 CO2 is an essential ingredient in photosynthesis. So then what do you think will happen if we plot the rate of photosynthesis versus CO2 concentration? Well, if you think the rate of photosynthesis will increase with CO2 concentration, then you're not wrong. That's exactly what happens, but not linearly. It's somewhat like what happens with light. When you increase the amount of light, what happens to the rate of photosynthesis? Similar thing happens. If you don't remember what happens when light is increased, then I would suggest you go back to our video called Factors of Photosynthesis-Light and take a look at it. A similar thing happens here. Hence here, the rate initially increases linearly and then finally it levels off. Because there are at that point other limiting factors like light, for example, or temperature, which are not good enough. Let's say there is not enough light intensity or let's say there is the temperature is too low or too high. And hence at this point, even if you increase the CO2 concentration, the rate of photosynthesis won't increase. But at low CO2 concentrations, the rate of photosynthesis does increase. And in fact, what you are looking at is the graph for C3 plants. You might remember that there are two types of plants with regards to photosynthesis, C3 plants and C4 plants. So this is what happens with C3 plants. What do you think will happen with C4 plants? What will the graph for C4 plants look like? In order to predict that, we need to recall the differences between C3 and C4 plants. How they utilize carbon dioxide. So first let's look at the C4 plants. What exactly they do as far as carbon dioxide is concerned. So this is a mesophil cell in a C3 plant and this is the stoma. So carbon dioxide enters through stomata and then enters the mesophil cell where it enters the Kelvin cycle. And you know that at the end of the Kelvin cycle, what is produced is carbohydrates. This is typically what happens in C3 plants. Now what happens that is so special in C4 plants? Again through this stomata the carbon dioxide enters and inside the mesophil cells it enters not the Kelvin cycle but what is called the C4 cycle. So the C4 cycle actually starts in the mesophil cell and continues in the bundle sheet cell. If you remember bundle sheet cells are the cells that surround the vascular bundle. That is a bundle that has xylem and loam. So it starts here, this is the C4 pathway and then continues in the bundle sheet cell and then things get recycled here and then go back to the mesophil cell. And what is released here is carbon dioxide. And then here the carbon dioxide enters the Kelvin cycle in the bundle sheet cell and as usual carbohydrates are formed. So why go through all this hassle of first entering the C4 pathway then again releasing the CO2 in the bundle sheet cell and then having the Kelvin cycle in the bundle sheet cell? Well the answer is photo respiration. Photo respiration is a wasteful pathway that happens in C3 plants and the C4 plants avoid this by going through this elaborate pathway, the C4 pathway. So in effect what happens is carbon dioxide is concentrated in C4 plants in the bundle sheet cell. That means at the same atmospheric concentration of carbon dioxide in C3 and C4 plants the C4 plants are more sensitive to CO2 concentration. That's because CO2 is concentrated in the bundle sheet cell where the Kelvin cycle is occurring. So what does this mean with regards to our graph? So in C4 plants as you have seen the photosynthesis is more sensitive to CO2 concentration. That means at low CO2 concentrations themselves the rate of photosynthesis is higher than what you would expect in a C3 plant. So what does it mean for our graph? It means that at low CO2 concentrations themselves the rate of photosynthesis starts rising steeply in the beginning itself and then it levels off but much earlier than C3 plants. So that means so look here where it has started leveling off here. So it turns out that this is around 0.36% of CO2 at which the C4 graph starts leveling off. That means it's saturated at that concentration of CO2. Whereas if you look at C3 over here it's far from leveling off it's at this concentration of CO2. The rate of photosynthesis in C3 plants is only about half or even less than half of what it could achieve. Now can you tell me what the concentration of CO2 is in the atmosphere? The atmospheric concentration of CO2 is around 0.04%. So this is close to the percentage of CO2 where the C4 graph starts leveling off. That means at atmospheric CO2 concentrations the C4 plants are already operating at close to the maximum of their rate of photosynthesis. Whereas C3 plants they're operating at about half or less than half of their maximum rate of photosynthesis. So who is more efficient? Naturally the C4 plants and it's all because of their elaborate mechanism of the C4 pathway and the CO2 concentrations being high in the bundle sheet cells where the Kelvin cycle takes place.