Cellular respiration: how glycolysis, the citric acid cycle, and oxidative phosphorylation turn nutrients into energy.

Energy is the currency your body spends every day. Whether you’re chasing a PB on the track, painting a long ride, or just keeping up with kids and chores, your cells are quietly converting fuel into the power you rely on. The mechanism that makes this possible is called cellular respiration—a continuous loop with three main stages. Think of it as a well-oiled trio that keeps turning as long as you’ve got fuel and oxygen.

The big picture: a three-part rhythm you can count on

Cellular respiration isn’t a one-and-done event. It’s a seamless cycle, a bit like a relay race where each leg hands off energy to the next. The three stages—glycolysis, the citric acid cycle (also known as the Krebs cycle), and oxidative phosphorylation (the electron transport chain happening in the mitochondria)—work together to turn glucose and other fuels into ATP, the cell’s main energy molecule. When you hear “three stages,” you might picture them as separate rooms, but they’re really more like connected rooms with doors that stay open. The energy plants—mitochondria—do the heavy lifting, and even when one segment slows down, another can compensate.

Stage 1: Glycolysis — sugar’s first stop, right in the cytoplasm

Let’s start with glycolysis, the opener. This stage happens in the cell’s liquid interior, the cytoplasm. Here, a single glucose molecule—the familiar six-carbon sugar—gets split into two molecules called pyruvate. It’s a brisk process, because your body loves quick energy when a sprint is involved or when you’re just starting a workout.

A couple of neat facts to keep in mind:

• Net ATP: glycolysis nets a small but meaningful amount of energy—about 2 ATP per glucose, plus a handful of electron carriers.

• Electron carriers: NAD+ grabs electrons and becomes NADH, a loaded shuttle that will feed the next two stages.

• Oxygen isn’t required to start glycolysis. That’s why you can still power a burst of effort with glycolysis even when oxygen is scarce (leading to lactate formation in the muscle for short-term energy).

You might wonder, where does it all go from here? Pyruvate is the bridge. In the presence of oxygen, pyruvate moves into the mitochondria to feed the next stage. Without enough oxygen, glycolysis slows its pace, and the body uses alternative routes for energy—this is the moment where nutrition and metabolism choices really matter for performance and recovery.

Stage 2: The citric acid cycle — the mitochondria’s busy kitchen

Inside the mitochondria—the cell’s powerhouse—the pyruvate from glycolysis is transformed into a molecule called acetyl-CoA. This is the ticket to the citric acid cycle, the second stage. Here, acetyl-CoA enters a cyclical process that keeps churning as long as fuel streams in.

Two quick takeaways:

• The cycle isn’t about producing a ton of ATP directly. It’s about generating high-energy carriers—NADH and FADH2—that will later fuel the big ATP factory.

• Carbon bookkeeping matters: the cycle releases carbon dioxide, a reminder that much of your energy work is tied to breaking bonds and rearranging atoms.

If you’re picturing energy as money, glycolysis is your basic wage, while the citric acid cycle is a clever side hustle that keeps the reserves full. For athletes, the rate at which the cycle runs influences endurance and how long you can sustain a given effort. More efficient cycling of these substrates often means you can rely on different fuel sources at various intensities, which has direct implications for diet and training.

Stage 3: Oxidative phosphorylation — the grand finale and the energy floodgate

The third stage is where the real ATP boom happens. Oxidative phosphorylation happens along the inner mitochondrial membrane in a process driven by the electron transport chain (ETC). NADH and FADH2—those carriers formed earlier—donate their electrons to the chain. As electrons move along the chain, protons are pumped across the membrane, creating a gradient. Think of it as charging a battery: the proton gradient stores potential energy, and ATP synthase uses that energy to manufacture ATP from adenosine diphosphate (ADP) and phosphate.

A few practical points:

• Oxygen is the final electron acceptor. Without it, the chain backs up, and ATP production falls off a cliff. That’s why oxygen availability matters so much in sports and daily activity.

• The typical ATP yield from oxidative phosphorylation is around 26–28 ATP per glucose, but that number isn’t carved in stone. It varies with cell type, substrate availability, and other factors.

• Water is the byproduct of the final electron transfer to oxygen, which is pretty neat when you think about how metabolism quietly completes the circle of life.

Putting the three stages together: the energy loop in action

When you string glycolysis, the citric acid cycle, and oxidative phosphorylation together, you get a robust energy system. Glucose starts the process, is broken down and channeled into a series of energy transfers, and finally leaves you with a steady stream of ATP. The cycle is continuous: as long as substrates are available and oxygen is present, your cells keep churning out energy. In real life, that means sustained effort, whether you’re pushing through a long run, lifting weights, or even engaging in focused mental tasks that still demand nucleotide turnover in the brain.

Rhetorical digression: why this matters for nutrition coaching

If you’re advising clients—whether endurance athletes, strength trainees, or everyday movers—the details of cellular respiration illuminate a lot of practical strategies. Carbohydrate availability, for example, shapes how smoothly glycolysis can proceed. When carbohydrates are plentiful, the body can rapidly replenish the glucose pool and keep glycolysis humming. That’s why pre- and post-workout meals often emphasize carbs, to refill glycogen stores and support ongoing energy production.

But it isn’t all carbs. The body is clever at using fats as a fuel when carbohydrate supply is more modest or during longer, steady efforts. Fat oxidation becomes a bigger piece of the energy puzzle as exercise duration increases and intensity wanes. This interplay between carbohydrate and fat use is a central theme in personalized nutrition plans, and it hinges on your client’s training status, body composition, and even micro-nutrient status that keeps the mitochondria happy and healthy.

A practical matrix you can use with clients

• For short, high-intensity efforts: ensure adequate carbohydrate availability to support glycolysis and quick ATP production.

• For longer, moderate efforts: promote a blend of carbs and fats, training the body to oxidize fat more efficiently without starving glycolysis of needed fuel.

• For recovery days: emphasize protein and a balanced mix of fats and carbs to replenish substrates and support mitochondrial repair and growth.

Real-world twists and tangents worth noting

You’ll sometimes hear people talk about mitochondria as if they’re just powerhouses churning away. They are, but they’re also sensitive hubs. Nutrition, sleep, and stress all factor into mitochondrial health. Chronic stress can blunt mitochondrial efficiency, while adequate sleep supports repair and adaptation. Micronutrients like B vitamins, iron, magnesium, and CoQ10 play roles in the pathways that shuttle electrons and build ATP. In practice, a caring nutrition plan doesn’t just chase macros; it helps the cellular machinery function smoothly at a daily tempo.

Common myths (and what’s actually true)

• Myth: You only burn carbs when you’re sprinting. Truth: Even at rest, you’re burning a mix of fuels, and the mix shifts with activity and conditioning.

• Myth: Fat is a lazy fuel. Truth: Fat oxidation is a powerful energy source, especially during endurance tasks, and mitochondria become more efficient at tapping fat with consistent training.

• Myth: All ATP comes from the same place. Truth: There’s a dynamic balance. Glycolysis handles quick bursts; the citric acid cycle and oxidative phosphorylation handle sustained energy and high-yield production.

A few notes on the “why” behind the numbers

If you’re curious about the numbers behind ATP yield, you’ll find they’re surprisingly context-dependent. The exact ATP count per glucose can vary based on cell type, the presence of shuttle systems, and what other molecules in the cell are doing. The takeaway isn’t a single static number but a sense of proportion: glycolysis supplies some ATP and NADH early, the citric acid cycle boosts carrier counts, and oxidative phosphorylation converts those carriers into a sizable ATP harvest. This is why nutrition and training plans are personalized. One athlete might derive more benefit from higher carb intake around workouts, while another optimizes performance by improving fat oxidation through endurance training and steady energy availability.

Closing thoughts: the continuous cycle you can count on

Cellular respiration is more than a textbook concept. It’s the living architecture behind how energy flows through your body every day. From the moment you wake up and take a breath to the last rep you squeeze out at the gym, this cycle hums along, adapting to your activity, your fuel choices, and your body’s needs. For nutrition coaches, understanding the three stages helps translate science into real-world advice: when to load carbs, how to structure meals around workouts, and how to support mitochondrial health with the right mix of nutrients and lifestyle habits.

So next time you hear someone say energy comes from somewhere inside the body, you can nod and say, “Yep—the three-stage loop in action.” Glycolysis starts the spark, the citric acid cycle keeps the momentum going, and oxidative phosphorylation delivers the grand finale of ATP. It’s a quiet, efficient system—one that makes every bite of food and every minute of training count toward stronger energy, better performance, and healthier living.