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Every cell needs a constant supply of energy just to stay alive — to build molecules, to contract muscles, to move substances and to keep warm. That energy is released by respiration, a process happening in every cell, every second of your life. This lesson, part of Topic B1 of OCR Gateway Combined Science, covers respiration as an exothermic process, the difference between aerobic and anaerobic respiration, the word equations you must know, and what the released energy is used for. It connects directly to photosynthesis (the next lesson) and to the role of mitochondria you met earlier in the topic.
By the end of this lesson you should be able to state that respiration is exothermic and happens in all living cells, write the word equation for aerobic respiration, describe anaerobic respiration in animals and in plants/yeast, and explain what the released energy is used for.
This lesson builds AO1 (recall of the respiration word equations and where respiration occurs) and AO2 (applying the aerobic/anaerobic distinction to explain what happens in muscles during hard exercise).
Respiration is the process that transfers energy from glucose so that cells can use it. Three points define it:
Exam Tip: Respiration is not the same as breathing. Breathing (ventilation) moves air in and out of the lungs; respiration is the chemical reaction inside cells that releases energy. Examiners take marks off for confusing the two — never write "breathing" when you mean cellular respiration.
A common misconception worth fixing straight away: respiration does not "make" or "create" energy. Energy cannot be created. Respiration transfers the energy already stored in glucose into a usable form.
Respiration is the central energy-releasing reaction in metabolism — the sum of all the chemical reactions happening in a cell or organism. Metabolism includes reactions that build large molecules (such as joining glucose into starch or cellulose, or amino acids into proteins) and reactions that break large molecules down. The building reactions need an input of energy, and that energy is supplied by respiration. So respiration is, in a sense, the engine that powers all the other chemistry of life: without a continuous supply of energy from respiration, a cell could not build new molecules, move substances or stay organised, and it would quickly die. This is why respiration must go on every second in every living cell, even while you sleep.
The energy released by respiration is put to use throughout the organism:
| Use of energy | Example |
|---|---|
| Movement / muscle contraction | Walking, a heartbeat, a plant's guard cells changing shape |
| Building larger molecules from smaller ones | Making proteins from amino acids; making starch or cellulose in plants |
| Active transport | Moving mineral ions into root hair cells against a concentration gradient |
| Keeping warm (in mammals and birds) | Maintaining a constant body temperature |
Aerobic respiration is respiration using oxygen. It takes place mainly in the mitochondria and releases a large amount of energy from each glucose molecule — far more than anaerobic respiration does. This is the cell's main and most efficient way of releasing energy.
The word equation is one you must memorise:
glucose+oxygen→carbon dioxide+water
The balanced symbol equation is C6H12O6+6O2→6CO2+6H2O, but the word equation is the essential one for combined science.
So aerobic respiration takes in glucose and oxygen and produces carbon dioxide and water, transferring energy as it does so.
flowchart LR
A["Glucose"] --> C["Aerobic respiration<br/>(in mitochondria, uses oxygen)"]
B["Oxygen"] --> C
C --> D["Carbon dioxide"]
C --> E["Water"]
C --> F["Energy transferred<br/>(large amount)"]
Exam Tip: Learn the aerobic equation in both directions — a question may give you the products and ask for the reactants, or the other way round. The reactants are glucose + oxygen; the products are carbon dioxide + water.
Anaerobic respiration is respiration without oxygen. It happens when cells cannot get enough oxygen — for example in hard-working muscles during vigorous exercise. It releases much less energy per glucose molecule than aerobic respiration, because the glucose is only partially broken down, but its advantage is that it does not need oxygen.
Importantly, anaerobic respiration produces different products in animals and in plants/yeast.
In animal cells (such as muscle), anaerobic respiration produces lactic acid:
glucose→lactic acid
During hard exercise, lactic acid builds up in the muscles, which can cause fatigue and the familiar burning feeling. The body has to break it down afterwards, and that requires oxygen.
In plant cells and in yeast, anaerobic respiration is called fermentation and produces ethanol and carbon dioxide:
glucose→ethanol+carbon dioxide
This process is hugely important industrially: it is the basis of brewing (the alcohol in beer and wine) and baking (the carbon dioxide makes bread rise). In bread-making the yeast respires anaerobically, and the carbon dioxide it gives off is trapped in the dough as bubbles, so the loaf rises; the small amount of ethanol produced evaporates during baking.
Exam Tip: The product of anaerobic respiration depends on the organism. Animals → lactic acid. Plants and yeast → ethanol + carbon dioxide (fermentation). Mixing these up is a frequent error — keep them firmly apart.
Anaerobic respiration in muscle matters most during vigorous exercise, when your muscles need energy faster than your lungs and circulation can deliver oxygen. The muscles then top up their energy supply by respiring anaerobically, but at the cost of producing lactic acid. As lactic acid builds up it makes the muscles feel tired and ache, and eventually they cannot contract as well — this is muscle fatigue.
The lactic acid does not stay forever. After exercise you carry on breathing deeply and quickly for a while, taking in extra oxygen. This extra oxygen is used to break down the lactic acid (it is gradually removed, largely by being transported to the liver and converted back into glucose). The amount of extra oxygen the body needs after exercise to deal with the lactic acid is called the oxygen debt. It is why a sprinter is still panting hard for a minute or two after the race has finished — they are "repaying" the oxygen debt.
During exercise your body makes several changes so that it can deliver oxygen and glucose to the muscles faster and remove carbon dioxide and heat. Your heart rate and the force of each heartbeat increase, pumping blood (and so oxygen and glucose) around the body more quickly. Your breathing rate and the depth of each breath increase, so more oxygen is taken into the blood and more carbon dioxide is removed. The blood vessels supplying the muscles widen, so more blood flows to where it is needed. All of these responses support a higher rate of aerobic respiration. When even this increased supply cannot keep up — during very intense effort — the muscles make up the shortfall with anaerobic respiration, accepting the build-up of lactic acid and the oxygen debt that must later be repaid. This is a good example of how respiration links to the whole organism, a theme you will develop in later topics.
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