Plants Don’t Produce Oxygen (O2) From Carbon Dioxide (CO2)

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Robert Pavlis

Everyone is aware of the high carbon dioxide levels in the air and its contribution to global warming. And many of you have heard that plants can convert this CO2 into oxygen. Plants are important in controlling the CO2 level, but they don’t convert CO2 into O2.

Did you know that plants also take in oxygen and produce carbon dioxide, just like animals?

All of this is a bit academic, but very interesting if you really want to understand plants.

Plants Don’t Produce Oxygen (O2) From Carbon Dioxide (CO2)
Plants Don’t Produce Oxygen (O2) From Carbon Dioxide (CO2), Photo source: Science Sparks

Understanding Photosynthesis

All plants, including algae and cyanobacteria carry out photosynthesis. This is a process where CO2 and water are converted to sugars and oxygen. It takes place in special cell structures called chloroplasts which are small organs inside plant cells. Each one of these chloroplasts is a small chemical manufacturing plant that contains an important molecule called chlorophyll.

Chlorophyll is able to capture sunlight and turn it into useable chemical energy. This energy is then used to produce sugars from CO2 and water. Chlorophyll is a special molecule that is able to trap blue and red light, and reflect most of the green light. That is why plants look green.

Why is sugar so important? Sugar is the building block for making many of the more complex chemicals found in a plant. It is also the vital energy source to do everything that takes place in a plant.

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Since photosynthesis requires light, it is only active during the day. Plants don’t use CO2 or make food energy in the dark.

Is Carbon Dioxide Converted to Oxygen?

Much of the popular press describes photosynthesis in very simple terms and says that plants convert CO2 into O2. This simplification is incorrect.

The reaction that occurs is a two step process.

Step 1:

In step 1, light energy is captured by chlorophyll and used to break up water into free electrons, hydrogen ions (protons) and oxygen gas.

2H20 –> 4 e + 4 H+ + O2

Most of this oxygen is then expelled from the plant, making it available to animals. The oxygen in the air and the oxygen which we breathe comes from water, not CO2.

Step 2:

Step 2 is actually a complex set of reactions called the Calvin Cycle which takes the electrons from step 1, combines it with CO2 to form sugars.

Plants Use Oxygen Just Like Animals

Most gardeners understand that plants produce O2. What many don’t know is that plants also use oxygen just like animals.

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The sugars are an energy source for all living things. It takes energy to build larger molecules, like proteins and carbohydrates which are key building blocks for animals to grow. They get this energy through a process called respiration.

During respiration, oxygen is combined with sugar to produce energy, water and CO2. This reaction is also a multi-step process and occurs in animals and plants.

When leaves respire they get the needed oxygen from one of two sources. Some is absorbed through the stomata of the leaves and some comes from photosynthesis.

In roots, the required oxygen comes mostly from the soil. This is why roots need a significant amount of air around them. When they are overwatered they can’t get enough oxygen and they die.

Green herbaceous stems have stomata and respiration can occur as in leaves. Woody stems have openings called lenticels which allow the movement of gases in and out of the plant.

lenticels on a birch tree
lenticels on a birch tree

A key difference between respiration and photosynthesis is that the former occurs all of the time, day and night, while photosynthesis only occurs in light.

How Can Plants Add Oxygen to Earth?

I’ve just discussed two processes that go on in plants. One produces oxygen and the other uses it up. If that is true, why do plants add a net amount of oxygen to the air?

Photosynthesis produces about 10 times more oxygen than respiration consumes. On balance, plants are net producers of oxygen.

Anaerobic Respiration

The respiration discussed above is more correctly called aerobic respiration because it occurs in the presence of oxygen. There is a similar process that does not require oxygen, called anaerobic respiration. In this process sugars are converted to energy, alcohol and CO2.

This kind of respiration is used by yeast in a fermentation process, by anaerobic bacteria and it takes place in Bokashi. This process is less efficient in producing energy, but it does mean some organisms can live in low oxygen environments, like very heavy clay soil.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

9 thoughts on “Plants Don’t Produce Oxygen (O2) From Carbon Dioxide (CO2)”

  1. Really let us see 6CO2+6H2O processed with light C6H12O6+6O2 yes CO2 is converted to O2. The bottom line it does convert CO2 to oxygen. As to one of the comments it is said rainforest produce 28% to 1/3 of the oxygen and the oceans produce around 70%. There are some quote of higher percentage from the oceans. There are studies showing that low CO2 in the atmosphere in the recent past has caused inhibiting growth of plant life. The is a old saying the truth is found in the details.

    Reply
    • If you took the time to read the post you would know why what you say is incorrect!

      You are right about one thing – the truth is found in the details!

      Reply
  2. It’s pedantic to insist plants don’t make oxygen from carbon dioxide. Whilst of course it’s correct what you assert in dissecting photosynthesis the way you do, NORMAL people simplify the process to be green plants turning carbon dioxide and water into sugar and oxygen using solar energy. Photosynthesis can not occur with one of the three agents that are CO2, H2O and light missing. Can it now? Well done for showing you understand A level biology, though!

    Reply
  3. It would seem to me that lawns, prairie grass, seaweed, etc do the same thing as forests to provide oxygen. What’s all the fuss about deforestation?

    Reply
  4. Yes. It’s not about this post, but I have two questions.
    1. I have a 4×12 raised bed. Would planting clover be a good crop to plant in the fall? My zip code is: 25526
    2. What fertilizer would you recommend for my garden, I have my garlic in. I am going to plant peppers l, tomatoes, and lettuce this year. My soil is mostly potting soil bagged from Miracle Grow with some bagged compost, vermiculite and peat moss all mixed in. At the end of summer I emptied my cucumber pots and flower pots in the garden

    Reply
  5. It’s amazing the extent to which this process can be misunderstood. You might have made the point more strongly that net oxygen production depends on the fixed carbon being sequestered in some way, either in a long-lived tree, or organic soil, or some other kind of buried sediment.

    Reply

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