Welcome back to Bogobiology!
In this video, we’ll be discussing how C3, C4 and CAM plants perform photosynthesis and
avoid performing the wasteful process of photorespiration.
We’ll go over some practical applications for applying this information in your research
and/or in your business.
Ok, here we go.
The sun powers nearly all forms of life on earth, either directly or indirectly, via
the process of photosynthesis.
Photosynthesis is the solar-powered process that plants use to make sugars.
Energy from the sun jumpstarts the process of “remixing” the components of carbon
dioxide and water into useful organic compounds like sugars, starches, and other products
containing carbon.
We call this “remix” process “carbon fixation.
Photosynthesis consists of 3 major phases ; Photolysis, the Light Dependent Reactions
and the Light Independent Reactions, also known as the Calvin Cycle.
If you’d like a more detailed overview of any of these processes, there’s a photosynthesis
review video linked in the description.
The calvin cycle is where we’ll focus our attention today, because it’s where C3 C4
and CAM plants really have specialized processes.
In the calvin cycle of photosynthesis, we take carbon from carbon dioxide and we remix
its components to make organic molecules.
Remember, in the scientific context “organic” just means “a thing containing carbon”.
Carbon dioxide enters the leaf through tiny openings called stomata.
The new carbons from that carbon dioxide are added to other carbons that are already in
the cycle to make new molecules.
The enzyme that does this is called Rubisco, and it’s thought to be the most common protein
on earth.
Its full name is Ribulose-1,5-bisphosphate carboxylase-oxygenase, so you can see why
most folks (except maybe its parents) use its nickname.
The 1-carbon co2 combines with a 5-carbon molecule called RUBP and after a few intermediate
steps it ultimately forms 2 3- carbon molecules called PGA.
The PGA combines with a few other key reagents to form PGAL (also called G2P or GALP in some
parts of the world) and we siphon off a couple of them to make into glucose.
The remaining PGAL are re-invested into the calvin cycle to keep it turning.
PGA with its 3 carbons is why this standard photosynthesis process is called “C3”.
Most plants in the world are C3 plants, and a few common examples include wheat, rice,
beans, spinach, most fruiting plants, and cannabis.
The C3 photosynthesis process works very, very well assuming that there is an ample
supply of CO2 available to keep feeding into the Calvin Cycle.
Ideally, RuBisCO will pair up with or “Fix” CO2 and keep the Calvin Cycle turning.
If, for any reason, there isn’t enough carbon dioxide to occupy rubisco, it’s very flexible
and will happily pair up with oxygen instead.
That wouldn’t be a huge deal EXCEPT that (obviously) oxygen doesn’t contain any carbon,
and plants need to get carbon from somewhere in order to keep photosynthesizing.
Plants DO have a work around where they can slowly recoup carbon to keep the cycle turning,
but it’s not ideal This process is called photo-respiration or,
if you want to get fancy, the “oxidative photosynthetic calvin cycle”.
The details of all the reactions are outside the scope of this video (there are an additional
dozen steps that aren’t shown here) but the idea is that the plant is able to use
2 additional organelles, peroxisomes and mitochondria, to generate 3-carbon PGA.
That new PGA can feed into the Calvin Cycle and keep it turning.
This process does allow the plant to keep photosynthesizing, but it is maddeningly inefficient
compared to the regular calvin cycle.
Even under the best of conditions, it produces 25% less product, creates lots of waste in
the form of hydrogen peroxide and ammonia which the plant has to deal with, and it’s
also energy inefficient because it costs ATP.
C3 Plants are more likely to do photorespiration when it’s hot.
This is for two reasons; first because in hot weather, c3 plants close their stomata
to avoid drying out.
Second, because RuBisCO, the enzyme, has a greater affinity for oxygen at higher temperatures.
New co2 isn’t coming in and eventually the supply will run out.
Unfortunately, C3 plants have no special adaptations to avoid photorespiration; they mostly rely
on the temperature staying relatively low.
If the temperature goes up more than a few degrees, the rate of photosynthesis in C3
plants falls dramatically.
Since most food crops are C3 plants, if global temperatures continue to rise, it is likely
to have huge implications on food production.
While some photorespiration is almost inevitable, it’s in a plant’s (and a grower’s!)
best interest to avoid it, if at all possible.
If crops are only photosynthesizing at 75% capacity, they won’t be able to produce
nearly as many leaves, flowers, buds, fruits or seeds.
We’re now going to begin comparing C3 plants to C4 and CAM plants.
C4 and CAM plants are specialists in avoiding photorespiration, and have a number of mechanisms
to help them do it.
We’ll be keeping track of C3, C4 and CAM photosynthesis process, leaf structure, example
plants, key cells and ideal temperature range in a table that looks like this.
We’ve already reviewed the “standard” c3 photosynthesis process; co2 enters through
the stomata, is fed into the calvin cycle and produce sugars.
C4 plants are a bit more crafty; they have the process of carbon fixation and the calvin
cycle in two different locations, which keeps Rubisco from being tempted by oxygen
C4 plants thrive in hot environments and common examples include corn and sugarcane.
Here’s how they do it.
In C4 plants, Co2 enters the leaf via the stomata, and combines with a 3-carbon product
called PEP or “phosphoenol pyruvate”.
The fixation forms a 4-carbon product called oxaloacetate.
The enzyme that does this, PEP carboxylase, will not bind with oxygen the way RuBisCO
does.
Oxaloacetate gets converted into another 4-carbon molecule called malate.
This 4-carbon molecule is why this process is called “c4”.
Here’s where the plant gets really smart; the malate gets transported out of the palisade
mesophyll cells and into the leaf’s bundle sheath cells to power
the Calvin cycle.
Because carbon fixation occurs in one cell and the Calvin cycle occurs in another, rubisco
doesn’t come into contact with oxygen and the plant can avoid wasteful photorespiration.
When 4-carbon malate arrives in the bundle sheath cells, it is broken down forming 1-carbon
CO2, which Rubisco can feed into the calvin cycle.
The 3 leftover carbons are converted into a molecule called pyruvate, which then gets
transported back into the palisade mesophyll cells.
Finally, it gets converted back into the starting 3-carbon molecule, PEP.
This part of the process requires some ATP The enzyme PEP carboxylase can go to work
on PEP, combine it with carbon dioxide and the cycle can start again.
Now let’s move on to CAM plants.
“CAM” stands for crassulacean acid metabolism, and it has some similarities to the C4 pathway.
CAM Plants live in the desert, and include pineapples and succulents like aloe and agave
CAM plants prevent photo respiration by doing their carbon fixation in advance during the
night, and then using the products to perform photosynthesis during the day
During the night, the plant can open its stomata without risking drying out, allowing carbon
dioxide (and some oxygen) to diffuse into the leaf.
The enzyme PEP carboxylase is not tempted by oxygen, so it can add the carbons from
CO2 onto PEP to form oxaloacetate, and then malate, or other organic acids.
The plant stores these acids in vacuoles for use later on.
During the day, the plant does have to keep its stomata closed, but it’s already stored
up the products that it needs.
The malate can then be transported out of the vacuoles, split and split into pyruvate
and carbon dioxide.
As in C4, the carbon dioxide powers the calvin cycle, while the pyruvate is re-formed into
PEP so the cycle can begin again.
Similar to C4, this specialized process does cost some ATP, but it still works out in the
plant’s favor because it avoids the wasteful process of photorespiration.
Now let’s talk about leaf structure.
When we look at the leaf structure of C3, C4 and CAM plants, we can see how their anatomy
helps each of them to perform their own versions of photosynthesis.
Most plants in the world are C3, so they have different variations of this leaf structure
on the left.
C4 plants have what is called “kranz” anatomy, meaning “wreath”.
The palisade mesophyll cells are clustered in a ring around the bundle sheath cells.
The two cell types need to be close together because the bundle sheath cells perform part
of c4 photosynthesis.
CAM plants live in the desert, and thus have to store large amounts of water whenever they
have the chance.
They store the water in tissue called aquiferous parenchyma, which other plants don’t need.
Similar to C4 plants, they have mesophyll cells in wreaths next to the bundle sheath
cells, but unlike c3 or C4 plants, CAM plants generally have smaller vascular bundles scattered
around the perimeter of the leaf rather than a few larger ones.
Hopefully now you can fill in the rest of this summary table; we’ve covered the processes,
leaf structure, example plants, key cells and the ideal climate for each one.
So what does this mean for people trying to grow certain crops?
How can we capitalize on this information?
The goal is to maximize the rate of photosynthesis and minimize photorespiration by optimizing
the conditions.
This might seem obvious, but plants that are struggling to photosynthesize efficiently
won’t grow as big, will have fewer flowers and yield less fruit, fewer seeds, and fewer
sugars and other useful compounds.
Optimizing conditions will look different depending on which type of plant you’re
trying to grow; C3, C4 or CAM.
As we’ve seen, these plants have different ideal conditions, so it’s very hard to grow
them all in the same place and get maximum yield.
You’re better off either separating them, or picking one to specialize in.
Since C3 plants are most common, we’ll use those as an example, but the conditions can
easily be tweaked to accommodate c4 or cam plants
Assuming there is ample sunlight and water, the two major factors that control the rate
of photosynthesis (ie how quickly the plant is able to churn out these carbon-rich compounds)
are temperature and Carbon Dioxide saturation, and it’s important to understand exactly
why.
At low temperatures, photosynthesis is limited by enzymes, which get too cold to function
at full capacity.
At lower temperatures, molecules move more slowly, reducing the likelihood of enzyme-substrate
collisions.
Low temperatures are also thought to limit plant’s ability to take up water and nutrients
through their roots, even if the temperature does not dip to freezing.
At ideal temperatures, available CO2 is the limiting factor.
RuBisco is not saturated most of the time.
In ambient conditions, some of it is standing around without CO2, or is bound to oxygen.
At high temperatures, membranes become deformed, and enzymes denature.
When membranes deform, ion leakage occurs, gradients don’t work as well, the electron
transport chain doesn’t work as well, photosynthesis rate decreases
The enzyme that is thought to be by far the most sensitive is NADP Glyceraldehyde Phosphate
Reductase.
This enzyme is the one that converts PGA into PGAL during the calvin cycle.
Without it, production of sugars can’t occur.
If temperatures fall within the ideal range, adding some Co2 to the local environment can
be beneficial and can further boost the rate of photosynthesis.
This should be done carefully, however.
Carbon dioxide can be expensive, it’s not awesome for the environment, and there are
diminishing returns on investment after a certain point.
The graph on the right shows the relationship between the rate of photosynthesis and the
level of Co2 in the area (assuming ideal temperatures).
Bumping the Co2 concentration from ambient air concentration of 340ppm to about 800-900
ppm gives photosynthesis a boost of almost 20% in ideal conditions.
However, above this point, you’ll notice that the curve flattens out and no matter
how much more CO2 you add, the plants don’t photosynthesize substantially faster.
It should also be noted that letting CO2 levels dip BELOW 340 is more detrimental than boosting
co2 is beneficial.
If time or resources are limited, focusing on MAINTAINING ambient levels of CO2 via ventilation
is a far better area to focus on than trying to add additional CO2.
Outdoors wind can do this, and indoors simply adding a fan to a greenhouse can really help.
If you do choose to add CO2 to a greenhouse, there is often a window of time when it will
do the most good.
adult plants often don’t need a constant stream of excess CO2 for their entire lifespan
in order to do well.
Often, adding CO2 just in the first 2-3 weeks as the plants begin to produce buds is the
most beneficial for eventual yield.
This gives the plant a boost in production of starches and sugars just at the exact moment
that it’s constructing useful products like flowers, fruits or seeds.
That pretty much wraps up our discussion of C3 C4 and CAM Photosynthesis and Photorespiration.
Thanks again for watching, see you next time!
Bye!