Photosynthesis: A Deceptively Complex Process
Written By: Arman Momeni
In elementary school, we are taught a rather simple equation for photosynthesis; it follows the general construct that carbon dioxide and water – with help from the sun’s energy – form to make glucose (sugar) and oxygen. While that equation is accurate, in reality, photosynthesis is a multifaceted and rather intriguing process.
Photosynthesis, contrary to what the equation says, is not a single-step process, rather two steps or two processes, each with multiple steps themselves, that all combine to help plants (or any autotrophs) make their own food.
The two stages of photosynthesis are the light reactions (the “photo” part) and the Calvin cycle (the “synthesis” part). The light reactions are the first stage of the process, occurring on the *thylakoid membranes of *chloroplast, and they convert solar energy into chemical energy (ATP and NADPH); oxygen is released as a waste product from the light reactions. The second stage of photosynthesis is the Calvin cycle, which preserves atmospheric CO2 and through reduction, transforms fixed carbon into carbohydrate.
Thylakoid: pouch-like sacs that are bound to the membrane of the chloroplasts in a plant cell. They contain a pigment, chlorophyl, which absorbs different wavelengths of light and reflects others.
Chloroplast: an organelle within the cells of plants that is the main site of photosynthesis.
Light Reactions:
Light-dependent reactions occur, as stated above, in the thylakoid membrane of chloroplasts and require the presence of sunlight to occur. During the light-dependent reactions, the sunlight (AKA energy from the sun) is converted into chemical energy. Light reactions follow a few simple steps, which are outlined as follows.
First, the chlorophyll in the plants absorb sunlight and transfer it to photosystems, which are structural and functional protein complexes involved in photosynthesis. The chlorophyll gets excited by the sunlight and transfers its electrons through an electron transport chain. Water is then used to donate hydrogen ions and electrons, but in return, through a redox reaction, produces oxygen. The electrons and protons (hydrogen ions) are used to produce NADPH, which is the reduced form of NADP. ATP and NADPH are both produced through the light -reactions and are energy storage and electron carrier/donor molecules that are used in the following stage of photosynthesis: the Calvin cycle. Finally, the chlorophyl molecules regain the electrons they lost from the water molecule at the beginning of the reaction.
Calvin Cycle:
The Calvin cycle is the second part of photosynthesis and is called a cycle, because the material that it started with is regenerated throughout the process. The Calvin cycle is anabolic, which means that it builds carbohydrates from smaller molecules and uses energy.
The Calvin cycle, however, does not produce glucose directly. Every turn of the Calvin cycle produces G3P, which is why it takes two turns of the Calvin cycle to make 1 glucose; 2 G3P combine to form a glucose molecule. The Calvin cycle has three main phases: carbon fixation, reduction, and regeneration.
In the first phase, carbon fixation, the Calvin cycle incorporates each CO2 molecule by attaching it to a five-carbon sugar known as RuBP. Rubisco, an enzyme, catalyzes the attachment of RuBP to CO2. The product of the reaction is a six-carbon intermediate that is energetically unstable, so it immediately splits in half, forming two molecules of 3-phosphoglycerate.
In the second phase, reduction, each molecules of the 3-phosphoglycerate receives a phosphate group from ATP. And then, a pair of electrons are donated by NADPH, which causes the 3-biphosphoglycerate (the new 3-phosphoglycerate with an added phosphate group) to lose 2 phosphate groups, becoming G3P (the molecule that is half of a glucose molecule)
In the third phase, regeneration, a series of complex reactions cause the carbon skeleton of 5 G3P molecules (which were produced in the reduction phase) to turn into three molecules of RuBP – and thus, the cycle continues, starting from the very beginning, carbon fixation.
Works Cited:
Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Photosynthesis. Available from: https://www.ncbi.nlm.nih.gov/books/NBK9861/#
Urry, L. A., Cain, M. L. 1., Wasserman, S. A., Minorsky, P. V., Reece, J. B., & Campbell, N. A. (2017). Campbell biology. Eleventh edition. New York, NY, Pearson Education, Inc.
