Respiration · Process
Aerobic respiration
The reaction that keeps you alive is the photosynthesis equation run backwards, at body temperature, without a flame — and its main product leaves through your mouth without you noticing.
Start here
Someone loses 10 kg of fat. Where does the 10 kg go?
The mass has to go somewhere — matter is not destroyed, as you established in Conservation of mass. Ten kilograms left a person's body, and almost nobody, including most adults, can say by which exit.
Which way did it leave?
Most of it was breathed out as carbon dioxide, and the rest left as water. Fat is mostly carbon and hydrogen; respiration joins that carbon to oxygen and the result is a gas you exhale. You lose weight through your lungs. Nothing was converted into energy — energy is transferred, and the atoms all still exist somewhere.
At the bench · weigh both sides
The books have to balance
ledger only
The energy transferred is not part of either total.
Glucose respired
A banana
About 25 g of sugar, once the starch in it has been digested.
Goes in
- Glucose25.0 g
- Oxygen26.7 g
Comes out
- Carbon dioxide36.7 g
- Water15.0 g
Total in 51.7 g
Total out 51.7 g
Energy transferred 390 kJ
Which way each product leaves
Carbon dioxide
Dissolves into the blood, is carried to the lungs, and diffuses into the alveoli. You breathe it out without noticing — this is the exit almost nobody names.
Water
Some is used by the body, some leaves in urine and sweat, and a surprising amount leaves as vapour in your breath. Breathe on a cold window and that is respiration you are looking at.
Energy
Not an exit at all. It is transferred to the cell to do work, and what is not used usefully warms you — which is why a room full of people gets hot.
Two of those three have mass and are in the ledger. The third has none, which is exactly why it is listed separately — energy is not a substance leaving the body.
The word summary
Photosynthesis, read from right to left.
glucose + oxygen
carbon dioxide + water
energy is transferred from the glucose to the cell
Where
In the mitochondria
Almost every cell has them, and cells that work hardest have the most. Not in the lungs, not in the blood — in the cells themselves. The red blood cell is the exception you met in Specialised cells, and having none is why it cannot use the oxygen it carries.
When
Continuously
There is no off switch. A cell that stops respiring for more than a few minutes dies, which is what makes this the reaction the others depend on.
What for
Every other process
Movement, keeping warm, building new molecules, active transport, nerve signals. The energy is not stored again — it is spent.
Key fact
Aerobic respiration is glucose + oxygen giving carbon dioxide + water, releasing energy the cell can use. It happens in the mitochondria of almost every cell, continuously, and it is a chemical reaction — not breathing, which is the muscular job that supplies it. The red blood cell is the exception you met in Specialised cells, and it is why it cannot use its own cargo.
Think again
“Respiration is just slow burning.”
The overall equation is the same as burning sugar, and that is where the resemblance stops. Set fire to glucose and it releases everything it has in one uncontrolled rush at several hundred degrees — useless to a cell, and fatal to it. Respiration takes the same molecule apart in a long series of small enzyme-controlled steps at 37 °C, capturing the energy in usable amounts along the way, and it needs no flame and no spark to start. The comparison is worth making precisely because of where it breaks: a fire is a reaction that has escaped control, and a cell is the opposite. The other half of the confusion is the word itself. Respiration is a chemical reaction inside cells; breathing is the muscular job that delivers the oxygen for it, and that distinction is the subject of The gas exchange system, which is the lesson to revisit if it still feels blurred.
“The fat is converted into energy, so the mass disappears.”
Mass cannot turn into energy in a chemical reaction — that is the whole content of Conservation of mass, and biology does not get an exemption. Look at the ledger above: every atom that goes in comes out, and the two totals are identical to the gram. What the glucose or fat supplies is a store of chemical energy, and respiration transfers that energy to the cell while rearranging exactly the same atoms into carbon dioxide and water. The carbon you exhale today was in your breakfast, and before that in a plant, and before that in the air. Weigh the reactants and weigh the products and nothing has gone missing; the only thing that has moved is energy, from a store to where it was needed.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · The summary
Which is the word summary for aerobic respiration?
Rung 2 · The one that catches people
A person loses 10 kg of fat over a year. Where has almost all of that mass gone?
Rung 3 · Explain the exercise change
When you run, you breathe faster and deeper and your heart beats harder. Explain why, starting from what your muscle cells are doing.
Rung 4 · Take it somewhere new
Germinating peas are sealed in a flask with a thermometer, alongside a second flask of peas that have been boiled and cooled. Both are left overnight. Predict what happens to the temperature, the oxygen and the carbon dioxide in each flask, and explain why the boiled peas are there.
Key note
Glucose + oxygen gives carbon dioxide + water, and energy is transferred to the cell. It happens in the mitochondria of almost every cell, all the time, in small enzyme-controlled steps at body temperature. The red blood cell is the exception, and having no mitochondria is why it cannot use its own cargo. No mass is lost: every atom in the reactants appears in the products, which is why weight lost as fat leaves mostly through the lungs.
Going further
Mitochondria are where the reaction happens, and how many a cell has tells you what it does for a living. A heart muscle cell, contracting every second of your life without a break, is packed with them — by some counts a third of its volume. A skin cell has a fraction of that. Sperm cells carry a spiral of mitochondria wrapped around the base of the tail, powering the only journey they will ever make, and you met that in Gametes and fertilisation. Stranger still, mitochondria have their own small loop of DNA and divide on their own schedule, which is why the accepted explanation is that they began as free-living bacteria absorbed by a larger cell about a billion and a half years ago and never left. Every breath you take supplies oxygen to what were once someone else's cells.
Before this lesson
Connects to
At GCSE this becomes
- The balanced symbol equation, ATP as the energy currency, and the metabolic rate calculations behind respirometer data.
Where to next
Ask Mr Badmus AI
Want to check where the atoms in your breakfast ended up?
If any of this is about you or someone you know, talk to someone you trust — a parent or carer, a teacher, your school nurse, a pharmacist or your GP. Out of school hours: Childline — 0800 1111, free and confidential.
The ledger is calculated from the balanced equation and rounded, so the two totals match to the gram by construction. Real bodies respire fats and proteins as well as glucose, the proportions shift with diet and activity, and some of the water made is retained rather than exhaled. Energy figures are approximate values per gram of glucose.
Lesson content © MrBadmusAI.