Photosynthesis: Sugar as Food
- Outline the stages of photosynthesis.
- Describe the chloroplast and its role in photosynthesis.
- List the steps of the light reactions.
- Describe the Calvin cycle.
Plants and other autotrophs make food out of “thin air”—at least, they use carbon dioxide from the air to make food. Most food is made in the process of photosynthesis. This process provides more than 99% of the energy used by living things on Earth. Photosynthesis also supplies Earth’s atmosphere with oxygen.
An overview of photosynthesis:
Stages of Photosynthesis
Photosynthesis occurs in two stages, which are shown in Figure below.
- 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.
The two stages of photosynthesis are the light reactions and the Calvin cycle. Do you see how the two stages are related?
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.) Look at the Elodea plant in photograph (a) of Figure below. If you could look at a single leaf of this plant under a microscope, you would see small green ovals, like those in photograph (b). These small green ovals are chloroplasts.
An Elodea plant (a) lives in the water. Viewed under a microscope (b), each Elodea leaf contains many chloroplasts, like those shown here.
Figure below shows the components of a chloroplast. Each chloroplast contains neat stacks called grana (singular, granum). The grana consist of saclike 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. You can take a video tour of a chloroplast at the link below.
A chloroplast consists of thylakoid membranes surrounded by stroma. The thylakoid membranes contain molecules of the green pigment chlorophyll.
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. You can follow the process in the figure as you read about it below. You can also watch an animation of the light reactions:
This figure shows the light reactions of photosynthesis. This stage of photosynthesis begins with photosystem II (so named because it was discovered after photosystem I). Find the two electrons (2 e-) in photosystem II, and then follow them through the electron transport chain to the formation of NADPH in Step 5. In Step 6, where do the hydrogen ions (H+) come from that help make ATP?
Steps of the Light Reactions
The light reactions occur in several steps, all of which take place in the thylakoid membrane, as shown in Figure above.
- 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 (2H+). 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.
Summary of Stage I
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 Figure below as you read about it in this section. You can also watch an animation of the Calvin cycle at this link:
The Calvin cycle begins with a molecule named RuBP (a five-carbon sugar, Ribulose-1,5-bisphosphate) and uses the energy in ATP and NADPH from the light reactions. Follow the cycle to see what happens to all three of these molecules. Two turns of the cycle produce one molecule of glucose (called sucrose in the figure). In this diagram, each black dot represents a carbon atom. Keep track of what happens to the carbon atoms as the cycle proceeds.
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.
Summary of Stage II
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. The Calvin Cycle is discussed at:
- Most autotrophs make food using photosynthesis. This process occurs in two stages: the light reactions and the Calvin cycle.
- Both stages of photosynthesis take place in chloroplasts. The light reactions take place in the thylakoid membranes, and the Calvin cycle takes place in the stroma.
- The light reactions capture energy from sunlight, which they change to chemical energy that is stored in molecules of NADPH and ATP. The light reactions also release oxygen gas as a waste product.
- The reactions of the Calvin cycle add carbon (from carbon dioxide in the atmosphere) to a simple five-carbon molecule called RuBP. These reactions use chemical energy from NADPH and ATP that were produced in the light reactions. The final product of the Calvin cycle is glucose.
Lesson Review Questions
1. What are the stages of photosynthesis? Which stage occurs first?
2. Describe the chloroplast and its role in photosynthesis.
3. Summarize what happens during the light reactions of photosynthesis.
4. What happens during the carbon fixation step of the Calvin cycle?
5. During which stage of photosynthesis is glucose made?
6. The first living things appeared on Earth at least a billion years before photosynthetic organisms appeared. How might the earliest organisms have obtained energy before photosynthesis evolved? What process could they have used to make food?
7. Explain the role of the first electron transport chain in the formation of ATP during the light reactions of photosynthesis.
8. Explain what might happen if the third step of the Calvin cycle did not occur.
9. Plants release oxygen during the day but not during the night. Explain why.
Points to Consider
All living things need to break down glucose to make ATP for energy. Cellular respiration is the process in which this occurs.
- How do you think cellular respiration occurs? What steps do you think might be involved?
- How many molecules of ATP do you think cells get from a single molecule of glucose?