38.1: Photosynthesis and respiration

Photosynthesis and its sister reaction respiration are key biogeochemical processes in the Earth system. The simplified chemical equation for photosynthesis is:

\[\ce{CO2} +\ce{H2O} \rightarrow \ce{C6H12O6} + \ce{O2} \nonumber\]

(Requires a lot of energy input, from the Sun)

In other words, the Sun’s energy can cause carbon dioxide to react with water, producing glucose (a sugar) and free oxygen. This reaction is facilitated by living organisms, principally cyanobacteria, algae, and plants {LINK TO PLANT CASE STUDY}. The glucose is the point of the reaction for these organisms, while the oxygen is just an incidental waste product.

For respiration (or its inorganic equivalent, combustion), just flip the equation around and run it in reverse:

\[\ce{C6H12O6} + \ce{O2} \rightarrow \ce{CO2} + \ce{H2O}\nonumber\]

(Releases a lot of energy)

Figure \(\PageIndex{1}\): Photo by Rosmarie Voegtli. (CC-BY; via Flickr.)

Every green plant you have ever seen is actively doing photosynthesis while sunlight shines on its leaves, and the same is true for primordial cyanobacteria and floating phytoplankton in the oceans. Collectively, these organisms are powerful agents of geochemical change: they extract \(\ce{CO2}\) from the atmosphere, shucking off its oxygen as waste, and use its carbon to build their bodies.

Today, this effect can be measured in real time: photosynthesis in the northern hemisphere summer pulls about 6 ppm of \(\ce{CO2}\) out of the air. That’s about 1.5% of the total atmospheric pool of \(\bf{\ce{CO2}}\)! There is not as much land in the southern hemisphere (and much of it is desert), so the \(\ce{CO2}\) drawdown effect is not as large during the austral summer. It is most pronounced between April and September of each year.

Figure \(\PageIndex{2}\): The ‘leaf sheep’ is an exception to the rule: a photosynthesizing animal. This 1-cm-long sea slug hosts symbiotic algae within its flesh. (CC-BY; Photo by prilfish, via Flickr.)

When animals eat algae or plants, they aren’t doing it to be mean. They are doing it because they will die if they don’t. Unlike autotrophic plants, animals are heterotrophs – a word that means they need to eat. Eating is an act whose intention is to grab some of the plants’ stored energy and use it to drive the animal’s own life: moving, growing, reproducing. As a general rule, animals are not capable of photosynthesis. Eating is their only option. Curiously though, in multiple lineages, some animals have figured out how to host photosynthetic cyanobacteria or algae within their bodies as endosymbionts.

Let’s circle back now to photosynthesis. Driven by the energy in sunbeams, plants suture carbon dioxide to water, making glucose, and ejecting much of the oxygen as waste. But trees are not made of glucose. Instead, they bulk up by converting glucose into cellulose via this reaction:

\[\ce{C6H12O6} + \ce{C6H12O6} + \ce{C6H12O6} + \ce{C6H12O6}, \ etc. \ \rightarrow \ce{(C6H10O5)}_n + \ce{H2O} \nonumber\]

Cellulose is a much more durable compound. Non-woody plants such as grasses are capable of projecting upward from the ground because cellulose in their cell walls provides structural support. Along with lignin, cellulose makes up the bulk of trees and other woody plants. It’s wood. Wood makes up the bulk of the planet’s biomass (450 Gt of carbon, about ~82% of the total).

Wood is capable of being oxidized by organisms, but not especially rapidly. Fungi and bacteria can break it down, given enough time. Depending on the species of wood and levels of moisture and ambient oxygen, this decomposition can take a year or several centuries. Herbivores such as termites and cows host symbiotic bacteria in their guts which can break down cellulose more rapidly, but that’s not a trick that humans can pull off! If we decide to oxidize wood quickly, we burn it.

Figure \(\PageIndex{3}\): Bonfires are hot because of stored solar energy. What you can’t see in this photo is the \(\ce{CO2}\) and \(\ce{H2O}\) vapor that’s being released. (Callan Bentley photo.)

We have probably all experienced fire. When you enjoy a bonfire, campfire, or the radiant heat of a woodstove, you’re reversing the photosynthesis reaction via the secondary product of wood. All that warmth, all that orange and yellow light, all those loud crackling noises – these are various forms of energy that are released as the bonds holding together cellulose and lignin are broken. For most wood available to most people, that energy was captured in the past few years or decades.

The same is also true when we burn ancient plant biomass – though not “wood” or “cellulose” any more, ancient plants can provide long-term stores of ancient carbon. Let us next examine how these “fossil fuels” form, and what the implications are of their formative processes, specifically for the atmosphere and biosphere.