Chapter 12: Respiration and Lipid Metabolism

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Welcome back to the Deep Dive.

Today we're really getting into the engine room of plants, how they generate power from stored fuel respiration, and how they pack away energy and carbon as fats and oils that's lipid metabolism.

That's right.

And our guide for this is a really solid chapter from Plant Physiology and Development, the sixth edition.

It lays it all out.

Yeah, it's comprehensive.

Most folks think plants, they think photosynthesis, right?

Capturing sunlight.

And that's huge, obviously.

But respiration, that's how they use the energy day and night to actually live and grow.

It's absolutely essential.

And lipids, they're more than just cooking ingredients for us.

For plants, they're super concentrated energy reserves and key structural parts.

Exactly.

So our mission today is to kind of unpack all that free.

We want to look at the core processes, see what's uniquely planty about them, how they're controlled, and why it all matters in the real world, all straight from the source material.

It's fascinating because plants do oxidize compounds for energy, much like animals, but they have these extra pathways,

these metabolic tricks up their sleeve that give them amazing flexibility.

So photosynthesis stocks the pantry, making sugars and stuff.

Respiration is raiding the pantry.

Pretty much.

It's carefully taking those stored compounds, breaking them down step by step, and releasing the energy needed for everything else, building walls, absorbing nutrients, you name it, and lipid metabolism.

That's like building the high energy survival rations, especially important for getting a seed started.

Okay.

Let's dive into that respiratory toolkit first.

What exactly is happening when a plant breathes, metabolically speaking?

Well, aerobic respiration, the main type we're talking about, is essentially controlled oxidation.

It's taking reduced organic compounds.

Sugars are key, but not the only ones reacting them with oxygen and breaking them down.

And the goal is?

Two main things.

Releasing energy, mostly captured as ATP,

and generating carbon skeletons.

These are the building blocks the plant uses to synthesize almost everything else it needs.

And you said it's not just sugars they burn?

No, definitely not.

Sucrose is the main sugar transported around, but they can tap into glucose, fructose, other sugars, organic acids, even intermediates, straight from photosynthesis, like trios, phosphates.

The source material has a figure 12 .1, I think, showing this web.

And yeah, if needed, they'll break down stored lipids and proteins too.

The overall reaction, sucrose plus oxygen, gives CO2 and water.

It feels like photosynthesis run backwards.

Conceptually, yes.

It's releasing the energy that photosynthesis stored.

And the amount of energy released is substantial, a really large negative delta G, meaning it's very favorable.

Which is why it can't happen all at once, right?

Like a tiny explosion.

Exactly.

That would cook the cell.

So plants, like us, use a series of carefully controlled steps organized into major pathways.

This allows them to capture that energy efficiently and safely.

What are those main pathways?

You've got four major players.

Glycolysis,

the oxidative pentose phosphate pathway, that's a bit of a mouthful.

We can call it OPPP, the citric acid cycle, often called the Krems cycle.

And finally, oxidative phosphorylation.

And importantly, they're not isolated.

They're constantly exchanging molecules.

And they shuffle electrons around using specific carriers.

They do.

Key players are NAD plus and FAD.

They accept electrons during oxidation steps, becoming NADH and FADH2.

Think of them as rechargeable batteries or electron taxis, carrying energy potential.

So the big ATP payout comes when these carriers unload their electrons.

That's the core of it.

The energy stored in NADH and FADH2 is released when they're oxidized by the electron transport chain.

This released energy, quite a bit per molecule, is then harnessed to make ATP.

The book mentions a theoretical yield, something like 60 ATP per sucrose, but the actual number can vary.

And we need to remember it's not just about ATP.

Crucial point.

These pathways are crossroads.

Intermediates get pulled out at various stages to build amino acids, nucleotides, lipids, pigments, basically all the complex molecules a plant needs.

So it's this constant balancing act.

Generate energy and provide building materials.

Okay, let's tackle the first big one.

Glycolysis.

The sugar splitting pathway.

Right.

Glycolysis literally means sugar splitting.

It takes carbohydrates, usually starting from sucrose or its products, converts them into hexose phosphates, splits those into three carbon triose phosphates, and eventually oxidizes these down to pyruvate.

Along the way, it makes a bit of ATP directly and captures some electrons in NADH.

Where does this happen?

Primarily in the cytosol, but plants are special.

They also have a parallel glycolytic pathway running inside their plastids.

And it's super important when oxygen is low.

Absolutely critical.

In waterlogged soils, for instance, oxygen can run out.

Glycolysis can still run, becoming the main source of ATP, though it needs fermentation pathways to recycle the NADH it produces.

Fermentation itself is very inefficient, energy -wise, but it keeps glycolysis ticking over.

You mentioned plant -specific features.

What makes their glycolysis different?

Several things.

The regulation is unique.

They use sucrose as the main input, not glucose, typically.

They have alternative enzymes that offer bypasses or different reactions, and like I said, that parallel pathway in plastids.

Plus, the end products aren't just pyruvate.

They can readily make mallet, too.

How does sucrose get into the game?

Two main routes.

One uses an enzyme called invertase to chop sucrose into glucose and fructose, which then get phosphorylated using ATP.

The other uses sucrose synthase, which splits sucrose using UDP to yield fructose and UDP glucose.

This UDP glucose can then be converted to glucose -1 -phosphate.

Does it matter which route?

Well, the invertase route is irreversible, while the sucrose synthase route is reversible.

The source mentioned studies with transgenic plants showing that while both contribute, there's some redundancy.

If one is knocked out, the other can often pick up the slack.

It provides metabolic flexibility.

Okay, so sugar gets in, gets phosphorylated, then what?

The initial phase uses ATP, actually 2 to 4 ATP per sucrose, to phosphorylate the sugars and then splits the 6 -carbon molecule into two 3 -carbon molecules, the triose phosphates.

There are a couple of irreversible steps here, like hexokinase and phosphofructokinase, PFK, which are important control points.

PFK is a classic control point, isn't it?

It is, but plants have another trick here.

Besides the standard ATP -dependent PFK, they also have a PPi -dependent PFK.

PPi is pyrophosphate.

This enzyme uses PPi instead of ATP, and its reaction is reversible.

It adds another layer of control and flexibility.

Studies suggest it contributes to the flow, but isn't absolutely essential if the ATP one is working.

Okay, after the split, we get the energy payoff phase.

Right.

The triose phosphates are oxidized.

This is where NAD plus gets reduced to NADH.

Phosphate is added, forming a high -energy compound, 1 -phospho -3 -bisphosphoglycerate.

This molecule has a phosphate group it really wants to donate.

And that leads to ATP.

Yes, through substrate -level phosphorylation.

An enzyme called phosphoglycerate kinase transfers that high -energy phosphate directly to ADP, making ATP.

Since you get two triose phosphates per hexose, you get ATP back here.

It happens again later, too.

Distinguish that from the oxidative phosphorylation we mentioned earlier.

Good point.

Substrate -level phosphorylation is a direct transfer of phosphate from a substrate molecule to ADP.

Oxidative phosphorylation is indirect.

It uses the energy released from electron transport to create a proton gradient, which then drives ATP synthesis.

Glycolysis makes ATP directly in these steps.

What happens next?

The molecule gets rearranged, water is removed, and you end up with phosphenolpyruvate, or PP.

PP is another really high -energy phosphate compound, extremely high -energy of hydrolysis.

Setting up another ATP -making step.

Exactly.

Pyruvate kinase transfers the phosphate from PATE to ADP, making more ATP again via substrate -level phosphorylation.

This is another key, irreversible, regulated step.

And this gives us pyruvate, the classic end product.

But you said plants can make malate.

Yes, this is a big difference.

Plants have an enzyme called PPP carboxylase, which is very active in the cytosol.

It takes PP and combines it with bicarbonate dissolved CO2 to make oxaloacetate, a four -carbon molecule.

Instead of making pyruvate.

It can happen alongside pyruvate formation, or sometimes instead of it, depending on conditions.

This oxaloacetate is then usually quickly reduced to malate using NADH.

This malate can be stored in the vacuole or transported into the mitochondria to feed the citric acid cycle.

So glycolysis can feed the mitochondria via pyruvate or malate.

Correct.

Experiments with plants engineered to have less pyruvate kinase showed they by using this PP carboxylasomalite wrap more heavily, it gives them options.

What about running glycolysis backwards, gluconeogenesis?

Yes, plants do that too.

It's essentially reversing many of the glycolytic steps to synthesize sugars from non -carbohydrate precursors, like organic acids or amino acid breakdown products.

This is absolutely vital, for instance, during seed germination in oilseed plants.

They break down stored fats into components that enter gluconeogenesis to make the sugars the seedling needs to grow before it can photosynthesize.

And I saw a note about enzymes clustering near mitochondria.

Yeah, an interesting observation.

Under conditions of high energy demand, some glycolytic enzymes seem to physically associate with the outer mitochondrial membrane.

The hypothesis is this might channel pyruvate directly into the mitochondrion, making energy production more efficient, and possibly keeping it separate from cytosolic pathways using glycolytic intermediates for biosynthesis.

It suggests a higher level of spatial organization than we might think.

Fascinating.

Okay, that's glycolysis covered.

What about the other sugar pathway, the oxidative pentose phosphate pathway, OPPP?

Right, the OPPP.

It's an alternative route for oxidizing glucose, specifically glucose 6 -phosphate.

It happens in both the cytosol and plastids, though the plastidial pathway is often dominant.

Is it mainly about breaking down sugar for energy too?

Not primarily.

While it does oxidize glucose, its main roles are different.

The initial steps are oxidative and irreversible.

Glucose 6 -phosphate is oxidized, a carbon is lost as CO2, and importantly, it generates two molecules of NADPH for each glucose 6 -phosphate entering.

NADPH, not NADH.

Correct.

NADPH is the key product here.

Its reducing power is used mainly for biosynthetic reactions building things rather than feeding the electron transport chain for ATP like NADH.

What kind of biosynthesis?

Lots of things.

Fatty acid synthesis relies heavily on NADPH, especially in plastids.

Nitrogen assimilation needs it.

It's also crucial for detoxification and dealing with oxidative stress -regenerating antioxidants like glutathione.

So the OPPP is a major source of NADPH, particularly in non -photosynthetic tissues or in chloroplasts when it's dark.

Does it contribute much to overall sugar breakdown?

Estimates vary.

Maybe 10 -25 % depending on the tissue and conditions.

Its activity can change a lot based on the plant's needs.

Any other important products besides NADPH?

Yes.

It also produces vital precursor molecules.

One is Arespros -4 -phosphate, which is needed to make aromatic compounds, think lignin for wood, flavonoids, pigments, protectants, and phytoalexins, defense compounds.

The pathway activity often increases when plants are wounded or attacked, likely to boost production of these defense molecules.

It also makes ribose -5 -phosphate, the precursor for nucleotides, DNA, RNA, and ATP itself, although the source notes that in plants, other routes might supply this too.

How is the OPPP controlled?

It's sensitive to the cell's redox state.

The very first enzyme, glucose -6 -phosphate dehydrogenase, is strongly inhibited if NADPH levels are high relative to NADP -plus all the ways.

Basically, if the cell already has plenty of reducing power in the form of NADPH, it slows down the pathway that makes more.

Makes sense, right?

Yeah, feedback inhibition.

What about light?

In chloroplasts, OPPP is generally inhibited in the light.

This is partly due to redox regulation via the ferredoxin -thyridoxin system, and also because photosynthesis itself generates a lot of NADPH.

The Calvin cycle running in the light actually provides intermediates that can run the reversible part of the OPPP backwards, allowing synthesis of erythrose -4 -phosphate even when the oxidative steps are off.

In non -green plastids, it's less sensitive to light.

Okay, so we've split sugars in glycolysis and generated NADPH and precursors in the OPPP.

Now we get to the citric acid cycle.

Indeed.

Also known as the Krebs cycle, or TCA cycle.

This is stage two of aerobic respiration occurring inside the mitochondria.

It takes the acetyl -CoA derived from pyruvate, which came from glycolysis, and completely oxidizes its acetyl group carbons to CO2.

Let's talk mitochondria for a second.

They're like mini power plants.

They really are.

Complex organelles.

They have their own DNA and ribosomes, so they're semi -autonomous.

Structurally, you have a smooth outer membrane that's pretty permeable, and then a highly folded inner membrane.

Those folds are called cristae.

The inner membrane is the real barrier, and where the electron transport chain will get to is located.

Inside the inner membrane is the matrix.

And the matrix is where the citric acid cycle happens.

Exactly.

The matrix is densely packed with enzymes, including all the citric acid cycle enzymes, plus mitochondrial DNA, ribosomes, and other molecules.

It's thought to be quite organized, maybe with enzymes forming complexes called metabolons to channel intermediates efficiently.

So pyruvate from glycolysis needs to get into the matrix first.

Yes.

It crosses the outer membrane easily, but needs a specific transporter protein to get across the impermeable inner membrane.

Once inside, it encounters the pyruvate dehydrogenase complex.

What does that do?

This large enzyme complex carries out a crucial step,

oxidative decarboxylation.

It removes one carbon from pyruvate as CO2, transfers electrons to NAD plus to make NADH, and attaches the remaining two -carbon acetyl group to coenzyme A, forming acetyl CoA.

And acetyl CoA is what enters the cycle proper.

Correct.

Acetyl CoA, two carbons, joins with oxaloacetate, four carbons, catalyzed by citrate synthase to form citrate, six carbons, hence the name citric acid cycle.

Citrate is then isomerized to isocitrate.

Then more oxidation and CO2 release.

Yep.

Two key steps involve oxidative decarboxylation.

Isocitrate is oxidized, releasing CO2 and producing NADH, yielding alpha -ketoglutarate, or two -oxoglutarate.

Then alpha -ketoglutarate is oxidized, releasing another CO2 and producing another NADH, yielding succinyl CoA.

So that's two more CO2s released per acetyl CoA.

So for one pyruvate that entered the mitochondria, we've now released all three of its carbons as CO2.

Exactly.

One CO2 from pyruvate dehydrogenase and two from the cycle itself.

What happens to the remaining four carbon molecules, succinyl CoA?

The energy in its thioester bond is used to make ATP directly another substrate -level phosphorylation step, catalyzed by succinyl CoA synthetase.

This produces succinate.

Interestingly, plants typically make ATP here, while animals make TTP.

Then succinate gets oxidized.

Yes, by succinate dehydrogenase.

This enzyme is unique because it's actually part of the inner mitochondrial membrane and is also complex, too, of the electron transport chain.

It oxidizes succinate to fumarate and transfers the electrons to FAD, making FADH2.

FADH2, not NADH here.

Right.

Then fumarate is hydrated to malate by fumarase.

And finally, malate is oxidized back to oxaloacetate by malate dehydrogenase, producing one more NADH.

This regenerates the oxaloacetate needed to accept the next acetyl CoA, completing the cycle.

So let's tally the mitochondrial output from one pyruvate.

Okay, inside the mitochondrion, three molecules of CO2 are released, four molecules of NADH are generated, one from pyruvate dehydrogenase, three from the cycle.

One molecule, FADH2, is made by succinate dehydrogenase complex 2, and one molecule of ATP is made directly by succinyl CoA synthetase.

You mentioned plant -specific features earlier.

Anything else in the cycle?

A key one is the mitochondrial malic enzyme.

We talked about malite coming from the cytosol

carboxylase.

Well, mitochondria also have this enzyme that can decarboxylate malate directly to pyruvate, releasing CO2 and making NADH or sometimes NADPH.

Why is that useful?

It provides another way to feed carbon into the cycle or even bypass parts of it.

It allows plants to metabolize malite completely, which is important for balancing organic acid levels, like during fruit ripening.

It adds another layer of metabolic flexibility.

It means the cycle can run, even if intermediates are being pulled out for other things.

Which brings us to anaplerotic reactions.

Yes, that's the term for reactions that replenish cycle intermediates when they're withdrawn for biosynthesis.

For example, alpha -ketoglutarate is the starting point for glutamate and other amino acids.

If lots of it is pulled out, the cycle could stall.

The PEP carboxylase pathway -making malite is a major anaplerotic route in plants, feeding carbon back into the cycle.

Okay, we've fully oxidized the carbon, we've generated ATP directly, and a lot of NADH and FADH2.

Now, the big energy payoff.

Mitochondrial electron transport and ATP synthesis.

This is where the energy temporarily stored in NADH and FADH2 gets converted into the bulk of the ATP.

It's called oxidative phosphorylation because the oxidation of these carriers is coupled to the phosphorylation of ADP to ATP using oxygen as the final electron acceptor.

And it all happens on that inner mitochondrial membrane.

Electron transport chain, ETC.

It's a series of multi -protein complexes embedded in the inner membrane.

Think of them as stepping stones for electrons.

Electrons from NADH and FADH2 are passed sequentially along the chain, moving from higher energy carriers to lower energy ones.

And energy is released at each step.

Exactly.

And crucially, three of these complexes, complex 1, complex 3, and complex 4, use that released energy to pump protons, H +, from the mitochondrial matrix across the inner membrane into the intermembrane space.

Creating a proton gradient.

Precisely.

This pumping action builds up a higher concentration of protons in the intermembrane space than in the matrix.

This creates an electrochemical gradient, a difference in charge and concentration across the intermembrane.

This gradient is called the proton motive force, and it represents stored energy, like water, behind a dam.

Walk us through the complexes.

Where do NADH and FADH2 drop off their electrons?

Okay.

NADH delivers its electrons to complex I, NADH dehydrogenase.

As electrons pass through complex 1, protons are pumped.

Electrons are then transferred to a small, mobile lipid carrier in the membrane called ubiquinone, or coenzyme Q.

What about FADH2?

FADH2, generated by succinate dehydrogenase, which is complex 2, transfers its electrons directly to ubiquinone as well.

Importantly, complex 2 itself does not pump protons.

So, ubiquinone collects electrons from both complex 1 and complex 2?

Yes.

The reduced ubiquinone, ubiquinol, then diffuses through the membrane and delivers electrons to complex 3, the cytochrome BC1 complex.

As electrons move through complex 3, more protons are pumped across the membrane.

And from complex 3?

Electrons are passed to another mobile carrier, a small protein called cytochrome C, located in the intermembrane space.

Cytochrome C then ferries electrons to complex 4, cytochrome C oxidase.

The final step.

Complex 4 is the terminal oxidase.

It accepts the electrons from cytochrome C and catalyzes the final reaction, the reduction of molecular oxygen O2 to water, H2O.

This step is vital.

It consumes the oxygen we breathe.

And complex 4 also pumps protons across the membrane.

So, complexes 1, 3, and 4 are the proton pumps?

Correct.

They establish the proton motive force.

You mentioned similarities to photosynthesis earlier.

Yeah, ubiquinone and complex 3 in mitochondria are functionally quite similar to plastoquinone and the cytochrome B6F complex in chloroplasts.

Both involve quinone carriers and a cytochrome complex that pumps protons using a q -cycle mechanism.

Nature reuses good designs.

But plants have extra bits in their ETC, right?

The alternative pathways.

They do.

This is super important for plants.

Besides the main complexes, their inner mitochondrial membrane contains additional NADH dehydrogenases and oxidases that offer bypass routes.

A key feature is that these alternative pathways generally do not pump protons.

Meaning the energy isn't conserved as ATP.

Exactly.

When electrons flow through these rats, the energy released during electron transport is primarily dissipated as heat instead of being stored in the proton gradient.

What are these bypasses?

There are alternative NADH dehydrogenases that can accept electrons from NADH, both in the matrix and from the cytosol side, and pass them to ubiquinone by passing complex I.

And then there's the very famous alternative oxidase, or AOX.

AOX.

We mentioned it for heat generation.

Yes.

In thermogenic plants like the voodoo lily, AOX activity skyrockets, burning fuel rapidly just to generate heat to attract pollinators.

It's dramatic.

But its main role isn't usually making heat.

Probably not in most tissues.

AOX provides a bypass around complexes 3 and 4.

It takes electrons directly from the ubiquinone pool and transfers them to oxygen, making water.

Since it bypasses two proton -pumping sites, 3 and 4, the energy yield is much lower, essentially zero ATP from this branch.

Why have it then?

Seems wasteful.

It's about flexibility and safety.

Imagine a situation where the cell has plenty of carbohydrates and reducing power, NADH, FADH2.

But maybe ATP demand is low because growth is slow.

The main ETC could get backed up, over -reduced.

And that's dangerous.

An over -reduced ETC, especially at complex ion 3, can leak electrons directly to oxygen, creating harmful reactive oxygen species, ROS, superoxide, hydrogen peroxide, which can damage cellular components.

So AOX acts like a safety valve.

Exactly.

By providing an alternative route for electrons to flow to oxygen, AOX can drain electrons from the ubiquinone pool, preventing the main chain from becoming over -reduced and minimizing ROS production.

It's induced by many stresses, cold, drought, pathogen attack conditions where metabolic balance might be perturbed.

It sacrifices maximum ATP efficiency for stability and stress tolerance.

Are there other non -proton pumping pathways?

Yes.

Plants also have uncoupling proteins, UCPs, in their inner membrane.

These are similar to those found in mammalian brown fat.

They essentially create a regulated leak for protons, allowing them to flow back into the matrix without going through the ATP synthase.

Also releasing energy as heat and reducing ATP yield.

Like AOX, UCPs help dissipate the proton gradient, reduce the risk of ROS formation when the gradient is too high, and contribute to metabolic flexibility.

They are also often induced by stress.

So these alternative routes, AOX, UCPs, alternative NADHD hydrogenases, give plants ways to manage electron flow, balance redox state, and cope with stress, even at the cost of lower ATP production.

That's the key takeaway.

Plants seem to prioritize metabolic flexibility and safety over maximizing energy efficiency in many situations, likely because they often have abundant energy from photosynthesis anyway.

Okay, so the proton gradient is built up with or without these bypasses.

How does that gradient actually make ATP?

Through the action of complex V, the FF1 ATP synthase.

This is a remarkable molecular machine, also embedded in the inner membrane.

It acts like a turbine.

How so?

The proton motive force drives protons to flow back down their concentration gradient from the inner membrane space into the matrix.

But they can only do this through a specific channel in the foe part of the ATP synthase.

This flow of protons causes a part of the enzyme to rotate.

A literal rotation.

Yes, a physical rotation.

This rotation is transmitted to the F1 part of the enzyme, which protrudes into the matrix.

The rotating F1 subunit causes conformational changes in the catalytic sites, which drives the synthesis of ATP from ADP and inorganic phosphate, pi.

So the proton flow powers the ATP synthesis machine.

That's the chemiosmotic theory.

Peter Mitchell's chemiosmotic theory.

The energy from electron transport is temporarily stored as a proton gradient, and that gradient energy is then used by ATP synthase to make ATP.

It also means ATP synthesis is tightly coupled to electron transport if there's no proton gradient.

For example, due to uncouplers poking holes in the membrane, ATP synthesis stops, even if electrons are flowing.

And the gradient drives other transports, too.

Moving ATP into the matrix and ATP out, moving phosphate in, moving pyruvate in these, rely on specific transporter proteins in the inner membrane, and many of them utilize the proton motive force, either the charge difference or the pH difference, to power the transport.

Getting ATT out costs a bit of the gradient energy, too.

So adding it all up, how much ATP do you get per sucrose, realistically?

Well, the theoretical maximum, assuming everything flows through the main proton pumping pathways and accounting for transport costs, is estimated in the source book to be 60 ATP per sucrose molecule.

That's way, way more than the 2 or 4 ATP you net from glycolysis alone or via fermentation.

Aerobic respiration captures a significant chunk, maybe around 40 -50 % of the energy stored in that sugar molecule.

Let's touch briefly on the genetics and regulation.

Mitochondria have their own DNA.

They do, but it's a relatively small genome compared to the nucleus.

It encodes some essential subunits of the respiratory complexes, IV, and proteins for making cytochromes, but the vast majority of mitochondrial proteins, hundreds of them, including all the citric acid cycle enzymes, the alternative pathway proteins, transporters are encoded by nuclear genes made in the cytosol and then imported into the mitochondrion.

So it requires coordination between two genomes.

Absolutely.

And it's complex.

How the expression of nuclear and mitochondrial genes is coordinated isn't fully understood.

Interestingly, natural mutations or rearrangements in plant DNA can sometimes lead to defects, like cytoplasmic male sterility, CMS, where pollen development fails.

This is actually exploited in agriculture to create hybrid varieties more easily.

How is all this respiratory activity controlled in the short term?

It's regulated at multiple levels.

A key factor is the availability of ADP and phosphate.

If ATP levels are high and ADP is low, ATT synthase slows down.

This causes the proton gradient to build up, which in turn slows down the electron transport chain because it becomes harder to pump more protons.

This is often called bottom -up control.

The demand for ATP pulls the process along.

What about controlling the input?

Yes, key enzymes are regulated.

Pyruvate dehydrogenase, the gatekeeper for acetyl CoA entering the cycle, is regulated by phosphorylation.

It's switched off when phosphorylated.

High levels of NADH and ATP promote this phosphorylation, slowing things down when energy is abundant.

Conversely, high pyruvate inhibits the kinase that phosphorylates it, keeping PDH active when substrate is available.

And within the citric acid cycle?

High levels of NADH can directly inhibit key enzymes like isocitrate dehydrogenase and alpha -ketoglutarate dehydrogenase.

Also, intermediates can exert feedback control.

For example, high levels of citrate in the cytosol can inhibit phosphofructokinase back in glycolysis, slowing the whole supply line down.

It's an integrated system.

And don't forget the connection to other pathways.

Respiration isn't isolated.

Glycolysis, OPPP, and the citric acid cycle provide essential carbon skeletons for synthesizing amino acids, lipids, pigments, hormones, pretty much everything.

A lot of the carbon entering these pathways gets diverted for biosynthesis rather than being fully oxidized for ATP.

The whole thing is embedded within the cell's larger metabolic network.

Okay, let's bring this out to the whole plant level.

How does respiration vary in different parts of the plant, or under different conditions?

Rates vary enormously.

Growing tissues like root tips or developing buds have incredibly high respiration rates because they need lots of ATP and building blocks.

Mature tissues generally have lower maintenance respiration.

Rates depend on species, age, and critically the environment, light, temperature, oxygen availability, water, nutrients.

Does respiration stop when photosynthesis is happening?

Not at all.

Mitochondria are definitely active in the light.

They're needed to supply ATP to the cytosol for various processes, and crucially, they provide carbon skeletons like alpha -ketoglutarate, which is essential for assimilating nitrogen coming from nitrate reduction, which happens in the light.

Also, photorespiration, a process linked to photosynthesis, actually feeds metabolites into the mitochondria in the light, generating NADH there.

There is a lot of interplay and metabolite exchange between chloroplasts and mitochondria during the day.

What about things like fruit ripening?

Many fruits, like apples, bananas, tomatoes, show a characteristic surge in respiration called the climacteric rise as they ripen.

This is associated with major metabolic changes, starch or acids being converted to sugars, texture changes, aroma production, and it's often triggered by the hormone ethylene.

Interestingly, the alternative oxidase pathway, AOX activity, often increases during this climacteric phase.

How do environmental factors like oxygen and temperature affect the rate?

Oxygen is absolutely required as the final electron acceptor.

While the enzyme using it, complex IV, is very efficient, oxygen diffusion into tissues can become limiting, especially in bulky organs like potato tubers, dense seeds, or in waterlogged soils where oxygen levels plummet.

Plants adapted to wet soils often have specialized tissues, aranchyma or structures, pneumatophores and mangroves, to get air down to the roots.

Respiration rates generally increase with temperature, roughly doubling for every 10 degrees C rise, that's the Q10 concept, up to an optimum usually around 30, 40 degrees C, after which rates decline as enzymes start to denature.

This temperature dependence is why refrigeration works to preserve fruits and vegetables.

It dramatically slows down the respiration and spoilage.

But there are complexities like low temperature sweetening in potatoes where cold storage boosts sugar levels.

What about CO2?

We hear a lot about rising atmospheric CO2.

High CO2 concentrations, like the 3 -5 % sometimes used in controlled atmosphere storage, can directly inhibit respiration to some extent.

The effect of the current rise in atmospheric CO2, around 420 ppm now projected to go much higher, is less clear.

Lab studies often don't show strong direct inhibition at, say, 700 ppm.

However, some studies in whole ecosystems suggest that plants grown under elevated CO2 might respire less per unit of biomass, but the mechanisms and long -term implications for the global carbon cycle are still being actively researched and debated.

It's a complex picture.

OK, massive topic, respiration.

Let's switch gears now to the other major focus, lipid metabolism,

storing energy and building structures with fats and oils.

Right, lipids are vital for plants.

We mostly think of them as energy storage, the oils and seeds like sunflower or canola or fats in fruits like avocado.

But they are also the fundamental building blocks of all cell membranes.

Why use lipids for storage instead of just starch?

Energy density.

Lipids are much more reduced chemically than carbohydrates, meaning they store significantly more energy per gram, about 40 kilojoules per gram compared to roughly 16 for starch.

This makes them a very compact way to store fuel, perfect for a small seed that needs to pack enough energy to get established.

The trade -off is that synthesizing lipids costs more energy up front.

Besides storage fats, trisoclycerols, what other lipids are key?

The big ones are the structural lipids.

Phospholipids and glycolipids.

These are the main components of membranes.

They have that characteristic structure, a polar head group and non -polar fatty acid tails that allows them to form bilayers.

Plants also make waxes for the cuticle -covering leaves and other lipid -derived molecules like sterols and carotenoids.

Tell us more about the storage lipids, the trisoclycerols.

These are basically composed of a glycerol molecule with three fatty acids attached, esterified to it.

The fatty acids are typically long chains, usually 16 or 18 carbons, and they can be saturated.

No double bonds like steric acid or unsaturated.

One or more double bonds like oleic or linoleic acid, whether it's an oil, liquid at room temp, or fat, solid, depends on the degree of saturation.

More double bonds means more kinks, lower melting point, has liquid oil.

Where are these stored in the seed?

Inside specialized organelles called oil bodies, sometimes called spherosomes or oleosomes, These are unique structures found in the cytoplasm of storage cells, like in the cotyledons or endosperm.

They consist of a core of trisoclycerol surrounded by just a single layer, a monolayer of phospholipids, embedded with specific proteins called oleosins.

A monolayer, not a bilayer.

Yes, just half a bilayer.

The hydrophobic fatty acid tails of the phospholipids face inwards towards the oil, and the polar heads face outwards to the cytosol.

The oleosins stabilize the structure and prevent the oil bodies from fusing together into one giant unmanageable blob.

They actually form by butting off the endoplasmic reticulum, ER, as lipids accumulate between the two layers of the ER membrane.

What are the membrane lipids?

These are the polar glycerolipids.

They form the basic structure of all cell and organelle membranes.

They are amphipathic, having both a water -loving polar head, and water -fearing hydrophobic tails part.

This allows them to spontaneously form bilayers in water, creating a barrier.

There are two main categories based on the head group.

Glyceroglycolipids, which have sugar head groups, and glycerophospholipids, which have phosphate -containing head groups.

Do different membranes have different lipids?

Very much so.

Chloroplast membranes, for example, are dominated by glyceroglycolipids.

They make up about 70 % of total leaf lipids.

Other membranes, like the plasma membrane or ER membrane, primarily use glycerophospholipids.

The specific mix of lipids helps define the properties and function of each membrane.

How are the fatty acid building blocks themselves made?

Fatty acid synthesis is a fastening process.

In plants, it occurs primarily inside the plastids, unlike animals, where it's mainly cytosolic.

It's a cyclic process where two carbon units derived from acetyl -CoA are repeatedly added to a growing fatty acid chain.

Is there a carrier molecule involved?

Yes.

The growing chain is attached to a small protein called acyl carrier protein, or ACP.

All the intermediates are attached to ACP.

The enzymes involved are thought to form a multi -enzyme complex called fatty acid synthase, which helps channel the intermediates efficiently from one reaction to the next.

What's the committed step?

The first committed step is the synthesis of malonyl -CoA from acetyl -CoA and bicarbonate CO2, catalyzed by acetyl -CoA carboxylase.

This is a major regulatory point controlling the overall rate of fatty acid synthesis.

And the cycle itself.

Ammonyl -CoA gets transferred to ACP, making malonyl -ACP.

Acetyl -CoA also provides the initial two carbons.

These condense, then the keto group is reduced, water is removed, and the double bond is reduced, all while attached to ACP.

This adds two carbons.

The cycle repeats, adding two carbons at a time from malonyl -ACP until you typically reach 16 or 18 carbons.

What are the main products coming out of the plastid?

Primarily palmitoyl -ACP, 16 .0, and stear -oil -ACP, 18 .00, which can be desaturated right there in the plastid to form oleoil -ACP, 18 .1.

These acyl -ACPs are then usually cleaved, and the fatty acids are exported from the plastid, typically as acyl -CoA esters, to be used elsewhere.

Can they be modified further, like adding more double bonds?

Yes.

Further desaturation often happens after the fatty acid has been incorporated into a lipid, like phosphatidylcholine, PC, in the ER membrane, or monogalactosyldiacylglycerol, MgDG, in the chloroplast.

Specific desaturase enzymes located in the ER or chloroplast introduce double bonds at precise positions to create fatty acids like linoleic acid, 18 .2, and linoleic acid, 18 .3.

So building the final lipids involves both plastids and the ER?

Absolutely.

It's a cooperative effort.

The basic fatty acids are made in the plastid.

They are then exported and used for assembling complex lipids in both the plastid itself, mainly chloroplast membrane lipids like MgDG, DgDG, and PG, and very significantly in the ER membrane, where phospholipids like PC, PE, PI, and the storage triacylglycerols are made.

How does that assembly work?

The general starting point is glycerol -3 -phosphate.

Fatty acids, either from acyl -ACP in the plastid or acyl -CoA in the ER, are attached to the first two positions of the glycerol backbone, forming phosphatidic acid, Pa.

Pa is a central intermediate.

From Pa, different head groups can be added or the phosphate removed to make diacylglycerol, which is then used to make other lipids.

There are prokaryotic and eukaryotic pathways.

Yes, reflecting the evolutionary origins.

The prokaryotic pathway happens entirely within the plastid, using acyl -ACPs directly, to make lipids like PG and some of the galactolipids.

The eukaryotic pathway involves exporting fatty acids from the plastid to the ER, where they are incorporated as acyl -CoAs, into lipids like PC and PE.

These ER -synthesized lipids can then sometimes be modified and even transported back to the chloroplast to contribute to chloroplast membranes.

The relative contribution of these two pathways varies between plant species.

And finally, putting it all together to make those storage triacylglycerols in oilseeds.

Right.

The fatty acids exported from the plastid as acyl -CoAs are used in the ER.

Enzymes called siltransferases add these fatty acids onto glycerol -3 -phosphate and then onto DAG, diacylglycerol, to finally produce triacylglycerol, TAG.

This TAG accumulates within the layers of the ER membrane and, as we said, buds off to form the oil bodies, ready for storage and eventual mobilization during germination.

Wow.

Okay, that was a really comprehensive journey from breaking down sugars to building fats.

It really covers the core energy and carbon handling of the plant cell, doesn't it?

We went from glycolysis and the OPPP splitting sugars and providing NADPH through the citric acid cycle, fully oxidizing carbons in the mitochondria, to the electron transport chain capturing that energy as ATP, always noting those unique plant features like the alternative respiratory pathways.

And we saw how tightly regulated it all is responding to energy demand, redox, state, stress, and the need for building blocks.

Plus how lipids are synthesized through that intricate dance between plastids and the ER, serving as both vital energy stores and essential membrane components.

Hopefully this deep dive gives you a clearer picture of these fundamental processes.

This isn't just textbook stuff.

It's the machinery that allows plants to grow, survive environmental challenges, produce the food we eat, and shape entire ecosystems.

Understanding this metabolism is key to understanding plant life.

Absolutely.

And maybe a final thought to leave you with.

We kept seeing how plants use these alternative, sometimes less energy efficient pathways like AOX or UCPs.

They seem willing to sacrifice maximum ATP yield for metabolic flexibility and stress resilience.

It makes you wonder, what other subtle, perhaps unexpected, metabolic tradeoffs are plants constantly making to navigate their complex world tradeoffs that impact their survival, their interactions, and ultimately our planet?

Something to think about.