C3, C4, CAM Photosynthesis

By Martin Schweig (April 2009)

My first interest in succulent plants developed because of their unique physical differences to most other botanical species. What I did not realize was how different they were in many other aspects of their existence.

Their basic biochemical process is somewhat different from the chemistry of most other plants. To survive in a dry environment with irregular or little rainfall, succulent plants must store water in their leaves, stems or roots. These plants often show specific adaptations in their metabolism.

As we know, plants produce food by photosynthesis, which is the bonding together of carbon dioxide with water to make sugar and oxygen using the sun’s energy. Sugar contains the stored energy and serves as the raw material from which other compounds are made.

What I was not aware of is that there are at least three different pathways in which photosynthesis can occur to achieve the same results. They are known as C3, C4 and CAM, because the first chemical made by the plant is a three- or four-chain molecule.

C3 (normal conditions)
C4 (high temperature/high water/high light availability)
CAM (high temperature/low water availability)

CAM stands for crassulacean acid metabolism, after the plant family in which it was first discovered. It is essentially a means of isolating in time the carbon dioxide intake from sunlight-fueled photosynthesis. Acid is stored at night within the plant so that during the day it can be turned into sugars by photosynthesis.

All plants can use C3 photosynthesis, and some are able to use all three types. However, C4 and CAM do not exist in the same plant. It is interesting to note that the only cacti to use C3 photosynthesis is the primitive pereskia.

C4 and CAM photosynthesis are both adaptations to arid conditions, because they are more efficient in the conservation of water. CAM plants are also able to “idle,” thus saving energy and water during periods of harsh conditions. CAM plants include many succulents such as Cactaceae, Agavacea, Crassulaceae, Euphorbiaceae, Liliaceae, Vitaceae (grapes), Orchidaceae and bromeliads.

CAM plants take in carbon dioxide during the night hours, fixing it within the plant as an organic acid with the help of an enzyme. During the daylight hours, CAM plants can have normal C3 metabolism, converting carbon dioxide directly into sugars or storing it for the next day’s metabolism for use in the evening.

With the sun’s energy during daylight, the stored organic acid is broken down internally with the help of enzymes to release carbon dioxide within the plant to make sugars. The stomata (pores) can be open during the evening when the temperature is lower and humidity relatively higher.

During the day, the stomata can remain closed, using the internally released carbon dioxide and thus sealing the plant off from the outside environment. This is probably a six to 10 times more efficient way to prevent water loss compared to normal plant respiration. This modified effect seems to work best when there is a considerable difference between daytime and nighttime temperatures.

C4 plants can photosynthesize faster under a desert’s extreme heat than C3 plants, because they use extra biochemical pathways and anatomy to reduce photorespiration. Photorespiration basically occurs when the enzyme (rubisco) that grabs carbon dioxide for photosynthesis grabs oxygen instead, causing respiration that blocks photosynthesis and thus causes a slowing of the production of sugars.

The majority of plants fall into the C3 category and are best adapted to rather cool, moist temperatures and normal light conditions. Their stomata are usually open during the day.

When conditions are extremely arid, CAM plants can just leave their stoma closed night and day, and the organic cycle is fed by internal recycling of the nocturnally fixed respiratory carbon dioxide. Of course, this is somewhat like a perpetual motion machine, and because there are costs in running this machinery, the plant cannot CAM-idle for very long. This idling does, however, allow the plants to survive dry spells and recover quickly when water is again available. This is quite unlike plants that drop their leaves and go dormant during dry spells.

The following comparison of photosynthesis and respiration may be helpful.

A note of interest: It is not only plants that can photosynthesize. The sea slug looks somewhat like a leaf because it contains chlorophyll-rich plant structures called chloroplasts. These structures are extracted from the algae that it eats. Researchers have recently found that this creature has acquired from the algae at least one gene necessary for photosynthesis. The sea slug can photosynthesize on its own for months. This is a first among animals.