Light and Dark Reaction of photosynthesis?

Question

I'm having a hard time understanding light and dark reactions because all the sites I've gone through provide different equations. Some say photolysis is H20 = H+ + O2 while some present it as H20 = H+ + OH-. Moreover some have given that NADP+ + H+ = NADPH and some have stated that NADPH+H+ = NADPH2 Could someone provide me with a better, more detailed explanation of both the light and dark phases with the equation of all the reactions that occur during the process, as well as the site of occurrence.

Answer 1

I can see your frustration if you meet errors such as NADPH₂ but that is the price you pay for approaching as complex a subject as photosynthesis without a good biochemistry textbook. Even the on-line versions (e.g. Berg et al.) are unsatisfactory because of their layout. You will have to sit down and spend a couple of hours on the topic — all you can expect from a site like this is clarification of key points.

That said I'll offer some clarification. The first thing I'll say is that focusing on chemical equations is not the way to understand photosynthesis. It will demonstrate that the law of conservation of matter applies, but otherwise is a bit like doing a chemical analysis of a cell to try to understand how it works. This is because:

• There are two quite separate reactions in photosynthesis. As the products of one are (more strictly can be) used in the second, overall equations cancel them out, and so ignore key features of the process.
• Photosynthesis is concerned with the transformation of energy as much as molecular structure. Chemical equations do not really address this.
• The generation of energy involves the establishment of a gradient of hydrogen ions across a biological membrane — something your chemical equation does not encompass.

So what are the key ideas to bring to the text book you are going to purchase?

The overall objective is to make sugar from carbon dioxide

This poses a problem, not particularly because of the chemical reactions, but because these reactions require energy of two kinds:

• To join the carbons from the carbon dioxide into C-C bonds. This requires ATP, the free energy change of its hydrolysis to ADP being coupled to ‘drive’ the formation of a C-C bond.
• To reduce the oxygen atoms of carbon dioxide to OH groups. The biochemical molecule that is used for this reduction is NADPH, so ‘energy’ is needed to reduce its oxidized form, NADP⁺. (I won’t go into why this can be thought of as energy — if interested you need to read about the free energy equivalents of redox couples.)

The sun provides the energy

The energy for creating the reducing NADPH and the ATP comes from the sun in a complex series of reactions (these do not involve carbon dioxide). Light of a particular wavelength is harvested and used to break a water bond so as to separate charge in a reaction that I will deliberately not balance:

(1) H₂O + sunlight → O₂ + H

A complex series of reactions transfer the H (more strictly an electron) between different molecules, eventually reducing NADP+ to NADPH. At the same time an equally complex process builds up a H⁺ concentration difference across a membrane which provides the energy to convert ADP to ATP (see also Where do the H+ ions come from in light reactions?).

This is what is called the light reaction of photosynthesis because it requires light. It occurs in the thylakoids of the chloroplast, particularly at the membrane, although the products are in the stroma.

After energy has been harvested from the sun chemical synthesis can proceed

Having harvested the energy of the sun the plant has the wherewithal to make sugar from carbon dioxide. This is referred to as the ‘dark reaction’ because it does not require light. But its name can be misleading to the neophyte as the process does not have to take place in the dark. The overall unbalanced equation is:

(2) CO₂ + NADPH + ATP → C₆H₁₂O₆ + NADP⁺ + ADP

This is actually a complex series of enzyme-catalysed reactions (known as the Calvin Cycle) and a triose is the initial product. It occurs in the stroma of the chloroplast.

Answer 2

Light Reaction (also known as light dependent reaction)

The light reaction uses chlorophyll (which is the main photosynthetic pigment) to capture light, and then uses the light energy to make ATP and NADPH. Water is also broken apart in this process so the electrons can be extracted, yielding hydrogen ions and oxygen gas. The stimulation of chlorophyll releases ATP. Furthermore, hydrogen is formed from the conversion of NADP+ into NADPH.

2H2O + 2 NADP+ + 3ADP + 3Pi → O2 + 2 NADPH + 3ATP

Dark Reaction (light independent reaction, or also known as calvin cycle) The enzyme RuBP carboxylase catalyses the attachment of C02 to the 5C compound ribulose biphosphate. The unstable 6C compound then breaks down into 2 3C compounds called glycerate-3-phosphate (GP). The GP molecules are phosphorylated by ATP and reduced by NADPH+H+. The GP molecules are then converted into triose phosphates (TP) called glyceraldehyde phosphate. From the 6 molecules of TP which are created, only 1 is used to form/create half of a sugar (this means 2 cycles are required to construct a full sugar). The other 5 "unused" molecules of TP are reorganised in stacks in order to regenerate the stacks of RuBP. The reorganisation requires ATP.

3 CO2 + 6 NADPH + 5 H2O + 9 ATP → glycerate-3-phosphate (G3P, or GP) + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi