What is Photosynthesis?

Photosynthesis (from the Ancient Greek combining form φωτω-, phōtō-, from φῶς, phôs, ‘light’, and σύνθεσις, synthesis, ‘a putting together’:  ‘a putting together with light’) is the process by which plants, algae, cyanobacteria, and a few other bacterial species convert the light energy of photons and the carbon from carbon dioxide into the chemical energy of carbohydrates. These carbohydrates (sugars) are stored as a food source — in short, photosynthesisers make their own food with the help of sunlight.

Metabolically, all photosynthesisers are both phototrophs, with their energy source coming from photons, and autotrophs, with their carbon source coming from inorganic carbon.

There are other organisms which are phototrophs, and others again which are autotrophs, but only photosynthesising organisms are both (and can be called photoautotrophs).

The Two Types of Photosynthesis

There are two types of photosynthesis: anoxygenic (not producing oxygen), and oxygenic (producing oxygen).

All anoxygenic photosynthesisers are bacterial, and believed to be amongst the first life on Earth appearing at least 3.5 billion years ago. These microbes are with us to this day, and include the green sulfur bacteria and the purple bacteria.

These were the only photosynthesisers in existence for about 1.5 billion years, until cyanobacteria appeared some 2 billion years ago. These organisms, also still with us to this day, were the first to produce oxygen as a by-product of photosynthesis. This led to the gradual oxygenation of Earth’s atmosphere during a period known as the Great Oxygen Event (or Oxygen Catastrophe depending on your point of view).

The atmosphere prior to their appearance was nothing like today’s. It was predominately nitrogen gas (N₂) and carbon dioxide (CO₂), with traces of water (H₂O), methane (CH₄), carbon monoxide (CO) and hydrogen gas (H₂). Cyanobacterial activity was the sole means by which this atmosphere slowly became oxygenated, driving both a mass extinction of many anaerobic lifeforms (those which grow in the absence of oxygen) and the evolution of multicellular life (impossible without oxygen, owing to the amount of energy required).

Oxygenic photosynthesis is by far the dominant form today, and performed by all cyanobacteria, algae and plants.

The Formula for Photosynthesis

All photosynthesis, whether anoxygenic or oxygenic, can be written with the basic formula:
CO₂ + 2H₂A + photons → [CH₂O]_n + H₂O + 2A

where:
1. A is typically S (sulfur) in anoxygenic, and always O (oxygen) in oxygenic photosynthesis; and
2. [CH₂O]_n is a carbohydrate.

With reference to the Metabolic Classifications chapter, this formula shows us that:
1. the carbon from CO₂, an inorganic molecule, is converted into carbohydrate, an organic molecule. Here, inorganic carbon is the carbon source, making this an example of autotrophy;
2. the A atoms are oxidised from ionic form to elemental form. (Please read Understanding Redox (Reduction-Oxidation) Reactions for a deeper explanation of this process). Here, A ions are losing electrons, making the H₂A compound the electron donor. A is typically sulfur or oxygen, which are both inorganic, making this an example of lithotrophy;
3. photons are present. These are the energy source, making this an example of phototrophy.

Photosynthesisers are not only photoautotrophs, they are also photolithoautotrophs.

Anoxygenic Photosynthesis

Anoxygenic photosynthesis typically involves sulfur, and substituting S for A into the basic formula above gives:
CO₂ + 2H₂S + photons → [CH₂O]_n + H₂O + 2S

The carbohydrate is glucose, which to fit the above equation can be written as [CH₂O]₆, but is more commonly written as C₆H₁₂O₆. Thus we have:
CO₂ + 2H₂S + photons → C₆H₁₂O₆ + H₂O + 2S

Both sides of a chemical formula must balance, therefore:
6CO₂ + 12H₂S + photons → C₆H₁₂O₆ + 6H₂O + 12S

H₂S is hydrogen sulfide, or ‘rotten-egg gas’. Thus we see that some of life’s first photosynthesisers were — and still are — using the energy from light and the electrons from sulfide (S^2-) ions to convert carbon dioxide into glucose food, whilst producing pure (elemental) sulfur (S) as a by-product!

Other anoxygenic photosynthesisers make use of hydrogen gas (A = null), ferrous (Fe²⁺) iron ions (A = Fe), and even arsenic salts (A = HAsO₄^2-) as electron donors in their conversion of carbon dioxide to carbohydrate.

Oxygenic Photosynthesis

Today this is the most widespread form of photosynthesis, simply because of the sheer biomass these organisms comprise. Unlike anoxygenic photosynthesisers, which are all bacterial, oxygenic photosynthesisers include microbial cyanobacteria, unicellular algae, and multicellular algae and plants. Cyanobacteria are the only bacterial representatives of this group.

All oxygenic photosynthesisers use water as their electron source, thus substituting O for A in the above basic formula gives:
6CO₂ + 12H₂O + photons → C₆H₁₂O₆ + 6H₂O + 6O₂

There is far more going on behind the scenes than this deceptively simple formula implies, but nonetheless, this formula is the reason why Earth’s atmosphere is 21% oxygen gas today.

The Importance of Photosynthesisers

As an Energy Input via Phototrophy

The Law of Conservation of Energy states that energy can neither be created nor destroyed. It can only be transformed or transferred from one form to another.

In other words, living things cannot make their own energy, but must acquire it somehow. Each organism must continuously input energy into its system from somewhere, so as to drive that system.

For photosynthesisers — and all phototrophs for that matter — that ’somewhere’ is sunlight.

Sunlight constantly bathes and warms the Earth, but it is not bestowing some ‘life-force’ to the planet whilst doing so. Rather, phototrophs are constantly extracting energy from its photons, which then drives metabolic pathways for their growth and reproduction. They are the sole means by which photon energy is converted into the chemical energy all other organisms can then access by either eating those phototrophs, or organisms which have eaten phototrophs, or both.

Phototrophs are the ones to continuously pump external solar energy into the greater system that is Life on Earth — should they cease to be, then so too will that energy input come to an end and the vast bulk of life disappear.

[Not all life necessarily, but certainly all terrestrial and aquatic macroorganisms would die off. Many microbial species would also become extinct, but many more would be unaffected so long as the sun and Earth’s atmosphere are still present. The micro and macro life around ocean floor hydrothermal vents will persist for as long as the water remains and those vents continue to pump out heat and minerals. (This is a completely different ecological system worth exploring later.)]

As a Food Input via Autotrophy

The Law of Conservation of Mass states that the quantity of mass in a closed system must remain constant over time. This means that mass can neither be created nor destroyed. It can only be rearranged/converted into new forms, with no net loss or gain.

In other words, living things cannot create from nothing the carbohydrates, lipids and proteins they need to live, grow and reproduce — they must continually obtain a source of carbon (and other elements) from which to synthesise these. These carbon (and other) atoms must enter the organism in a form the organism can make use of.

Photosynthesisers — and all autotrophs for that matter — obtain their carbon from carbon dioxide. (Photoautotrophs use photon energy while chemoautotrophs use chemical energy to do this). That carbon goes into the carbohydrates autotrophs can then use as food, and those carbohydrates, along with the autotrophs themselves, become food in turn for other organisms — that food becomes their carbon source. Those organisms in turn become the carbon source for still others, and so on. 

Autotrophs are the sole means by which inorganic carbon is converted into organic forms all organisms can use. Without autotrophs, there is no mechanism by which otherwise inaccessible carbon in carbon dioxide becomes the accessible carbon all life needs.

As an Input of Inorganic Materials via Lithotrophy

Anoxygenic photosynthesisers — and all lithotrophs for that matter —  use inorganic substances such as hydrogen gas, iron and sulfides as their electron donor sources. This makes the hydrogen, iron and sulfur atoms available in forms other organisms can use, and contributes to nutrient cycling within their environments.

The oxygenic photosynthesisers use water as their inorganic electron donor. Here, the oxygen atoms in water are oxidised to oxygen gas, the one gas essential for all macro life, and which makes it possible at all. (Another topic to explore later!)

A Bridge Between Inorganic and Organic

Life is organic, but all life can be traced back to organisms which convert at least one inorganic material into an organic one.

For example, many food chains begin with phototrophs converting light energy into the chemical energy of organic carbohydrates. (Many, but not all, as people learn at school. Some food chains, as with deep sea hydrothermal vent ecosystems, begin with chemotrophs converting the chemical energy of inorganic minerals rather than light. But it’s still inorganic in origin.)

Food chains also begin with autotrophs converting inorganic carbon dioxide into organic carbohydrates. And the electrons which drove their manufacture most likely came from inorganic materials such as sulfur and water. (Most autotrophs are also lithotrophs, though there are organoautotrophs which use organic compounds as electron donors.)

Photosynthesisers are a unique all-in-one package of phototroph, lithotroph and autotroph, and in a way are the complete bridge between inorganic materials and organic life.