Photosynthesis

On this page we will focus on the photosynthesis portion of the energy cycle, the conversion of water and carbon dioxide into oxygen and glucose using the process photosynthesis. Photosynthesis is the process by which plants secure energy from the sun and create useful chemicals.

Photosynthesis portion of the the energy cycle diagram. [67008.jpg]

This process can be over-simplified by using this equation. Solar energy plus water and carbon dioxide are converted into carbohydrate and oxygen. There are some problems with this oversimplification.

Solar energy + H₂O + CO₂ Ž "CH₂O" + O₂

One is that the carbohydrate that is made contains six carbons rather than just one, so we must have six water molecules, six carbon dioxides, and also make six molecules of oxygen.

Energy + 6 H₂O + 6 CO₂ Ž "(CH₂O)₆" + 6 O₂

Another problem is that if you look at this equation, you might infer that the two hydrogens bond to the carbon and one of the oxygens comes out to combine with the oxygen from the water to make the oxygen molecule.

However, studies done using radio-labeled oxygen (O*) resulted in these findings. Both of the oxygen atoms in the oxygen molecule were radio-labeled oxygen. The oxygen in the carbohydrate is unlabeled and is from the carbon dioxide, but another water molecule is also formed that does not have radio-labeled oxygen.

It turns out that there are two water molecules involved in this process. The oxygen from both of them ends up in the oxygen molecule. Of the two oxygen atoms from the carbon dioxide, one stays with the carbon, one ends up with some hydrogen in a water molecule.

Energy + 2 H₂O* + CO₂ Ž "CH₂O" + H₂O + O*₂

So, this initial equation, although very useful at summarizing the reaction, does not give a complete picture of what happens in photosynthesis.

Solar energy + H₂O + CO₂ Ž "CH₂O" + O₂

Energy and Oxidation Aspects

Let's explore the energy and oxidation aspects of this reaction. There are some energy diagrams in Exercise 6 in your workbook that you can use to take notes.

One thing that you should be very much aware of is that the reaction is an endothermic reaction. It requires energy to change the reactants (water and carbon dioxide) into the products (carbohydrate and oxygen). It is solar energy that is used to drive this endothermic reaction.

Reaction diagram for photosynthesis. [67010.jpg]

Also note that if you look at the oxidation states of the elements involved, there is a change for both the carbon and the oxygen. Some oxygen ends up being oxidized, going from a -2 state to a zero state. Carbon on the other hand is reduced from the +4 state to a zero state. So there are oxidation and reduction aspects to this reaction.

Oxidation States
in H₂O	H	+1	in "CH₂O"	H	+1
in H₂O	O	-2		O	-2
in CO₂	C	+4		C	0
in CO₂	O	-2	in O₂	O	0

Photosynthesis can be separated into the photo part and the synthesis part.

The photo part of photosynthesis involves the oxidation of oxygen, specifically the oxygen from the water. The photo part provides us with reducing power (in the 4Hˇ) and energy and also oxygen for animals to breathe.

The synthesis part of photosynthesis involves the reduction of carbon, specifically, it's the carbon contained in carbon dioxide. The synthesis part uses the hydrogens and electrons that were generated in the photo part to reduce carbon to form carbohydrates and also water.

Reaction diagram for photosynthesis with separate diagrams for the photo part and the synthesis part. [67012.jpg]

I should point out that both the photo and the synthesis parts of photosynthesis are multiple step reactions. Quite a number of steps are involved in separating hydrogen and electrons from oxygen in water. Similarly, quite a number of steps are involved in removing oxygen from carbon dioxide and incorporating hydrogen and additional electrons into the compound. Also, the carbon dioxide is not used to create a new compound as such, but instead, the carbon is incorporated into existing carbon-containing compounds in a way that makes them larger than they were before.

The energy that's absorbed in the photosynthesis reactions can be released later (in respiration) in a sequence of steps which change the carbohydrate back to carbon dioxide and give off hydrogen and electrons, which, in turn, combine with oxygen (that we get from plants) to release energy as it's needed in the body.

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E-mail instructor: Sue Eggling

Clackamas Community College
Š2001, 2003 Clackamas Community College, Hal Bender