Friday, January 8, 2016

Photosynthesis Overview

One of the defining characteristics of plants is their ability to produce their own food via photosynthesis. Photosynthesis is also responsible for all of the food we eat! Almost all of the biological carbon on this planet is generated by photosynthesis. This carbon is created from carbon dioxide, which is a waste product we breath out. And the oxygen that we breath in, we also have photosynthesis to thank for that. After all, oxygen is a waste product of photosynthesis. But how does this all happen? And where does it happen inside the plant? Let's find out!

When thinking about photosynthesis, many people associate it with the green color of plants. This green color is from chlorophyll, a pigment utilized in one small section of photosynthesis. Photosynthesis occurs within the chloroplast organelle inside plant cells. Within chloroplast are stacks of thylakoid membranes (in green below), which house all the photosynthetic machinery. In fact, chloroplasts have their own genome and most of the important photosynthetic proteins are created in-house, so to speak.


Chloroplast By SuperManu • CC BY-SA 3.0 • Wikipedia
Photosynthesis is typically broken down into two phases: light-dependent and light-independent. You may have heard of these as the light and dark reactions of photosynthesis. The trouble with this nomenclature is that the dark reactions occur only IN the light, they just do not USE light directly. Thus the naming change to light-dependent and light-independent. Each of these reactions take place either on, or inside, the thylakoid membranes.

Photosynthesis by The Awkward Yeti
The overall reaction of  photosynthesis: light + carbon dioxide + water -> glucose + oxygen, exactly as depicted in the awesome The Awkward Yeti cartoon above.
  • Light-dependent = light + water -> oxygen + ATP (cellular energy) + NADPH (electron carrier). 
  • Light-independent = ATP + NADPH + carbon dioxide -> glucose
Thus both light-dependent and light-independent reactions play important and linked roles in photosynthesis. Let's take a quick look at both "sides" individually.

Light-dependent


Light-dependent reactions occur on the thylakoid membrane. Being on the membrane is critical because the key to ATP creation is the establishment of a proton gradient. Just like water will only flow downhill, molecules can "flow down" a membrane from areas of higher to lower concentration. In this case, pumping protons to side A results in a lower concentration on side B. These protons then be passed through from side A to side B via a special protein that spans the membrane called ATP synthase. ATP synthase, just like its name sounds, generates ATP. But how are these protons continually passed to side A (lumen) so that side B (stroma) always has less? This is accomplished by the photosystems.
Thylakoid membrane 3 by Somepics • CC BY-SA 4.0 • Wikipedia
Photosystem I (PSI) and photosystem II (PSII) both capture light at slightly different wavelengths with special chlorophyll molecules. An electron inside these special and specific chlorophylls is excited by the light and "jumps" out of the chorophyll into Q. Q carries the electron out of PSII and into the plastoquinone pool. This leaves the chorophyll without an electron, a very dire situation! This electron is replaced by breaking up water and taking an electron from it. After 4 of these cycles, a single molecule of oxygen and 4 protons are generated.

The excited electron takes a ride through a few electron carriers, including cytochrome b6f. Cytochrome b6f releases the proton that was riding along with the electron into the lumen, while passing the electron to plastocyanin. Plastocyanin will ferry the electron to photosystem I.

As stated earlier, PSI has special chlorophyll molecules that can be excited by light. This excited electron jumps up in energy and is passed along to create NADPH. But unlike PSII, photosystem I does not have an oxygen evolving complex to replace this jumping electron. It is replaced by the electron carried in by plastocyanin. The replacement electron originated in photosystem II.

The movement of electrons from photosystem II -> photosystem I -> NADPH is called the Z scheme.

Z scheme - personal illustration

It is traditionally drawn as I did above, which looks more like an N! But when it was originally described, prior to the addition of energy as the X axis, it was drawn as a Z from PSII --> PSI.

Light-independent


Also known as the Calvin cycle, after Melvin Calvin whose work in uncovering this pathway earned him the Nobel Prize in Chemistry. The Calvin cycle is a great big circle during which carbon dioxide is fixed into biologically usable carbon. The major player in the carbon fixation game is the enzyme RuBisCO. RuBisCO takes a 5 carbon molecule and attaches carbon dioxide creating a 6 carbon compound. This 6 carbon compound is then broken in half. RuBisCO can fix carbon dioxide but it can also use oxygen as well in a process called photorespiration. We will come back to photorespiration in a later post.

Calvin Cycle - personal illustration


The 3 carbon molecule generated by RuBisCo is called 3-phosphoglycerate. With a couple of enzymes, 3-phosphoglycerate is rearranged into glyceraldehyde 3-phosphate. This requires the usage of ATP and NADPH, both of which are generated in the light-dependent reactions.

The cycle part of the Calvin cycle is the many steps required to regenerate the initial 5 carbon molecule used by RuBisCo. A host of enzymes are required for the combination and rearrangement of carbon compounds to produce the initial 5 carbon compound (hence the many arrows on that part of the cycle). For each glucose molecule, the Calvin cycle has to turn 6 times using up 18 ATP, 12 NADPH, and 6 carbon dioxide.

Sum it Up

Photosynthesis is the real circle of life (sorry Lion King). Plants take carbon dioxide and fix it into glucose, while releasing oxygen as waste. Animals breath in oxygen, consume glucose, and release carbon dioxide as waste. Just another one of Nature's many recycling factories.

References

Falkowski and Raven, 2007. Aquatic Photosynthesis: Princeton University Press. ISBN: 9781400849727
Lodish et al., 2012. Molecular Cell Biology:W.H. Freeman & Company. ISBN: 9781464119132.

Vinyard DJ, Ananyev GM, Charles Dismukes G (2013) Photosystem II: The Reaction Center of Oxygenic Photosynthesis. Annual Review of Biochemistry 82 (1):577-606. doi:doi:10.1146/annurev-biochem-070511-100425

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