Most of us learn that plants need sunlight, water and carbon dioxide to grow, but I didn’t realise how amazing the process actually is until I started looking into what happens inside a leaf.

Plants are basically able to capture energy coming from the Sun and turn it into chemical energy that can be stored and used later. Most photosynthesis in plants happens inside tiny structures called chloroplasts. Inside them is chlorophyll, the pigment that absorbs light and also helps give plants their green colour.

The plant takes water up through its roots, while carbon dioxide from the atmosphere enters the leaves through microscopic openings called stomata. Using energy from light, the plant converts carbon dioxide and water into carbohydrates. Oxygen is released as part of the process. The sugars produced can then be transported around the plant and used as a source of chemical energy or as material for growth.

One thing I found really surprising is that the oxygen released during photosynthesis comes from water molecules, not from the carbon dioxide. So some of the oxygen around us exists because photosynthetic organisms used sunlight to split water during the light-dependent reactions. That feels almost like a science-fiction technology, except plants, algae and some bacteria have been doing it for an incredibly long time.

Another thing that surprised me is that forests are only part of the story. We often hear about trees producing oxygen, but scientists estimate that roughly half of the oxygen produced on Earth comes from the ocean, mainly from microscopic photosynthetic organisms such as phytoplankton. Some are so tiny that we cannot see them without a microscope, yet together they have an enormous influence on the planet.

Phytoplankton are also at the base of most marine food webs. That means photosynthesis happening inside microscopic organisms floating near the ocean surface can eventually help support everything from tiny zooplankton to fish and much larger marine animals.

There is even a tiny photosynthetic bacterium called Prochlorococcus. NOAA describes it as the smallest known photosynthetic organism, yet estimates that it may be responsible for up to 20% of the oxygen produced in Earth’s biosphere. It is strange to think that something far too small to see with your eyes can have an effect on such an enormous scale.

Photosynthesis also connects directly to the carbon cycle. Photosynthetic organisms remove carbon dioxide from their surroundings and use its carbon to build organic molecules. In the oceans, phytoplankton therefore play an important role not only in food webs and oxygen production, but also in the movement of carbon through Earth's systems.

And plants don’t actually just “make oxygen for us.” Plants need oxygen too. Their cells carry out cellular respiration to release usable energy from organic molecules, just as our cells do. Photosynthesis captures and stores energy, while cellular respiration helps release energy for cellular processes.

What I find coolest about all of this is the scale. Something happening inside structures so small that we need microscopes to study them affects forests, oceans, food webs, the carbon cycle and the atmosphere of an entire planet.

A leaf looks incredibly ordinary.

But chemically, it is a solar-powered system that captures energy from a star about 150 million kilometres away and uses that energy to help build living matter.

That makes looking at a leaf slightly different once you know what is happening inside it.