Crassulacean acid metabolism

Some plants live in very dry lands.

They must save their water to stay alive. At night, they open tiny holes in their leaves. This lets them take in air. They store this air as a sour acid.

During the day, the holes stay shut. This keeps the water inside the plant. The plant uses the stored acid to make food. This way, they can grow in the hot sun. It is a clever way to live!

Some plants live in very dry places. They have a special way to make food. This way is called CAM photosynthesis.

Most plants open tiny holes in their leaves during the day. These holes are called stomata. But opening holes in the hot sun lets water out. This can make a plant dry up.

CAM plants do things differently. At night, they open their stomata. They take in a gas called carbon dioxide. The plant turns this gas into a sour acid. It stores this acid in tiny parts of its cells called vacuoles.

During the day, the plant keeps its stomata shut. This helps the plant save water. The plant then moves the stored acid to other parts of the cell. There, it turns the acid back into carbon dioxide. The plant uses that gas to make food in the sunlight.

Many cacti use this way to live. Some plants, like the pineapple, use it too. This helps them grow even when water is hard to find.

Some plants have a clever way to survive in very dry places. This special way of making food is called Crassulacean acid metabolism, or CAM for short.

Most plants open tiny holes in their leaves during the day to breathe. These holes are called stomata. However, opening stomata in the hot sun lets water escape. This is called evapotranspiration. For many plants, this causes a huge loss of water. CAM plants avoid this hard job by changing when they breathe. This allows them to grow in places that are far too dry for other living things.

CAM works like a two-part cycle that uses time to save water.

At night, when the air is cool, the plant opens its stomata. Carbon dioxide enters the leaves and moves into the mesophyll cells. The plant turns this gas into a four-carbon acid called malic acid. It then stores this acid in large storage bubbles called vacuoles. During the day, the plant shuts its stomata tight to keep moisture inside. The stored malic acid moves to the chloroplasts, which are the plant's food-making parts. There, the acid turns back into carbon dioxide to be used for photosynthesis.

Scientists have been studying these strange plant habits for a long time.

In 1804, a researcher named de Saussure made the first observations about this. Later, in 1812, Benjamin Heyne noticed something interesting in India. He saw that the leaves of a plant called Bryophyllum tasted sour in the morning. By the afternoon, the leaves tasted plain again. This happened because the acid was being used up. Other scientists like E. Aubert and H. M. Richards studied these changes further in the late 1800s and early 1900s. The name CAM was likely created by Ranson and Thomas in 1940.

This way of working is very common in certain plant families.

For example, over 99% of the 1,700 known species in the Cactaceae family use CAM. This includes almost all cacti that grow edible fruits. The name CAM comes from the Crassulaceae family, which includes jade plants and Sedum. Some plants are "obligate," meaning they only use CAM. Others are "inducible," meaning they can switch to CAM if it gets too dry. Even some trees, like those in the genus Clusia, can use this method. Some aquatic plants also use CAM when carbon dioxide is hard to find in water.

You can think of CAM like a grocery shopping trip. Most plants shop for food while the sun is out. But for a CAM plant, the sun is too hot to go outside. Instead, they wait until the cool night to go out and collect their supplies. They bring the carbon dioxide home and store it in the pantry, which is the vacuole. When the sun comes up, they stay safe inside and cook the food they collected earlier. This smart trick helps them stay hydrated even in the heat of the desert.

Crassulacean acid metabolism, commonly known as CAM photosynthesis, is a specialized carbon fixation pathway. This biological process evolved in various plant species as a critical adaptation to arid environments.

While most plants perform photosynthesis and gas exchange during the daylight hours, CAM plants separate these two tasks by time. This temporal separation allows them to collect carbon dioxide at night and process it during the day. By doing this, they can survive in habitats that would otherwise be too dry for most vegetation. This mechanism is essential for water conservation in plants that face extreme heat and limited moisture.

The mechanism of CAM relies on a precise two-part cycle involving the stomata and specialized cell structures.

During the night, when temperatures are lower, the plant opens its stomata, which are tiny pores on the leaf surface. Carbon dioxide diffuses into the mesophyll cells and undergoes a reaction with phosphoenolpyruvate (PEP). This reaction is facilitated by the enzyme PEP carboxylase (PEP-C), which is highly active at night. The resulting organic acid, malic acid, is then transported into large storage bubbles called vacuoles. During the daylight hours, the plant closes its stomata to prevent evapotranspiration, which is the loss of water vapor. The stored malate is then released from the vacuoles and transported to the chloroplasts. There, an enzyme cleaves the malate to release carbon dioxide, which is then used in the Calvin cycle to build carbohydrates.

Plants utilize this metabolism in several distinct ways depending on their environment.

Some are classified as "obligate CAM plants," meaning they rely exclusively on this pathway for photosynthesis. These are often divided into "strong CAM" and "weak CAM" species based on how much acid they can store. Other plants exhibit "inducible CAM," allowing them to switch between standard photosynthesis and CAM depending on environmental stress, such as drought or high salinity. There is also a group known as "CAM-cycling" plants. These species do not open their stomata at night but instead recycle the carbon dioxide produced during their own respiration. This flexibility helps plants survive in regions where water availability changes frequently.

The history of discovering this process spans nearly two centuries of botanical research.

The first observations were recorded by de Saussure in 1804. In 1812, Benjamin Heyne noted that the leaves of the Indian plant *Bryophyllum* were acidic in the morning but tasteless by the afternoon. This change in acidity was a direct result of the plant consuming its stored malic acid. Later, researchers such as E. Aubert in 1892 and H. M. Richards in 1915 provided more detailed studies on these gas interchanges. The specific term "Crassulacean acid metabolism" was likely coined by botanists Ranson and Thomas in 1940. They observed these cycles in the succulent family Crassulaceae, which includes plants like jade and Sedum.

The efficiency of CAM is most evident when looking at the survival statistics of desert flora. For example, over 99% of the 1,700 known species in the Cactaceae family utilize CAM. This includes almost all cacti that produce edible fruits. Standard plants can lose as much as 97% of the water they absorb through their roots due to transpiration. CAM plants avoid this massive loss by keeping their stomata closed during the hottest parts of the day. This adaptation is so successful that it has evolved convergently many times across different plant lineages. It is not a single evolutionary line but a repeated solution to the problem of aridity.

Biochemically, the process is a complex dance of enzymes and energy molecules. At night, the enzyme PEP carboxylase kinase (PEP-C kinase) phosphorylates PEP carboxylase to enhance its ability to create oxaloacetate. This oxaloacetate is then transformed into malate by the enzyme NAD+ malate dehydrogenase. During the day, the plant must manage the breakdown of this malate. Depending on the species, malate is cleaved into pyruvate and carbon dioxide by either the malic enzyme or PEP carboxykinase. The pyruvate can then be used to recover PEP through a high-energy step involving ATP, ensuring the cycle can repeat the following night.

CAM is not limited to desert landscapes; it also appears in unexpected ecosystems. Some aquatic plants, such as those in the genus *Isoetes*, use CAM to manage carbon dioxide levels. In water, carbon dioxide diffuses 10,000 times slower than it does in the air. These aquatic species capture carbon at night when competition from other photosynthetic organisms is lower. This process is most intense during summer months when competition for carbon is highest. Whether in a dry desert or a crowded pond, CAM represents a sophisticated way for life to manage its most precious resources.