Introduction to photosynthesis

Nature of light


Light reactions

Calvin cycle and C4

Introduction to photosynthesis

Photosynthesis is a complex series of reactions that evolved millions of years ago, but as far as we know has remained pretty conservative over the long period.

There are a number of potentially good reasons for this:

The first advantage of this system is that the starting reactants are cheap and relatively plentiful.
Both CO2 and H2O diffuse into the plant, and except in extreme environments can be obtained without
a large investment energetically. This is not to say that a greater supply doesn't increase productivity,
but that on the average the materials are available and relatively cheaply obtained.

Second, CO2 is stable and contains little chemical energy, so lots of energy can be deposited into these compounds. At the same time, the carbohydrates produced are also stable and nonreactive, thus once energy is gained it can be held in storage for long periods of time. For long-lived species ( or their seed) this is an important consideration.

Third, everything involved as starting initials and final products are nontoxic ( at least at the levels involved).

The energetics of photosynthesis.

The basic reaction involved in photosynthesis is :

nCO2 + nH20 -> (CH2O)n + nH2O

What we have here is the reduction of the C of CO2 from the +4 oxidation state to the +0 oxidation state.

The reverse of this reaction is a combustion reaction (the one we and plants use in our bodies to produce ATP), that produces large amounts of energy. Thus it makes sense that plants need energy to do the reverse of the combustion reaction.

Plants have to break the bonds of two stable compounds, CO2 and H2O, rearrange electrons,and produce two compounds which are less stable relative to the first two. It would not be profitable for the plants to do this using their own energy. Instead plants use what an energy source that is readily available to them, light.

The process of photosynthesis essential involves removing e- from the oxygen in water and redistributing them around the carbons which come from the CO2. This is a redox reaction. The two half reactions are:

2nH2O ->nO2 + 4ne- + 4nH+ (oxidation)

which leads to production of ATP + NADP

This reaction uses the suns energy to remove electrons from the H2O in Photosystem II of the light reactions. Again this is not something that is easily done. We have seen before that oxygen tends to hold on to electrons tightly. It is the same in this case.

Location: chloroplast membranes

4nH+ + 4ne- + nCO2 -> (CH20)n + nH2O (reduction)


This reaction is what essentially happens in the dark reaction.

Location: stroma


A last critical consideration of the process is the structure that is necessary for these reactions to occur.


A chloroplast is a series of membranes that create compartments that are separate from each other. Hence the membranes can be used to create chemical gradients between compartments. Chloroplasts create different proton concentrations between the thylakoid space, and the stroma. The pH of the thylakoid space is around 5, and the pH of the stroma is around 8.

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