GCSE/iGCSE Biology - Year 1 Week 11
- Sallyann Clark
- 1 day ago
- 7 min read
Do Cells Breathe?
How Does Your Body Actually Get Energy Out of Food?
The Big Question This Week
"If glucose contains energy, how does your body actually get that energy out of it and into a form it can use?"
Write this in your learning journal as a title before you begin. Don't look anything up yet, just think. If you prepared this question at the end of Week 10, go back and read what you wrote.
Before You Begin — Predict!
Write down whether you think these are true or false, and why:
"Breathing and respiration are just two words for the same thing."
"Your muscles can only produce energy when there's enough oxygen available."
Hold onto your predictions. We will return to them at the end of the week.
Story of the Week
Here's a question that trips a lot of people up at first: does a plant "breathe"?
Does a bacterium?
The answer depends entirely on which of two very different things you mean by "breathe." There's breathing, the physical, mechanical act of moving air in and out of lungs, which only some animals do. And there's respiration, a chemical process happening inside every single living cell, all the time, in every organism on Earth, whether or not it has lungs at all. Mixing these two up is one of the most common confusions in GCSE biology, so it's worth getting the distinction crystal clear right from the start of this week.
Respiration is how a cell releases the energy locked inside glucose and transfers it into a more immediately usable form: a molecule called ATP, which powers almost everything a cell does: muscle contraction, active transport, building new molecules, and much more. Most of the time, this happens with the help of oxygen, in a process called aerobic respiration, which releases a large amount of energy efficiently. But sometimes, during intense exercise, or in organisms living in low-oxygen environments, cells switch to anaerobic respiration, a faster but far less efficient backup process that doesn't need oxygen at all.
This week we look at both pathways, follow the journey from glucose to ATP inside the mitochondria, and meet a scientific theory about combustion and breathing that was believed by some of history's greatest scientists for over a century — and turned out to be completely, spectacularly wrong.
Video Session One
While you watch, look out for:
Why mitochondria are described as the "powerhouse of the cell," and what molecule they mainly produce
The overall word equation for aerobic respiration, and how it compares to the equation for photosynthesis you met last week
Why aerobic respiration releases so much more energy than anaerobic respiration
Draw the word equation for aerobic respiration in your journal: glucose + oxygen → carbon dioxide + water (+ energy released).
Underneath it, write one sentence explaining how this equation compares to photosynthesis's equation from Week 9.
Video Session Two
While you watch, look out for:
What happens to a cell's ability to make ATP when oxygen isn't available
The difference between alcoholic fermentation (in yeast and some plant cells) and lactic acid fermentation (in animal muscle cells)
Why anaerobic respiration, despite being much less efficient than aerobic respiration, is still useful to have as a backup option
⚠️ Common Misconception Alert
A very common mix-up: students sometimes think anaerobic respiration is simply "respiration without oxygen, but otherwise the same as aerobic respiration." It isn't. Anaerobic respiration in human muscle cells produces lactic acid instead of carbon dioxide and water, releases far less energy per glucose molecule, and cannot continue indefinitely; lactic acid builds up and eventually needs to be broken down once oxygen becomes available again. It's a genuinely different chemical pathway, not just a lower-oxygen version of the same one.
Here is a good place to stop if you need to.
Reading Assignment
Booklet Readings 18 and 19 accompany this week, going into more detail on oxygen debt during exercise and a persistent popular myth about lactic acid that even some textbooks have got wrong.
Hands-On Activity — Measuring the Effect of Exercise on Breathing and Pulse Rate
What you need: A stopwatch, and space to do some light exercise safely (star jumps or brisk stepping on the spot work well). If you have any heart or breathing condition, check with a parent or guardian before doing vigorous exercise, and stop immediately if you feel unwell.
What to do:
At rest, sitting quietly, count your pulse for 15 seconds (feel for it at your wrist or neck) and multiply by 4 to get beats per minute. Also count your breaths for 15 seconds and multiply by 4 for breaths per minute. Record both.
Do one minute of moderate exercise (e.g. star jumps or stepping on the spot).
Immediately after stopping, measure your pulse and breathing rate again using the same method.
Rest, and measure again every minute for five minutes, recording how long it takes both rates to return to your resting values.
In your journal, plot a simple graph of pulse rate (y-axis) against time (x-axis), marking the point where exercise stopped.
Write two or three sentences explaining, in terms of the body's need for oxygen and removal of carbon dioxide, why both breathing rate and pulse rate increase during exercise and take time to return to normal afterwards.
Watch Cognitio - Exercise and Oxygen Debt.
The Brilliant Mistake — Stories Science Gets Wrong
This Week's Story: The Fire-Stuff That Didn't Exist
For roughly a hundred years, from the late 1600s onward, many of Europe's leading scientists believed that a mysterious, weightless substance called phlogiston was released whenever something burned, or, they reasoned, whenever an animal breathed. According to phlogiston theory, wood, coal, and living tissue all contained phlogiston, and burning or breathing was simply the process of that phlogiston escaping into the air. A candle would eventually go out inside a sealed jar, phlogiston theorists explained, because the air inside had become "saturated" with phlogiston and could absorb no more.
It was an elegant idea, and it explained a genuinely wide range of observations reasonably well, which is exactly why it survived for so long. In 1774, the English scientist Joseph Priestley isolated a gas that made candles burn unusually brightly and kept mice alive far longer than ordinary air. Priestley, a committed believer in phlogiston theory to the end of his life, called this new gas "dephlogisticated air", air with all its phlogiston removed, he reasoned, which was therefore extra hungry to absorb more.
It took the French scientist Antoine Lavoisier, working with many of the same experimental results Priestley had already produced, to see the situation completely differently. Lavoisier proposed that there was no phlogiston at all. Instead, he argued, this new gas, which he named oxygen, was actively combined with substances during burning and breathing, not released from them.
Lavoisier carried out careful quantitative experiments, weighing substances precisely before and after burning, and showed that burned materials actually gained mass by combining with oxygen from the air, the exact opposite of what phlogiston theory predicted. He extended the same reasoning to breathing itself, showing that animals consume oxygen and produce carbon dioxide, establishing, for the first time, that respiration is fundamentally a form of slow combustion happening inside the body.
Journal Prompt:
Priestley discovered the very gas that disproved his own theory, yet he never abandoned phlogiston theory. Why do you think a scientist might hold onto a familiar idea even in the face of contradicting evidence?
Lavoisier's key tool was careful, precise weighing. Why might a simple change in measurement approach (weighing things precisely, rather than just observing them) be enough to overturn a century-old theory?
Phlogiston theory explained a lot of real observations, just with the wrong underlying cause. Can you think of another idea from earlier in this course that also "worked" reasonably well for a long time despite being fundamentally wrong?
Scientist Portrait
Antoine Lavoisier (1743–1794)
Add Antoine Lavoisier to your Scientist journal. Draw or print a portrait. Beneath it, write: "He weighed his way to the truth and showed that breathing, like burning, is really about oxygen — not a mysterious fire-stuff escaping."
Return to Your Predictions
Go back to the two statements from the start of the week:
"Breathing and respiration are just two words for the same thing."
"Your muscles can only produce energy when there's enough oxygen available."
Rewrite each one now, using what you've learned this week.
The Big Question — Final Answer
Return to this week's big question:
"If glucose contains energy, how does your body actually get that energy out of it and into a form it can use?"
Write a paragraph in your learning journal. A strong answer will mention aerobic respiration, ATP, and briefly explain what happens instead during anaerobic respiration.
Practice Questions — 10 Question Set
Write the word equation for aerobic respiration.
Name the organelle where most aerobic respiration takes place.
What is ATP, and why is it important to cells?
Explain the difference between breathing and respiration.
Write the word equation for anaerobic respiration in human muscle cells.
Give one reason why anaerobic respiration releases much less energy than aerobic respiration.
In the exercise practical, explain why pulse rate and breathing rate both increase during exercise.
What is meant by an "oxygen debt," and why does it occur after intense exercise?
Joseph Priestley discovered oxygen but still believed in phlogiston theory. What did Antoine Lavoisier show that phlogiston theory could not explain?
Name the two different products of anaerobic respiration in yeast compared with human muscle cells.
Week 11 Checklist
Before moving to Week 12, tick off each item:
Watched Crash Course Biology #27: Cellular Respiration
Drawn and labelled the word equation for aerobic respiration
Watched Amoeba Sisters: Fermentation
Completed the exercise/pulse rate practical and plotted your results
Read the phlogiston/Lavoisier story and answered the journal prompts
Added Antoine Lavoisier to your Scientist journal
Rewritten your two predictions from the start of the week
Answered the 10 practice questions
Written a final answer to the Big Question
Looking Ahead — Week 12 Preview
Next week we zoom back in on plants and look at how they exchange gases with the world around them — and why the leaves that let carbon dioxide in are also, unavoidably, letting precious water escape at the same time.
Prepare by writing this question in your journal: "If a plant needs to let carbon dioxide in to survive, how does it stop itself from drying out in the process?"
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