Where Does Energy Come From?
Your body is constantly working — your heart beats, your muscles move, your brain thinks. All of this activity requires energy. But where does that energy actually come from? The answer is cellular respiration, the process by which living cells release energy stored in glucose.
Glucose is a type of sugar that comes from the food you eat. When you digest carbohydrates — bread, rice, fruit — your digestive system breaks them down into glucose. That glucose travels through your bloodstream to every cell in your body, where it is used as fuel.
Aerobic Respiration
The most efficient form of cellular respiration is aerobic respiration, which means it requires oxygen. It takes place mainly in the mitochondria — the "powerhouses" of the cell. The process breaks down glucose completely, releasing a large amount of energy, and produces carbon dioxide and water as waste products.
The simplified equation is:
Glucose + Oxygen → Carbon dioxide + Water + Energy
This is why you breathe in oxygen and breathe out carbon dioxide. Your lungs are supplying the oxygen needed for respiration, and getting rid of the carbon dioxide waste produced by it.
Every cell in your body carries out cellular respiration — including your brain cells, muscle cells, and skin cells. Even plant cells respire. Respiration never stops; if it did, the cell would quickly run out of energy and die.
Anaerobic Respiration
Sometimes cells need energy faster than oxygen can be delivered. During intense exercise, your muscle cells may run short of oxygen but still need energy. In this situation, cells switch to anaerobic respiration (without oxygen). This produces energy much faster, but in much smaller amounts, and it produces lactic acid as a by-product.
Lactic acid building up in your muscles is what causes the burning feeling and soreness during and after hard exercise. When you rest, your body uses oxygen to break down the lactic acid and get rid of it — this is called "repaying the oxygen debt."
Yeast cells also carry out anaerobic respiration, producing ethanol (alcohol) and carbon dioxide instead of lactic acid. This process is called fermentation and is used in baking (the CO₂ makes bread rise) and in making drinks like beer and wine.
Notice how photosynthesis and aerobic respiration are almost opposites: photosynthesis uses CO₂ and water to make glucose and O₂, while aerobic respiration uses glucose and O₂ to release energy and produce CO₂ and water.
Respiration vs Breathing
A very important distinction: breathing and cellular respiration are not the same thing. Breathing (or ventilation) is the physical act of moving air in and out of your lungs. Cellular respiration is the chemical process inside cells that releases energy from glucose. Breathing supplies oxygen for respiration and removes the carbon dioxide it produces, but they are different processes.
Do not confuse respiration with breathing! Breathing is a physical movement; cellular respiration is a chemical reaction inside cells. All living things respire, but not all living things breathe with lungs (plants, fungi, and bacteria respire too).
Real world: bread-making (yeast fermentation)
Baker's yeast cells carry out anaerobic respiration in bread dough: glucose → ethanol + CO₂ + energy. The CO₂ bubbles become trapped in the dough, making it rise. Baking then drives off the ethanol. Without understanding anaerobic respiration in yeast, there would be no leavened bread, beer, or wine — fermentation industries rely on this biochemistry.
Real world: exercise and lactic acid
A 400 m sprinter runs faster than oxygen can be delivered to muscles. Muscle cells switch to anaerobic respiration, producing lactic acid. The burning sensation and fatigue that follow are caused by this lactic acid build-up. After the race, the athlete breathes heavily to supply the oxygen needed to break down the lactic acid — "repaying the oxygen debt."
Real world: scuba diving oxygen supply
A scuba diver's tank contains compressed oxygen (or air). At depth, the diver's cells are constantly consuming oxygen via aerobic respiration: glucose + O₂ → CO₂ + H₂O + energy. A diver using a 12-litre tank at 40 bar has roughly 480 litres of air to breathe. Knowing metabolic oxygen consumption rates allows dive operators to calculate safe dive times.
Practice Problems
Write the word equation for aerobic respiration.
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What is the difference between aerobic and anaerobic respiration?
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In which organelle does aerobic respiration mainly take place?