C3, C4 & CAM Plants: Videos & Practice Problems

Plants utilize different photosynthetic pathways to adapt to their environments. C3 plants, the most common, undergo a single round of carbon fixation in mesophyll cells, making them susceptible to photorespiration, especially in hot conditions. C4 plants, like corn, perform two rounds of carbon fixation in separate cells (mesophyll and bundle sheath), minimizing photorespiration. CAM plants, such as cacti, also fix carbon twice but do so at different times—stomata open at night to store CO2, allowing photosynthesis during the day. These adaptations enhance efficiency in energy use and carbon fixation under varying environmental conditions.

C3, C4 & CAM Plants

In this video, we're going to begin our introduction to the differences between C₃, C₄, and CAM plants. It's important to recall from our previous lesson videos that photorespiration is actually a bad thing for plants. Because photorespiration, as recalled from our previous lesson videos, wastes energy in the form of ATP and NADPH to make carbon dioxide gas or CO₂, which is really the opposite of what photosynthesis does, which is consume or use carbon dioxide. Ultimately, photorespiration makes photosynthesis inefficient, and that's why it is a bad thing that plants want to try to avoid or minimize. Now, here in this lesson, we're saying that in hot temperatures, it actually turns out that the C₃ plants specifically are most susceptible to photorespiration. However, some other plants have evolved a solution for photorespiration; this includes the C₄ and the CAM plants.

The C₄ and the CAM plants have evolved a slightly different variation in the way they conduct photosynthesis that allows them to avoid or minimize photorespiration. This allows C₄ and CAM plants to better withstand hot temperatures that C₃ plants are not able to withstand. Once again, C₄ and CAM plants are able to better withstand hot temperatures because they've evolved a solution to be able to minimize or avoid photorespiration. Moving forward in our course, in our next video, we're going to talk more about the differences between C₃ plants, C₄ plants, and CAM plants. But for now, let's take a look at this image down below that we have here, which is really a map of our lesson on photosynthesis. Already in our previous lesson videos, we've talked about the light reactions and the Calvin Cycle, which are the two stages of photosynthesis.

Because we've already discussed these, that's why they're grayed out in this image here. We did talk about the light reactions and the Calvin cycle reactions, under the conditions where the stomata were in an open position, where gas exchange was able to occur. Also, in our previous lesson videos, we discussed how in hot temperatures, plants will close their stomata to prevent dehydration. But when they close their stomata, they also prevent gas exchange. By preventing gas exchange, this leads to photorespiration. Here in this video, we're telling you that it's actually the C₃ plants that are most susceptible to photorespiration. We're indicating that we're calling it C₃ photorespiration. Again, C₃ photorespiration is a bad thing for plants as it wastes energy and produces carbon dioxide. So, plants want to be able to avoid or minimize photorespiration, and some plants have actually been able to evolve a solution. This includes the C₄ and the CAM plants, which have evolved a solution for photorespiration to be able to withstand hot temperatures and avoid photorespiration.

You can see here in our image that we have some plants that are able to avoid photorespiration, and this includes again the C₄ plants and the CAM plants. Notice that right next to the C₄ plants, we have this C₄ bomb, and later in our next video, we're going to talk about exactly how this C₄ bomb can help you remember C₄ plants. Then also notice that next to the CAM plants, we have this camel wearing some pajamas. Again, in our next video, we're going to talk about how thinking of a camel wearing pajamas can help you remember CAM plants. But for now, this here concludes our brief introduction to C₃, C₄, and CAM plants. In our next video, we're going to talk more about the differences between C₃, C₄, and CAM plants. So, I'll see you all in that video.

Comparing 3 Types of Photosynthetic Plants

In this video, we're going to be comparing the 3 types of photosynthetic plants, which are the C3 plants, the C4 plants, and the CAM plants. And so it turns out that the vast majority of plant species are actually C3 plants. The C3 plants perform the standard photosynthesis process that we already covered in our previous lesson videos. They will perform the light reactions and the Calvin cycle exactly as we discussed them in our previous lessons. There's nothing new here; it's just review information from our previous lessons. The reason that C3 plants are called C3 plants is that the very first stable molecule produced during the Calvin cycle is a 3-carbon intermediate. Also, during C3 photosynthesis, C3 plants have only one round of carbon fixation. Carbon dioxide gas is affixed to another molecule just one time, and that is during the Calvin cycle. Additionally, in C3 plants, the light reactions and the Calvin cycle both occur in the same exact cell, the mesophyll cell.

Turning our attention to the C4 plants, these plants and the CAM plants have both evolved slightly different variations in the way they perform photosynthesis, allowing them to withstand hot temperatures by avoiding or minimizing photorespiration. C4 plants are named for the very first molecule formed being a 4-carbon intermediate. One major difference between C3 and C4 plants is that C4 plants use 2 carbon fixation rounds instead of just 1. Unlike C3 plants where the light reactions and the Calvin cycle reactions occur in the same cell, with C4 plants, the light reactions will occur in one cell type and the Calvin cycle reactions in a different cell type. This separation allows C4 plants to minimize photorespiration. The 4-carbon intermediate is crucial in supplying additional carbon dioxide when CO2 levels drop due to the closing of stomata in warmer and drier temperatures. In C4 plants, we see two different cell types being utilized: the mesophyll cells conduct the light reactions, and the bundle sheath cells perform the Calvin cycle.

Lastly, the CAM plants, like C4 plants, form a 4-carbon intermediate and have two rounds of carbon fixation. However, unlike both C4 and C3 plants, the CAM plants have their light reactions and Calvin cycle reactions occur within the same cell, like C3. What makes CAM plants unique is that these two carbon fixation rounds occur at different times of the day. Their stomata open during the cool nights to collect CO2, which is stored as 4-carbon intermediates. During the hot days, the stomata close, but the stored 4-carbon intermediates supply the CO2 needed for the Calvin cycle. This adaptation makes CAM plants highly efficient in hot, arid environments similar to deserts.

This introduction compares the three types of photosynthetic plants: C3, C4, and CAM. We'll be able to get some practice applying the concepts we've learned here as we move forward in our course. So, I'll see you all in our next video.

A plant that opens its stomata only at night is a

• A. C₂ plant
• B. CAM plant
• C. C₃ plant
• D. C₄ plant

Answer: (B)

Here’s what students ask on this topic:

What are the main differences between C3, C4, and CAM plants?

C3, C4, and CAM plants differ primarily in their methods of carbon fixation and their adaptations to environmental conditions. C3 plants, the most common, perform a single round of carbon fixation in mesophyll cells, producing a 3-carbon intermediate. C4 plants, like corn, perform two rounds of carbon fixation in separate mesophyll and bundle sheath cells, producing a 4-carbon intermediate that helps minimize photorespiration in hot conditions. CAM plants, such as cacti, also fix carbon twice but do so at different times of the day. They open their stomata at night to reduce water loss and store CO₂ as a 4-carbon intermediate, which is used during the day for photosynthesis.

Why are C3 plants more susceptible to photorespiration?

C3 plants are more susceptible to photorespiration because they perform a single round of carbon fixation in mesophyll cells, producing a 3-carbon intermediate. In hot and dry conditions, C3 plants close their stomata to prevent water loss, which also limits CO₂ intake. This leads to an increase in oxygen levels inside the leaf, causing the enzyme Rubisco to fix oxygen instead of CO₂. This process, known as photorespiration, wastes energy and reduces the efficiency of photosynthesis, making C3 plants less efficient in hot climates.

How do C4 plants minimize photorespiration?

C4 plants minimize photorespiration by performing two rounds of carbon fixation in different cell types: mesophyll and bundle sheath cells. The first round occurs in mesophyll cells, where CO₂ is fixed into a 4-carbon intermediate. This intermediate is then transported to bundle sheath cells, where it releases CO₂ for the Calvin cycle. This spatial separation ensures a high concentration of CO₂ around Rubisco, reducing its interaction with oxygen and thereby minimizing photorespiration. This adaptation allows C4 plants to thrive in hot and dry environments.

What is the role of stomata in CAM plants?

In CAM plants, stomata play a crucial role in minimizing water loss while allowing for efficient photosynthesis. CAM plants open their stomata at night when temperatures are cooler and humidity is higher, reducing water loss. During this time, they take in CO₂ and store it as a 4-carbon intermediate. During the day, when temperatures are higher, the stomata close to conserve water. The stored CO₂ is then released from the 4-carbon intermediate and used in the Calvin cycle for photosynthesis. This temporal separation of carbon fixation helps CAM plants survive in arid environments.

Why do CAM plants open their stomata at night?

CAM plants open their stomata at night to minimize water loss while still allowing for CO₂ intake. In arid environments, daytime temperatures are high, leading to significant water loss if stomata are open. By opening their stomata at night when temperatures are cooler and humidity is higher, CAM plants reduce water loss. They store the absorbed CO₂ as a 4-carbon intermediate, which is used during the day for photosynthesis when the stomata are closed. This adaptation allows CAM plants to efficiently manage water and survive in dry conditions.