Organic compounds

Figure 6-11 above illustrates how the light reactions in the thylakoid and the Calvin cycle in the stroma actually work together as one continuous cycle—photosynthesis. Amazingly, this complicated process can occur in every one of the thousands of chloroplasts present in a plant if the conditions are right.

Recall that water is split during the light reactions, yielding electrons, protons, and oxygen as a byproduct. Thus, the simplest overall equation for photosynthesis, including both the light reactions and the Calvin cycle, can be written as follows:

In the equation above, (CH2O) represents the general formula for a carbohydrate. It is often replaced in this equation by the carbohydrate glucose, C6H12O6, giving the following equation:

6CO2 + 6H2O "ght energy C6H12O6 + 6O2

However, glucose is not actually a direct product of photosynthesis. Glucose is often included in the equation mainly to emphasize the relationship between photosynthesis and cellular respiration, in which glucose plays a key role. Just as the light reactions and the Calvin cycle together create an ongoing cycle, photosynthesis and cellular respiration together also create an ongoing cycle.

The light reactions are sometimes referred to as light-dependent reactions, because the energy from light is required for the reactions to occur. The Calvin cycle is sometimes referred to as the light-independent reactions or the dark reactions, because the Calvin cycle does not require light directly. But the Calvin cycle usually proceeds during daytime, when the light reactions are producing the materials that the Calvin cycle uses to fix carbon into organic compounds.

PLANT CELL figure 6-11

I Photosynthesis is an ongoing cycle.

PLANT CELL figure 6-11

I Photosynthesis is an ongoing cycle.

Quick Lab

Analyzing Photosynthesis

Materials disposable gloves, lab apron, safety goggles, 250 mL Erlenmeyer flasks (3), bromothymol blue, 5 cm sprigs of elodea (2), water, drinking straw, plastic wrap, 100 mL graduated cylinder


1. Put on your disposable gloves, lab apron, and safety goggles.

2. Label the flasks "1," "2," and "3." Add 200 mL of water and 20 drops of bromothymol blue to each flask.

3. Put the drinking straw in flask 1, and blow into the blue solution until the solution turns yellow. Repeat this step with flask 2.

4. Put one elodea sprig in flask 1. Do nothing to flask 2. Put the other elodea sprig in flask 3.

5. Cover all flasks with plastic wrap. Place the flasks in a well-lighted location, and leave them overnight. Record your observations.

Analysis Describe your results. Explain what caused one of the solutions to change color. Why did the other solutions not change color? Which flask is the control in this experiment?

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