Biochemistry

Photosynthesis: The chemical process.....

This equation for photosynthesis is a deceptively simple summary of a complex process. In reality photosynthesis is not just one reaction but two processes each with multiple steps. The two stages are known as the light reactions (photo part) and the dark reactions (the synthesis part, also known as the Calvin Cycle). These two stages of converting carbon dioxide and water into glucose are almost the same for all plants, algae and photosynthesizing bacteria. In the following section, both stages will be described and hopefully it will become clear how the food we eat can be created from the light of the sun.

Light Reactions

Light energy from the sun is converted into useful chemical tools through the light reactions of photosynthesis. After light is absorbed in the chloroplasts by the pigments, the pigment will boost one of its electrons to a higher energy level. Of course, a lone pigment would simply excite and then give off its energy as heat resetting itself for another absorption. This process is replaced by gathering several pigments together to form an attennae complexwithin the chloroplasts,which is similar to a funnel that gathers water from a large area and concentrates it at the center. At the center of this complex will be one pigment, most likely, chlorophyll a. The position of this pigment allows it to receive all the energy the other pigments received from the light and thus excite one electron to a very high energy state. This is the energy that is used to power the breakdown of carbon dioxide into nutrition for the plant in Calvin Cycle, or dark reactions. The combination of the attenae complex and the reaction center make up a photosystem, the light harvesting complexes of the thylakoid membrane.

Noncyclic Electron Flow:There are two types of photosystems, Photosystem I(PSI) and Photosystem II(PS II), which absorb light at 700 and 680 nm, respectively. These photosystems also work together to gather and convert the light into a storable form of chemical energy called ATP(adenine triphosphate) and the creation of a reductant (electron carrier) called NADPH. This cooperative process of energy production is called non-cyclic photophosphorylation and the transfer of electrons from water to NADPH, known as the Hill Reaction, is noncyclic electron flow. Light is absorbed at PS II, causing the photosystem to grab electrons from water and excite them to a primary acceptor. A primary acceptor is a molecule that can hold the high energy electrons. The electrons now travel down a chain of enzymes called an electron transport chain. This electron transport chain creates a battery of sorts (an electrochemical gradient), which powers a small factory that can store the energy difference in the thylakoid membrane, much like a battery. The electrons finally end at PSI and are excited again by another photon to the second primary acceptor. The electrons end up on feredoxin. Feredoxin is an iron containing molecule that can carry electrons. Feredoxin delivers the electrons to the final enzyme called NADP+ reductase, whose job is to make NADPH. NADPH is simply the way the cell transports electrons to other cycles in order to create bigger molecules for storage.

Cyclic Electron Flow
If the plant needed more energy instead of storing it away in other forms, the light reactions of photosynthesis have another cycle for that purpose. Cyclic photophosphorylation, involves only PSI. PSI electrons are excited by a photon to the second primary acceptor which transfers the electrons to feredoxin. If the cell needs ATP for the Calvin cycle, then the electrons will be diverted to cytochrome Q instead of NADP⁺ reductase. Cytochrome Q is part cytochrome complex, which produces ATP. Thus all of the electrons are used for creating ATP, satisfying the needs for more energy for the cell.

Dark Reactions

The NADPH and ATP, that are created/stabilized through the light reactions are used primarily in the fixation of carbon dioxide to make glucose. Glucose is simply a stable molecule that can store a great deal of energy that plants and animals can break down and use. Plants are able to create this powerbar of the cellular world through the dark reactions known as the Calvin cycle. The dark reactions can occur at any time of day, but are so called because they are not directly dependent on light.

Calvin Cycle:
Melvin Calvin was a professor of chemistry at the University of California at Berkeley when he and his colleagues elucidated the pathway of carbon fixation into glucose. Calvin was able to determine the path of carbon by exposing at various times and illuminations, the unicellular alga Chlorella to 14CO₂ a radioactive isotope of carbon. Then they quickly dropped the alga into a pot of boiling alcohol to stop the process but still preserve the labeling of the isotope. Calvin and his colleagues were able to determine that fixation of carbon occurred in three stages that make up one cycle . These are presented below:

Stage 1: A very slow enzyme named Rubisco takes an incoming CO₂ molecule and attaches it to a five carbon sugar called Ribulose Bisphosphate (RuBP). Rubisco is the first step and is just an abbreviation for ribulose bisphosphate carboxylase. Unfortunately this enzyme is very slow and thus the plant counteracts this by having a lot of it, as it is the most abundant enzyme on earth. The 6 carbon sugar formed from this union is very unstable and breaks into two identical smaller units called 3-Phosphoglycerate (3-PGA).

Stage 2: This stage is mostly a series of chemical reactions that the 3-Phosphoglycerate goes through, where it picks up another phosphate ion, and two electrons from NADPH. Finally, it ends with a new name and new structure called Glyceraldehyde 3-Phosphate(G3P). Remember, we have two of these, one travels off to form glucose while the other renters the cycle in the stage 3. So, it takes two turns of the cycle in order to create one molecule of glucose.

Stage 3: This is a series of complex reactions that regenerates the G3P back into the five carbon sugar, Ribulose Bisphosphate, which waits for another CO₂ molecule to come around and start the cycle all over again.

For one molecule of G3P, the Calvin cycle will use 9 molecules of ATP and 6 molecules of NADPH. Both of these are regenerated by the light reactions. Therefore, the light reaction or the dark reactions alone can not make sugar from CO₂, yet they work together feeding off each other to store energy effectively as glucose and later as starch.