2 - Biochemistry and photochemistry of terrestrial photosynthesis: a synopsis

Summary

Introduction

Photosynthesis is typically understood as the light-dependent production of sugar from carbon dioxide (CO₂). The endosymbiotic chloroplast is the cellular location for most of this metabolism in plants, but some additional metabolism occurs in the cytosol to make the sugars that will be transported around the plant, mainly sucrose and also sugar alcohols, such as sorbitol and manitol. There are many processes that can properly be called photosynthesis, but a core set of processes underlie most of the considerations in this book. This chapter will provide an overview of those processes, and many topics covered in this chapter are the subject of more in-depth chapters later on. This chapter begins by describing the initial capture and temporary storage of light energy as highly reactive molecules (nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP)) on carbon. By reducing (i.e., by adding electrons to) carbon from its most oxidised state (CO₂) to the status of sugars (CH₂O)_n, the energy initially stored as NADPH and ATP can be stored on the carbon. Additional energy can be stored on each carbon atom by reducing it fully, as happens in the synthesis of oils (R-CH₂-R), but this is generally not considered when describing photosynthesis. Finally, issues surrounding uptake of the CO₂ will be addressed.

Photochemistry Synopsis

Photochemistry, the capture of light energy and its conversion to chemical energy suitable for reducing CO₂ to sugar, is the source of nearly all energy available to living things. Energy captured by absorbing molecules is stored as the high-energy intermediates NADPH (reducing power) and ATP (sometimes called the energy currency of the cell).