Chapter 8: Photosynthesis: The Carbon Reactions

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Welcome back to the Deep Dive.

Today we're getting into something truly fundamental.

I mean, how life on earth really works at its base.

How plants build everything from, well, sunlight and air.

Yeah, it's all about transforming and circulating molecules and the energy driving it all.

That comes from the sun.

Captured by photosynthetic organisms, right?

Exactly.

And the scale is just staggering.

Think about three times 10 to the 21 joules of solar energy captured every single year globally.

Wow.

And that energy is used to fix what was the carbon number?

Around two times 10 to the 11 tons of carbon pulled straight out of the atmosphere

That's incredible.

Building the base of pretty much every food chain.

It really is the biosphere's engine.

And this whole ability, it goes way back.

We're talking over a billion years ago.

You had these free living cells, heterotrophs, that basically engulfed ancient cyanobacteria.

The endosymbiosis is that.

Precisely.

Primary endosymbiosis.

That led to the group called archipelastidae, green algae, land plants, red algae, the whole lot.

And it wasn't just getting a photosynthesizer inside.

It was like integrating it, right?

Merging functions.

Totally.

New metabolic pathways popped up.

Things like oxygenic photosynthesis, actually releasing oxygen and storing energy as starch.

Real game changers.

We talked about the first part before, the light reactions.

Capturing light energy, splitting water.

Generating ATP and NADPH, yeah, on the thylakoid membranes inside the chloroplast, the crucial chemical energy.

So this deep dive is about what happens next.

The carbon reactions, as you call them.

Right.

These happen in the chloroplast stroma, that fluid -filled space.

And they use that ATP and NADPH from the light reactions to turn atmospheric CO2 into sugars.

And you mentioned calling them carbon reactions is better than the old dark reactions term.

Definitely.

Because they aren't happening in the dark, necessarily.

They stop without the products of the light reactions.

No ATP, no NADPH, no light -triggered signals.

These reactions shut down.

They depend on light indirectly.

Okay, got it.

So our plan for today is to unpack that main CO2 fixing cycle, look at big inefficiency plans, deal with photo respiration, and then explore the clever ways some plants get around it, like C4 and CAM.

And finally, how the sugars made are packaged up as starch or sucrose.

We'll pull out the key stuff for you.

All right, let's jump in.

Where does carbon fixation actually start?

It starts with the absolute workhorse, the Calvin -Benson cycle.

This is the main pathway.

This is the pathway.

The dominant way autotruffs fix CO2, found everywhere from cyanobacteria to giant trees, it's job.

Reduce carbon from CO2, where it's highly oxidized, down into sugars.

And we know about this thanks to some pretty clever experiments back in the 50s.

Oh yeah, pioneering work.

Calvin, Benson, Bassam, and their colleagues used radioactive carbon tracers to figure out the steps.

Really ingenious stuff.

So this cycle happens in the stroma.

What are the main stages?

Okay, three key phases, all tightly linked.

First is carboxylation.

Adding the carbon.

Exactly.

CO2 combines with a five -carbon molecule, ribulose, 105 -bisphosphate.

Let's call it RoBP.

It also needs water.

This makes a temporary six -carbon thing that instantly splits into two molecules of a three -carbon compound, three -phosphoglycerate, or three -PGA.

Okay, so CO2 comes in, joins RoBP, makes two three -PGAs, phase one.

Then comes reduction.

This is where the energy goes in.

Using the ATP and NADPH from the light reactions.

You got it.

The three -PGA is converted into three carbon sugars, specifically.

Glyceraldehyde, three -phosphate, G3P, and its isomer.

These are called triose phosphates.

Think of it as adding energy to build them up.

Right.

And the third phase?

Regeneration.

Here's the thing.

Most of those triose phosphates made in phase two don't actually leave the cycle.

Oh, they stay in.

Yeah, five out of every six, basically.

They go through this complex shuffle of reactions, rearranging carbons to rebuild the original five -carbon RoBP acceptor molecule.

And this regeneration step needs more ATP.

So you put CO2 in, use energy to make triose phosphates, then use most of them plus more energy just to get back to the starting point, RoBP.

That's the cycle.

When it's running smoothly at steady state, the carbon coming in as CO2 equals the carbon leaving as those useful triose phosphates.

Ah, so some triose phosphate does leave.

Exactly.

That's the net product.

That triose phosphate can be used right there in the chloroplast to make starch for storage or get shipped out to the cytoplasm to make sucrose.

Sucrose for transport around the planet.

Right.

Fuel for growth or storage elsewhere.

Okay.

Let's zoom in on that first step again.

Carboxylation.

The enzyme is crucial, you said.

Absolutely critical.

It's Rubisco.

Rubulose 1 -philophyte -5 -bisphosphate -carboxylase -oxygenase.

Rubisco.

Okay.

The name even hints at a problem.

Carboxylase -oxygenase.

We are definitely coming back to that oxygenase part.

It's problematic.

But for now, its main job, the carboxylase part, is to grab CO2 and stick it onto RoBP, making those two 3 -PGA molecules.

Got it.

Then the reduction phase.

3 -PGA gets energy added.

Yep.

First, ATP is used by an enzyme called 3 -phosphoglycerate kinase to add another phosphate, making 1 -phol -3 -bisphosphoglycerate.

Then NADPH is used by NADP glyceraldehyde 3 -phosphate dehydrogenase to reduce that molecule, swapping the phosphate for a hydrogen, basically.

And boom, you get glyceraldehyde 3 -phosphate G3P at triose phosphate.

Right.

Now, that regeneration phase sounds like a bit of a maze.

Shuffling carbons.

It is complex on paper.

Think of it like this.

You take five molecules of those three carbon triose phosphates.

That's 15 carbons total.

Okay.

Five times three equals 15.

Right.

Then a whole series of enzymes, aldolysis, transketolysis, phosphatases, cut and paste carbon bits between different sugar phosphate molecules.

It's like molecular Lego.

The end result is rearranging those 15 carbons into three molecules of the five -carbon RoBP.

Which is also 15 carbons.

Three times five.

So the carbons balance.

Exactly.

The carbons are conserved.

Five triose phosphates go in, three RoBPs come out, ready to grab more CO2.

So if five out of six triose phosphates go into regeneration,

the sixth one is the profit.

That's the net gain.

For every three CO2 molecules that enter the cycle and go through the whole process, one molecule of triose phosphate is the actual output used by the plant.

And the energy bill for that.

Fixing three CO2s into one triose phosphate.

It costs six NADPH and nine ATP from the light reactions.

Wow.

So per CO2 molecule, that's two NADPH and three ATP.

A significant investment.

It really is.

Photosynthesis is powerful, but it's not free.

You mentioned an induction period earlier.

What's that about?

Like a warm -up time?

Kind of, yeah.

When the lights first come on, the cycle doesn't instantly hit top speed.

There's a lag, maybe a few minutes.

The enzymes need activating.

The levels of all the intermediate molecules need to build up.

Get the pipeline full.

Exactly.

Initially, the triose phosphates produced are mainly plowed back into regenerating ruby P until the whole system reaches a steady state where input matches output.

Makes sense.

You need the machinery running smoothly.

Okay.

So regulation is key.

How does the plant switch this on and off and keep it balanced?

Super important.

You don't want this energy guzzling cycle running in the dark, right?

So there's regulation at multiple levels.

Long -term, the plant can adjust how much of enzyme it makes, but there's also rapid control, switching enzymes on or off within minutes when the light changes.

And that rapid control is often linked directly to the light reaction.

Very often.

Take Rubisco itself.

It actually needs to be activated, and it's surprisingly slow for an enzyme.

Slow, but it's so important.

It is, and its activation is kind of neat.

A different CO2 molecule, an activator CO2, has to bind a specific lysine residue on the enzyme.

This forms something called a carbamate.

Okay.

And this carbamate is then stabilized by a magnesium ion Mg2 plus carvarbinate.

Only when Rubisco is in this carbamylated magnesium -bound state can it actually bind the ruby P and the substrate CO2 to do the reaction.

And magnesium levels change with light, right?

We'll get to that.

Right.

But what if other things get stuck to Rubisco?

Good point.

Sometimes sugar phosphates, inhibitors can bind to Rubisco and block it, even when it's activated or trying to get activated.

That's where another protein comes in.

Rubisco Activase.

Yeah.

It uses ATP energy to basically pry those inhibitory sugars off Rubisco, allowing it to get properly activated by CO2 and Mg2 plus box.

Clever.

What other light signals are involved?

The ferredoxin -theodoxin system sounds important.

Oh, it's major.

It's like a redox signaling chain connecting the light reactions to several key enzymes in the stroma.

How's it work?

Okay.

Light reduces ferredoxin at the thylakoids.

Reduced ferredoxin then passes electrons to an enzyme, ferredoxin -theodoxin reductase.

That enzyme then reduces a small protein called thyridoxin.

Reducing thyridoxin means breaking a disulfide bond, SS bond within it.

So thyridoxin gets charged up with electrons from light.

Sort of, yeah.

This reduced thyridoxin then goes and finds specific target enzymes in the Calvin -Benson cycle, like fructose, 1 -6 -bisphosphatase,

and it reduces their disulfide bonds.

Breaking that SS bond acts like an on -switch for these enzymes, increasing their activity dramatically.

So light ferredoxin and thyridoxin gore activates key cycle enzymes.

That's the chain.

In the dark, it all reverses.

Thyridoxin gets oxidized, the enzymes get oxidized back, and their activity drops.

It ensures the cycle runs efficiently only when light energy is available and helps prevent wasteful processes.

You also mentioned light -dependent ion movements, that magnesium connection.

Yes.

During the light reactions, protons, H +, are pumped into the thylakoid space, making it acidic.

To balance the charge, magnesium ions, Mg2 +, move out of the thylakoid space and into the stroma.

So in the light, the stroma gets less acidic, higher pH, and has more magnesium.

Exactly.

The pH goes from maybe 7 up to 8, and Mg2 +, concentration increases.

And guess what?

Many Calvin -Benson cycle enzymes, including ribisco and those regulated by thyridoxin, work much better at that higher pH and need that higher Mg2 +, concentration.

So the very environment in the stroma changes in the light to favor the cycle.

Neat.

It's another layer of control.

And one more thing.

Supermolecular complexes.

Enzymes sticking together.

Some of them, yeah.

In the dark, enzymes like glyceraldehyde -3 -phosphate dehydrogenase and phosphorbucanase can combine to another protein called Cp12, forming a big inactive complex.

Huddling together to keep quiet?

Kinda.

Then, in the light, reduced thyridoxin comes along again.

It breaks disulfide bonds in Cp12 and one of the enzymes, causing the whole complex to fall apart, releasing the active enzymes.

Wow.

So many checks and balances tied to light.

Okay, let's tackle the elfin in the room you mentioned.

Photorespiration.

Ribisco's dark side.

Yes, photorespiration.

It stems from that dual name.

Carboxylase deoxygenase.

Ribisco isn't perfectly specific for CO2.

It makes mistakes.

You could say that.

Oxygen O2 is structurally similar enough to CO2, and it's obviously around in the chloroplast.

So sometimes, Ribisco grabs an O2 molecule instead of a CO2 molecule and adds that to ruby P.

Uh oh.

No.

What happens then?

Instead of getting two useful 3 -PGA molecules, the oxygenation reaction produces one molecule of 3 -PGA, which is fine, it can enter the Calvin cycle, but also one molecule of a two -carbon compound called 2 -phosphoglycolate.

And 2 -phosphoglycolate is bad news.

Very bad news.

It's toxic.

It inhibits other important enzymes.

The plant absolutely has to deal with it.

So photorespiration is the cleanup process.

Exactly.

It's a metabolic pathway, also called the C2 oxidative photosynthetic carbon cycle, specifically designed to metabolize that toxic 2 -phosphoglycolate, salvage some of the carbon, and detoxify it.

And it's complicated.

It involves multiple parts of the cell.

Oh yeah.

It's a real tag team effort between three different organelles, the chloroplast, the leaf peroxisome, and the mitochondrion.

Okay, let's try and follow the molecule.

Starts in the chloroplast with Ribisco making 2 -phosphoglycolate.

Right.

An enzyme quickly removes the phosphate, making glycolate.

Glycolate then exits the chloroplast.

Where does it go?

To the peroxisome.

Inside the peroxisome, an enzyme called glycolate oxidase uses oxygen to convert glycolate into glyoxylate.

This step produces hydrogen peroxide, H2O2.

Which is also nasty stuff.

It is.

But peroxisomes are packed with another enzyme, cavalase, which immediately breaks down the H2O2 into water and oxygen.

Very efficient cleanup.

Okay, so now we have glyoxylate in the peroxisome.

Right.

Glyoxylate then gets an amino group added, transamination, using glutamate as the donor, turning it into the amino acid glycine.

Glycine then leaves the peroxisome.

And heads for the mitochondria.

Yep.

This is where a key step happens.

Two molecules of glycine enter the mitochondrion.

Two.

Two.

A complex set of enzymes works on them.

The outcome is one molecule of another amino acid, serine, which has three carbons, plus one molecule of CO2 is released, and one molecule of ammonia, NH4 +, is released.

It also generates some NADH.

Ah, so that's where carbon is lost.

Yeah.

Put in two carbon glycines, total four carbons, and you get out one three -carbon serine and one CO2.

Precisely.

That CO2 release is the respiration part of photorespiration.

It's a loss of fixed carbon.

And that released ammonia is also toxic and needs dealing with later.

Okay.

So serine leaves the mitochondrion.

And goes back to the peroxisome.

Back again.

Back again.

In the peroxisome, serine is converted via hydroxypyruvate into glycerate.

Glycerate.

That sounds familiar.

It should.

Glycerate then finally returns to the chloroplast.

Full circle almost.

Almost.

Inside the chloroplast, an enzyme uses ATP to add a phosphate back onto glycerate, turning it into three -phosphoglycerate.

Molecule from the Calvin cycle.

Exactly.

So the pathway managed to salvage three out of the four carbons that originally entered as two molecules of glycolate from the Rubusco mistake, and return them to the Calvin cycle as three PGA.

But it cost energy, ATP here, NADH was made in the mitochondria, needs oxidizing, oxygen was consumed, CO2 was lost.

And that toxic ammonia released in the mitochondria has to be quickly refixed back into amino acids in the chloroplast, which costs more ATP and reducing power.

Veridoxin.

So yeah, it's an expensive salvage operation.

And you said environmental conditions affect how much this happens.

CO2 versus O2 levels.

Temperature.

Hugely.

It all comes down to the competition at Rubusco's active site.

The ratio of CO2 to O2 concentration is critical.

Higher O2 or lower CO2 means more photorespiration.

Right.

And temperature plays a big role.

As temps rise, Rubusco's tendency to react with O2 increases more than its tendency to react with CO2.

Plus, CO2 becomes less soluble in water than O2 as it gets warmer.

A double whammy.

Triple, actually.

High temperatures often cause plants to partially close their stomata, the pores in their leaves, to save water.

But that also restricts CO2 entry, further lowering the internal CO2 -O2 ratio.

So hot conditions really favor photorespiration in many plants.

Absolutely, which is a major reason why photosynthetic efficiency can drop significantly in hot climates for standard or C3 plants.

It makes sense that this whole photorespiration pathway became more important as Earth's atmosphere changed, O2 going up, CO2 coming down over geological time.

Exactly.

Rubusco's oxygenase flaw was probably always there, but it only became a major liability needing this complex salvage pathway when oxygen levels rose significantly.

The enzymes involved are a patchwork, with origins tracing back to cyanobacteria and other bacterial ancestors.

Do cyanobacteria handle it differently?

They do, yes.

Their photorespiration pathway is simpler, happens entirely within their cell, uses different enzymes, and importantly avoids that costly release and refixation of ammonia.

Still releases CO2, though.

So photorespiration isn't just waste and salvage, it connects to other things.

Definitely.

It's tied into nitrogen metabolism, all those amino acids moving around, cellular redox state, that H2O2 production, maybe even signaling.

Plants engineered to lack parts of the pathway often show severe growth defects in normal air, highlighting its importance even if it seems wasteful.

Which leads to the idea of engineering it, making it better.

Exactly.

It's a huge target for crop improvement.

Since tweaking Rubusco itself has proven really hard, scientists are trying to introduce alternative, more efficient bypass pathways into plant chloroplasts.

Like borrowing pathways from bacteria.

Precisely.

Some approaches install bacterial enzymes that can convert glycolate directly to glycerate inside the chloroplast.

This releases the CO2 right next to Rubusco, giving it a good chance of being immediately refixed.

And it avoids all the costly transport and ammonia cycling between organelles.

And does it work?

Early results in model plants like Arabidopsis and even some crops are really promising.

These engineered plants can grow faster and produce more biomass, especially under conditions that favor photorespiration.

It shows real potential.

A fascinating application of understanding the biochemistry.

Okay, so photorespiration is a problem driven by Rubusco and atmospheric conditions.

This leads us to carbon concentrating mechanisms, or CCM's plants, fighting back.

That's a good way to put it.

As atmospheric CO2 dropped and O2 rose over millions of years, making photorespiration more problematic, some plants evolved these CCMs.

They're basically ways to actively pump and concentrate inorganic carbon, CO2 or bicarbonate, right around Rubusco.

Boosting the CO2 level locally to outcompete the oxygen.

Exactly.

They create a high CO2 environment specifically where the Calvin cycle is happening, minimizing that wasteful oxygenous reaction.

And the two big ones in land plants are C4 and CAM.

Right.

Both involve fixing CO2 initially with a different enzyme before handing it off to Rubusco.

Let's do C4 photosynthesis first.

This uses spatial separation.

Correct.

C4 is famous in highly productive plants like corn, sugarcane, sorghum.

It evolved independently many times.

The C4 name comes from the discovery that the first stable carbon products were four carbon acids like mallet and aspartate, not the three carbon, three PGA seen in most plants, which we call C3 plants.

And they have that special leaf structure.

Kranz Anatomy.

Yes, Kranz means wreath in German.

C4 leaves have two distinct photosynthetic cell types arranged in rings.

An outer layer of mesophyll cells surrounds an inner layer of bundle sheath cells, which themselves surround the vascular tissue, the veins.

Like a wreath around the vein.

Exactly.

This structure is key.

It separates the initial CO2 capture in the mesophyll from the final fixation by Rubusco in the bundle sheath.

Okay, walk us through the C4 cycle with these two cell types.

Right.

It's basically five stages coordinating between the cells.

One, fixation to mesophyll.

CO2 enters a mesophyll cell, gets converted to bicarbonate, H2O3.

Then an enzyme called PEPcase, phosphenolpyruvate carboxylase, attaches that bicarbonate to a three carbon molecule called PP.

This makes a four carbon acid oxaloacetate.

Critically, PEPcase loves bicarbonate and isn't bothered by oxygen at all.

So step one is grab CO2H2O3 efficiently in the outer cells using PEPcase.

Oxaloacetate is quickly converted to other four carbon acids, usually malate or aspartate.

Two, transport.

These four carbon acids are then transported from the mesophyll cell into a neighboring bundle sheath cell.

Shipping the captured carbon inward.

Yeah, got it.

Three, decarboxylation in bundle sheath.

Inside the bundle sheath cell, enzymes break down the four carbon acid releasing CO2.

This builds up a really high concentration of CO2 right there.

Creating that CO2 rich environment for Rubusco.

Exactly.

That concentrated CO2 is then fixed by Rubusco via the normal Calvin Benson cycle, which operates in the bundle sheath chloroplasts.

The high CO2 swamps the oxygen, massively reducing photorespiration.

Four,

transport.

The three carbon molecule left over after releasing CO2, like pyruvate, is transported back out to a mesophyll cell.

Five, regeneration in mesophyll.

Back in the mesophyll, another enzyme, PPDK, pyruvate phosphate tignase, uses ATP energy to convert that three carbon molecule back into PP, ready to grab another bicarbonate.

Ah, so there's an extra energy cost here in the C4 part too.

Yeah.

Regenerating PP.

There is.

That PPCK step is energetically quite expensive.

It effectively costs two ATP molecules for every CO2 molecule shuttled through this C4 pump.

That's on top of the three ATP and two NADPH needed by the Calvin cycle itself in the bundle sheath.

So C4 is actually more energy hungry per CO2 fix than C3.

Per CO2, yes.

But in hot, bright conditions where C3 plants lose a lot of carbon and energy to photorespiration, C4 plants come out way ahead because they avoid most of that waste.

The energy investment pays off.

Makes sense.

And these mesophyll and bundle sheath cells are specialized, right?

Different enzymes, maybe different chloroplasts?

Absolutely.

Pepcase is only in the mesophyll.

Rubisco, mainly in the bundle sheath.

The decarboxylating enzymes are in the bundle sheath.

Even chloroplast structure can differ.

With bundle sheath, chloroplasts sometimes having less of the water splitting part of the light reactions, PSII, since they need less NADPH relative to ATP compared to mesophyll.

Fascinating specialization.

And like the Calvin cycle, these C4 enzymes are regulated by light too.

Yes.

Key enzymes like Pepcase and PPDK are switched on by light dependent modifications, often phosphorylation, ensuring the C4 pump runs in sync with the Calvin cycle.

So the big advantage for C4 is minimizing photorespiration, especially when it's hot, and maybe saving water.

Exactly.

Because Pepcase is so good at grabbing HCO3, C4 plants can achieve high rates of photosynthesis even with their stomata partially closed.

This significantly improves water use efficiency, which is a huge plus in drier or hotter climates, where C3 plants would be struggling with both photorespiration and water loss.

That's why C4 grasses, for example, dominate tropical savannas.

Okay, that's C4 spatial separation.

Now, crassulation acid metabolism, CAM.

This one uses time, right?

Correct.

CAM is the strategy used by plants in really arid environments.

Cacti, succulents, pineapples, many orchids.

It's all about extreme water conservation.

Think thick leaves, waxy coatings, big internal water storage.

And the trick is opening stomata only at night.

That's the key.

At night, when it's cooler and more humid, SCIEM plants open their stomata and let CO2 in.

Just like in C4 mesophyll cells, Pepcase in the cytoplasm fixes the CO2, as by garbinate, onto peep, forming oxaloacetate, which is then usually reduced to malate, a four carbon acid.

But instead of shipping it to another cell.

They pump this malate into the huge central vacuole within the same cell.

All night long, they accumulate malic acid in the vacuole.

The cell sap becomes quite acidic by morning.

They're storing the CO2 captured overnight as acid.

Precisely.

They bank it.

Then, during the day, when it's hot and dry, they slam their stomata shut, tightly closed.

Saving water.

Maximally.

Then, they transport the stored malate out of the vacuole back into the cytoplasm.

Enzymes then decarboxylate the malate, releasing CO2 inside the cell, behind those closed stomata.

Creating a high CO2 concentration internally, just like in C4 bundlesheath cells.

Exactly the same principle.

This concentrated CO2 is then fixed by Rubisco, via the Calvin -Benson cycle running in the chloroplast during the day, powered by sunlight.

Photorespiration is suppressed because of the high internal CO2, and very little water is lost because the stomata are shut.

So, Pepcase works at night, stores acid.

Rubisco works during the day, using the stored acid CO2.

Temporal separation.

That's the essence of it.

It's a 24 -hour cycle.

And again, Pepcase activity is regulated, often high at night and low during the day, typically controlled by phosphorylation, linked to the plant's internal circadian rhythm.

It sounds like an extreme adaptation, but very effective for deserts.

Incredibly effective.

And CAM isn't always all or nothing.

Some plants are facultative CAM, meaning they might photosynthesize like a normal C3 plant when water is plentiful, but switch to CAM mode when drought or salt stress hits.

Very flexible.

Wow.

Okay, we've covered a lot of ground.

From the basic Calvin -Benson cycle, the engine fixing CO2, to the problem of photorespiration caused by Rubisco's little quirk.

And the elaborate salvage pathway plants evolved.

And then these amazing adaptations, C4 and CAM, using spatial or temporal separation to concentrate CO2 and beat photorespiration, especially under stress.

It really shows the incredible biochemical and anatomical plasticity of plants, evolving solutions to atmospheric changes and environmental challenges over millions of years.

And none of these cycles work in isolation, right?

They're all tightly linked to the light reactions for energy, transport systems moving molecules around, even nutrient metabolism like nitrogen.

Completely interconnected.

It's a whole system.

Understanding this stuff isn't just, you know, interesting plant biology.

It feels really relevant.

Agriculture, climate change.

Absolutely.

Knowing how C3, C4 and CAM plants work helps us predict how different crops or ecosystems might respond to rising CO2 levels or changing temperatures.

And it underpins those efforts we talked about to engineer photosynthesis for better yields.

Right.

So thinking about how finely tuned these systems are, C3 versus C4 versus CAM to current conditions,

it leaves you wondering, doesn't it?

Well,

as our climate changes, CO2 going up, but also more heat, maybe shifts in water availability, how does that affect the balance?

Will C4 plants always have the edge in the heat if CO2 is also higher, which might help C3 plants by reducing photorespiration somewhat?

How will CAM plants fare?

That's the multi -billion dollar question, really.

The interactions are complex.

And thinking about the future, how might our ability to actually engineer these pathways, like those photorespiration bypasses or maybe even aspects of C4 into C3 crops, play into feeding everyone on the changing planet?

Lots to think about there.

Indeed.

Plenty still to explore and understand.