Cell Respiration

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1Summary 重點摘要

Respiration oxidises glucose step by step to release energy: glycolysis nets 2 ATP in the cytoplasm, the aerobic pathway produces much more in the mitochondrion through the Krebs cycle and oxidative phosphorylation, and the anaerobic pathway yields lactic acid in muscle or ethanol and carbon dioxide in yeast.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: What respiration isRespiration goes on in every living cell all the time: food molecules such as glucose are oxidised step by step, releasing their chemical energy to be captured and transferred as ATP for muscle contraction, active transport, cell division and more. Note that respiration is not the gas exchange in the lungs. The overall equation for aerobic oxidation is: glucose + oxygen → carbon dioxide + water + energy. For example, brain cells burn enormous amounts of energy and depend on a constant supply of glucose and oxygen from the blood to keep respiring.
2. Step 2: The route at a glanceThe oxidation of glucose runs in three great sections: first glycolysis in the cytoplasm, needing no oxygen; then the Krebs cycle and finally oxidative phosphorylation, both in the mitochondrion and requiring oxygen. With plenty of oxygen all three sections run and the yield is high; without it only glycolysis can continue, followed by the anaerobic pathway. For example, think of glucose as one large banknote: glycolysis breaks it into smaller change, and the mitochondrion converts the small change into handy little units of ATP the cell can spend at any moment.
3. Step 3: Glycolysis — starting in the cytoplasmGlycolysis takes place in the cytoplasm and needs no oxygen: one six-carbon glucose molecule is first split into two three-carbon triose phosphates, which are each oxidised to pyruvate. The process nets 2 ATP directly and banks energy in the carrier NADH for later steps. This section is the common starting point of both the aerobic and anaerobic pathways. For example, even during hard exercise when oxygen is scarce, glycolysis in muscle cells keeps running at full speed — one reason the body can burst into action for a short time.
4. Step 4: Pyruvate enters the mitochondrionWhen oxygen is plentiful, pyruvate is transported into the mitochondrion, where it loses one carbon atom as carbon dioxide; the remaining two-carbon unit joins coenzyme A to become acetyl-CoA. This step links glycolysis to the Krebs cycle. For example, the mitochondrion's inner and outer membranes divide the reactions into separate compartments where enzymes are arranged in orderly sequence, and materials are processed station by station like a production line — a classic example of organelle specialisation.
5. Step 5: The Krebs cycleIn the mitochondrial matrix, acetyl-CoA combines with a four-carbon compound to form a six-carbon compound; a series of reactions then strips the two extra carbons off as two molecules of carbon dioxide, regenerating the four-carbon compound so the cycle continues. Along the way the energy carriers NADH and FADH₂ are produced, along with a little ATP; most of the energy is still banked in the carriers. For example, the Krebs cycle is like a merry-go-round: acetyl-CoA hops on for one turn, and carbon dioxide and loaded carriers hop off at each station.
6. Step 6: Oxidative phosphorylationThe final section runs on the inner mitochondrial membrane: NADH and FADH₂ hand over the energy captured earlier, which is used to build large amounts of ATP from ADP and phosphate; having released their energy, NAD and FAD are regenerated and return to the earlier stages to keep carrying. Oxygen serves as the final acceptor here, combining with hydrogen to form water and keeping the whole line running. For example, faster breathing and a faster heartbeat during exercise deliver oxygen to the mitochondria in time to sustain this high-yield stage.
7. Step 7: The anaerobic pathway — muscle and yeastWhen oxygen runs short, the mitochondrial production line stalls and pyruvate cannot enter the Krebs cycle, so cells switch to the anaerobic pathway: muscle cells convert pyruvate to lactic acid, while yeast converts it to ethanol and carbon dioxide. The anaerobic pathway regenerates the energy carriers so glycolysis can continue, netting 2 ATP. Built-up lactic acid makes muscles ache and tire; the carbon dioxide from yeast fermentation raises bread, and the ethanol goes into brewing. For example, dough slowly rising in a warm place is yeast releasing carbon dioxide by anaerobic respiration.
8. Step 8: Aerobic versus anaerobicThe key contrasts: in site, the aerobic pathway uses the cytoplasm plus the mitochondrion while the anaerobic pathway stays in the cytoplasm; in products, the aerobic pathway ends with carbon dioxide and water, the anaerobic pathway gives lactic acid in muscle or ethanol and carbon dioxide in yeast; in yield, the aerobic pathway produces large amounts of ATP per glucose while the anaerobic pathway nets only 2. For example, a 100-metre sprint relies mainly on the fast anaerobic pathway, while a long-distance runner depends chiefly on the high-yield aerobic pathway — which is why the two events are trained differently.

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8Recap Recap

• Respiration oxidises glucose step by step, transferring the energy as ATP for every cell activity.
• Glycolysis runs in the cytoplasm without oxygen, netting 2 ATP and two pyruvate molecules per glucose.
• The aerobic pathway runs in the mitochondrion: the Krebs cycle releases carbon dioxide and energy carriers, oxidative phosphorylation builds large amounts of ATP, and oxygen is the final acceptor forming water.
• The anaerobic pathway gives lactic acid in muscle (aching, oxygen debt) and ethanol plus carbon dioxide in yeast (fermentation).
• The aerobic pathway yields far more energy than the anaerobic one, but the anaerobic pathway supplies energy quickly over short bursts.