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Photosynthesis: The Light Reaction and Products Simplified

 

Photosynthesis Overview

The light reaction and its products are an important step in photosynthesis. Photosynthesis is the process that harnesses light energy to produce carbohydrates, and is found in over 100,000 plants species on Earth. Photosynthesis is an essential process of life because the carbohydrates produced by photosynthesis are a stable form of energy that can be used by the plants themselves, or other organisms as a source of energy. The production of carbohydrates at the end of photosynthesis is not a simple process and occurs in 2 different stages known as the Dark and Light Reaction.

The Light Reaction of photosynthesis should be properly called the Light Dependent Reaction because this is the only stage that captures energy in light. The dark reaction, also known as the Light Independent Reaction, functions independently of light and cannot capture energy. The Dark Reaction does however use products of the Light Reaction to function.

 

How Light is Absorbed

The Light Reaction occurs along the membrane of a plant specific organelle known as chloroplast. I’m sure you’re aware that chloroplast is the green colored organelle responsible for carrying out photosynthesis in plant cells.  There is nothing special about the color green or even green light. Chloroplast is actually green because the primary colors red and blue are absorbed by the organelle leaving the color green to be reflected back to our eyes. So the only significance about the color green is it is reflected, and that’s how we recognize chloroplast containing organisms.

Chloroplast also has 2 membranes around it, making it one of the organelles that are thought to have originated from bacteria. This theory is known as the endosybiosis and will be covered in a future article.

 

Chlorophyll may appear green but it is actually a pigment that absorbs red and blue light. The green light is reflect back into our eyes. Notice no light is needed for the dark reaction.

But the main structures in the chloroplast are columns of tiny sacks where photosynthesis actually occurs known as thylakoids. So just starting from square one, mainly red and blue light will pierce the membrane of the cell, and then the light will pierce membrane of chloroplast to reach the thylakoids where the light reaction takes place.

Now that you know where in a cell photosynthesis occurs, what exactly is going on inside these plant cells? What is the sunlight doing to the thylakoids in chloroplast that allows it to make carbohydrates (such as glucose) and oxygen that animals like us need to survive?

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Steps of the Light Reaction

 

Photosystems “Capture” Light

Complex molecules known as photosystems are able to capture the energy in light at 2 different wavelengths. There are also 2 light harvesting complexes within the thylakoid membrane. These light capturing complexes are called as Photosystem II(PS2) and Photosystem I(PS1).  Photosystem II contains primary electron acceptor P680 because it absorbs light at 680 nano-meters, and Photosystem I contains the primary electron acceptor P700 because it absorbs light at 700 nano-meters.

PS2 allows hydrogen to be pumped into the lumen which then powers the synthesis of ATP. The passing of electrons from PS1 powers creation of NADPH. Both products are used in the Dark Reaction or Calvin Cycle of Photosynthesis.

Flow of Energy from Photosystem 2

The process of photosynthesis begins with energy from light being absorbed by photosystem II (PS2). PS2 powers the pumping of H ions into the lumen of the cell. This process is similar to the electron transport chain in the mitochondria of cells. The difference is that hydrogen is either removed from water or pumped into the center(lumen) of thylakoids.  This buildup of hydrogen creates an electrochemical gradient that powers the synthesis of ATP.

Flow of Hydrogen atoms from the Stroma

The stroma is the outer region of the chloroplast. When electrons are passed from Photosystem II to Photosystem I, they power the movement of H to be released into the lumen or central compartment of the thylakoids. When enough H atoms build up in the lumen an electrochemical gradient is created. The electrochemical gradient means there is a higher concentration of H atoms inside the lumen rather than in the stroma. The movement of the H atoms powers the synthesis of ATP is the same manner observed in the electron transport chain.

Flow of Energy from Photosystem 1

Energy from light that strikes Photosystem I normally causes electrons to become exited and travels to the enzyme NADP reductase to make NADPH. NADPH is very similar to NADH produced in the Citric Acid Cycle. NADPH of the chloroplast and NADH of the mitochondria are both sources of electrons.

The process described above is called the non-cyclic reaction. Sometime the electron will be used to pump more H into the lumen and create more ATP. This is known as the cyclic reaction. The process cycles back to the previous function.

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Products of the Light Reaction

After harnessing energy from mostly red and blue wavelengths of light in Photosystem I and Photosystem II, the following product are created from the light reaction:

NADPH – Used in the dark reaction of photosynthesis.

ATP – Used in the dark reaction of photosynthesis.

 

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