Stages of Photosynthesis Photosynthesis occurs in two stages, which are shown in Figurebelow.
Stage I is called the light reactions. This stage uses water and changes light energy from the sun into chemical energy stored in ATP and NADPH (another energy-carrying molecule). This stage also releases oxygen as a waste product.
Stage II is called the Calvin cycle. This stage combines carbon from carbon dioxide in the air and uses the chemical energy in ATP and NADPH to make glucose.
Before you read about these two stages of photosynthesis in greater detail, you need to know more about the chloroplast, where the two stages take place.
The Chloroplast: Theater for Photosynthesis The “theater” where both stages of photosynthesis take place is the chloroplast. Chloroplasts are organelles that are found in the cells of plants and algae. (Photosynthetic bacteria do not have chloroplasts, but they contain structures similar to chloroplasts and produce food in the same way.)
Figurebelow shows the components of a chloroplast. Each chloroplast contains neat stacks called grana (singular, granum). The grana consist of sac-like membranes, known as thylakoid membranes. These membranes contain photosystems, which are groups of molecules that include chlorophyll, a green pigment. The light reactions of photosynthesis occur in the thylakoid membranes. The stroma is the space outside the thylakoid membranes. This is where the reactions of the Calvin cycle take place.
Photosynthesis Stage I: The Light Reactions The first stage of photosynthesis is called the light reactions. During this stage, light is absorbed and transformed to chemical energy in the bonds of NADPH and ATP. Steps of the Light ReactionsThe light reactions occur in several steps, all of which take place in the thylakoid membrane, as shown in Figurebelow.
Step 1: Units of sunlight, called photons, strike a molecule of chlorophyll in photosystem II of the thylakoid membrane. The light energy is absorbed by two electrons (2 e-) in the chlorophyll molecule, giving them enough energy to leave the molecule.
Step 2: At the same time, enzymes in the thylakoid membrane use light energy to split apart a water molecule. This produces:
two electrons (2 e-). These electrons replace the two electrons that were lost from the chlorophyll molecule in Step 1.
an atom of oxygen (O). This atom combines with another oxygen atom to produce a molecule of oxygen gas (O2), which is released as a waste product.
two hydrogen ions (2 H+). The hydrogen ions, which are positively charged, are released inside the membrane in the thylakoid interior space.
Step 3: The two excited electrons from Step 1 contain a great deal of energy, so, like hot potatoes, they need something to carry them. They are carried by a series of electron-transport molecules, which make up an electron transport chain. The two electrons are passed from molecule to molecule down the chain. As this happens, their energy is captured and used to pump more hydrogen ions into the thylakoid interior space.
Step 4: When the two electrons reach photosystem I, they are no longer excited. Their energy has been captured and used, and they need more energy. They get energy from light, which is absorbed by chlorophyll in photosystem I. Then, the two re-energized electrons pass down another electron transport chain.
Step 5: Enzymes in the thylakoid membrane transfer the newly re-energized electrons to a compound called NADP+. Along with a hydrogen ion, this produces the energy-carrying molecule NADPH. This molecule is needed to make glucose in the Calvin cycle.
Step 6: By now, there is a greater concentration of hydrogen ions—and positive charge—in the thylakoid interior space. This difference in concentration and charge creates what is called a chemiosmotic gradient. It causes hydrogen ions to flow back across the thylakoid membrane to the stroma, where their concentration is lower. Like water flowing through a hole in a dam, the hydrogen ions have energy as they flow down the chemiosmotic gradient. The enzyme ATP synthase acts as a channel protein and helps the ions cross the membrane. ATP synthase also uses their energy to add a phosphate group (Pi) to a molecule of ADP, producing a molecule of ATP. The energy in ATP is needed for the Calvin cycle.
By the time Step 6 is finished, energy from sunlight has been stored in chemical bonds of NADPH and ATP. Thus, light energy has been changed to chemical energy, and the first stage of photosynthesis is now complete.
Photosynthesis Stage II: The Calvin Cycle The second stage of photosynthesis takes place in the stroma surrounding the thylakoid membranes of the chloroplast. The reactions of this stage can occur without light, so they are sometimes called light-independent or dark reactions. This stage of photosynthesis is also known as the Calvin cycle because its reactions were discovered by a scientist named Melvin Calvin. He won a Nobel Prize in 1961 for this important discovery. In the Calvin cycle, chemical energy in NADPH and ATP from the light reactions is used to make glucose. You can follow the Calvin cycle in Figurebelow as you read about it in this section.
Steps of the Calvin Cycle The Calvin cycle has three major steps: carbon fixation, reduction, and regeneration. All three steps take place in the stroma of a chloroplast.
Step 1: Carbon Fixation. Carbon dioxide from the atmosphere combines with a simple, five-carbon compound called RuBP. This reaction occurs with the help of an enzyme named RuBisCo and produces molecules known as 3PG (a three-carbon compound, 3-Phosphoglyceric acid).
Step 2: Reduction. Molecules of 3PG (from Step 1) gain energy from ATP and NADPH (from the light reactions) and re-arrange themselves to form G3P (glycerate 3-phosphate). This molecule also has three carbon atoms, but it has more energy than 3PG. One of the G3P molecules goes on to form glucose, while the rest of the G3P molecules go on to Step 3.
Step 3: Regeneration. The remaining G3P molecules use energy from ATP to form RuBP, the five-carbon molecule that started the Calvin cycle. This allows the cycle to repeat.
The Calvin cycle takes over where the light reactions end. It uses chemical energy stored in ATP and NADPH (from the light reactions) and carbon dioxide from the air to produce glucose, the molecule that virtually all organisms use for food.