Marisa Morby logo

Photorespiration: the opposite of photosynthesis

To the best of our knowledge, the world is currently made up of almost 400,000 different species of plants. And I love to have plants in my garden, on my window sill, and in my house. That doesn't mean I knew that much about plants. My basic understanding was that plants produce their own food through photosynthesis. That process allows them to convert sunlight and CO2 carbon dioxide into glucose and the waste they release is O2 (oxygen). Win-win for all of us oxygen breathing animals.

And then I learned about photorespiration—where plants release CO2. Not great.

Photorespiration is a phenomenon that happens when land plants, specifically what scientists call C3 plants, become too hot and too dry. C3 plants currently make up about 80% of plants on earth and the majority of trees.

So, here's the scene: it's a really hot, dry, sunny summer day. Perfect conditions for photorespiration not photosynthesis. The plant says to itself "Hey self, it's too hot. I don't like it. I'm starting to feel parched."

Then the plant will close its stomata to prevent water loss and protect itself. The stomata are basically pores on the leaves of plants (think about how your skin has pores. Plant leaves do, too, but they can open and close them). Great, water conservation for the plant.

Except, in order for a plant to complete photosynthesis, the stomata must be open, and this opening releases water from the plant. But the plant doesn't want to lose any water because it's hot, it's dry, and they don't know when it's going to stop.

So, the stomata closes to reduce water loss. But, in order for CO2 to be pulled in by the plant, the stomata must be open. Problem number one.

Problem number two, the plant can't just stop its process altogether, it must pull in something. Since it closed its stomata to preserve water, that means it can't pull in CO2. Since it can't pull in CO2, it instead starts pulling in O2. Pulling in O2 creates a buildup of oxygen.

The plant can't handle all this oxygen, so, similar to photosynthesis it creates a different product. During photosynthesis the light energy that's pulled in ultimately helps create two different products: glucose and oxygen. The glucose feeds the plant and the oxygen is released.

But during photorespiration the two products that are ultimately produced are a compound that goes back into the plant to help with photosynthesis and CO2. The CO2 gets released back into the air.

Join the Design & Nature Reimagined Newsletter