Ecology

Lesson 2.3

2.3.3 Photosynthesis Key Information, Biocoach, and NOVA Interactive

1. Formula - Memorize and be able to explain this key formula. Know the chemical names, numbers, and symbols.

1. 6CO2 + 6H2O  C6H12O6 + 6O2
2. In words, the formula reads, “Six molecules of carbon dioxide combine with six molecules of water in the presence of sunlight and yields/produces one molecule of glucose and six molecules of oxygen.”

2. Movement of Molecules - Carbon dioxide enters the leaf through tiny holes called stomata. Water usually enters the plant through roots and travels to the leaf through vascular tissues or veins (specifically the xylem). Glucose is used by the plant for energy and also for biosynthesis of larger molecules such as starch, cellulose, and fiber. Oxygen is breathed out by the plant through stomata as a waste product.

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FIgure 2.3.2

3. Rearrangement of Carbon Molecules - The carbon molecules from carbon dioxide in the air are rearranged during photosynthesis to form glucose molecules. The glucose molecules are then combined during biosynthesis to form larger organic molecules, such as starch, cellulose, and fiber. Other glucose molecules are changed back into CO2 and water during cellular respiration to provide energy for the plant.

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Figure 2.3.3 Glucose molecules have 6 carbon atoms in a chain or a ring structure.

4. Energy Flow - Light energy from the sun is transformed into chemical energy by the plant during photosynthesis. The chemical energy is stored in high energy carbon-hydrogen and carbon-carbon bonds in glucose molecules. The chemical energy stays in the plant or is used by the plant. If used, the plant must break chemical bonds and form ATP during cellular respiration, releasing heat, water, and carbon dioxide. The four main types of energy are heat, light, chemical, and motion. All are seen in plants (yes, plants move too!), but light and chemical energy are probably the most important to plants.
5. Factors Affecting Photosynthesis - Temperature, light intensity and light wavelength, and CO2 concentration all play a role in increasing or decreasing the rate of photosynthesis. First of all, there is an optimum temperature for the ideal photosynthetic rate for each plant. If it is too cold or too hot, the plant’s enzymes slow down or stop working. Secondly, as light intensity increases (such as moving from the shade to full sunlight), the rate of photosynthesis increases until it reaches a plateau. Different wavelengths of light (red, orange, yellow, green, blue, and indigo) will also produce different photosynthetic rates. Green typically produces low photosynthesis rates while red and blue usually produce higher rates of photosynthesis. Finally, at higher levels of CO2 concentration there are higher photosynthetic rates. As CO2 concentration increases, photosynthesis increases as well until it reaches a maximum amount and plateaus. How do we measure photosynthesis rates? We can count bubbles in Elodea or spinach leaves. Sometimes we even use CO2 or O2 gas sensors and then measure the rate of change.

Figure 2.3.4

For further reading about the factors affecting photosynthesis, please try out these demonstrations and virtual labs:

• Simple online demo from KScience
• Virtual Lab: Wavelength and Photosynthesis from Glencoe Biology
• Light intensity vs rate of photosynthesis, from Reading University
• http://www.sparticl.org click photosynthesis