Photosynthetic: Light reactions/Dark reactions

Today’s post is a biology lesson- we’ll dig a little bit deeper into the details of how photosynthesis works. We’ll divide the process into two useful categories: the light reactions and the dark reactions. As the name implies, the light reactions require light. The dark reactions are a misnomer because they don’t require darkness; they are merely light-independent*. The light reactions convert solar energy into energy-storing biochemical compounds that are subsequently used to power the light-independent reactions, which convert carbon dioxide into the organic molecules used for making sugars. All of these reactions occur in the chloroplast.

Light reactions

The molecular machines that perform the light reactions are located within the thylakoid membranes of plants. These chlorophyll-containing membrane protein complexes give the thylakoids their green color. Overall, these enzymes use the energy in sunlight to move electrons through the thylakoid membrane so that they can be stored in useful biochemical compounds. In chemical equation form, the reactions can be reduced to:

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

The useful electron-storing compounds are NADPH and ATP. The NADPH molecules serve as useful electron donors for the subsequent light-independent reactions. The ATP molecules are the basic energy currency of all living cells. Most of the energy-requiring biochemistry performed by cells involves breaking a phosphate group off of an ATP molecule to provide the energy. All of the electrons being stored on these molecules must come from somewhere. In this case**, they come from water. This is a great strategy, since here on the blue planet water is plentiful, but this is technically and thermodynamically very challenging. Because water is used to supply the electrons for these reactions, oxygen (O₂) is also produced.

Here’s a view of the enzymes and their cofactors that are responsible for the light reactions (looking into the plane of the thylakoid membrane):

The Enzymes of the Photosynthetic Light Reactions

There are two photosystems (PSI and PSII) that perform the energy-requiring steps. These complexes are also associated with antenna complexes to help them capture the light they need to perform their reactions. Photosystem II (PSII) resides at the beginning of the photosynthetic electron transfer chain. It uses light energy to pull electrons from two water molecules and transfer them to a chemical called plastoquinone (PQ). In this reaction O₂ is produced and along with the reduced form of PQ (aka plastoquinol, PQH₂). The PQH₂ diffuses within the thylakoid membrane to the cytochrome b₆f complex (Cyt b₆f). Cyt b₆f is a proton pump which uses some of the energy from the electron transfer reactions to create a proton (H⁺) gradient across the thylakoid membrane before passing electrons along to plastocyanin (PC). PC transfers electrons to Photosystem I (PSI) which requires an additional input of light energy before transferring electrons to Ferredoxin (Fd). Finally Ferredoxin-NADP⁺ Reductase (FNR) stores the electrons on NADPH. The H⁺ gradient created across the thykaloid membrane (acidic thylakoid lumen vs. basic stroma) provides the driving force for ATP synthesis by ATP synthase.

If this seems like a lot of steps to make two chemicals (ATP and NADPH), you are right. However, each electron transfer step allows these enzymes to do useful work, like making that  H⁺ gradient. Multiple electron transfer steps also provides the opportunity for intricate regulation of the system. The light reactions are the engine of the cell, and all of these components can be finely tuned depending on environmental conditions and the needs of the cell.

Dark reactions Light-independent reactions

The light-independent reactions perform carbon fixation in the chloroplast stroma. In this process, carbon dioxide in the air is ‘fixed’ in the form of organic compounds, which can be used to make glucose. This is an energy-intensive process that uses the products of the light reactions (NADPH and ATP) to provide the necessary energy and electrons.

In chemical reaction form, it looks like this:

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

Carbon Fixation Overview

The carbon fixation reactions are actually a cycle, which breaks down into three stages. In the first step CO₂ is fixed by Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase) to convert one molecule of ribulose 1,5-bisphosphate (R1,5BP) into two molecules of 3-phosphoglycerate (3PG). In the next stage, 3PG is converted to hexose sugars (i.e. sugars that contain 6 carbons, like glucose) via the molecule glyceraldehyde-3-phosphate (G3P). In the last stage, R1,5BP is regenerated so the cycle can start again.

 

So wait, what’s our net gain? Isn’t this process supposed to be about making useful compounds? Yes. Each round of the cycle makes 2 G3P molecules with 6 total carbon atoms between them, but 5 of those carbons must be used to regenerate the R1,5BP used by Rubisco. As the equation states above, for every 3 CO₂ molecules that feed into the cycle (or every 3 turns of the cycle), you get one G3P which can be used to make hexose sugars. All of this is at the expense of 6 NADPH and 9 ATP molecules for those three rounds around the cycle. This may seem inefficient to the point of futile and unreasonably expensive in terms of energy, but it’s the only game in town when it comes to using CO₂ to make sugars. No wonder so many organisms choose to acquire their carbon by eating it rather than fixing it.

If you still knew all of this information already, then congratulations again. Be sure to thank your high school science teachers and collegiate freshman biology instructors. Stay tuned for the graduate-level versions describing how all of these enzymes work and the other flavors of photosynthesis. If on the other hand, you are in information overload. Hang in there. Just use the link to this post on the ‘Basics’ page for your reference when we discuss more details.

Johnna

*FYI- researchers that study the light-independent reactions of carbon fixation do not appreciate their work being called ‘the dark side.’ The biochemical pathway that comprises the light-independent reactions is also known as the Calvin Cycle or Calvin-Benson Cycle or Calvin-Benson-Bassham Cycle after the scientists that elucidated its details.

**Yes, there are other varieties of photosynthesis that occur, but you are not yet ready for these Padawan learner. Stay tuned, you will.

Useful references:

Light reactions:

Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Chapter 19, The Light Reactions of Photosynthesis. Available from: http://www.ncbi.nlm.nih.gov/books/NBK21191/

http://en.wikipedia.org/wiki/Light-dependent_reactions

Light-independent reactions:

Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Chapter 20, The Calvin Cycle and the Pentose Phosphate Pathway. Available from: http://www.ncbi.nlm.nih.gov/books/NBK21194/

Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Section 20.1, The Calvin Cycle Synthesizes Hexoses from Carbon Dioxide and Water. Available from: http://www.ncbi.nlm.nih.gov/books/NBK22344/

http://en.wikipedia.org/wiki/Light-independent_reactions