Aerobic and Anaerobic Respiration

Every cell needs a constant supply of energy to stay alive — to build molecules, contract muscles, move substances and keep warm. That energy is released by respiration, a process that happens in every cell, every second of your life. This lesson, part of Topic B1 of OCR Gateway Science A, 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 from earlier in the topic.

By the end 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 the uses of the energy released.

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What Is Respiration?

Respiration is the process that transfers energy from glucose so that cells can use it. Three points define it:

• Respiration happens in every living cell, continuously.
• It is an exothermic reaction — it transfers energy to the surroundings (which is one reason your body stays warm).
• The energy released is needed for life processes such as movement, building larger molecules, active transport and keeping warm.

Exam Tip: Respiration is not the same as breathing. Breathing (ventilation) moves air in and out of the lungs; respiration is the chemical reaction in cells that releases energy. Examiners deduct marks for confusing the two — never write "breathing" when you mean cellular respiration.

A common misconception worth fixing now: respiration does not "make" or "create" energy. Energy cannot be created. Respiration transfers the energy already stored in glucose into a usable form.

Respiration and metabolism

Respiration is the central energy-releasing reaction in metabolism — the sum of all the chemical reactions happening in a cell or organism. Metabolism includes both 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. 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.

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What the Energy Is Used For

The energy released by respiration is used across 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

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Aerobic Respiration

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. This is the cell's main, most efficient way of releasing energy.

The word equation is one you must memorise:

$\text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$glucose+oxygen→carbon dioxide+water

(The higher-tier balanced symbol equation is $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$C6​H12​O6​+6O2​→6CO2​+6H2​O, but the word equation is essential for all tiers.)

So aerobic respiration takes in glucose and oxygen and produces carbon dioxide and water, transferring energy in the process.

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 — examiners may give you the products and ask for the reactants, or vice versa. The reactants are glucose + oxygen; the products are carbon dioxide + water.

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Anaerobic Respiration

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 it has the advantage of not needing oxygen.

Importantly, anaerobic respiration produces different products in animals and in plants/yeast.

In animal cells

In animal cells (such as muscle), anaerobic respiration produces lactic acid:

$\text{glucose} \rightarrow \text{lactic acid}$glucose→lactic acid

During hard exercise, lactic acid builds up in the muscles, which can cause fatigue and the familiar burning sensation. The body must later break it down, which requires oxygen.

In plant and yeast cells (fermentation)

In plant cells and in yeast, anaerobic respiration is called fermentation and produces ethanol and carbon dioxide:

$\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide}$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 releases is trapped in the dough as bubbles, making the loaf rise; 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 separate.

Exercise and oxygen debt

Anaerobic respiration in muscle is most important 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 causes the muscles to feel tired and to ache, and eventually they cannot contract as effectively — this is muscle fatigue.

The lactic acid does not stay forever. After exercise you continue to breathe 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. This is why a sprinter is still panting hard for a minute or two after the race has finished — they are "repaying" the oxygen debt.

The body's response to exercise

During exercise your body makes several changes to deliver oxygen and glucose to the muscles faster and to remove carbon dioxide and heat. Your heart rate and the strength 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. Blood vessels supplying the muscles widen so that 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.

Exam Tip: If asked how the body responds to exercise, the mark-worthy points are an increase in heart rate, breathing rate and breath depth — all to supply more oxygen and glucose to the muscles for respiration and to remove carbon dioxide.

Investigating respiration