How Your Body Turns Glucose Into Energy: Glycolysis, the TCA Cycle, and the Electron Transport Chain

After the phosphagen system, the next energy system to come online is glycolysis, the metabolic pathway that breaks down glucose to generate ATP. But glycolysis is really just the beginning of a longer story. The full picture runs from glucose all the way through the TCA cycle and the electron transport chain, ending with oxygen accepting the final electron and water being produced as a byproduct.

Let’s walk through all of it.

Glycolysis: The 10-Step Breakdown of Glucose

Glycolysis occurs in the cytosol and converts one glucose molecule (C₆H₁₂O₆, a 6-carbon molecule) into two molecules of pyruvate (a 3-carbon molecule). The pathway has two phases: a preparatory phase that costs ATP, and a payoff phase that generates it.

The full sequence of intermediates is:

Glucose → Glucose-6-phosphate → Fructose-6-phosphate → Fructose-1,6-bisphosphate → Dihydroxyacetone phosphate + Glyceraldehyde-3-phosphate → 1,3-bisphosphoglycerate → 3-phosphoglycerate → 2-phosphoglycerate → Phosphoenolpyruvate (PEP) → Pyruvate

Key enzymes and regulatory steps:

The first phosphorylation, glucose to glucose-6-phosphate, is catalyzed by hexokinase (specifically hexokinase 2 in muscle, glucokinase in the liver), using one ATP. Adding a phosphate group traps the glucose inside the cell: the charged, bulky molecule can no longer pass back through the membrane, so the cell doesn’t have to expend energy to retain it.

The liver’s glucokinase behaves differently than muscle hexokinase. It is not inhibited by glucose-6-phosphate accumulation, and it is regulated by hormones like glucagon. This makes sense given the liver’s role: maintaining blood glucose levels for the brain regardless of what the rest of the body is doing.

The second phosphorylation, fructose-6-phosphate to fructose-1,6-bisphosphate, is catalyzed by phosphofructokinase 1 (PFK1). This is the committed step of glycolysis. Once glucose passes through PFK1, there is no turning back. It is committed to completing the pathway. PFK1 is the primary regulatory valve of glycolysis and is allosterically controlled by several signals:

• AMP and ADP activate PFK1. Low energy signals the cell to run glycolysis harder.
• Fructose-2,6-bisphosphate (produced by the related enzyme PFK2) is a potent activator.
• High ATP, low fructose-2,6-bisphosphate, low pH (high H⁺), and citrate all inhibit PFK1, signaling that energy is sufficient or that the TCA cycle is backed up.

Fructose-1,6-bisphosphate is then cleaved into two 3-carbon molecules. Dihydroxyacetone phosphate is converted into a second molecule of glyceraldehyde-3-phosphate, meaning from this point forward, every step happens twice per glucose.

The conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate by GAPDH (glyceraldehyde-3-phosphate dehydrogenase) is critical. This is a redox reaction. Electrons are removed from glyceraldehyde-3-phosphate and donated to NAD⁺, producing NADH. An inorganic phosphate (no ATP required) is added to the molecule simultaneously. This is significant: you are capturing energy in two forms at once.

The payoff phase then generates ATP through substrate-level phosphorylation, where high-energy molecules (1,3-bisphosphoglycerate and phosphoenolpyruvate) donate phosphate groups directly to ADP. Just as creatine phosphate is a higher-energy molecule than ATP and can drive ATP synthesis, these intermediates do the same.

• Net yield from glycolysis using glucose: 2 ATP + 2 NADH
• Net yield from glycolysis starting from glycogen: 3 ATP + 2 NADH (glycogen phosphorylase cleaves glucose from glycogen using inorganic phosphate, bypassing the hexokinase step and saving one ATP)

Glucose-6-Phosphate: The Hub of Multiple Pathways

It is worth pausing at glucose-6-phosphate, because it is not simply a stepping stone to pyruvate. It sits at the intersection of several pathways:

• Glycolysis, continuing toward pyruvate and ATP
• Glycogen synthesis, storing glucose for later
• Release to blood via a phosphatase enzyme in the liver, maintaining blood glucose
• Amino acid synthesis, contributing carbon skeletons
• Glycoconjugate production, building the glycocalyx and proteoglycans of the cell membrane
• The pentose phosphate pathway, producing NADPH for anabolic reactions and antioxidant regeneration (notably, reducing oxidized glutathione back to its active form), as well as ribose-5-phosphate for nucleotide synthesis

Once glucose-6-phosphate passes through PFK1 and becomes fructose-1,6-bisphosphate, all of these options are closed. That is what makes PFK1 the committed regulatory step.

The Fate of Pyruvate: Lactate or the TCA Cycle?

Pyruvate, the end product of glycolysis, has two main fates depending on conditions.

Lactate production: Lactate dehydrogenase (LDH) converts pyruvate to lactate while simultaneously converting NADH back to NAD⁺. This regeneration of NAD⁺ is the critical function. Without it, the GAPDH reaction cannot continue, NADH would accumulate, and glycolysis would grind to a halt.

Lactate production is not primarily driven by a lack of oxygen. It is driven by kinetics, the rate of ATP demand exceeds what the TCA cycle and electron transport chain can supply. By producing lactate and recycling NAD⁺, glycolysis can keep generating ATP at the speeds required for high-intensity work. In this way, lactate production actually sustains performance rather than causing fatigue. It is what allows glycolysis to remain the dominant ATP source when speed matters more than efficiency.

Entry into the TCA cycle: Under less urgent conditions, pyruvate enters the mitochondria via a specific pyruvate transporter (using a hydrogen ion as a cotransporter) and meets the PDH complex, pyruvate dehydrogenase.

The PDH Complex: Gateway to the TCA Cycle

The PDH complex is a fascinating multi-enzyme structure that uses five coenzymes, four of which are derived from vitamins: thiamine pyrophosphate (vitamin B1), lipoate, FAD (from niacin/B3), NAD⁺, and CoA (from pantothenic acid/B5).

Its job is to convert pyruvate (3 carbons) into acetyl-CoA (2 carbons) via oxidative decarboxylation, releasing one CO₂ and one NADH per pyruvate. Because two pyruvates are produced per glucose, the PDH complex yields 2 NADH and 2 CO₂ per glucose.

Acetyl-CoA is the universal entry point into the TCA cycle, whether the carbon comes from carbohydrates, fats, or certain amino acids.

The TCA Cycle: Extracting Electrons from Acetyl-CoA

The tricarboxylic acid (TCA) cycle, also called the citric acid cycle or Krebs cycle, takes place in the mitochondrial matrix. Acetyl-CoA (2 carbons) combines with oxaloacetate (4 carbons) to form citrate (6 carbons), catalyzed by citrate synthase. The cycle then proceeds through a series of transformations that regenerate oxaloacetate while extracting electrons and producing CO₂:

Citrate → Isocitrate → α-Ketoglutarate → Succinyl-CoA → Succinate → Fumarate → Malate → Oxaloacetate

Key steps and products per turn:

• Isocitrate → α-Ketoglutarate (isocitrate dehydrogenase): releases CO₂, produces NADH
• α-Ketoglutarate → Succinyl-CoA (α-ketoglutarate dehydrogenase): releases CO₂, produces NADH
• Succinyl-CoA → Succinate (succinyl-CoA synthetase/thiokinase): substrate-level phosphorylation produces GTP (equivalent to ATP)
• Succinate → Fumarate (succinate dehydrogenase): produces FADH₂, and this is the same enzyme as Complex II of the electron transport chain, embedded in the inner mitochondrial membrane
• Malate → Oxaloacetate (malate dehydrogenase): produces NADH

Per acetyl-CoA: 3 NADH, 1 FADH₂, 1 GTP, 2 CO₂

Because two acetyl-CoAs enter per glucose (via two pyruvates), the TCA cycle yields 6 NADH, 2 FADH₂, and 2 GTP per glucose, plus the 2 NADH from PDH.

Regulatory enzymes of the TCA cycle respond to energy status:

• Citrate synthase is activated by high oxaloacetate and high acetyl-CoA, and inhibited by high ATP and citrate.
• Isocitrate dehydrogenase and α-ketoglutarate dehydrogenase are activated by ADP and calcium, a direct link between muscle contraction and metabolic rate. The same calcium released from the sarcoplasmic reticulum to initiate a muscle contraction also accelerates the TCA cycle to meet the resulting energy demand.

These enzymes are used clinically and in research as markers of mitochondrial function. Measuring citrate synthase or α-ketoglutarate dehydrogenase activity gives an index of mitochondrial capacity.

The Electron Transport Chain: Making the Majority of Your ATP

The NADH and FADH₂ produced throughout glycolysis, PDH, and the TCA cycle carry high-energy electrons to the electron transport chain (ETC) on the inner mitochondrial membrane. The chain consists of four complexes plus ATP synthase (Complex V):

Complex I (NADH dehydrogenase): NADH donates 2 electrons and is oxidized to NAD⁺. The electrons enter the chain and drive the pumping of 4 protons from the matrix into the intermembrane space.

Complex II (succinate dehydrogenase): FADH₂ donates electrons here. Importantly, Complex II performs no proton pumping. This is why FADH₂ generates less ATP than NADH.

CoQ10 (ubiquinone): A mobile electron carrier that shuttles electrons from Complexes I and II to Complex III.

Complex III: Accepts electrons from CoQ10 and pumps 4 more protons. Electrons are passed to cytochrome C.

Complex IV (cytochrome oxidase): Accepts electrons from cytochrome C and pumps 2 protons. Critically, this is where oxygen serves as the final electron acceptor, combining 2 electrons, 2 protons, and ½ O₂ to produce water (H₂O). This is the fundamental reason oxygen is required for sustained energy production: it pulls electrons down the chain, keeping the entire system moving.

ATP Synthase (Complex V): The proton gradient built up across the inner mitochondrial membrane, positive outside and negative inside with a pH difference of approximately 0.75 or more, creates a powerful electrochemical driving force. Protons flow back through ATP synthase down this gradient, physically rotating the enzyme’s nanomotor. Every rotation of the motor requires 3 protons to synthesize one ATP, plus 1 proton used by the phosphate transporter to bring inorganic phosphate into the matrix. Total: 4 protons per ATP.

ATP accounting per reducing equivalent:

• NADH contributes electrons through Complexes I, III, and IV: 4 + 4 + 2 = 10 protons pumped, giving 10 / 4 = 2.5 ATP
• FADH₂ skips Complex I (Complex II pumps no protons): 4 + 2 = 6 protons pumped, giving 6 / 4 = 1.5 ATP

Total ATP from Complete Glucose Oxidation

Adding everything together:

Source	Yield	ATP Equivalent
Glycolysis (net)	2 ATP	2 ATP
Glycolysis NADH (×2)	2 NADH	5 ATP
PDH NADH (×2)	2 NADH	5 ATP
TCA NADH (×6)	6 NADH	15 ATP
TCA FADH₂ (×2)	2 FADH₂	3 ATP
TCA GTP (×2)	2 GTP	2 ATP
Total		32 ATP

The range typically cited, 30 or 32 ATP, reflects which shuttle system transports cytosolic NADH into the mitochondria. When the malate-aspartate shuttle is used (as in heart and liver), electrons enter at Complex I and the full 2.5 ATP per NADH applies. When the glycerol-3-phosphate shuttle is used (as in skeletal muscle and brain), electrons bypass Complex I and enter at Complex II, yielding only 1.5 ATP per cytosolic NADH, giving a total closer to 30 ATP.

Starting from glycogen instead of free glucose adds one more ATP (from the hexokinase step saved), bringing the total to 33.

Beta-Oxidation of Fatty Acids

Fats enter the same final common pathway, acetyl-CoA into the TCA cycle into the ETC, but their preparation is different.

Long-chain fatty acids (longer than 12 carbons) cannot cross the inner mitochondrial membrane directly because the CoA group is too large. They are transported as acyl-carnitine conjugates via the carnitine palmitoyltransferase (CPT1/CPT2) system. CPT1 on the outer membrane attaches carnitine, and CPT2 on the inner membrane removes it and reattaches CoA. The carnitine is recycled back across in an antiport mechanism.

This is relevant to carnitine supplementation: because carnitine is recycled and rarely rate-limiting in healthy individuals, supplementation is unlikely to meaningfully increase fatty acid oxidation unless there is a specific deficiency.

Beta-oxidation then sequentially cleaves 2-carbon acetyl-CoA units from the fatty acid chain, with each cleavage producing one FADH₂ and one NADH. For palmitic acid (16 carbons), 8 acetyl-CoAs, 7 FADH₂, and 7 NADH are produced from beta-oxidation alone.

Counting through the TCA cycle for all 8 acetyl-CoAs and subtracting the 2 ATP activation cost, palmitic acid yields approximately 106 net ATP, significantly more than glucose per molecule, though requiring more oxygen per carbon.

The Creatine Phosphate Shuttle: Connecting Mitochondria to Muscle

ATP is a large molecule. Diffusing from the mitochondria all the way to the myosin ATPase or SERCA pumps at the sarcomere takes time. Creatine phosphate is smaller and diffuses faster.

A creatine kinase enzyme located in the intermembrane space of the mitochondria converts ATP leaving the matrix into creatine phosphate immediately. Creatine phosphate then carries the energy to where it is needed, regenerates ATP locally, and the resulting creatine diffuses back to the mitochondria, where ADP re-enters the matrix and drives ATP synthase. This elegant shuttle system efficiently couples oxidative phosphorylation to cytosolic energy demands.

Uncoupling Proteins and Metabolic Efficiency

Not all proton flow through the inner mitochondrial membrane produces ATP. Uncoupling proteins (UCP1, also called thermogenin) allow protons to bypass ATP synthase and flow directly into the matrix, dissipating the gradient as heat instead.

This is especially prominent in brown adipose tissue, which is rich in mitochondria with uncoupling protein, allowing newborns and cold-adapted individuals to generate heat directly.

There is an important counterintuitive point here: a “more efficient” metabolism in the common sense, someone who burns through food without gaining weight, likely has more uncoupling activity, not less. They are extracting less ATP per electron, dissipating more energy as heat. The thermodynamics of efficiency work the other direction from what is often assumed.

Reactive Oxygen Species and Antioxidant Defense

Occasionally, electrons leak prematurely to oxygen at Complex I or Complex III, forming the superoxide radical rather than water. At low levels, superoxide may serve signaling functions. At high levels, it contributes to oxidative stress.

The body manages this through enzymes like superoxide dismutase (which converts superoxide to hydrogen peroxide) and glutathione peroxidase (which neutralizes hydrogen peroxide). This is why the pentose phosphate pathway, producing NADPH to regenerate reduced glutathione, is metabolically important even when its direct ATP contribution is minimal.

The RISP (Rieske Iron-Sulfur Protein) in Complex III is a site of particular interest in this regard and has been the subject of research into how molecular hydrogen may interact with mitochondrial function.

The Bigger Picture

Everything described here, glycolysis, the TCA cycle, the electron transport chain, and beta-oxidation, is simply thermodynamics at work. Oxygen, as the most powerful final electron acceptor, pulls electrons down the chain and makes the entire process favorable. Every electron can be traced. Every proton can be accounted for. The stoichiometry maps perfectly.

Understanding this system in full is what allows us to interpret fatigue, training adaptation, substrate utilization, and the real mechanisms behind performance, without hand-waving.