATP, forming 1,3 - bisphosphoglycerate.
- Then a pair of electrons from NADPH reduces
1,3 - bisphosphoglycerate forming
glyceraldehide 3- phosphate (G3P).
- Then a pair of electrons from NADPH reduces
1,3 - bisphosphoglycerate forming
glyceraldehide 3- phosphate (G3P).
3. Regeneration of RuBP
- The five molecules of G3P are rearranged into three molecules of RuBP by spending three molecules of ATP.
- RuBP is ready to receive carbon dioxide again and the cycle continues.
- RuBP is ready to receive carbon dioxide again and the cycle continues.
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