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Guide to Understanding Photosynthesis Light Dependent Reactions

Updated on July 8, 2012

Light Dependant Photosynthesis

In highschool, photosynthesis has always been one of the most confusing topics to students. Really, it isn't hard at all. It appears hard because there isn't really a definite step to it because nearly all the processes happen at the same time. In this tutorial, I will explain photosynthesis from easy to harder.

The Easy Stuff

- Photosynthesis is the process in which plants convert light (radiant) energy into chemical energy, glucose.

Most of the oxygen we breath is actually a by-product of privative algae rather than our backyard trees, however that doesn't mean the trees are useless, they prevent runoff as well as food and habitat for animals.

- Photosynthesis takes place in the chloroplasts of plants. Chloroplasts are organelles that have once been thought to be independent from the plant cell due to the endosymbiosis theory.

Endosymbiosis Theory

The endosymbiosis theory was proposed by Lynn Margulis. The theory states that chloroplasts and mitochondria were once organisms living freely in the environment until it was engulfed by another cell. In other words, another cell "ate" the chloroplast and mitochondria and used its energy generating potential for its own gain.

Evidence of this theory includes:

- 70S ribosomes (All ribosomes that belong to a eukaryotic cell is 80S)

The 70S ribosomes are identical to the 70S ribosomes in bacteria and other proparyotic cells which suggests that chloroplasts and mitochondria were once prokaryotic cells that were engulfed by another cell

- Triple membrane

The triple membrane suggests that the golgi vesicle that engulfed the prokaryotic cell then became a new membrane part of the chloroplast

-  Circular DNA

This is excellent evidence. All bacteria have circular DNA (DNA with only exons) while all eukaryotic cells have linear DNA (exons + introns). The chloroplast and mitochondria acts as if it is a cell working all by itself without taking orders from the DNA within the cell nucleus. In other words, imagine if a shark ate a cat and the cat was living inside the dog.

Photosynthesis Has a few Basic Reactants and Products


- Water (H20)

- Sunlight (Photons)

- Carbon Dioxide (CO2)


- O2 (By-product)

- NADPH/ATP (Light Dependant)

- Glucose (The Goal made in the Light Independent reactions not discussed here)

- Metabolic Water

Photosynthesis - The Process

Now we are getting into the actual process of photosynthesis. Drink some grape juice or take a breather. Take some time to look at the picture above. This is easy stuff. If there's a term you do not understand, google it. Here we Go.

Light Dependant Reactions

The Light Dependant reactions occur in the membrane of the thylakoid membrane of a chloroplast and the lumen of the thylakoid. A stack of thylakoids are called a grana.

As we zoom up on the membrane of one of these pancake stacks, we get something that looks like the image above.

Chlorophyll, Accessory Pigments, and Electrons

Light is constantly hitting the thylakoid stacks (grana). We are now beginning in Photosystem 2 (It's called Photosystem 2 because it was discovered second). As light is hitting the thylakoid, the pigment, Chlorophyll is absorbing the energy from the photons. Chlorophyll absorbs the red and blue light and reflects green light which is why we see it. Accessory Pigments, which are seen in autumn in a vibrant array of red and yellow, also absorb light, but only in small amounts. The amount it does absorb are transferred to chlorophyll and once chlorophyll absorbs enough energy, it will pop off 2 electrons. In order to replenish these electrons, the chlorophyll uses light (photons) to split water into Oxygen and Electrons. The Oxygen is then released as a by-product and the chlorophyll keeps the electrons. The kept electrons are stored in the PEA (Primary electron acceptor) which is a protein that holds the electron.

Electron Transport Chain (ETC)

Immediately after the electron is popped off of chlorophyll and held by the Primary Electron Acceptor, it moves down the ETC (electron transport chain) down proteins called a cytochrome - in this case, it is cytochromes. Cytochromes shuttle electrons much like electricity moving through a power cord. It is a known fact that when energy is transported in one direction, energy is produced; however, it still isn't understood how it is so. Anyway, the electrons move down the electron transport chain; MEANWHILE, the movement generates a little bit of energy in the form of ATP (adenosine tri-phosphate) which activates the proton pump to pump hydrogen ions into the lumen of the thylakoid.

From here: there are two processes that happen at the exact same time - two pathways, two different journeys.

1. The continuation of the Hydrogen

2. The Electron that has moved down the ETC ends up in Photosystem 1

The Continuation of the Hydrogen

Hydrogen has just been been pumped into the lumen of the thylakoid. Everything moves from a high to low concentration. As hydrogen is pumped into the lumen of the thylakoid, the concentration of hydrogen ions are higher than the concentration outside of the lumen. The hydrogen ions then flow throw a channel protein called ATPase aka ATPSynthatase aka ATPsyntase. This yields a LARGE amount of ATP. This is once of the goals of the light independent reactions. 

The Electron that has moved down the ETC ends up in Photosystem 1

The electron after photosystem 2 ends up in photosystem 1. The chlorophyll in photosystem 1 doesn't need to split water to get electrons because there's one from photosystem 2 that it can use. Anyway, the electron then moves to the Primary Electron Acceptor (just like in Photosystem 2) and down the Electron Transport Chain (just like in Photosystem 2). Now, the electron moves to the protein called ferredoxin which combines NADH, the electron, and a Hydrogen to produce NADPH. NADPH is the other goal of the light independent reactions.

Both ATP and NADPH created in the light independent reactions of photosynthesis is then used in the light-independent reactions which I will discuss in a the next guide.


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      Ryan 5 years ago

      Thank you so much for making this! My text book does a terrible job of explaining how photosynthesis works.

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