Plant DecodedHow does photosynthesis work?
The most important chemistry on Earth happens in a leaf the thickness of paper.
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Short answer
Photosynthesis is how plants turn light into food. Leaves capture sunlight with chlorophyll, use that energy to split water, and build sugar out of carbon dioxide from the air. Oxygen is the leftover, released into the air we breathe. In short: carbon dioxide plus water plus light makes sugar plus oxygen.
What goes in and what comes out?
A plant needs three things for photosynthesis: light, water and carbon dioxide. Water comes up from the roots through the xylem. Carbon dioxide drifts in through tiny pores on the leaf called stomata. Light simply lands on the leaf.
What comes out is sugar, which the plant uses as fuel and building material, and oxygen, which escapes back out through the same pores. Written as chemistry, it looks like this:
6CO2 + 6H2O + light energy → C6H12O6 + 6O2
That’s six molecules of carbon dioxide and six of water becoming one molecule of glucose and six of oxygen.
Where does it happen?
Inside leaf cells are small green structures called chloroplasts. They’re packed with chlorophyll, the pigment that absorbs light.
Chlorophyll soaks up red and blue light really well but reflects most of the green. That reflected green light is what reaches your eyes, and it’s why leaves look green.
The two halves of photosynthesis
The light reactions. Chlorophyll captures light energy and uses it to split water molecules. This is where the oxygen comes from. The energy gets stored in two short-lived carrier molecules, a bit like charging batteries.
The Calvin cycle. The plant spends that stored energy fixing carbon dioxide from the air into sugar. This half doesn’t need light directly, but it relies on the batteries the light reactions charged, so in practice it runs during the day.
How efficient is a leaf?
Less than you might expect. Even in theory, photosynthesis can only turn about 4.6% of the sun’s energy into plant material in most plants, and around 6% in plants like corn and sugarcane. Real crops in a field usually manage far less. A household solar panel converts roughly 20% of sunlight into electricity.
But leaves do something solar panels can’t: they build themselves, repair themselves and turn the energy into food. That’s a pretty good trade.
Why it matters to you
Almost every bite of food you eat traces back to photosynthesis, either directly from plants or through the animals that ate them. The oxygen in the air came from it. Even coal, oil and gas are ancient sunlight, captured by plants millions of years ago.
For the curious
Below the surface: the enzyme that runs the world
The enzyme that grabs carbon dioxide in the Calvin cycle is called Rubisco, and it’s often described as the most abundant protein on Earth. Plants make so much of it because it’s surprisingly slow and a bit clumsy.
Rubisco sometimes grabs oxygen instead of carbon dioxide by mistake. Cleaning up after that mistake, a process called photorespiration, wastes energy, especially in hot, dry weather.
Some plants evolved workarounds. Corn and sugarcane are C4 plants: they concentrate carbon dioxide around Rubisco so it makes fewer mistakes. Cacti and pineapples use CAM photosynthesis: they open their pores at night to take in carbon dioxide while losing less water, then finish making sugar in the daytime. Improving Rubisco, or giving more crops these tricks, is one of the big goals in crop science.
How sure are we?
Quick questions
Do plants photosynthesise at night?
No. The light reactions need light. CAM plants take in carbon dioxide at night, but they still finish making sugar during the day.
Why are leaves green?
Chlorophyll absorbs red and blue light and reflects most green light, so green is the colour we see.
Do plants breathe?
Yes. Plants respire all the time, using oxygen to burn sugar for energy. During the day, photosynthesis usually outpaces it.
Sources
- Zhu X-G, Long SP, Ort DR (2008). What is the maximum efficiency with which photosynthesis can convert solar energy into biomass? Current Opinion in Biotechnology 19(2): 153–159. Read the paper
- Taiz L, Zeiger E, Møller IM, Murphy A (2015). Plant Physiology and Development, 6th edition. Sinauer Associates.
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