What are two phases of photosynthesis

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Biology question

Phases of Photosynthesis

Phase One: Light energy
The absorption of light is the first step in photosynthesis. Plants have special organelles to capture the energy of light. There are two energy storage molecules, which are NADPH and ATP, which are produced to be used in the light independent reactions once the energy is captured. Chloroplasts capture light energy in photosynthetic organisms. They contain two main compartments essential to photosynthesis. The first compartment is called the Thylakoid. Thylakoids are flattened saclike membranes that are arranged in stacks. Light dependent reactions take place there. When they forms stacks they form grana. The second important compartment is called the stroma, fluid filled place that is outside the grana. Light absorbing colored molecules called pigments are found in the thylacoid membranes of chloroplasts. The most common types of chlorophylls are a and b. Chlorophylls absorb most strongly in the violet-blue region and reflect light in the green region. Carotenoids absorb light in the blue and green regions, while reflection in the yellow orange and red regions.
Thylakoid membranes have a large surface area, which holds electron-transporting molecules and two types of protein complexes called photosystems. (I & II).
• First the light energy excites electrons in photosystem II, causing the water molecule to split, releasing an electron into the electron transport system, a proton into a thylakoid space and oxygen as a waste product
• exited electrons move from photosynthesis II to an electron acceptor molecule in the thylakoid membrane
• electron acceptor transfers the electrons to photosystem I
• In the light photosystem I transports the electrons to a protein(ferrodoxin)
• Ferrodoxin transfers the electrons to the electron carrier NADP+ forming NADPH.
ATP is produced in conjunction with electron transport by the process of chemiosmosis. Chemiosmosis is the mechanism by which ATP is produced as the result of the flow of electrons down a concentration gradient.

Phase Two: The Calvin Cycle
In the Calvin Cycle energy is stored in organic molecules such as glucose. In contrast to the process of light energy, this cycle is stable enough to store chemical energy for long periods of time. The reactions of Calvin cycle are also referred to as the light independent reactions.
• In carbon fixation, six carbon dioxide molecules combine with six 5-carbon compounds to form twelve 3-carbon molecules (3-PGA). The joining of carbon dioxide with other organic molecules is called carbon fixation.
• The chemical energy stored in ATP and NADPH is transferred to the 3-PGA to form high-energy molecules called glyceraldehydes 3-phosphates (G3P). ATP supplies the phosphate groups while NADPH supplies H ions and electrons.
• An enzyme called rubisco converts the remaining G3P molecules to new molecules. These molecules combine with new carbon dioxide molecules to continue the cycle

Plants use the sugars formed during the Calvin cycle both as a source of energy and as building blocks for complex carbohydrates, including cellulose, which provides structural support for the plant.


Phases of photosynthesis: there r two phases of photosynthesis;
1)Light reaction
2)Dark reaction
1)LIGHT REACTION:When sunlight falls on leaves ,it is absorbed by chlorophyll.The solar energy is utilized to brake water molecule into O2 and H this is called photolysis and O2 is released during this process.As this process takes place only in sunlight, it is called Light reaction.
When solar energy is converted into chemical energy during light reaction ,two compounds are formed which are:
NADPH2 (Nicotinamide Adenine Dinucleotide Phosphate)
ATP(Adenosine Tri-Phosphate)
2)DARK REACTION:This reaction occurs in the dark. Neither light energy nor chlorophyll are needed for the dark reaction.In it plants utilizes the energy from ATP and H from NADPH2 ,water combines with CO2 to form carbohydrates. Thus the solar energy is converted into chemical energy
to form glucose.This is called Dark reaction.Dark reaction was studied by a scientist Melvin Calvin, thus it is also called as Calvin's Cycle.


Photosynthesis occurs in two stages. The first stage, light-dependent reactions or photosynthetic reactions (also called the Light Reactions) captures the energy of light and uses it to make high-energy molecules. The second stage, or light-independent reactions (also called the Calvin-Benson Cycle, formerly known as the Dark Reactions) uses high-energy molecules to capture and chemically reduce carbon dioxide to make the precursors of carbohydrates,


Light Reaction and Dark Reaction.

In the light reaction, light splits water into hydrogen and oxygen. NADPH2 is formed to be used during the Dark Reaction. The oxygen binds with each other and is released as O2. ATP is also formed during the light reaction to be used in the dark.

During the dark reaction, enzymes use the energy from NADPH2 and ATP to convert CO2 into glucose and water.

Xpke92 these phases are called the light reactions and the dark reactions. The light reactions take place in the presence of light. The dark reactions do not require direct light, however dark reactions in most plants occur during the day.
I have attached a picture where photsynthesis occurs in light reaction stage.


Light reaction occurs in the grana and the site of the dark phase happens in the stroma