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.

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

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.

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

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.

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.

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).

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.

Science Focus