CHAPTER 8 INTRODUCTION

CORE CONCEPTS

8.1 Photosynthesis is the major pathway by which energy and carbon are incorporated into carbohydrates.

8.2 The Calvin cycle is a three-step process that results in the incorporation of carbon dioxide into carbohydrates.

8.3 The light-harvesting reactions use sunlight to produce ATP and NADPH required by the Calvin cycle.

8.4 Photosynthesis faces several challenges to its efficiency.

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 topic, the Flemish chemist and physiologist Jan Baptist van Helmont (1580–1644) found that the initial 300 pounds of 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 more than 150 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 entirely is that the other half of his tree had been created out of thin air.

The process that allowed Van Helmont’s tree to increase in mass using substances pulled from the air is called photosynthesis. Photosynthesis is a biochemical process for building carbohydrates from sunlight and carbon dioxide (CO₂) taken from the air. These carbohydrates are used as both structural components of the plant and as a source of energy used to produce ATP.