Aerobic and Anaerobic Respiration

Respiration is the chemical process that transfers energy from glucose so that living cells can use it. It happens continuously in every living cell, every second of every day — in animals, plants, fungi and bacteria alike.

Two points cause most of the lost marks on this topic, so fix them now:

• Respiration is exothermic — it transfers energy to the surroundings. Some of that energy is released as heat, which is why active organisms feel warm.
• Respiration is not breathing. Breathing (ventilation) moves air in and out of the lungs. Respiration is a chemical reaction inside cells. They are different things with similar-sounding names.

Respiration: an exothermic reaction, occurring continuously in living cells, that transfers the energy an organism needs for its living processes.

The energy transferred by respiration supplies all the energy needed for life. Organisms use it for three things named by the specification:

• Movement — muscle contraction in animals.
• Keeping warm — maintaining body temperature in mammals and birds.
• Building larger molecules — chemical reactions that join smaller molecules into bigger ones (for example, amino acids into proteins).

Aerobic respiration uses oxygen to break glucose down completely. The word "aerobic" means "with air/oxygen". It takes place mainly inside the mitochondria — the organelles often called the powerhouse of the cell.

The word equation is:

glucose + oxygen → carbon dioxide + water

The symbol equation is:

$C_6{H}_{12}{O}_6+6O_2\to 6C{O}_2+6H_{2}O$

You should recognise these chemical symbols: $C_6{H}_{12}{O}_6$ (glucose), $O_2$ (oxygen), $C{O}_2$ (carbon dioxide) and $H_{2}O$ (water).

Because glucose is broken down completely when oxygen is available, aerobic respiration transfers a large amount of energy from each glucose molecule. This is the cell's preferred route whenever enough oxygen is supplied.

Cells with high energy demands — such as muscle and liver cells — contain large numbers of mitochondria, because mitochondria are where aerobic respiration happens.

Examiners expect the aerobic symbol equation to be balanced — the same number of each type of atom on both sides. Counting the atoms is a quick check you can do in the exam.

Take the equation ${\text{C}}_6{\text{H}}_{12}{\text{O}}_6+6{\text{O}}_2\to 6{\text{CO}}_2+6{\text{H}}_2\text{O}$ and count each element on each side:

Element	C₆H₁₂O₆	6O₂	6CO₂	6H₂O	Left = Right
Carbon	6	0	6	0	6 = 6 ✓
Hydrogen	12	0	0	12	12 = 12 ✓
Oxygen	6	12	12	6	18 = 18 ✓

Working through it:

• Carbon: 6 in glucose; on the right, 6 in the 6 CO₂. 6 = 6.
• Hydrogen: 12 in glucose; on the right, 6 × 2 = 12 in the 6 H₂O. 12 = 12.
• Oxygen: glucose has 6, plus 6 O₂ gives 6 × 2 = 12, total 18; on the right, 6 CO₂ gives 6 × 2 = 12 plus 6 H₂O gives 6, total 18. 18 = 18.

Every element balances, so the equation is correct. The big numbers in front (the coefficients) multiply everything in that molecule — a common slip is to forget that the 6 in front of $O_2$ means 12 oxygen atoms, not 6.

When muscles work hard during vigorous exercise, the blood cannot deliver oxygen fast enough. The muscle cells switch to anaerobic respiration — respiration without oxygen.

In animal muscle cells the word equation is:

glucose → lactic acid

Notice what is missing compared with aerobic respiration: there is no oxygen used, and the products are different — only lactic acid is made, with no carbon dioxide or water.

The key idea the specification tests is energy yield. In anaerobic respiration the oxidation of glucose is incomplete — glucose is only partially broken down. Because it is not fully broken down, much less energy is transferred from each glucose molecule than in aerobic respiration.

Anaerobic respiration in muscles transfers much less energy per glucose molecule than aerobic respiration, because the glucose is only partially broken down (incomplete oxidation).

So anaerobic respiration is a short-term backup: it keeps energy flowing when oxygen runs low, but it is far less efficient and it produces lactic acid as a by-product.

Plant cells and yeast (a fungus) can also respire anaerobically, but they produce different products from animal muscle. The word equation for plant and yeast cells is:

glucose → ethanol + carbon dioxide

In yeast, this process has a special name: fermentation.

Compare the two anaerobic routes side by side — they share the lack of oxygen and the low energy yield, but the products differ:

Cell type	Word equation	Products
Animal muscle	glucose → lactic acid	lactic acid
Plant / yeast	glucose → ethanol + carbon dioxide	ethanol + carbon dioxide

Like anaerobic respiration in muscles, fermentation only partially breaks down glucose, so it transfers much less energy than aerobic respiration.

Fermentation: anaerobic respiration in yeast, producing ethanol and carbon dioxide from glucose.

(Extra context — not required by AQA 8461.) "Ethanol" is the alcohol found in alcoholic drinks; you do not need any chemistry detail about it beyond the word equation.

Fermentation in yeast matters far beyond the biology classroom: it is the basis of two huge food and drink industries. The specification asks you to know its economic importance in making bread and alcoholic drinks.

The same reaction — glucose → ethanol + carbon dioxide — is used for both, but a different product is the one we want:

Industry	Useful product of fermentation	What it does
Bread	carbon dioxide	The CO₂ gas forms bubbles that make the dough rise, giving light bread.
Alcoholic drinks	ethanol	The ethanol is the alcohol in beer and wine.

Worked walk-through — why bread rises:

1. Yeast is mixed into the dough with sugar (glucose).
2. With little oxygen inside the dough, the yeast respires anaerobically (ferments).
3. Fermentation releases carbon dioxide gas.
4. The gas is trapped as bubbles in the stretchy dough, so the dough expands and rises.
5. Baking kills the yeast and evaporates most of the small amount of ethanol, leaving the airy texture.

In brewing and winemaking the roles flip: the ethanol is the valuable product, and the carbon dioxide is what makes some drinks fizzy.

The specification asks you to compare aerobic and anaerobic respiration on three points: the need for oxygen, the products, and the relative amount of energy transferred. This table pulls everything together.

Feature	Aerobic respiration	Anaerobic respiration (muscles)	Anaerobic respiration (plant/yeast)
Oxygen needed?	Yes	No	No
Main location	Mitochondria	Cytoplasm of muscle cells	Cytoplasm of plant/yeast cells
Products	carbon dioxide + water	lactic acid	ethanol + carbon dioxide
Breakdown of glucose	Complete	Incomplete	Incomplete
Relative energy	Large amount per glucose	Much less per glucose	Much less per glucose

Worked walk-through — justifying the energy difference:

Suppose a sprinter and a jogger each use one glucose molecule.

1. The jogger has enough oxygen, so respires aerobically: glucose is broken down completely to CO₂ and water.
2. The sprinter's muscles outrun their oxygen supply, so they respire anaerobically: glucose is broken down only partially to lactic acid.
3. Complete breakdown releases more of the energy stored in the glucose; partial breakdown leaves much of that energy locked in the lactic acid.
4. Therefore the aerobic route transfers much more energy per glucose molecule — the anaerobic route is faster to start but far less efficient.

Exam phrasing to use: anaerobic respiration transfers less energy per glucose molecule because oxidation of glucose is incomplete. Both halves of that sentence earn marks.

1. Calling respiration "breathing"

Respiration is a chemical reaction inside cells that transfers energy from glucose. Breathing (ventilation) is the movement of air into and out of the lungs. Using "respiration" to mean breathing loses marks.

2. Saying respiration is endothermic

Respiration is exothermic — it transfers energy to the surroundings, including some as heat. Photosynthesis is the endothermic process; do not mix them up.

3. Forgetting why anaerobic transfers less energy

The mark is for the reason: glucose is only partially broken down (incomplete oxidation). "It makes less energy" without the reason is an incomplete answer.

4. Mixing up the anaerobic products

Muscle cells produce lactic acid only. Plant and yeast cells produce ethanol + carbon dioxide. Swapping these, or adding carbon dioxide to the muscle equation, is a frequent error.

5. Unbalancing the symbol equation

Each coefficient multiplies the whole molecule: $6O_2$ is 12 oxygen atoms, and $6H_{2}O$ is 12 hydrogen and 6 oxygen atoms. Check that carbon (6), hydrogen (12) and oxygen (18) all balance on both sides.

6. Confusing the use of fermentation in bread and brewing

In bread the useful product is carbon dioxide (it makes dough rise). In alcoholic drinks the useful product is ethanol. The reaction is the same; only the product we want changes.