Chapter 2, Section 1

Chapter 2, Section 1

The Two Stages of Photosynthesis

Photosynthesis is a complex process. During photosynthesis, plants and some other organisms use energy from the sun to convert carbon dioxide and water into oxygen and sugars. The process of photosynthesis is shown in Figure 3. You can think of photosynthesis as taking place in two stages: capturing the sun’s energy and producing sugars. You’re probably familiar with many two-stage processes. To make a cake, for example, the first stage is to combine the ingredients to make the batter. The second stage is to bake the batter. To get the desired result—the cake—both stages must occur in the correct order.

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Figure 3Two Stages of Photosynthesis Photosynthesis has two stages, as shown in the diagram. Interpreting Diagrams Which stage requires light?

Stage 1: Capturing the Sun’s Energy

The first stage of photosynthesis involves capturing the energy in sunlight. In plants, this energy-capturing process occurs mostly in the leaves. Recall that chloroplasts are green organelles inside plant cells. The green color comes from pigments, colored chemical compounds that absorb light. The main photosynthetic pigment in chloroplasts is chlorophyll.

Chlorophyll functions in a manner similar to that of the solar “cells” in a solar-powered calculator. Solar cells capture the energy in light and use it to power the calculator. Similarly, chlorophyll captures light energy and uses it to power the second stage of photosynthesis.

Stage 2: Using Energy to Make Food

In the next stage of photosynthesis, the cell uses the captured energy to produce sugars. The cell needs two raw materials for this stage: water (H₂O) and carbon dioxide (CO₂). In plants, the roots absorb water from the soil. The water then moves up through the plant’s stem to the leaves. Carbon dioxide is one of the gases in the air. Carbon dioxide enters the plant through small openings on the undersides of the leaves called stomata (stoh muh tuh) (singular stoma). Once in the leaves, the water and carbon dioxide move into the chloroplasts.

Inside the chloroplasts, the water and carbon dioxide undergo a complex series of chemical reactions. The reactions are powered by the energy captured in the first stage. These reactions produce chemicals as products. One product is a sugar that has six carbon atoms. Six-carbon sugars have the chemical formula C₆H₁₂O₆. Recall that sugars are a type of carbohydrate. Cells can use the energy in the sugar to carry out important cell functions.

The other product of photosynthesis is oxygen (O₂), which exits the leaf through the stomata. In fact, almost all the oxygen in Earth’s atmosphere was produced by living things through the process of photosynthesis.

The Photosynthesis Equation

The events of photosynthesis can be summed up by the following chemical equation:

Notice that the raw materials—six molecules of carbon dioxide and six molecules of water—are on the left side of the equation. The products—one molecule of a sugar and six molecules of oxygen—are on the right side of the equation. An arrow, which you can read as “yields,” connects the raw materials to the products. Light energy, which is necessary for the chemical reaction to occur, is written above the arrow.

What happens to the sugar produced in photosynthesis? Plant cells use some of the sugar for food. The cells break down the sugar molecules to release the energy they contain. This energy can then be used to carry out the plant’s functions. Some sugar molecules are converted into other compounds, such as cellulose. Other sugar molecules may be stored in the plant’s cells for later use. When you eat food from plants, such as potatoes or carrots, you are eating the plant’s stored energy.

Figure 4Stored energy When you eat a carrot, you obtain energy stored during photosynthesis.