What is photosynthesis?

A recent commentary in Nature prompts me to revisit an old bugaboo. The commentary discusses some recent work on CO₂ fixation in plants [A genetic switch drove photosynthesis in plants¹]. It begins with,

Photosynthesis, which uses energy from the Sun and carbon dioxide from the atmosphere to create carbohydrates, might be the most influential set of biochemical reactions on the planet.

Here's the problem. That's not a very good definition of photosynthesis. I discuss a much better definition in a post from seven years ago: Scientists confused about photosynthesis. A better definition is that photosynthesis is the process by which light energy is captured and converted to chemical energy. The direct products of photosynthesis are ATP and reducing equivalents such as NADPH. These cofactors are used to drive all sorts of reactions in the cell including DNA synthesis, protein synthesis, lipid synthesis, and carbohydrate synthesis.

This is very obvious when you examine photosynthetic bacteria but, unfortunately, photosynthesis was initially studied in large plants where much of the chemical energy produced by photosynthesis is used to fix CO₂ and make carbohydrates. This led to the widespread belief that photosynthesis is all about making carbohydrates.

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1. I'm using the title from the printed version of the journal. The web version has a different title. (I don't know why.)