Definition
Photosynthesis is the chemical process in which plants use the energy in sunlight to turn water and carbon dioxide into glucose, a nutrient that fuels life, and oxygen. It is the most basic energy-making system on Earth, supplying the food and the breathable oxygen that nearly every living thing depends on.
The Amazing Cooking Inside a Leaf Kitchen
Look at a plant leaf up close under a microscope, and you'll see it packed with green grains called chloroplasts, tiny kitchens. These kitchens soak up the light energy pouring down from the sun and get ready to cook.
The ingredients are very simple. There's just water, pulled up from the roots underground, and carbon dioxide, breathed in through tiny pores (stomata) on the underside of the leaf. Borrowing the power of sunlight, the plant breaks these simple ingredients apart and puts them back together as glucose, a rich, concentrated nutrient.
There is one interesting leftover. When the plant splits hydrogen off water and uses it to make food, what remains is oxygen. To the plant, it's just a byproduct of cooking. But to animals on Earth, it's the most precious gift of all, because it lets them breathe.
Catching Light, Then Building Sugar
Photosynthesis isn't one single reaction. It's a precise two-stage process. The first stage runs only when sunlight is shining: the light reactions. A pigment called chlorophyll grabs the light and splits water, and the power released in the process fills up the plant's biological energy, like charging a battery.
The second stage uses that stored energy to build the real nutrient: the dark reactions (the carbon fixation reactions). The name makes it easy to think they happen only at night. But "dark" just means they don't use light directly. They use the energy battery charged earlier to bundle carbon dioxide into sugar. In reality, they mostly happen during the day, while that battery keeps getting recharged.
The plant can burn this glucose right away for energy, or turn it into starch, which is easier to store. The rice, potatoes, and sweet potatoes we enjoy at dinner are concentrated lumps of solar energy, sunlight that plants packed up and stashed away.
To Be More Precise: Plants Breathe, Too
Many people think plants only breathe in carbon dioxide and breathe out oxygen during the day. But look a little closer and you'll find that plants breathe day and night, just like us. Living cells need to take in oxygen and give off carbon dioxide to get the energy they need to do anything at all.
On a bright, sunny day, photosynthesis runs several times faster than respiration. From the outside, it just looks like the plant is pouring out oxygen and soaking up carbon dioxide. In fact, it is running both factories, photosynthesis and respiration, at the same time, at full speed.
When the dark of night comes, the photosynthesis factory pulls down its shutters, and only respiration keeps going. Add up the whole day, and the oxygen released by photosynthesis is more than the oxygen used up by respiration. The plant uses the leftover nutrients to grow, and together with plankton in the ocean, it becomes a star player in supplying oxygen to Earth's atmosphere.
Common misconceptions
Plants don't breathe. They only do photosynthesis.
Plants are living things, too, so they breathe around the clock, taking in oxygen and giving off carbon dioxide. During the day, though, photosynthesis produces far more than respiration uses, so it just looks like they only give off oxygen.
Where you meet it
A potted plant on your windowsill grows thick and leafy in the sun thanks to nutrients made by photosynthesis.
The rice and bread we eat as staples are lumps of carbohydrates that plants made and stored using sunlight.
In one sentence
Photosynthesis is nature's great process in which plants turn sunlight, water, and carbon dioxide into glucose, the food of life, and oxygen.