Course 4.2Lesson 1 of 3

Breaking down glucose

Respiration releases the energy of food in every living cell. Glucose is first split into pyruvate in the cytoplasm. Without oxygen, yeast makes ethanol and carbon dioxide, and our muscles make lactic acid. With oxygen, in the mitochondria, pyruvate becomes carbon dioxide and water, and much more energy is released. The energy is stored as ATP.

about 25 min1 simulation1 practice question1 Python program5 questions to check yourself

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Why do cells need respiration?

You eat food, and then you can run, think and keep warm. All of these need energy. The energy is locked in the food, mostly in glucose. Every living cell breaks glucose down to release this energy. This process is called respiration.

Respiration goes on in every cell of every living thing. It goes on in plants and in animals, by day and at night. It is not the same as breathing. Breathing moves air into and out of the body. Respiration is the breakdown of food inside the cells.

The first step: glucose to pyruvate

Glucose is broken down in steps. The first step is the same in all organisms.

It takes place in the cytoplasm of the cell.
A glucose molecule has six carbon atoms. It is split into two molecules of pyruvate, with three carbon atoms each.
A little energy is released.

What happens to the pyruvate next depends on whether oxygen is present.

A flow chart of respiration. Glucose, a 6-carbon molecule, is split in the cytoplasm into pyruvate, a 3-carbon molecule, with a little energy. Three arrows lead from pyruvate. With no oxygen, in yeast, it becomes ethanol and carbon dioxide with a little energy. With a lack of oxygen, in our muscle cells, it becomes lactic acid with a little energy. With oxygen, in the mitochondria, it becomes carbon dioxide and water with much more energy. A note says that one glucose gives about 2 ATP without oxygen and about 38 ATP with oxygen.

Without oxygen: anaerobic respiration

Respiration without oxygen is called anaerobic respiration.

In yeast. Yeast cells change pyruvate into ethanol, a kind of alcohol, and carbon dioxide. This is called fermentation. The bubbles of carbon dioxide make bread dough rise.
In our muscles. During hard exercise, oxygen cannot reach the muscle cells fast enough. Then pyruvate changes into lactic acid. Lactic acid builds up in the muscles and causes cramps.

Anaerobic respiration releases only a little energy. The glucose is not broken down fully, so ethanol and lactic acid still hold much of its energy.

With oxygen: aerobic respiration

Respiration with oxygen is called aerobic respiration. It takes place in the mitochondria. There, pyruvate is broken down fully into carbon dioxide and water, and much more energy is released.

\displaystyle \text{glucose} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water} + \text{energy}

\displaystyle \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2 \rightarrow 6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + \text{energy}

Most of the energy that we use comes from aerobic respiration. That is why we must breathe in oxygen all the time.

ATP: the energy currency of the cell

The energy released in respiration is not used at once. The cell stores it in a molecule called ATP (adenosine triphosphate). ATP works like money. The cell earns it in respiration and spends it on its work.

When ATP is broken down, a fixed amount of energy is released.
The cell spends this energy to contract muscles, to make proteins and to send nerve messages.
From one glucose molecule, anaerobic respiration makes about 2 ATP.
Aerobic respiration makes about 38 ATP. This is the value used in school books; newer measurements give a little less.

So, from the same glucose, aerobic respiration gives about $38 \div 2 = 19$ times as much energy.

You should find that $10$ glucose molecules give $380$ ATP when all of them meet oxygen, but only $20$ ATP when none of them do.

Try it in Python

The program works out the ATP from $12$ glucose molecules, with oxygen and without it.

You should see $456$ ATP with oxygen and $24$ ATP without it, so oxygen gives $19$ times as much. The last line stays the same for any number of glucose molecules, because each molecule gives the same amount.

Practise with new numbers

Common mistakes

Thinking respiration means breathing. Breathing moves air. Respiration releases energy from food inside the cells.
Thinking plants do not respire. Plants respire all the time, by day and at night, just like animals.
Mixing up the products. Yeast makes ethanol and carbon dioxide. Our muscles make lactic acid and no carbon dioxide.
Putting the first step in the mitochondria. Glucose is split into pyruvate in the cytoplasm. Only the steps with oxygen happen in the mitochondria.

Remember these

Respiration releases energy from food, mostly glucose, in every living cell.
Glucose is split into pyruvate in the cytoplasm.
Without oxygen, yeast makes ethanol and carbon dioxide, and muscles make lactic acid. Little energy is released.
With oxygen, in the mitochondria, pyruvate becomes carbon dioxide and water, with much more energy.
The energy is stored as ATP, the energy currency of the cell.