CAM Plants Definition
CAM – short for “Crassulacean Acid Metabolism” – is a method of carbon fixation evolved by some plants in dry circumstances.
In most plants, the stomata – which are like tiny mouths that take in oxygen all along the surfaces of their leaves – open during the day to take in CO2 and release O2.
Plants must take in CO2 because they use it as a source for carbon atoms to build sugars, proteins, nucleotides, and the other building blocks of life. They must also release waste O2, which is the biproduct that is left over after the carbon atom from has CO2 been incorporated into a sugar.
Most plants open their stomata during the day because that is when energy is received from the Sun. The energy from the Sun is harvested by the chloroplasts and used to make ATP and NADPH. These short-term energy storage molecules are then used to power the fixation of carbon into sugar.
In plants living in very dry environments, however, dangerous amounts of water can be lost if the stomata are open during the hot, dry days. During the night, which tends to be much cooler in dry environments, far less water is lost by opening the stomata.
In order to meet their needs to combine the Sun’s energy with CO2 from the air, CAM plants take in CO2 at night and store it in the form of a four-carbon acid called “malate.” Then the malate is released during the day, where it can be combined with the ATP and NADPH created by the Sun’s energy.
This allows the plants to conserve their water by closing their stomata during the hot daytimes.
The name “Crassulacean Acid Metabolism” comes from the Crassula plant, which was the first place that CAM metabolism was discovered and studied.
Steps of CAM Photosynthesis
1. CAM photosynthesis begins at night, when the plant’s stomata open and CO2 gas is able to diffuse into the cytoplasm of CAM mesophyll cells.
In the cytoplasm of those cells, the CO2 molecules encounter hydroxyl ions, OH−, which they combine with to become HCO3 the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase).
CO2 + OH− → HCO3
2. The PEP carboxylase enzyme catalyzes the following reaction to add the CO2 to a molecule called phosphoenolpyruvate (PEP).
PEP + HCO3− → OXALOACETATE
3. Oxaloacetate then receives an electron from NADH and becomes a molecule of malate. This reaction is catalyzed by the enzyme Malate Dehydrogenase (MDH). That reaction looks like:
OXALOACETATE + NADPH + MDH → MALATE + NADP+
Interestingly, malate dehydrogenase catalyzes a reversible reaction, meaning that it can either add electrons to oxaloacetate, or take electrons away from molecules of malate.
4. Malate is now stored in vacuoles within the plant cells, until the sun rises and photosynthesis begins. When that happens, malate enters the Calvin Cycle, just like 3-phosphoglycerate would in a plant using a 3-carbon, or “C3” pathway for carbon fixation.
Examples of CAM Plants
CAM metabolism is common in plants that live in hot, dry environments where water is difficult to gain and conserve. Examples include:
The stereotypical “desert plant” is the cacti. These plants, which look very different from your average leafy green, are ideally designed to survive in deserts.
Typical cacti have a rounded shape, which minimizes the surface area through which they can lose water during the day. Many also have spines to stab any animals that might want to eat them and consume their delicious water.
It makes sense, then, that cacti would also make use of the CAM cycle to prevent them from opening their stomata and losing water during the day!
Agave – a plant which has become popular because it is used to make tequila and the sweet agave nectar – also uses CAM to survive in desert environments.
It looks more like a leafy green plant than a cactus, but like cacti, it has developed thick flesh to reduce its surface area and conserve water, and spines along the edges of its leaves to discourage animals from eating them.
Clusia pratensis is a flowering tree that lives on the dry plains of Panama in Central America.
It is one of many plants which are “facultative CAM plants” – those that can use CAM respiration under hot, dry conditions, but which can also perform normal “C3” carbon fixation.
Other facultative CAM plants that can switch between two modes of carbon fixation include Calandrinia polyandra, Mesembryanthemum crystallinum, Portulaca oleracea and Talinum triangulare.