What is the CO2 offset point

Photosynthesis factor carbon dioxide

Dependence of the photosynthesis rate on the CO2 concentration

I assume that you have read the page "Photosynthesis factor light intensity" or at least the lexicon article "Light compensation point". Then you will surely understand the graphic above quickly.

If no If carbon dioxide is dissolved in the water, the waterweed can of course not carry out photosynthesis. The gross photosynthesis therefore has the value zero. So that the plant still has ATP available for its various metabolic processes, it has to aerobically oxidize glucose, which of course leads to a consumption leads to oxygen. The net photosynthesis, i.e. what the skilled experimenter can measure in the end, is therefore in the negative range.

Gross photosynthesis

The photosynthesis taking place in the plant.

Net photosynthesis

Measurable photosynthesis, measured via the release of oxygen or the consumption of carbon dioxide. The net photosynthesis can also have negative values ​​if the influence of breathing is greater than the influence of gross photosynthesis.

Net photosynthesis = gross photosynthesis - respiration

If you increase the CO2-Concentration in the water, for example by pouring some carbon dioxide-containing mineral water into the beaker (basic experiment see: Bubble counting method), then the plant begins with photosynthesis (if enough light is irradiated, of course). Above a certain carbon dioxide concentration, just as much oxygen is released through photosynthesis as is consumed by the constant breathing. The carbon dioxide concentration (or the carbonic acid concentration) at which this happens is referred to as CO2-Compensation point (analogous to the light compensation point).

CO2-Compensation point

The carbon dioxide concentration at which the measurable net photosynthesis has the value zero.

Interestingly, the CO2-Saturation point (analogous to the light saturation point) far above the carbon dioxide concentrations prevailing on earth. The gardeners like to take advantage of this by fumigating their greenhouses with carbon dioxide. In this way they increase the natural CO, which is around 0.038%2-Concentration of the air and thus increase the photosynthesis rate of your crops.

Now another interesting task for the exam writers and high school graduates among you:


A green plant is hermetically sealed in a container that has no carbon dioxide in the air. Water and light, on the other hand, are sufficiently available, as are minerals. What will happen over time?

Proposed solution

The plant will not perish! Since there is no carbon dioxide available, it will initially only breathe in order to ensure its energy supply. However, the plant should be equipped with enough reserve substances such as starch, otherwise it will die. When the plant now breathes, carbon dioxide is automatically released. However, since the vessel is hermetically sealed, the carbon dioxide collects in the air. Above a certain concentration, the plant can carry out photosynthesis and obtain new reserve substances from the carbon dioxide, which it then uses up again in its respiration in order to release carbon dioxide again, which it then needs again for photosynthesis, and so on. In other words - the CO2-Concentration in the glass levels off at a more or less constant value. As soon as there is too much carbon dioxide, the plant increases its photosynthesis, and so does the CO2- Salary is going down. Then again too little CO2 present, breathing becomes more noticeable and the CO2- Salary rises again.