Photosynthesis: what it is, how it works and why it fuels life on earth

By JV Chamary

Published: Saturday, 30 July 2022 at 12:00 am

In this expert guide, we look at photosynthesis – the process by which plants and other organisms convert carbon dioxide, water and light into food, creating oxygen along the way. So what exactly is photosynthesis, and how does it work?

What is photosynthesis?

Photosynthesis is one of nature’s most important processes. Powered by sunlight, it uses carbon dioxide (CO2) to produce carbohydrates, the food that ultimately fuels most life on Earth.

The process also releases a byproduct, oxygen (O2), which has become vital for burning carbs to generate energy through aerobic respiration.

Photosynthesis enables an organism to harness solar power to synthesise its own food – carbohydrate molecules – from carbon dioxide in the atmosphere.

Which species can photosynthesise?

Besides plants and microbes such as cyanobacteria, photosynthetic organisms also include algae – a diverse group containing everything from single-celled species like Chlorella to multicellular seaweeds that form kelp forests.

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Photomicrograph of a freshwater clustering single-celled Chlorella species. © NNehring/Getty

In plant cells, photosynthesis occurs inside chloroplasts – descendants of cyanobacteria that were integrated into plants over 2 billion years ago through endosymbiosis.

When and where does photosynthesis take place?

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Green chloroplasts, carrying chlorophyll, in plant cells of anacharis or waterweed, Egeria densa. © NNehring/Getty

While you might think it only happens during daytime, the process actually has two stages: light reactions that capture solar energy (the ‘photo’ half of the word) and dark reactions that can occur day or night to make food (‘synthesis’).

Dark reactions take place in a microbe or chloroplast’s internal fluid, whereas light reactions occur in membranes called thylakoids that are folded to maximise the surface area exposed to sunlight. Leaves are often large, flat structures to catch light, but other green parts of a plant (like stems) also photosynthesise.

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Why do leaves have pores?

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Stomata and guard cells in the leaf epidermis of a spiderwort. © Ed Reschke/Getty

Leaf pores are a double-edged sword: the openings allow a plant to suck-up the carbon dioxide it needs to make food, but they also let precious water escape. While early plants had to cope with losing water passively through pores that were just holes, modern species have structures called stomata that actively adjust an aperture’s size in response to physiological or environmental cues. Stomata therefore help a plant to minimise water loss, balanced with gaining carbon for photosynthesis.

How do organisms capture light?

The cells of photosynthetic organisms convert light to chemical energy through photosystems – a combination of pigments, enzymes and other molecules. The core component of a photosystem is the green pigment chlorophyll.

After being hit by photons (particles that make up light), electrons in chlorophyll absorb enough energy to break free, which starts a chain reaction in which energised electrons are passed between intermediates to generate the molecules ATP and NADPH, whose chemical bonds carry the energy that’s later released to make food. In plants, chlorophyll’s lost electrons are replaced by splitting water, releasing oxygen.

Is chlorophyll the only pigment?

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Red coralline algae in Point Fermin Park in San Pedro, CA. © Pam Susemiehl/Getty

No, but it’s the best at absorbing sunlight near the blue end of the visible spectrum, where photons have greater energy. Many organisms, especially aquatic species, also have accessory pigments such as phycobilin that are more efficient at harnessing photon energies toward the red end of the spectrum, allowing those wavelengths to be absorbed if water has filtered-out blue light. The distinctive colour of red algae is explained by the presence of large amounts of phycobilin.

So how is food made from carbon dioxide?

Like making drinks in a slushie machine that continually rotates and is refilled with flavouring and ice, photosynthetic cells produce carbohydrates by recycling them and replenishing the supply of carbon by regularly adding carbon dioxide (and usually water). That involves a circular series of reactions popularly called the Calvin cycle, which is driven by the energy-carrying molecules ATP and NADPH.

The first step in that cycle involves carbon fixation: adding or ‘fixing’ carbon dioxide (a volatile gas) to a stable compound, a reaction catalysed by the enzyme RuBisCO. Because it constitutes an estimated 3 per cent of the total mass in leaves, and most of Earth’s biomass is plant matter, RuBisCO is arguably the most abundant protein on the planet. And because the enzyme’s activity ends-up producing more than 90 per cent of organic carbon, RuBisCO is one of nature’s most important proteins.

Main image: Large green hosta leaves. © Maryna Terletska/Getty