Photosynthesis: What is it and how does it work?

Photosynthesis is all around us. It's happening under our feet, above our heads and in the sunlit zones of aquatic environments. But what exactly is photosynthesis? Why is it so important? And, when did it evolve? Answers to these questions, and more, are below.

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For those who missed it, check out these five astonishing plant adaptations or find out whether plants are conscious.

What is photosynthesis?

Photosynthesis is the process by which carbohydrate molecules are synthesised. It's used by plants, algae and certain bacteria to turn sunlight, water and carbon dioxide into oxygen and energy, in the form of sugar. It’s probably the most important biochemical process on the planet.

Essentially, it takes the carbon dioxide expelled by all breathing organisms and reintroduces it into the atmosphere as oxygen.

The rate of photosynthesis is affected by light intensity, the concentration of carbon dioxide, water supply, temperature and availability of minerals. The process takes place entirely in the chloroplasts, and it's the chlorophyll within the chloroplasts that make the photosynthetic parts of a plant green.

Photosynthesis is important too, elsewhere in the biosphere. Both marine and terrestrial plants remove carbon dioxide from the atmosphere, and some of this is precipitated back out, as shells made of calcium carbonate, or buried as organic matter in soil.

Without photosynthesis, the carbon cycle could not occur, and we would soon run out of food. Over time, the atmosphere would lose almost all gaseous oxygen, and most organisms would disappear.

How does photosynthesis work?

Plants require light energy, carbon dioxide, water and nutrients. These ingredients come from both the adjacent atmosphere and the soil.

Phase 1

Plants absorb sunlight through the two top layers of their leaves, the cuticle and epidermis. These layers are thin, so light can travel through them easily. Carbon dioxide is brought in from the atmosphere, and at the same time, water is drawn up from the soil, into the body of the living plant.

Phase 2

Just beneath the cuticle and epidermis are the palisade mesophyll cells. These specialised cells are vertically elongated and arranged closely together to maximise light absorption.

Below the palisade mesophyll cells is the spongy mesophyll tissue, which is loosely packed for efficient gas exchange. As gases move in and out of these cells, they dissolve in a thin layer of water that covers the cells.

Chloroplasts within the cells of the aquatic waterweed plant, elodea. © Getty images

Phase 3

Inside the palisade mesophyll cells are the chloroplasts, lots of them. They contain chlorophyll, molecules that don’t absorb green wavelengths of white light. Instead, they reflect it back to us, giving plants their green colour.

Phase 4

Inside the chloroplast is where the magic happens. A light-dependent reaction takes place, where energy from the light waves is absorbed and stored in energy-carrying ATP molecules.

Then, in a light-independent reaction (the Calvin Cycle), ATP is used to make glucose, a source of energy. Water is oxidised, carbon dioxide is reduced, and oxygen is released into the atmosphere.

Oxygen is released via stomata in the leaves, microscopic pores that open to both let in the carbon dioxide, and release oxygen (and water vapour).

What is the equation for photosynthesis?

Photosynthesising organisms form the base of the food chain.

Carbon dioxide + water (with light energy) = glucose + oxygen

As well as the light energy, carbon dioxide and water, plants also need nutrients, which they get from the soil. These nutrients are released again, or recycled, when the plant tissue dies and begins decomposing in the soil.

Oxygen in the form of gas molecules (O₂) is actually a by-product of photosynthesis, but it's responsible for the oxygen in the air that keeps us alive. Plants also release energy and water to the atmosphere through respiration.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The balanced equation takes it a little further. Six carbon dioxide molecules and six water molecules (the reactants) are converted into one sugar molecule (C₆H₁₂O₆₎ and six oxygen molecules, via the light energy captured by the chlorophyll.

Photosynthesis and the food chain

During photosynthesis, energy passes through the system, and you can think of photosynthesis as an energy flow system, tracing the path of solar energy through the ecosystem. This energy is stored by the primary producers, the photosynthesising organisms. As these organisms are eaten and digested by the primary consumers, chemical energy is released and this is used to power new biochemical reactions.

At each level of energy transformation throughout the food chain, some energy is lost as waste heat. In addition, a significant amount of the energy input to each organism is used in respiration, to maintain the body of that organism. This energy is not stored for use by other organisms higher up the food chain. This is one of the reasons why both the number of organisms and their total quantity of living tissue decrease as you go further up the food chain.

When did photosynthesis start?

The evolution of photosynthesis had immense consequences for the Earth. As organic matter from photosynthetic life was buried in the strata, carbon was removed from the atmosphere allowing oxygen to accumulate.

Evidence suggests that photosynthetic organisms were present around 3.2 to 3.5 billion years ago, in the form of stromatolites. Stromatolites are laminated microbial structures (generally an alternation between light and dark laminae), usually formed by cyanobacteria and algae, and are the oldest known fossils, and therefore the earliest evidence of life on Earth.

Stromatolites are layered, calcareous mounds secreted from cyanobacteria and trapped sediment. They are the oldest known fossils and the earliest evidence of life on Earth. They are still being formed today, and these ones are from Lake Thetis, Western Australia © Getty Images

As this early oxygen diffused into the upper atmosphere (the stratosphere), solar radiation transformed the oxygen molecules into ozone, which created the stratospheric ozone layer. And of course, as the ozone layer absorbs most of the Sun's ultraviolet radiation (UV-B), it plays an important role in protecting human health, so it's unlikely that life would have flourished without this protective shield.

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