Glycerol Phosphate Shuttle & Aspartate - Malate Shuttle

Glycerol Phosphate Shuttle & Aspartate - Malate Shuttle

The names Glycerol Phosphate Shuttle and Aspartate–Malate Shuttle describe how electrons from cytosolic NADH are transported into the mitochondria. Understanding the meaning of each word makes the mechanism easier.

1. Glycerol Phosphate Shuttle Word-by-word meaning

Glycerol

• from Greek glykys (γλυκύς) = sweet

• glycerol is a 3-carbon alcohol backbone found in fats and phospholipids.

Phosphate

• from Greek phōs (φῶς) = light

• phoros = bearer
  Originally referring to phosphorus compounds that glow.
  In biochemistry it means a PO₄³⁻ group attached to a molecule.

Shuttle

• English word meaning something that moves back and forth carrying something (like a weaving shuttle).

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Meaning of the whole term

Glycerol-phosphate shuttle

= a transport system that uses glycerol-3-phosphate to carry electrons into the mitochondria.

Simple idea:

Cytosolic NADH
      ↓
glycerol-3-phosphate carries electrons
      ↓
mitochondria
      ↓
electron transport chain

It is common in:

• brain

• skeletal muscle

2. Malate–Aspartate Shuttle Word-by-word meaning

Malate

• from Latin malum = apple
  Malic acid was first isolated from apples.

Aspartate

• derived from asparagine, which was first isolated from asparagus.

Shuttle

• again means a carrier moving molecules back and forth.

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Meaning of the whole term

Malate–Aspartate shuttle

= a transport system that uses malate and aspartate to transfer electrons from NADH into mitochondria.

Simple idea:

Cytosolic NADH
      ↓
oxaloacetate → malate
      ↓
malate enters mitochondria
      ↓
electrons transferred to mitochondrial NADH

This shuttle is common in:

• liver

• heart

• kidney

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3. Why these shuttles exist

The inner mitochondrial membrane cannot transport NADH directly

So cells move the electrons, not the NADH molecule.

Cytosolic NADH
      ↓
Shuttle system
      ↓
Mitochondrial NADH
      ↓
Electron transport chain
      ↓
ATP production

4. Key difference

Shuttle	Molecules used	ATP yield
Glycerol-phosphate shuttle	glycerol-3-phosphate	less ATP
Malate-aspartate shuttle	malate & aspartate	more ATP

✅ Simple summary

• Glycerol-phosphate shuttle: uses glycerol-3-phosphate to carry electrons into mitochondria.

• Malate-aspartate shuttle: uses malate and aspartate to carry electrons into mitochondria.

Both are electron transport systems that allow glycolysis NADH to contribute to ATP production

why use the shuttle to transfer electrons? Here, we will explain how the nictotinamides in the reduced form or NADH that is formed during glycolysis needs to circumvent the transfer of electrons from the cytoplasm into the mitochondria.

1. when electron carriers are produced in the glycolysis, the reduced form of those electron carriers need to move the electrons now into the mitochondria.

2. An important concept is that NADH that is produced in the cytoplasm CANNOT be transported directly into the mitochondria.

This is different and in contrast with the NADH produced in the krebs cycle, which happens in the mitochondrial matrix.

-NADH from glycolysis cannot be transported directly into the mitochondria, but NADPH produced in the kreb’s cycle is already in the mitochondrial matrix.

Electrons are ultimately needed for cellular respiration, but produced in the cytoplasm, yet needed for mitochondrial electron transport chain system.

Therefore, those electrons cannot reach directly and use for the electron transport chain system.

There is no direct mitochondrial transporters for the exchange, especially for the NADH or the FADH two production

ATP from Cytosolic NADH

production of NADH in the kreb cycle happens in the mitochondria, becoming available for cellular respiration.

When it’s produced in the mitochondria, both NADH and FADH2 are readily available for the oxidative phosphorylation.

However, if the NADH is producing the cytoplasm, there needs to be a system that moves those electrons into the mitochondria.

Therefore, the way to circumvent this is by using one of the two shunts. This is where we’re going to contrast the fate of the mechanisms by which electrons from NADH have been produced via glycolysis are moved into the mitochondria for ATP production.

bottom line

NADH generated by the glycolysis leads to synthesis of 2 ATPs if the glycerol phosphate shuttle is used

NADH generated by Glycolysis leads to the synthesis of 3 ATPs if the Malate Aspartate Shuttle is used.

cells from different tissue types use one shunt, or the other, but not both. For example, malate aspartate shuttle is used in highly active tissues, including the muscle and the brain.

number of ATPs from total oxidation of a glucose molecule

the total yield for ATP production will depend on which shunt is used.

1. Starting from glycolysis, you can generate two ATPs directly. This is a fixed amount, but the two NADH that are produced will vary in the yield.

This yield will range 2-3 ATPs per NADH for a total of 4 to 6 ATPs. this is why you notice the two asterisk.

Following this breakdown, you have two pyruvates form two acetyl-CoAs and two NADH form six ATPs.

Now in the Kreb cycle, 2 acetyl-CoA → 6 NADH + 2 FADH2 + 2 GTP= 24 ATP

summed together, all of this will give a yield of 36 to 38 ATPs per glucose molecule.

depends on shuttle used

This slide is explaining why the total ATP yield from one glucose molecule can be either 36 or 38 ATP. The difference comes from which shuttle system transfers electrons from glycolysis into the mitochondria.

Let’s build the puzzle step-by-step.

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1. Glycolysis (cytosol)

From 1 glucose → 2 pyruvate

Products:

• 2 ATP (directly made)

• 2 NADH

The ATP is fixed:
✔ always 2 ATP

But the NADH is produced in the cytosol, and NADH cannot cross the inner mitochondrial membrane.

So the electrons must be transported using a shuttle system.

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2. Why the shuttle matters

The electrons from cytosolic NADH can enter the mitochondria using:

Malate–Aspartate Shuttle

Electrons enter the ETC as NADH

Yield:

1 NADH → ~3 ATP (older calculation)

So:

2 NADH × 3 ATP = 6 ATP

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Glycerol-3-phosphate Shuttle

Electrons enter the ETC as FADH₂

Yield:

1 NADH equivalent → ~2 ATP

So:

2 NADH × 2 ATP = 4 ATP

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Therefore glycolysis yields

Component	ATP
Direct ATP	2
NADH via malate shuttle	6
NADH via glycerol shuttle	4

Total glycolysis ATP:

6–8 ATP

That is why the slide shows:

6 or 8 ATP

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3. Pyruvate → Acetyl-CoA (Pyruvate Dehydrogenase)

Each pyruvate produces:

• 1 NADH

Since glucose produces 2 pyruvate:

2 NADH → 6 ATP

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4. Krebs Cycle (TCA cycle)

From 2 acetyl-CoA

Products:

• 6 NADH

• 2 FADH₂

• 2 GTP (ATP equivalent)

ATP equivalents:

Molecule	ATP produced
6 NADH	18 ATP
2 FADH₂	4 ATP
2 GTP	2 ATP

Total:

24 ATP

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5. Total ATP yield If Malate–Aspartate shuttle is used

Glycolysis → 8 ATP
Pyruvate → 6 ATP
TCA cycle → 24 ATP

Total:

38 ATP

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If Glycerol-3-phosphate shuttle is used

Glycolysis → 6 ATP
Pyruvate → 6 ATP
TCA cycle → 24 ATP

Total:

36 ATP

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Final idea

The difference (36 vs 38 ATP) happens because:

Cytosolic NADH from glycolysis
↓
Must enter mitochondria
↓
Uses a shuttle
↓
Different shuttle = different ATP yield

Shuttle	ATP produced from glycolysis NADH
Malate–Aspartate	more ATP
Glycerol-phosphate	less ATP

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✅ Simple summary

The total ATP from glucose is 36–38 ATP because the two NADH produced in glycolysis must enter the mitochondria through a shuttle, and the type of shuttle determines how much ATP those electrons produce.

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If you want, I can also show you the modern calculation (30–32 ATP) that most medical schools use today, which fixes a lot of the confusing numbers in this slide.

as an overview of the glycerol phosphate shuttle, when we start to look at the cytoplasm and compare that to the mitochondria. we start from the left and look at glycolysis producing that NADH at the bottom.

That NADH will serve as the coenzyme for an enzyme known as the glycerol triphosphate dehydrogenase .

in short, the glycerol-3-phosphate dehydrogenase reduces dihydroxyacetone phosphate (a glycotic intermediate), and converts it to glycerol three phosphate.

now that glycerol 3 phosphate has the ability to translocate to the mitochondria, when it’s converted back to the hydrocetone phosphate. This hydrocetone phosphate returns then to the cytoplasm.

Notice that in the mitochondria, the glycerol-3-phosphate dehydrogenase, opposes the direction of the reaction seen in the cytoplasm. what’s important here is that for the mitochondrial glycerol-3-phosphate dehydrogenase, the coenzyme is not another NADH molecule but rather a flaming adenine dinucleotype or FAD. This is the oxidized form, this is the substrate that is uses and forms FADH2, which is the reduced form. The fadh2 subsequently enters the electron transport chain system, transferring the electrons, and reconverting to FAD.

This shuttle is confusing at first, but the idea behind it is actually simple. Think of it as a delivery system for electrons from the cytoplasm into the mitochondria.

I’ll walk through it step-by-step in plain language.

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Big Idea (One Sentence)

The glycerol phosphate shuttle moves the electrons from NADH (made in glycolysis) into the mitochondria so they can be used to make ATP.

Why is this needed?

➡ NADH cannot cross the mitochondrial membrane.
So the cell transfers the electrons instead of the molecule itself.

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Step-by-Step Explanation 1. Glycolysis makes NADH in the cytoplasm

During glycolysis:

Glucose → Pyruvate

This produces:

• 2 NADH

These NADH molecules contain high-energy electrons that must go to the electron transport chain (ETC) to make ATP.

⚠ Problem:
NADH cannot enter the mitochondria.

So the cell needs a shuttle system.

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2. Cytoplasmic enzyme transfers electrons to DHAP

The enzyme:

Cytosolic glycerol-3-phosphate dehydrogenase

uses the NADH electrons.

Reaction:

DHAP + NADH → Glycerol-3-phosphate + NAD⁺

What happens here?

• NADH gives its electrons

• DHAP accepts them

Result:

DHAP becomes glycerol-3-phosphate

This regenerates NAD⁺, which glycolysis needs to continue.

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3. Glycerol-3-phosphate moves to the mitochondria

Now:

Glycerol-3-phosphate travels to the outer surface of the inner mitochondrial membrane.

There is another enzyme there.

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4. Mitochondrial enzyme removes the electrons

Enzyme:

Mitochondrial glycerol-3-phosphate dehydrogenase

This enzyme uses FAD instead of NAD⁺.

Reaction:

Glycerol-3-phosphate + FAD → DHAP + FADH₂

So:

• glycerol-3-phosphate loses electrons

• FAD gains electrons

Result:

FADH₂

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5. FADH₂ enters the electron transport chain

FADH₂ gives its electrons to the ETC.

But it enters at Complex II level.

Because of that:

• less ATP is produced

Each NADH from glycolysis → ~1.5 ATP instead of 2.5 ATP

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6. DHAP returns to the cytoplasm

The molecule becomes DHAP again.

DHAP goes back to the cytoplasm and the cycle repeats.

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Visual Flow (Simplified)

Cytoplasm:

NADH
↓
gives electrons to DHAP
↓
DHAP → Glycerol-3-phosphate

⬇ moves to mitochondria ⬇

Mitochondria:

Glycerol-3-phosphate
↓
gives electrons to FAD
↓
FAD → FADH₂
↓
Electron Transport Chain → ATP

DHAP returns to cytoplasm.

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Why This Shuttle Exists

Two main reasons:

1⃣ NADH cannot cross the mitochondrial membrane
2⃣ Glycolysis must regenerate NAD⁺

This shuttle solves both problems.

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Why It Produces Less ATP

Because the electrons end up on:

FADH₂ instead of NADH

Electron entry point:

Molecule	ETC Entry	ATP
NADH	Complex I	~2.5 ATP
FADH₂	Complex II	~1.5 ATP

So this shuttle costs about 1 ATP per NADH.

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Where This Shuttle Is Used

Common in:

• Brain

• Skeletal muscle

Other tissues (heart, liver) prefer the malate-aspartate shuttle.

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Simple Analogy

Think of this like a bucket brigade passing water.

You cannot move the bucket (NADH) across the wall,
so you pour the water (electrons) into another bucket.

That bucket carries the electrons across.

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✅ One-line summary

The glycerol phosphate shuttle moves electrons from cytoplasmic NADH to mitochondrial FAD so the electrons can enter the electron transport chain and produce ATP.

to overview the malate aspartate shuttle, we will do the same approach, when we are going to start focusing on glycolysis.

at the left, glycolysis produces the NADH that will then be used as a Coenzyme for a cytoplasmic enzyme known as malate dehydrogenase. Malate dehydrogenase produces malate. Malate has the ability to get into the mitochondria where it is converted to oxaloacetate by the mitochondrial version of the same enzyme.

Note that the coenzyme converts malate to oxaloacetate is the oxidized form of the nicotineamide adenine dinucleotide, that is found in the mitochondria.

This becomes a reduced NADH as a byproduct.

The NADH now has the ability to enter the electron transport chain. The fate of the oxaloacetate that is produced in the mitochondria will either serve the Krebs cycle or will serve to become aspartate.

Aspartate is formed by an amino transferase from oxaloacetate, which is then moved out of the mitochondria. Now the cytoplasmic aspartate is deaminated, meaning the amino group is removed to form a keto acid. In this case the ketoacid is oxaloacetate, now oxaloacetate in the cytoplasm will be the precursor for the malate that formed in the cytoplasm.

The cycle goes on and on with the production of certain amino acids along the way, you can see that in the cytoplasm, you have alpha ketoglutarate forming glutamate. This glutamate helps in the exchange of aspartate from the mitochondria out of the cytoplasm. then glutamate then donates the amino group to the oxaloacetate to form alpha ketoglutarate.

then mitochondria alpha ketoglutarate then helps the antiport malate into the mitochondria.

The malate–aspartate shuttle looks complicated in diagrams, but the main goal is actually the same as the glycerol phosphate shuttle:

👉 Move the electrons from cytoplasmic NADH (from glycolysis) into the mitochondria so ATP can be made.

The difference is how the electrons are transported.

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The Big Idea (one sentence)

The malate–aspartate shuttle transfers electrons from cytoplasmic NADH into the mitochondria by temporarily storing them in malate.

Unlike the glycerol shuttle, this one preserves NADH, so it produces more ATP.

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Step-by-Step Simple Explanation 1. Glycolysis produces NADH in the cytoplasm

During glycolysis:

Glucose → Pyruvate

This produces:

• 2 NADH

These electrons must reach the mitochondrial electron transport chain.

⚠ Problem:
NADH cannot cross the mitochondrial membrane.

So the cell transfers the electrons indirectly.

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2. Oxaloacetate accepts the electrons

In the cytoplasm:

Enzyme: malate dehydrogenase

Reaction:

Oxaloacetate + NADH → Malate + NAD⁺

What happens:

• NADH donates electrons

• Oxaloacetate accepts electrons

Result:

Malate is formed.

Malate can cross the mitochondrial membrane.

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3. Malate enters the mitochondria

Malate is transported into the mitochondria through a malate–α-ketoglutarate transporter.

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4. Malate becomes oxaloacetate again

Inside the mitochondria:

Enzyme: mitochondrial malate dehydrogenase

Reaction:

Malate + NAD⁺ → Oxaloacetate + NADH

So:

• Malate loses electrons

• NAD⁺ gains electrons

Result:

Mitochondrial NADH

This NADH directly enters the electron transport chain at Complex I.

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5. Oxaloacetate cannot leave the mitochondria

Another problem appears:

Oxaloacetate cannot cross the mitochondrial membrane.

So the cell converts it to something that can leave.

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6. Oxaloacetate becomes aspartate

Enzyme:

Aminotransferase (AST)

Reaction:

Oxaloacetate + Glutamate → Aspartate + α-ketoglutarate

So:

• Oxaloacetate gains an amino group

• It becomes aspartate

Aspartate can cross the membrane.

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7. Aspartate leaves the mitochondria

Aspartate moves back to the cytoplasm.

In the cytoplasm:

Aspartate → Oxaloacetate again.

Now the cycle is reset.

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Visual Flow (Simplified) Cytoplasm

NADH
↓
Oxaloacetate → Malate

⬇ enters mitochondria ⬇

Mitochondria

Malate → Oxaloacetate
↓
NAD⁺ → NADH

NADH → Electron Transport Chain

Oxaloacetate → Aspartate
↓
Aspartate returns to cytoplasm

Cycle repeats.

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Why This Shuttle Is Better for ATP

The electrons end up as mitochondrial NADH.

Shuttle	Electron carrier	ATP per NADH
Glycerol phosphate	FADH₂	~1.5 ATP
Malate–aspartate	NADH	~2.5 ATP

So this shuttle produces more ATP.

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Where It Is Used

Mostly in high-energy organs:

• Heart

• Liver

• Kidney

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Simple Analogy

Imagine you need to move electricity through a wall.

You cannot pass the wire (NADH) through the wall.

So you:

1⃣ store the electricity in malate
2⃣ carry malate through the wall
3⃣ convert it back into NADH inside the mitochondria.

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One-Line Summary

The malate–aspartate shuttle transfers electrons from cytoplasmic NADH into the mitochondria using malate and aspartate, producing mitochondrial NADH for ATP generation.

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✅ Since you're studying metabolism at a medical-school level, the real key exam trick is understanding why the two shuttles produce different ATP yields.

Now let’s go directly into the explanation of the two main pathways, let’s review the steps and the enzymes involved the glycerol phosphate shunt. How you will tell part one shunt over the other is the enzyme and the substrate at play. Glycerol-3-phoshpate dehydrogenase is found in the cytoplasm and in the mitochondria too, but depending on where you are looking, the direction of the reaction will be the opposite to the other.

the most crucial detail of the glycerol phosphate shunt is the electron carrier using the mitochondria is FAD+ to form FADH2. that’s why you produce 2 less ATP, since FADH2 yields up to two ATPs.

Let’s simplify what your slide is trying to say. The key point is how the glycerol-phosphate shuttle moves electrons from glycolysis into the mitochondria and why it produces less ATP.

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The Main Idea

During glycolysis, NADH is produced in the cytoplasm.
However:

❗ NADH cannot cross the mitochondrial membrane.

So the cell uses a shuttle system to move the electrons instead of the NADH molecule.

One of these systems is the glycerol-phosphate shuttle.

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Step-by-Step Simple Explanation 1. Glycolysis produces NADH

In the cytoplasm:

Glucose → Pyruvate

This produces:

• 2 NADH

These NADH molecules contain high-energy electrons that must reach the mitochondria.

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2. Cytoplasmic glycerol-3-phosphate dehydrogenase uses NADH

Enzyme:

Glycerol-3-phosphate dehydrogenase (cytoplasmic)

Reaction:

DHAP + NADH → Glycerol-3-phosphate + NAD⁺

What happens:

• NADH donates electrons

• DHAP accepts electrons

This produces glycerol-3-phosphate.

This step also regenerates NAD⁺, which glycolysis needs to continue.

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3. Glycerol-3-phosphate moves to the mitochondria

Glycerol-3-phosphate travels to the outer surface of the inner mitochondrial membrane.

Here there is another glycerol-3-phosphate dehydrogenase, but this one is mitochondrial.

Important detail:

⚠ The reaction goes in the opposite direction here.

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4. Mitochondrial enzyme transfers electrons to FAD

Reaction:

Glycerol-3-phosphate + FAD → DHAP + FADH₂

What happens:

• Glycerol-3-phosphate loses electrons

• FAD accepts the electrons

Result:

FADH₂

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5. FADH₂ sends electrons into the Electron Transport Chain

FADH₂ transfers its electrons to the electron transport chain (ETC).

But it enters at Complex II instead of Complex I.

Because of this:

• Less proton pumping occurs

• Less ATP is produced

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Why the Shuttle Produces Less ATP

Electrons from glycolysis normally produce NADH ATP yield.

But in this shuttle they become FADH₂ instead.

Electron carrier	Entry point in ETC	ATP produced
NADH	Complex I	~2.5 ATP
FADH₂	Complex II	~1.5 ATP

So:

Each cytoplasmic NADH produces about 1 ATP less.

Since glycolysis makes 2 NADH, the total is about 2 ATP less overall.

That is why older textbooks say:

• 38 ATP with malate-aspartate shuttle

• 36 ATP with glycerol phosphate shuttle

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Key Point From Your Slide

You identify the glycerol phosphate shuttle by two clues:

1⃣ The enzyme glycerol-3-phosphate dehydrogenase
2⃣ The mitochondrial electron carrier FAD → FADH₂

Those features distinguish it from the malate-aspartate shuttle.

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One-Sentence Summary

The glycerol phosphate shuttle transfers electrons from cytoplasmic NADH to mitochondrial FAD, producing FADH₂, which enters the electron transport chain and generates less ATP than NADH.