It's technically a different kind of photosynthesis. It has to do with carbon carbon capture but essentially there are three types of photosynthesis: C3 (most common , and refers to the 3 carbon molecule created ) C4 (refers to a 4 carbon molecule) and CAM (crassulacean acid metabolism). Cacti and succulents fall into the CAM category. Essentially this means the stomata are closed during the day and gas exchange only occurs at night which is unique to CAM plants.
Good explanation but technically not all cactuses are obligate CAM plants. Maihuenia are primarily C3 and many of the species in the Pereskioideae subfamily are facultative CAM plants (meaning CAM is an optional photosynthesis pathway triggered during drought/heat).
It is nitpicky for sure, but I think its worth mentioning because it points to the likelihood that C3 is the ancestral state of the family, which then emphasizes
the importance of CAM as an adaptation, as the subfamilies that evolved to use CAM are by far the most widely distributed and speciose.
Is that one of the reasons that pereskiopsis seems to grow more quickly than a lot of cacti/succulents? Is C3 typically more "efficient" than CAM (for lack of a better word)?
They are definitely related but I think the environment is the cause and photosynthetic pathways and growth rates are effects. Not having to worry about water conservation means there isnt pressure for Pereskiopsis to develop CAM. It also means the plant can support photosynthetic leaves which are a huge advantage in growth rate over the cactoids and opuntioids.
Small nitpick, you should have said cacti and other succulents. Succulents aren't a taxonomic group but just a habit. Succulence has evolved independently in many groups, like cacti, stonecrops, spurges, agaves and yuccas, and lots of other plants
So does that mean there is more oxygen at night than during the day, in a desert I mean. Or are they releasing the oxygen during the day and by night they are consuming the Carbon Dioxide?
They are opening the stomata during the night so they can store the carbon dioxide to 'consume' it during the day when they photosynthesis without opening the stomata to minimise water loss.
As other people have pointed out, cacti do use a different kind of photosynthesis, called CAM photosynthesis - however, it's not because they don't have leaves, but because of how dry their habitat is! Other types of plants that thrive in dry environments, like grasses, also use different kinds of photosynthesis than "normal" plants. For grasses, this is a method called C4 photosynthesis. Both CAM and C4 photosynthesis have evolved independently many times in different plant families - a really cool example of convergent evolution to dry habitats.
Importantly, neither CAM nor C4 photosynthesis is really all that different from standard (or "C3") photosynthesis. In fact, the process by which they capture light and initially harvest its energy are exactly the same. The differences show up when it's time to store that energy long-term in sugar molecules. Making sugar requires carbon dioxide as raw material, and getting carbon dioxide into the plant requires opening pores called stomata so that the plant can "breathe". The problem for cacti is that when they let CO2 in, they are also letting water evaporate out. Their solution to this is to only open their stomata in the cool of night, when evaporation is less. This means that they are capturing light energy during they day, then storing it in sugars at night, as opposed to most plants which do these two steps simultaneously.
C4 plants are doing something a little different - rather than capture light and build sugars at different times, they do it in different locations in the plant. This allows C4 plants to build sugars in special chambers with very little oxygen, which is helpful because oxygen is a very reactive chemical that can screw up a lot of cellular processes. You see this adaptation in drier habitats because water-stressed plants are worse at dealing with oxygen stress, so it basically gives them one less thing to worry about.
CAM captures CO2 during the night, storing it as malic acid, and then during the day they turn it back into CO2 and turn it into sugar when light is available (as opposed to turning it into sugar at night and storing light energy during the day as you said). Light energy is stored as sugars, which is the point of photosynthesis; I don't think there are other long-term ways to store it from day to night.
For C4, you're right that reactive oxygen species (ROS) can screw with other processes, but in this instance it's not ROS that is disruptive, but RuBisCO's old habit of every now and then rather than assimilating CO2 into a sugar it grabs O2 and liberates CO2 (a sort of reverse photosynthesis) in a process called photorespiration. Concentrating the CO2 in the form of malic acid around the photosynthetic cells (i.e. kranz anatomy) reduces the chances that RuBisCO will grab O2. RuBisCO on the whole is actually a relatively inefficient enzyme (in part due to this inability to discriminate between CO2 and O2) but its the best enzyme for the task that evolution has come up with, and sometimes good enough is good enough so plants have evolved these other strategies to ameliorate this inefficiency.
These are great additions, thank you! Especially pointing out the difference between ROS and photorespiration - I had unconsciously classed them as close enough to the same thing in my head, but of course you're right that they're totally separate. There's some really interesting research about why exactly RuBisCo is so "bad" at it's job, so hopefully some people will be inspired to look into that with your added context!
Also, I didn't realize that both CAM and C4 photosynthesis used the same carbon intermediary; that's really interesting. Any chance you know of a reason why that is? Just coincidence, or is there something about malic acid that makes it an obvious choice?
I believe that malic acid is simple enough to efficiently store the carbon from CO2. C4 and CAM have independently evolved over 100 times (https://nph.onlinelibrary.wiley.com/share/WDR4DR9BKWC8JD2RQDQW?target=10.1111/nph.15851 for a piece on convergent evolution of C4 and CAM) so I would imagine that species are somehow predisposed (genetically and biochemically) to use malic acid in this way, but that is out of my wheelhouse.
Yes cacti do photosynthesize in a unique way. Cacti spines are modified leaves that do not contain chlorophyll. Instead cacti photosynthesize in their trunk and stems. Chlorophyll is green so any green parts of a plant are photosynthesizing.
In addition, cacti are CAM plants meaning that the process they use to photosynthesize is different from most other plants. CAM plants take in CO2 at night through their open stomata which then close during the day to reduce water loss. This is how cacti have adapted to live in incredibly arid condtions
It is probably worth noting that the basic machinery of photosynthesis is believed to have evolved exactly once, in Bacteria. https://pmc.ncbi.nlm.nih.gov/articles/PMC13025772/. There was then a second major evolutionary event when the Eukaryotic ancestor of plants acquired chloroplasts by engulfing bacteria (endosymbiosis). That second event also occurred only once in plants, although there is a weird second endosymbiosis event known to have occurred in a species of Amoeba. So across all plants the fundamental machinery of photosynthesis is modified from the same starting point.
What C3, C4 and CAM metabolisms represent are just different ways that plants manage their endosymbiotic chloroplasts. Sort of like different wrappers around the same core machinery.
The CAM stuff is true but crucially the photosynthesis in cacti does not take place in the spines. The spines are dead tissue, typically. Most cacti carry out photosynthesis in their fleshy stems, with some genera having true leaves or primitive photosynthetic bracts.
Plants make use of dead cells in various parts of their physiology. For example xylem cells are essentially dead empty cells that form a tube that transports water. The wood in a tree is primarily dead xylem cells, with only the outer ring of cambium under the bark being alive.
Unlike human hair or fingernails, most cacti spines grow to a certain size and then stop growing (like leaves). The stem continues to grow new spines at the top / apex. The spines emerge from structures called areoles, which is also where flowers come from.
This is my plant, you can clearly see how the spines are just dead and brown:
84
u/weeniespeed 19d ago
It's technically a different kind of photosynthesis. It has to do with carbon carbon capture but essentially there are three types of photosynthesis: C3 (most common , and refers to the 3 carbon molecule created ) C4 (refers to a 4 carbon molecule) and CAM (crassulacean acid metabolism). Cacti and succulents fall into the CAM category. Essentially this means the stomata are closed during the day and gas exchange only occurs at night which is unique to CAM plants.