Crassulacean Acid Metabolism: Definition & Plants

Instructor: Lauren Posey

Lauren has taught intermediate reading in an English Language Institute, and she has her Master's degree in Linguistics.

Have you ever wondered how desert plants can keep from losing water? The answer lies in the process known as Crassulacean acid metabolism, which we will discuss in this lesson.

Unwanted Side Effects

You've probably heard of photosynthesis the method plants use to turn sunlight, water, and carbon dioxide (CO2) into energy. In this process, the stomata, or pores on the leaves of the plants, open to let CO2 in. Unfortunately, one side effect is that water can then escape through the open stomata.

In hot, dry environments water is scarce, and losing any water is a big problem. To deal with this plants that live in these environments have developed their own version of photosynthesis, which is more effective at keeping water in. This method is known as Crassulacean acid metabolism (CAM).

Desert plants have to preserve water
Cactus

The Solution

Instead of opening their stomata during the day, CAM plants have developed a way to open them at night instead. These plants absorb CO2 at night and turn it into malic acid, which they then store in their thick, waxy leaves or stalks. When the sun rises, the plant breaks the malic acid back down into CO2 so that it can be used in the regular photosynthetic process. This acid breakdown is where the 'acid' part of 'Crassulacean acid metabolism' comes from. The 'Crassulacean' part is because it was initially observed in the Crassulaceae family of plants.

Interestingly, the storage of CO2 as malic acid is something you can taste! If you were to eat the leaves of a CAM plant in the morning, they would be very acidic. However, as the day goes on and the malic acid is broken down, the acid taste goes away. (Of course, always check that a plant is really edible before trying this out!)

Essentially, CAM plants create energy the same way that non-CAM plants do. The main difference is that the breakdown of CO2 is on a time delay, so CO2 isn't absorbed at the same time that the rest of the photosynthetic process is taking place.

CAM plants open their stomata at night, and close them during the day
CAM plant stomata

Water Preservation

So how exactly does this process conserve water? Water loss in plants occurs through osmosis, which is when a substance moves from a higher concentration into a lower concentration to create an equal balance. At night, water vapor in the air is more concentrated, so water doesn't need to move from inside the leaves for there to be a balance.

During the day, at least in hot, dry climates, there is very little water in the air. If desert plants opened their stomata during the day, they would lose water at a rapid rate as the water evaporated into the drier surrounding air. By not having their stomata open during the day, plants can conserve water while still using photosynthesis to create energy.

CAM is not the most efficient method of growth, however. The delayed CO2 release means the photosynthetic process is slower and produces less energy per day. CAM plants have to use this method to survive in dry climates, but it means they grow very slowly. Most CAM plants are fairly small. Some, such as the giant saguaro cactus, can grow very large, but they grow extremely slowly.

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