Photosynthesis Part 2(Light and Dark reaction in chloroplast):

In the previous post, we learned about the mechanism of photosynthesis happens in chloroplast of mesophyll cell that occurs in two steps:

Light Reaction
Dark Reaction (Please visit my first post on photosynthesis)

The difference between Light and Dark Reactions:

Light Reaction

Dark Reaction

a) Sunlight is a source of energy.	a) Source of energy is ATP.
b) H₂O, ADP, H₃PO4 is used as raw materials in the Light reaction	b) CO_2, NADPH₂and ATP is used as raw materials in the Dark reaction.
c) Oxidation of water happens, and the product is Oxygen .	c) Reduction of Carbon dioxide happens and the product is Glucose.
d) In the granum of the chloroplast, the light reaction occurs.	d) In the stroma of the chloroplast, the dark reaction is occurs.
e) Sunlight is the driving force	e) ATP & NADPH₂ is a driving force .

Photochemical Reaction Or Light Reaction

This reaction occurs in the grana of the chloroplast and initiates the light-dependent activities. In 1939, some experiments were done on the chloroplast extracts from the leaves of Stellaria and Lamium suspended in a test tube with an electron acceptor; they saw that Oxygen is released because of photochemical splitting of water, and there is no CO₂ is consumed, and no carbohydrate is formed.

4Fe³⁺ + 2H₂O —————–→ 4Fe²⁺ + 4H⁺ + O₂

And this process is known as Photolysis in Photosynthesis. And this Experiment is Called as Hill Reaction.

Because in this step of photosynthesis, plants need Sunlight, i.e photon and only the oxidation of water occurs, so this reaction is known as The Light Reaction.

Hill experiment proves that Oxygen and electrons are released in photosynthesis, and later this electron is used in the reduction of carbon dioxide.

Photophosphorylation: When light energy is converted into chemical energy and stored in the form of ATP.
Photoreduction : When a hydrogen ion is formed in a light reaction and attached to NADP⁺ (an electron acceptor substance), forming NADPH.

The Two Functional Units of Chloroplast which is reposnsible for the harvesting of Photoenergy are :

a) PS I / Photoact I / Photosystem first

b) PS II / Photoact II / Photosystem second

Photosystem first:

Many types of molecules of chlorophyll -a(Chl-a-66o, Chl-a-670, etc.) participate in photosystem I as absorbents of wavelengths of light but do not work as the reaction center. When Chl-a-700 participates, it works as the reaction center for electron donation, and this reaction center is known as P700(p=pigment, & 700nm is wavelength).

Photosystem Second :

Chlorophyll-a-680, Chl-a-660, Chl-a-670, and Chl-b-650 molecules take part in this reaction, but the reaction center works as Chl-a-680. At this point, the electron is released, so P680 is known as the second pigment.

Photophosphorylation

Formation of ATP in the chloroplast in the presence of sunlight is called as photophosphorylation it occurs in two steps:

Cyclic photophosphorylation

Non-cyclic photophosphorylation

Cyclic photophosphorylation :

As the name of this process, i.e., cyclic, means something starts from somewhere and end on started point i mean to say that the movement of electrons from the P700 reaction center to (when NADP+ is totaly reduced) via the cytochrome (b6-f complex) and plastocyanin(b6-f complex)with the help of Fd(ferredoxin) returns to P700 in a cyclic way in this process ATP is formed during cycling and this is known as Cyclic photophosphorylation.

Non- Cyclic Photophosphorylation :

The transportation of electrons from the P680 reaction center to NADP+ in linear path way via pheophytin, cytochrome, PC, and enters the P700 reaction center; during this process, ATP is formed, and the whole process is known as non-cyclic photophosphorylation(because electrons are not returned to their starting reaction center , i.e., P680)

The Dark reactions or Photosynthetic Carbon reactions

After the formation of ATP, NADPH, and Oxygen, the further process of photosynthesis is started in the stroma of the chloroplast; that is, the reduction of Carbon Dioxide into Carbohydrates that’s why it’s also known as PCR. This step doesn’t require light, so it is also known as the Dark reaction. The reduction of CO₂ is processed in the following 3 ways:

C₃ Cycle(Calvin Cycle) :

The reduction of carbon dioxide into 3-phosphoglyceric acid(first stable product, i.e 3 carbon compound), and the plants are known as C₃ plants, where this pathway happens.

C₄ Cycle(Hatch-Slack Cycle) :

The plants that follow this pathway are known as C₄ plants, where the reduction of carbon dioxide into oxaloacetic acid (the first product is a 4-carbon compound) occurs.

CAM Cycle (Crassulacean acid metabolism):

This pathway is found in desert plants, i.e., succulents like Opuntia, Agave, etc. In these plants, the C₃ & C₄ cycle is runs in two times during day time C₃ cycle happens, and in night the C₄ cycle happens. During this cycle, the acidification and deacidification of malic acid occur. In succulent plants, CO₂ is stored in the form of malic acid at night by the C₄ cycle and converted in to carbohydrate in day time by calvin cycle.