UCSB Science Line
Sponge Spicules Nerve Cells Galaxy Abalone Shell Nickel Succinate X-ray Lens Lupine
UCSB Science Line
How it Works
Ask a Question
Search Topics
Our Scientists
Science Links
Contact Information
Since photosynthesis and cellular respiration are opposites, why don't plants use the waste products of each for the other? For example, why don't plants use the CO2 and H20 from cellular respiration in photosynthesis? Why do they give it off as waste? And same goes for the waste products of photosynthesis used in cellular respiration. All I'm asking is why, after 3.5 billion years of evolution, has this not been utilized?
Question Date: 2012-11-29
Answer 1:

Good question. The answer is that they do recycle, to some extent. Let’s start with CO2. Plants take it in, combine it with water, and use energy (which they get from light) to build molecules like sugar, starch, and wood. That’s photosynthesis. The sugar may be broken down quickly by cells needing energy, but the other molecules become part of the plant’s “body.” When you look at a flower or tree, most of what you see is carbon, oxygen, and hydrogen that are tied up in plant tissues. To do more photosynthesis, the plant needs fresh stores of CO2 and water.

Water is also evaporating all the time. Obviously, this happens more in dry climates. Every time the plant opens up the holes in the leaves (stomata) to move gases like oxygen and carbon dioxide, even more water evaporates. Desert plants have evolved adaptations to limit this, such as only opening the holes at night, and having a waxy coating to reduce their water loss.

Plants can use their waste oxygen when they’re breaking down the sugar in cellular respiration, so they are usually not limited by a lack of oxygen. They can’t hold onto oxygen long, though. The reason is a little complicated. Enzymes are biological catalysts— chemicals that speed up chemical reactions. Many enzymes are needed to catalyze all the reactions of photosynthesis and cellular respiration. Putting the CO2 together to make sugar uses an enzyme called rubisco. The problem is that oxygen “clogs” the active site on the enzyme rubisco so that it can’t do its job. This means that the plant has to let out the oxygen. Some plants have evolved ways to isolate the rubisco so that it’s not getting clogged up with oxygen. These are called C4 plants.

What parts of the world do you think have the most C4 plants?

Thanks for asking.

Answer 2:

Many bacterial ecosystems do exactly that: the waste products of one are the food for another; plants are not too smart! And they have not figured out how to do it. Give them another 3 or 4 billion years and I bet they will!

Answer 3:

This is a great question. Plants actually do use the oxygen they produce from photosynthesis for respiration and the carbon dioxide they produce from respiration for photosynthesis. However, (and fortunately for us!) plants use much less oxygen for their own respiration than they produce from photosynthesis, and they use much more carbon dioxide for photosynthesis than what they produce from respiration. So the net result is that they expel the extra oxygen, and take in some extra carbon dioxide. I hope this helps!

Answer 4:

Well, to begin with, plants have only been around for about 400 million years.

The answer is that there is only so much space in the interior of a leaf to store gasses like CO2 and water vapor, so the plant has to get rid of it - holding it in would increase the pressure and blow the leaf apart. Single-celled organisms have even less room to store excess gasses.

When a plant is actively photosynthesizing as well as respiring, they do use any gasses they have available in their internal pore spaces - which includes their waste products from respiration (and photosynthesis, too - that makes oxygen, which is a really nasty waste product!).

Click Here to return to the search form.

University of California, Santa Barbara Materials Research Laboratory National Science Foundation
This program is co-sponsored by the National Science Foundation and UCSB School-University Partnerships
Copyright © 2020 The Regents of the University of California,
All Rights Reserved.
UCSB Terms of Use