Redox and photosynthesis: A simple concept, a complicated procedure

Photosynthesis: nature’s natural way

Photosynthesis is such a fascinating process: imagine making your own food as long as you have a few ingredients like chlorophyll and sunlight, and you are basically set to be full for life! It is a simple concept where you never have to think about what you are going to eat for your next meal, because your body would have already made it. But have you ever wondered, behind this seemingly simple idea, what are the reactions that have to take place for photosynthesis to work?

If you are an Upper Secondary student revising for the next year or term, or a Lower Secondary student that decided to use your precious break to read up new chemistry topics (can’t relate, sorry) and are therefore more familiar with the textbook than I am, you would by now be familiar with what redox is and some examples of it in everyday life.

And just as you would have guessed (or looked at the answer), photosynthesis is indeed a redox reaction that occurs in chloroplasts found in plants. But before we get all technical, let us work it out for ourselves.

Redox in photosynthesis

From your Lower Secondary Chemistry knowledge, you would know that the chemical equation for photosynthesis is

Now let us determine which elements have undergone oxidation and reduction by calculating their oxidation states.

While determining the oxidation states of all the elements, you may face difficulty figuring out that of glucose. This is because of glucose’s cyclic nature – all the carbons are connected to one another, forming a ring (see diagram below). In addition, carbon does not have a fixed oxidation state or rule (e.g. oxygen is usually -2, and hydrogen is usually +1). It really depends on what compound it is and which elements it is connected to.

By using the known oxidation rules, let’s now try to figure out all the oxidation states of the respective elements.

QUESTION 3.1: What is the oxidation state of the respective elements?

When calculating oxidation state, the aim is to add up all the numbers to 0 (as all compounds have a total oxidation state of 0). Since hydrogen has an oxidation state of +1, and oxygen of -2:

6 x (-2) + 12 x (+1) + Carbon O.S. = 0

Carbon O.S = +12 – 12 = 0

The oxidation state of carbon is therefore 0.

QUESTION 3.2: How would you illustrate this in a graph? Try for yourself before revealing the answer!

Note: this is one way you can draw a graph representing oxidation states. Remember to include your graph title, y and x axis titles, and an appropriate scale.

Now you may be wondering: why is the oxidation state of carbon in glucose 0? This has to do with the electronegativity of carbon in relation to the other elements around it, or how likely it is to give up its electrons to oxygen or hydrogen. As a result of all the giving and receiving, it ends up having the same number of valence electrons as it originally had (as opposed to e.g. oxygen, which has an oxidation state of -2 because it receives two extra electrons). You can read more about carbon and electronegativity here.

Now, you can calculate the initial and final oxidation state of elements and thus determine which elements have been reduced or oxidised. Try to also illustrating the states in a similar graph as the one above!

QUESTION 4.1: What is the initial and final oxidation state of the elements, and which elements have been reduced or oxidised?

Remember, even if redox has taken place in a chemical reaction, it is still possible that there are some elements, such as hydrogen, that did not undergo reduction or oxidation. It is always wise to calculate everything before making a conclusion.

Element	Hydrogen	Oxygen	Carbon
Initial O.S	+1	-2	+4
Final O.S	+1	0	0
Reduced or oxidised?	No change	Oxidised	Reduced

QUESTION 4.2: Illustrate this in a similar graph.

Calculating the oxidation state of the elements is just one way to demonstrate that photosynthesis is a redox reaction. We can also show that redox has taken place using electron transfer.

QUESTION 5: Write the half-equations of the photosynthesis reaction using the chemical equation given above (Recall how to construct half-equations using water, hydrogen ions and electrons).

So there you have it – the process of plants making their food, or any organisms that have chlorophyll in their body, is really just redox taking place.

In actuality, however, the general equation is a simplified version of what really happens. How does the carbon dioxide get ‘converted’ into sugar? And water to oxygen?

What really happens (wait, I have to read more???)

Simply put, the photosynthesis reaction we are all familiar with actually consists of TWO redox reactions and a special molecule known as NADP+ and its reduced form, NADPH.

For photosynthesis, there are two main reactions that occur: the light dependent reaction and the light independent reaction. In the light dependent reaction, which occurs in the presence of light, it starts with the reactants such as water, NADP+, and other molecules that are required to start the reaction. Water is oxidised by NADP+ to form oxygen, and NADP+ is reduced by water to form NADPH. This is the first redox reaction.

In the light independent reaction, which does not require light, the NADPH that was produced from the light dependent reaction reacts with carbon dioxide: carbon dioxide is reduced by NADPH to form C₃H₆O₃-phosphate (which later becomes glucose), and NADPH is oxidised by carbon dioxide to once again form NADP+. This is the second redox reaction.

When these two reactions are put together, we get the general equation that we are familiar with:

Thus, it is slightly inaccurate to say that carbon dioxide oxidised water and water reduced carbon dioxide because both belong to two different reactions. It is due to the help of the intermediate molecule, NADPH, which is not shown in the general equation at all.

To finally summarise, photosynthesis is

1. A redox reaction
2. Wait, two redox reactions, and
3. An example of how complicated biological processes really are

Learning about these reactions are the first steps to really understanding and appreciating how chemistry can come to life in all that we see and experience. If the Chemistry textbook is not enough for you, you can find out more about redox in photosynthesis in the resources below.

Light-dependent reaction

Lumen Learning

Light-independent reaction: Calvin Cycle

Lumen Learning

Biology LibreTexts