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  • Writer's pictureCaden Leung

Photosynthesis: All You Need To Know

We’ve all heard of photosynthesis: it’s how plants make food. But in the process, what actually happens?

In this article, I’m going to be taking a deep dive into each phase of photosynthesis including how light-dependent and light-independent reactions are important.


First, what is photosynthesis?

It’s where plants turn carbon dioxide and water into glucose and oxygen using sunlight. This process takes place in the chloroplast.

Photosynthesis is broken down into 2 parts: light-dependent reactions and light-independent reactions.


Parts of a Chloroplast:

The chloroplast has a double membrane.

The main parts we are going to be focusing on are the thylakoids, which are the little green disks in the image, and the stroma, which is a fluid that surrounds the thylakoids.

Granum in this image just means a stack of thylakoids.


Light Dependent Reactions:

1) Here’s the weird part: Photosystem II, not I, will absorb the sunlight at 680 nm. The sunlight will be absorbed by the pigment known as chlorophyll.

2) Because the light energy will cause the loss of an electron from Photosystem II’s chlorophyll, water is oxidized to replace the lost electron, forming hydrogen ions and oxygen gas.

The oxygen will be kicked out as a byproduct and hydrogen ions will move down the electron transport chain and diffuse across the thylakoid membrane.

3) The electron now reaches Photosystem I. Photosystem I absorbs sunlight at 700 nm which excites the electron.

4) The electron then combines with NADP+ to form NADPH.

5) Don’t forget the hydrogen ions on the other side of the thylakoid membrane. They’re going to diffuse through a protein channel, ATP Synthase, to create ATP from ADP.

So the “results” of the light-dependent reactions are NADPH and ATP, which will be used in the light-independent reactions.


Calvin Cycle or Light Independent Reactions

The light-independent reaction is also called the Calvin Cycle. This process occurs in the stroma and doesn’t require any light.

Courtesy of Khan Academy

1. 3 carbon dioxide molecules are combined with RuBP, a five-carbon acceptor, to create a six-carbon molecule. The enzyme that helps with this reaction is known as rubisco. (So you have 3 6-Carbon molecules.)

2. Each 6-Carbon molecule is then split into two 3-carbon molecules, known as 3-PGA. (So because each 6-Carbon molecule splits into 2 3-Carbon molecules, we should have 6 3-PGA molecules in total)

3. ATP and NADPH are used to help transform the 6 3-PGA molecules into 3-carbon sugar known as G3P. The NADPH will lose an electron to make G3P. So we should have 6 G3P molecules.

4. 5 of the G3P molecules will be recycled to help regenerate the RuBP acceptor in step 1. Only 1 of the G3P molecules will be used toward glucose production. Therefore, 3 carbon dioxides (meaning 3 cycles) will only produce 1 G3P molecule. It takes 2 G3P to make one glucose.


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