Chapter 8 Introduction

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CHAPTER 8

Photosynthesis

Using Sunlight to Build Carbohydrates

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Core Concepts

  1. Photosynthesis is the major pathway by which energy and carbon are incorporated into carbohydrates.
  2. The Calvin cycle is a three-step process that uses carbon dioxide to synthesize carbohydrates.
  3. The light-harvesting reactions use sunlight to produce the ATP and NADPH required by the Calvin cycle.
  4. Challenges to the efficiency of photosynthesis include excess light energy and the oxygenase activity of rubisco.
  5. The evolution of photosynthesis had a profound impact on life on Earth.

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Walk through a forest and you will be struck, literally if you aren’t careful, by the substantial nature of trees. Where does the material to construct these massive organisms come from? Because trees grow upward from a firm base in the ground, a reasonable first guess is the soil. In the first recorded experiment on this question, the Flemish chemist and physiologist Jan Baptist van Helmont (1580–1644) found that the 200 pounds of dry soil into which he had planted a small willow tree decreased by only 2 ounces over a 5-year period. During this same period, the tree gained 164 pounds. Van Helmont concluded that water must be responsible for the tree’s growth. He was, in fact, half right: A tree is roughly half liquid water. But what he missed completely is that the other half of his tree had been created almost entirely out of thin air.

The process that allowed Van Helmont’s tree to increase in mass using material pulled from the air is called photosynthesis. Photosynthesis is a biochemical process for building carbohydrates using energy from sunlight and carbon dioxide (CO2) taken from the air. These carbohydrates are used both as starting points for the synthesis of other molecules and as a means of storing energy that can be converted into ATP through cellular respiration.