Photosynthesis is the process by which plants, algae, cyanobacteria, and a few other bacterial species convert light energy into chemical energy, which is stored as a food source. Photosynthetic organisms thus produce their own food, and are called autotrophs (’self-nourishment’). Those that must take in food, such as us, are heterotrophs (’other/different nourishment’).

Photosynthesis is a truly amazing process without which we and all other multicellular organisms would never have come to be. And the photosynthesis we are most interested in — oxygenic photosynthesis which occurs in plants, algae and cyanobacteria, as opposed to the anoxygenic photosynthesis in some bacteria — would never have developed at all, were it not for water.

Early Earth (believed to have formed over 4.5 billion years ago) had virtually no oxygen in its atmosphere, and the first life did not appear until at least 3.5 billion years ago. The first photosynthetic organisms probably appeared around this time, and performed anoxygenic photosynthesis, photosynthesis which does not produce oxygen.

It wasn’t until cyanobacteria evolved around two billion years ago that oxygen was produced from photosynthesis (oxygenic photosynthesis), leading to the oxygenation of Earth’s atmosphere. This Great Oxygen Event (or Oxygen Catastrophe depending on your point of view) would have made extinct most of the organisms of the time, which were all unicellular and anaerobic (not requiring oxygen for growth; oxygen in this case would have been poisonous for them). This oxygenation however is directly responsible for the evolution of all multicellular life that has ever existed on Earth.

Oxygenic photosynthesis is the most common today. It is a very complex process, but can be summarised in words as:

carbon dioxide + water + photons (particles of light, light energy) → glucose + oxygen + water

and chemically as:

6CO₂ + 12H₂O + photons → C₆H₁₂O₆ + 6O₂ + 6H₂O

You could say more long-windedly that the input of light energy causes six molecules of carbon dioxide to react with twelve molecules of water, to form one molecule of glucose (a simple carbohydrate which the plant stores as food/chemical energy), six molecules of oxygen gas which is released into the air that we and many other organisms then breathe, and six molecules of water.

(This equation doesn’t really occur. Carbon dioxide doesn’t react with water during photosynthesis, and glucose isn’t even the product. Rather, this equation is used in textbooks as a ‘good enough’ summary of the inputs and outputs of photosynthesis. In reality there are two very different sets of reactions going on, and these are described in this site’s From Soil to Fruit book chapter.)

As you can see here, oxygenic photosynthesis requires water, and produces oxygen. Anoxygenic photosynthesis does not require water, and does not produce oxygen. To this present day, many anoxygenic photosynthetic bacteria utilise hydrogen sulfide (’rotten egg gas’) instead of water to form glucose, and produce solid sulfur instead of oxygen as a by-product!

We (multicellular organisms) literally would not be here today were it not for the presence of water and those early cyanobacteria (or some similar organism that may have evolved had they not) producing the oxygen that kickstarted the whole process. And we won’t be here tomorrow should oxygenic photosynthesis cease to exist.

As plants are our special interest here, let’s now discuss photosynthesis as it directly relates to plants.

For starters, plants are the only photosynthetic organisms with specialised equipment dedicated to photosynthesis — leaves. (Though there are plants, for example cacti, that don’t have leaves and instead photosynthesise via other body parts such as the stems.)

In the leaves are special structures called chloroplasts, which contain chlorophyll a and chlorophyll b. These absorb red and blue light the most, and the green and near-green wavelengths the least. These green and near-green wavelengths are reflected back, and we see the leaves as green.

Carbon dioxide enters the leaves via pores called stomata (plural form of stoma), and water enters via the xylem. Xylem is a transport tissue, whose purpose is to conduct water from the roots to stems and leaves via capillary action for various biochemical processes including photosynthesis. (Note to self: these could be subjects for future posts.)

With carbon dioxide and water in place, what happens next in the chloroplast is incredibly involved and perhaps the subject of future posts if I run out of things to write about in winter!

(Update: please refer to the Photosynthesis section of the From Soil to Fruit book for those posts!)

But to gloss over all that, very simply, photons (light energy) excite electrons in chlorophyll a molecules, which become unstable and leave chlorophyll a. These electrons are replaced by the splitting of water molecules in the chloroplast into hydrogen and oxygen, and these in turn become excited by photons, and also leave chlorophyll a. The combined result is a chain reaction of electron flow which ultimately produces energy (stored as adenosine triphosphate (ATP) and sugars.

The oxygen from the water-splitting is a by-product not used by the plant, and exits the chloroplast to be released via the stomata into the atmosphere. The hydrogen stays in the chloroplast, where it combines with carbon dioxide to form sugar.

We can infer factors that affect photosynthesis and hence plant growth. Light intensity is an obvious one — not too much photosynthesis occurs at night! Latitude and altitude will also play into how much light is received during the day and over a year, and structures or other vegetation that cast shadows for long hours will affect sun-loving plants.

The wavelengths of light received influence photosynthesis. Plants grown commercially under cover often require special lightbulbs for optimum growth — your reading light just won’t cut it!

Photosynthesis increases when carbon dioxide levels increase, and it is routine to pump this gas into commercial greenhouses for increased growth.

Photosynthesis, like all other biological processes, requires enzymes to drive reactions (this is part of the complexity not shown and deliberately skipped over in this post). Enzymes are biological catalysts, and each one has its optimum temperature at which full operation occurs. Just a few degrees above or below this optimum is enough to slow the rate of operation, while lower or higher again will halt enzyme activity altogether.

Extremely high temperatures will denature enzymes, and in very extreme cases (such as a lack of water needed for its coolant and solvent properties) cause a plant to shut down and even die. (Likewise, a fever-induced high body temperature in a person is potentially dangerous — a body must keep a fine line between a slightly higher than normal temperature that kills invasive bacteria or viruses, and a too high temperature that shuts down biochemical reactions and leads to a body breaking down.)

Water, the miracle molecule, and photosynthesis, the miracle process by which light is converted into food, are the very backbone of life on this planet. Had oxygenic photosynthesis not appeared around two billion years ago, anoxygenic photosynthetic life would in all likelihood still be around, but would remain as simpler unicellular life to the present day.

It is oxygenic photosynthesis producing oxygen that enabled multicellular organisms such as jujube trees and people to evolve at all.

Next week I’ll cover a process even more important than photosynthesis — the nitrogen cycle. For if not for the nitrogen cycle there’d be no such thing as a plant in the first place.