Chapter 10.5, Problem 4CC

Summary Introduction

To find: The change in the relative abundance of C₃ versus C₄ and CAM species in a geographical region where the climate has become much hotter and drier, with no change in CO₂ concentration.

Introduction: A plant fixes carbon from the absorbed atmospheric CO₂ and turns this into sugar; this process is known as carbon fixation or the Photosynthetic carbon reduction (PCR) cycle. These reactions are independent of light, hence called dark reactions.

Thus, the pathway of assimilation of CO₂ during photosynthesis is mainly of three types.

1.) C₃ pathway_{: -} This pathway is also called as Calvin cycle. All photosynthetic plants show the Calvin cycle and followed by the plant when the first product of CO₂ fixation is a C₃ acid (3 Phosphoglyceric acids).

2.) C₄ pathway: - This pathway is evolved in the plant species which are adapted to live in hotter or high-temperature regions. They have a special type of leaf anatomy that can tolerate high temperatures. Also to minimize the photorespiration C₄ plants show two types of cells, outer mesophyll cells where they fix CO_2, and inner bundle sheath cells where they break down the C₄ acid (OAA) into C₃ molecule and also release CO₂. Thus, released CO₂ is entered into the Calvin cycle

3.) CAM pathway: - The plants which utilize the CAM pathway are mostly xerophytes. They have evolved a mechanism to adapt and survive in dry and hotter areas. They gather solar energy in the day and they also do not open their stroma in the day to prevent water loss. The stoma opens at night to absorb atmospheric CO_2. Then this CO₂ is converted into sugar (malate) which is stored until the day return.