What are the by-products of photosynthesis?

What are the by-products of photosynthesis?
Let’s start with some basic biology. Plants use sunlight to convert carbon dioxide and water into chemical energy that takes the form of organic compounds that they use for growth and reproduction. The process also produces other gases, called "byproducts," which include oxygen, and consumes water and carbon dioxide. This process is called photosynthesis, and its root is in Greek: phōs, "light", and synthesis, "putting together".

Since oxygen is one of the byproducts of photosynthesis and vital for all respiratory processes, this means that plants are “fueling” all aerobic life (literally meaning “living only in the presence of oxygen). This includes most living things you could name off the top of your head, from humans to insects to many microorganisms and even plants themselves. Moreover, plants fuel the biosphere by converting sunlight into chemical energy that other organisms can access by eating them. Therefore, plants are called primary producers.


Chemical overview

Plants and other organisms use sunlight to convert water into energy. Plants, cyanobacteria and a variety of algae rely on photosynthesis as their primary and often only source of energy, which earns them the name photoautotrophs.  Plants have unique structures inside them called chloroplasts, which contain pigments sensitive to light called chromophores, such as chlorophyll. When energy is being produced, green plants, cyanobacteria or algae release oxygen during the day and take in carbon dioxide at night.

Photosynthesis Crash Course

The light-dependent reactions and oxygen by-product

During the daytime, plants use light energy to split water molecules apart – the light-dependent reactions. This process takes place on the membrane of thylakoid membranes, coin-like structures inside chloroplasts. There, proteins strip the electrons from the water molecules and produce hydrogen ions (protons) and oxygen.  This means that oxygen is a by-product of photosynthesis.

Oxygen is produced when light energy is absorbed by chlorophyll molecules located within the leaf cell walls. Chlorophyll molecules contain photosensitive atoms, which become excited when exposed to light. In a process called charge separation, the energy of a photon is passed onto an electron. The chlorophyll passes around the electron to a series of molecular intermediates called an electron transport chain, and eventually, after many intermediary reactions, water is oxidized, resulting in O2 and H+. This by-product of a complex reaction is the source of most oxygen in the Earth’s atmosphere.

The light-independent reactions

Plants use the proton generated in the light-dependent reactions to make food during the Calvin cycle. This light-independent process happens in the stroma, the fluid-filled region outside of thylakoid membranes. Since it doesn’t need light, it can theoretically take place at night and is also called the dark reactions or the light-independent reactions, but in reality, the products of the light-dependent reactions are short-lived, so the two processes are tightly coupled. The proton is then used in a different series of reactions to form two further compounds that serve as short-term stores of energy, called nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP). Like the dollar is the currency of the US economy, ATP and NADPH are the “chemical energy currencies” of the cell’s “energy economy”.

ATP and NADPH are used to reduce CO2, turning it into organic compounds called carbohydrates, such as glucose. These simple carbohydrates are stored as more complex starches inside the plant cells, for future use. As the plant grows, these complex carbohydrates are converted back into sugars and then used by the plant for vital functions, such as building new tissues during growth or to be used as energy during hard times, such as winter or droughts.

The oxygen is usually released shortly after being produced, during the daytime, but some plants have a delay, such as certain succulents adapted to hot deserts which only open their stomata at night to avoid damaging heat and water loss.

Carbon dioxide is stored by photosynthesis – but can be released

Carbon dioxide absorbed from the atmosphere is used as a building block during photosynthesis. Plants use carbon dioxide to make carbohydrates, proteins, fats, and other organic compounds. Through various processes natural or artificial processes, the resulting organic matter can be stored long-term, which is why plants and algae play a key role in carbon sequestration.

When discussing this, we must understand that ecosystems depend on complex, intertwined flows and cycles, not simple linear processes. For example, during the day plants release oxygen and consume carbon through photosynthesis, but at night their cellular respiration releases carbon back into the atmosphere. But of these two flows, the one absorbing carbon is stronger, so they are net carbon sinks and net oxygen emitters.

For example, we can look at wetlands: terrestrial ecosystems flooded by water for significant, recurring periods of time. They are important carbon sinks because waterlogging prevents plant material from fully decomposing. This can store the carbon for hundreds of years, and the process sequesters over 44 million tons of carbon annually. Another example is the formation of soils, which can “bury” carbon on a geological timescale. Artificial processes can also speed up carbon sequestration. By pyrolyzing biomass, humans produce biochar, which can be stored in the soil for thousands of years and could represent a significant climate change mitigation – if done sustainably.

If plants are allowed to naturally decay, however, the stored carbon is mostly released back into the atmosphere. Humans can release tremendous amounts of CO2 into the atmosphere by draining peatlands, cutting down old-growth forests and destroying grasslands, while at the same time limiting the planet’s ability to re-absorb the carbon released. Unlike natural processes, we humans have self-awareness, and the scientific understanding of the consequences of our actions.


As conservation enthusiasts equipped with the understanding of photosynthesis, it's our duty to ensure that the ecosystems which allow all life to eat and breathe are protected. By understanding natural cycles and how energy and matter flow between systems, we obtain the tools needed to ensure a stable climate and a world that fosters abundance for all life. Armed with this knowledge, it's our duty to act to protect the living world.

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