Cellular Respiration & Energy

Every living cell needs energy to function — to move, divide, make proteins, pump ions, and stay alive. That energy comes from cellular respiration — the process of extracting energy from food molecules and storing it as ATP (adenosine triphosphate). ATP is the universal energy currency of the cell. It is like a rechargeable battery — loaded with energy and able to release it instantly where it is needed. Almost every energy-requiring process in the body — muscle contraction, nerve impulses, protein synthesis, active transport — is directly powered by ATP. The big picture equation: Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP) C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~30–38 ATP This is why you breathe in oxygen and breathe out carbon dioxide — it is the direct exhaust of your cells making energy. Three stages of cellular respiration: 1. Glycolysis — in the cytoplasm, no oxygen needed 2. Krebs cycle (Citric Acid Cycle) — in the mitochondrial matrix 3. Oxidative phosphorylation (Electron Transport Chain) — on the inner mitochondrial membrane The first stage can work without oxygen. Stages 2 and 3 require oxygen — they are aerobic. This is why your muscles hurt and fatigue when oxygen cannot be delivered fast enough during intense exercise.

Glycolysis (from Greek: glyco = sugar, lysis = splitting) is the first stage of cellular respiration. It breaks one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each). Key facts about glycolysis: - Happens in the cytoplasm — no mitochondria needed - Does NOT require oxygen (anaerobic) - Net production: 2 ATP molecules and 2 NADH (an electron carrier) - Very fast — can produce ATP almost instantly Because glycolysis does not need oxygen, it can supply energy even when oxygen is limited — like during the start of an intense sprint before the cardiovascular system catches up. What happens to pyruvate? Pyruvate is the end product of glycolysis. What happens next depends on whether oxygen is available: If oxygen IS available (aerobic respiration): Pyruvate enters the mitochondria and is converted to acetyl-CoA (releasing one CO₂). Acetyl-CoA then enters the Krebs cycle. If oxygen is NOT available (anaerobic respiration): Pyruvate is converted to lactate (lactic acid) in a process called lactic acid fermentation. This regenerates NAD⁺ (needed to keep glycolysis running) but produces no additional ATP. This is what happens in your muscles during intense exercise when oxygen cannot be delivered fast enough: - Glycolysis keeps running → lactic acid builds up - The drop in pH causes the "burning" sensation in muscles - You breathe heavily after exercise to repay the "oxygen debt" — processing the accumulated lactate back to pyruvate

The Krebs cycle (also called the citric acid cycle or TCA cycle) takes place in the matrix of the mitochondria. It extracts the remaining energy from the pyruvate produced by glycolysis. The key inputs and outputs (per glucose molecule = 2 cycles): - Inputs: 2 acetyl-CoA + water - Outputs: 4 CO₂ (exhaled), 6 NADH, 2 FADH₂, 2 ATP The CO₂ produced here is what you breathe out. NADH and FADH₂ are electron carriers — they are like energy "tokens" that are cashed in for ATP in the next stage. Why is it a "cycle"? The last product of the cycle regenerates the first molecule (oxaloacetate), allowing the cycle to repeat continuously — as long as acetyl-CoA is available. The Krebs cycle only produces 2 ATP directly — but it generates large amounts of NADH and FADH₂ which are worth far more ATP in the next stage. An important clinical connection: Thiamine (Vitamin B1) is a cofactor essential for the enzyme that converts pyruvate to acetyl-CoA (pyruvate dehydrogenase). Thiamine deficiency — most commonly from chronic alcohol misuse — impairs this step, starving cells of energy. This causes Wernicke's encephalopathy — a brain disorder with confusion, abnormal eye movements, and unsteady gait. It is a medical emergency treated with IV thiamine.

This is where the vast majority of ATP is made. Oxidative phosphorylation takes place on the inner mitochondrial membrane and uses the NADH and FADH₂ produced by glycolysis and the Krebs cycle. The Electron Transport Chain (ETC): NADH and FADH₂ donate their electrons to a series of protein complexes embedded in the inner mitochondrial membrane (Complexes I–IV). As electrons move through these complexes, they release energy — which is used to pump hydrogen ions (protons, H⁺) from the matrix into the space between the inner and outer mitochondrial membranes. This creates a proton gradient — a high concentration of H⁺ on one side of the membrane. The H⁺ ions want to flow back down their concentration gradient into the matrix, but they can only do so through a remarkable enzyme called ATP synthase. ATP synthase — a molecular turbine: ATP synthase is a rotary motor embedded in the membrane. As protons flow through it, they physically spin a rotor, and this mechanical energy is used to attach a phosphate group to ADP — producing ATP. This process is called chemiosmosis. This is one of biology's most extraordinary machines — and Peter Mitchell won the Nobel Prize in 1978 for discovering it. Final step: At the end of the ETC, the electrons are transferred to oxygen — reducing it to water. This is why you need oxygen to breathe — it is the final electron acceptor. ATP yield summary (per glucose): - Glycolysis: 2 ATP - Krebs cycle: 2 ATP - Oxidative phosphorylation: ~28–34 ATP - Total: ~30–38 ATP Clinical relevance: - Cyanide poisoning blocks Complex IV of the ETC — cells cannot make ATP → rapid death - Carbon monoxide poisoning blocks haemoglobin from delivering oxygen → ETC cannot run - Metformin (the most widely used diabetes drug) partially inhibits Complex I of the ETC in liver cells, reducing glucose production - Statins reduce CoQ10 (a carrier in the ETC) — a controversial potential cause of muscle side effects

The body is remarkably adaptable in how it generates energy depending on oxygen availability and energy demand. Three energy systems in exercise: 1. Phosphocreatine system (immediate — 0–10 seconds): No oxygen needed. Creatine phosphate donates its phosphate to ADP to instantly regenerate ATP. Powers explosive bursts — a 100m sprint, a single maximal weight lift. Runs out in about 10 seconds. (This is why creatine supplements are popular in sports — they increase phosphocreatine stores.) 2. Anaerobic glycolysis (fast — 10 seconds to 2 minutes): Glycolysis runs without oxygen, producing 2 ATP per glucose very rapidly. Produces lactic acid as a byproduct → burning sensation, fatigue. Powers activities like 400m sprint, intense interval training. 3. Aerobic respiration (sustained — beyond ~2 minutes): Full glycolysis + Krebs + ETC. Needs oxygen but produces ~30–38 ATP per glucose — far more efficient. Powers all sustained activity — distance running, cycling, daily life. Oxygen debt and recovery: After intense anaerobic exercise, you keep breathing hard even after stopping. This "excess post-exercise oxygen consumption" (EPOC) is needed to: oxidise accumulated lactate back to pyruvate, restore phosphocreatine stores, and pay back the oxygen "borrowed" from haemoglobin and myoglobin. Why cells cannot survive without oxygen: Without oxygen, only glycolysis runs → 2 ATP per glucose. The brain, heart, and kidneys have very high energy demands and deplete their limited glycolytic capacity within minutes. Brain neurons begin dying after just 4–6 minutes without oxygen — which is why cardiac arrest or drowning causes brain damage so rapidly. The heart (which relies almost exclusively on aerobic respiration) also fails quickly. This is the physiological urgency behind CPR and rapid defibrillation.