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.
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.
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.
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 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.
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.
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.
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.