How Does Photosynthesis Work?

Photosynthesis is a remarkable process that allows green plants, algae and some bacteria to convert carbon dioxide and light energy into chemical energy. This intricate biochemical process plays a vital role in sustaining life on Earth by providing oxygen and the foundation for food chains.

Light-dependent Reaction

Light-dependent reactions is the stage of photosynthesis where the captured solar energy turns into chemical energy.

The process begins when chlorophyll molecules, located in the chloroplasts of plant cells, absorb light energy from the sun. This absorbed energy splits water molecules (H₂O) into oxygen (O₂), hydrogen ions (H+) and electrons (e-).

The energy from the absorbed light is then used to create two essential molecules: adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). These molecules store and transport the energy needed for the next stage of photosynthesis: the light-independent reaction.

The Calvin Cycle

In the light-independent reactions, also known as the Calvin cycle, the chemical energy stored in ATP and NADPH is used to fix carbon dioxide (CO₂) into organic compounds. With carbon fixation, six molecules of carbon dioxide (CO₂) are captured from the atmosphere and combined with a five-carbon sugar molecule (RuBP) to form three-carbon compounds (3-PGA).

These three-carbon compounds undergo a series of chemical reactions facilitated by enzymes, consuming ATP and NADPH in the process. This leads to the formation of sugar molecule glyceraldehyde-3-phosphate (G3P).

Some of the G3P molecules produce glucose and other carbohydrates, serving as an energy source for the plant and are essential for growth and reproduction. To sustain the Calvin cycle, some of the G3P molecules regenerate RuBP. This step ensures a continuous supply of three-carbon compounds for further carbon fixation.

The energy available in sunlight is an untapped resource we've only begun to really get a handle on. Current photovoltaic-cell technology, typically a semiconductor-based system, is expensive, not terribly efficient and only does instant conversions from sunlight to electricity — the energy output isn't stored for a rainy day (although that could be changing: See "Is There a Way to Get Solar Energy at Night?").

But an artificial photosynthesis system or a photoelectrochemical cell that mimics what happens in plants could potentially create an endless, relatively inexpensive supply of all the clean "gas" and electricity we need to power our lives — and in a storable form, too.

To re-create the photosynthesis that plants have perfected, an energy conversion system has to be able to do two crucial things (probably inside of some type of nanotube that acts as the structural "leaf"): harvest sunlight and split water molecules.

Plants accomplish these tasks using chlorophyll, which captures sunlight, and a collection of proteins and enzymes that use that sunlight to break down H₂O molecules into hydrogen, electrons and oxygen (protons). The electrons and hydrogen are then used to turn CO₂ into carbohydrates, and the oxygen is expelled.

For an artificial system to work for human needs, the output has to change.

Instead of releasing only oxygen at the end of the reaction, it would have to release liquid hydrogen (or perhaps methanol) as well. That hydrogen could be used directly as liquid fuel or channeled into a fuel cell. Getting the process to produce hydrogen is not a problem, since it's already there in the water molecules. And capturing sunlight is not a problem — current solar-power systems do that.

The hard part is splitting the water molecules to get the electrons necessary to facilitate the chemical process that produces the hydrogen.

Splitting water requires an energy input of about 2.5 volts [source: Hunter]. This means the process requires a catalyst — something to get the whole thing moving. The catalyst reacts with the sun's photons to initiate a chemical reaction.

There have been important advances in this area in the last five or 10 years. A few of the more successful catalysts include:

Once perfected, these systems could change the way we power our world.

Fossil fuels are in short supply, and they're contributing to pollution and global warming. Coal, while abundant, is highly polluting both to human bodies and the environment. Wind turbines are hurting picturesque landscapes, corn requires huge tracts of farmland and current solar-cell technology is expensive and inefficient. Artificial photosynthesis could offer a new, possibly ideal way out of our energy predicament.

Fuel We Can Store

For one thing, it has benefits over photovoltaic cells, found in today's solar panels. The direct conversion of sunlight to electricity in photovoltaic cells makes solar power a weather- and time-dependent energy, which decreases its utility and increases its price. Artificial photosynthesis, on the other hand, could produce a storable fuel.

Multiple Output Options

And unlike most methods of generating alternative energy, artificial photosynthesis has the potential to produce more than one type of fuel. The photosynthetic process could be tweaked so the reactions between light, CO₂ and H₂O ultimately produce liquid hydrogen.

Liquid hydrogen can be used like gasoline in hydrogen-powered engines. It could also be funneled into a fuel-cell setup, which would effectively reverse the photosynthesis process, creating electricity by combining hydrogen and oxygen into water. Hydrogen fuel cells can generate electricity like the stuff we get from the grid, so we'd use it to run our air conditioning and water heaters.

One current problem with large-scale hydrogen energy is the question of how to efficiently — and cleanly — generate liquid hydrogen. Artificial photosynthesis might be a solution.

Methanol is another possible output. Instead of emitting pure hydrogen in the photosynthesis process, the photoelectrochemical cell could generate methanol fuel (CH₃OH).

Methanol, or methyl alcohol, typically derives from the methane in natural gas, and it's often added to commercial gasoline to make it burn more cleanly. Some cars can even run on methanol alone.

Skipping Harmful Byproducts

The ability to produce a clean fuel without generating any harmful byproducts, like greenhouse gasses, makes artificial photosynthesis an ideal energy source for the environment. It wouldn't require mining, growing or drilling. And since neither water nor carbon dioxide is currently in short supply, it could also be a limitless source, potentially less expensive than other energy forms in the long run.

In fact, this type of photoelectrochemical reaction could even remove large amounts of harmful CO₂ from the air in the process of producing fuel. It's a win-win situation.

But we're not there just yet. There are several obstacles in the way of using artificial photosynthesis on a mass scale.

While artificial photosynthesis works in the lab, it's not ready for mass consumption. Replicating what happens naturally in green plants is not a simple task.

Efficiency is crucial in energy production. Plants took billions of years to develop the photosynthesis process that works efficiently for them; replicating that in a synthetic system takes a lot of trial and error.

The manganese that acts as a catalyst in plants doesn't work as well in a man-made setup, mostly because manganese is somewhat unstable. It doesn't last particularly long, and it won't dissolve in water, making a manganese-based system somewhat inefficient and impractical.

The other big obstacle is that the molecular geometry in plants is extraordinarily complex and exact — most man-made setups can't replicate that level of intricacy.

Stability is an issue in many potential photosynthesis systems. Organic catalysts often degrade, or they trigger additional reactions that can damage the workings of the cell. Inorganic metal-oxide catalysts are a good possibility, but they have to work fast enough to make efficient use of the photons pouring into the system.

That type of catalytic speed is hard to come by. And some metal oxides that have the speed are lacking in another area: abundance.

In the current state-of-the-art dye-sensitized cells, the problem is not the catalyst; instead, it's the electrolyte solution that absorbs the protons from the split water molecules. It's an essential part of the cell, but it's made of volatile solvents that can erode other components in the system.

Advances in the last few years are starting to address these issues. Cobalt oxide is a stable, fast and abundant metal oxide. Researchers in dye-sensitized cells have come up with a non-solvent-based solution to replace the corrosive stuff.

Research in artificial photosynthesis is picking up steam, but it won't be leaving the lab any time soon. It'll be at least 10 years before this type of system is a reality [source: Boyd]. And that's a pretty hopeful estimate. Some people aren't sure it'll ever happen. Still, who can resist hoping for artificial plants that behave like the real thing?

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.