What is the process of photosynthesis and how does it work?

Plants have evolved to be highly efficient energy producers, harnessing sunlight for photosynthesis. This process converts sunlight, carbon dioxide, and water into fuel, releasing oxygen in the process. How does photosynthesis work? It is one of nature’s most brilliant energy systems.

Every year, using only sunlight, plants convert 1,102 billion tons (1,000 billion metric tons) of CO₂ into organic matter, providing energy for animals in the form of food. Remarkably, they do this with just 3 percent of the sunlight that hits Earth [source: Boyd].

For plants, algae, and some bacteria, "usable fuel" includes carbohydrates, proteins, and fats. In contrast, humans are in search of liquid fuels for cars and electricity for refrigerators. However, this doesn't mean photosynthesis can't help address our growing energy challenges. Scientists have been exploring ways to replicate the plant energy system, but with a different end result.

This article delves into the progress of artificial photosynthesis. We'll examine its goals, explore current methods being used to achieve it, and discuss the challenges that make it more complicated to design compared to other energy conversion systems.

How Photosynthesis Works

Photosynthesis is an extraordinary process that enables green plants, algae, and certain bacteria to transform carbon dioxide and light energy into chemical energy. This complex biochemical process is crucial for maintaining life on Earth, providing oxygen and forming the basis of food chains.

Light-dependent Reaction

The light-dependent reactions represent the phase of photosynthesis where solar energy is converted into chemical energy.

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

The energy harvested from the light is then used to produce two crucial molecules: adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH). These molecules store and carry the energy required for the next stage of photosynthesis: the light-independent reactions.

The Calvin Cycle

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

These three-carbon compounds go through a sequence of chemical reactions driven by enzymes, utilizing ATP and NADPH. This process results in the production of the sugar molecule glyceraldehyde-3-phosphate (G3P).

Some G3P molecules are used to create glucose and other carbohydrates, which provide energy for the plant and are crucial for its growth and reproduction. To keep the Calvin cycle going, some G3P molecules regenerate RuBP, ensuring a steady supply of three-carbon compounds for continued carbon fixation.

The Sun as a Resource

Sunlight holds vast untapped potential that we are just beginning to fully harness. Present photovoltaic technology, which generally relies on semiconductors, is costly, relatively inefficient, and only provides instant conversions from sunlight to electricity—without storing the energy for later use (though advancements are being made: See "Is There a Way to Get Solar Energy at Night?").

An artificial photosynthesis system, or a photoelectrochemical cell modeled after plant processes, could offer a virtually endless and cost-effective supply of clean "gas" and electricity to power our lives, with the added benefit of storing the energy for later use.

Artificial Photosynthesis Approaches

To replicate the photosynthesis process perfected by plants, an energy conversion system must achieve two key objectives (likely within a nanotube functioning as the structural "leaf"): harness sunlight and split water molecules.

Plants use chlorophyll to capture sunlight and a series of proteins and enzymes to utilize that sunlight to break down H₂O molecules into hydrogen, electrons, and oxygen (protons). The electrons and hydrogen are employed to transform CO₂ into carbohydrates, while oxygen is released as a byproduct.

For an artificial system to meet human needs, the outcome must be altered.

Rather than solely releasing oxygen at the end of the process, it would need to produce liquid hydrogen (or potentially methanol) as well. This hydrogen could be used directly as a liquid fuel or stored in a fuel cell. Generating hydrogen is not a challenge, as it is already present in water molecules. Capturing sunlight is also not an issue, as current solar power systems already do this.

The challenging part lies in splitting water molecules to obtain the electrons required for the chemical process that generates hydrogen.

Splitting water demands an energy input of approximately 2.5 volts [source: Hunter]. This necessitates a catalyst — a substance that initiates the entire process. The catalyst interacts with the sun's photons to kickstart a chemical reaction.

Significant progress has been made in this field over the past five to ten years. Some of the more effective catalysts discovered include:

When perfected, these systems could revolutionize the way we generate energy for the world.

Artificial Photosynthesis Applications

Fossil fuel reserves are dwindling, and their usage is accelerating pollution and global warming. Coal, though plentiful, is highly detrimental to both human health and the environment. Wind turbines mar scenic landscapes, corn farming requires vast areas of land, and current solar panel technology is both costly and inefficient. Artificial photosynthesis may provide a promising, if not ideal, solution to our energy crisis.

Fuel We Can Store

One advantage is that artificial photosynthesis has benefits over photovoltaic cells used in today's solar panels. Since photovoltaic cells convert sunlight directly into electricity, solar power becomes dependent on the weather and time of day, reducing its practicality and raising costs. In contrast, artificial photosynthesis could generate fuel that can be stored for later use.

Multiple Output Options

Unlike most alternative energy production methods, artificial photosynthesis could produce multiple types of fuel. By modifying the photosynthetic process, reactions involving light, CO₂, and H₂O could be adjusted to produce liquid hydrogen.

Liquid hydrogen could be utilized like gasoline in hydrogen-powered engines. It could also be directed into a fuel cell system, effectively reversing the photosynthesis process by generating electricity from the combination of hydrogen and oxygen into water. These hydrogen fuel cells can produce electricity similar to what we use from the grid, powering appliances like air conditioners and water heaters.

One of the current challenges with large-scale hydrogen energy is the efficient and clean production of liquid hydrogen. Artificial photosynthesis may offer a solution.

Methanol is another potential product of artificial photosynthesis. Instead of producing pure hydrogen, the photoelectrochemical cell might generate methanol fuel (CH₃OH).

Methanol, also known as methyl alcohol, is typically derived from methane in natural gas, and it is often blended with commercial gasoline to reduce emissions and make it burn cleaner. Some vehicles can even run solely on methanol.

Skipping Harmful Byproducts

One of the key advantages of artificial photosynthesis is its ability to create clean fuel without producing harmful byproducts like greenhouse gases. This makes it an ideal energy source for the environment. Unlike other energy sources, it doesn't require mining, farming, or drilling. Additionally, since water and carbon dioxide are abundant, this process could be a virtually limitless and potentially more affordable energy source in the long term.

In fact, this type of photoelectrochemical reaction could help eliminate significant amounts of harmful CO₂ from the atmosphere while producing fuel, making it a win-win scenario.

We are not quite there yet. There are still numerous challenges to overcome before we can utilize artificial photosynthesis on a large scale.

The Difficulties of Developing Artificial Photosynthesis

Though artificial photosynthesis functions in controlled environments, it's far from ready for widespread use. Mimicking the natural process in green plants is a complex undertaking.

Efficiency plays a key role in energy production. Plants spent billions of years perfecting the photosynthesis process that is highly effective for them; reproducing this in an artificial system involves a significant amount of experimentation and refinement.

Manganese, which serves as a catalyst in plants, doesn’t perform as well in artificial systems due to its instability. It tends to degrade quickly and doesn’t dissolve in water, making a manganese-based approach inefficient and impractical.

A major challenge is the complex and precise molecular structure in plants, which is extremely difficult to replicate in most synthetic systems.

Stability is a common problem in many photosynthesis models. Organic catalysts often break down or cause side reactions that harm the cell’s functionality. Inorganic metal-oxide catalysts show promise, but they need to react quickly to utilize the incoming photons effectively.

Achieving this level of catalytic speed is a challenge. Additionally, some metal oxides that are fast enough are deficient in another critical area: availability.

In the latest dye-sensitized cells, the real issue isn’t the catalyst; it’s the electrolyte solution responsible for absorbing the protons from the split water molecules. This vital part of the cell is made of volatile solvents that can wear down other components in the system.

Recent progress is beginning to tackle these challenges. Cobalt oxide, a stable, rapid, and plentiful metal oxide, shows promise. Researchers working on dye-sensitized cells have developed a non-solvent solution to replace the corrosive materials previously used.

Artificial photosynthesis research is gaining momentum, but it’s still a long way from practical use. It could take at least a decade before this kind of system becomes a reality [source: Boyd]. That’s an optimistic timeline. Some doubt it will ever come to fruition. Still, who wouldn’t wish for artificial plants that mimic the real ones?