Where does photosynthesis take place in a prickly pear cactus?
By Martin Schweig
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.
What part of the cactus photosynthesizes?
The >1500 cactus species all live in the Americas (well except one in Africa, probably dispersed there by birds) and mostly in arid environments. However, there are also many cacti that live in the tropical rainforests of Central and South America. At first this doesn’t make sense why a arid-adapted group would have a center of diversity in some of the world’s wettest habitats, until you look at the micro-habitats they live in, namely, up in the forest canopy. Rainforest cacti are almost all ephiphytes, meaning they grow on other plants, normally the branches of large trees. From plants’ perspective these environments are actually very dry, it is hot, there is no soil to hold water, and airflow from all directions desiccates. So the elegant adaptation that cacti evolved in the deserts gave them an advantage as tropical ephiphytes and when they arrived in these new habitats they thrived and diversified.
So why the spines? Why not just lose the leaves all together? The short answer is that spines are a defense against herbivores. Herbivory, or the consumption of plant matter, can be really bad news for a plant: get a bunch of your photosynthetic tissue eaten, have to grow it back, not enough resources left to produce seeds and reproduce, less offspring, lower fitness. Plants that get less damage from herbivores might have higher fitness so evolution should favor plants with traits that reduce herbivory. This is particularly true in environments where resources are scarce, like deserts, where regrowing tissue lost to herbivory is very difficult (this is called the ). Thus, plants in low resource environments, like cacti in deserts, invest very heavily in defenses, like big gnarly spines.
Photosynthesis occurs in a cactus stem or trunk
The most recognizable plant that uses this is the cactus, and the enzymes store the carbon dioxide as an acid during the night to prevent water loss, and then carry out the conversion in the daylight.
Does a cactus perform photosynthesis? How? | Yahoo …
With stomata open only at night when the temperature is lower and the relative humidity higher, the CAM plants use much less water than either or C4 plants. Some varieties convert to C3 plants at the end of the day when their acid stores are depleted if they have adequate water, and even at other times when water is abundant.
where does photosynthesis take place in a cactus
The main problem is that the back side of the plants in a window never gets enough light and they can lean or stretch.
Other than cost, the heat put out by the required level of lighting for cacti can be a huge problem and you will need to run a fairly robust system of fans both to stop the plants overheating and to stop the rest of your house overheating.
You might be better looking for types of cactus or other succulents that will grow well in your western window or outside without much sun.
Life of a Cactus Part 5: CAM Photosynthesis - …
The acidity was found to arise from the opening of their stomata at night to take in CO2 and fix it into malic acid for storage in the large vacuoles of their photosynthetic cells. It could drop the pH to 4 with a malic acid concentration up to 0.3M . Then in the heat of the day, the stomata close tightly to conserve water and the malic acid is decarboxylated to release the CO2 for fixing by the Calvin cycle. PEP is used for the initial short-term carbon fixation as in the , but the entire chain of reactions occurs in the same cell rather than handing off to a separate cell as with the C4 plants. In the CAM strategy, the processes are separated temporally, the initial CO2 fixation at night, and the malic acid to Calvin cycle part taking place during the day.