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Cellular respiration
How cells release energy from glucose in their mitochondria, the word equation for respiration, the difference between respiration and breathing, how activity changes the rate of respiration, testing for carbon dioxide with limewater, and respiration without oxygen.
Hold your breath and you can manage maybe a minute. Stop eating and you can last weeks. Both are the same problem, though: every cell in your body is constantly burning glucose with oxygen to release energy, and it never stops, even when you're asleep. Your brain alone uses about a fifth of the energy you get from food.
Why cells need energy
Every job a cell does needs energy: muscle cells contracting, nerve cells sending messages, cells building new proteins, dividing, pumping substances across their membranes, and keeping your body at 37 °C. That energy comes from food, but it can't be used straight from your lunch. Food is digested into small molecules such as glucose, a sugar, which the blood carries to every cell.
Cellular respiration
Cellular respiration is the chemical reaction cells use to release the energy stored in glucose. It happens mostly inside the mitochondria, and it needs oxygen:
glucose + oxygen → carbon dioxide + water + energy
- Glucose comes from digested food (in plants, from photosynthesis).
- Oxygen comes from the air you breathe in, carried by red blood cells.
- Carbon dioxide is a waste. The blood carries it to the lungs and you breathe it out.
- Water is also produced. Some leaves in your breath, which is why breath fogs up a cold window.
- Energy is released. Some is used by the cell; much of it is released as heat, which keeps you warm.
Plants respire too, all the time, in every living cell, day and night. In the daytime their photosynthesis is usually faster than their respiration, so they take in more carbon dioxide than they give out.
- Start with sleeping, then step up to walking and jogging. What happens to the flow of glucose and oxygen into the mitochondria?
- Watch the breathing rate and heart rate. Why do both rise together?
- Watch the limewater. Press "Fresh limewater" and compare how fast it goes milky when sitting and when jogging.
- Try sprinting. What changes when oxygen can't arrive fast enough?
Respiration is not breathing
In everyday speech "respiration" often means breathing, but in science they are different things.
| Breathing | Cellular respiration | |
|---|---|---|
| What it is | Moving air in and out of the lungs | A chemical reaction that releases energy from glucose |
| Where | Lungs, ribs, diaphragm | Inside every living cell, mainly in mitochondria |
| Which organisms | Animals with lungs | Every living organism, including plants and bacteria |
| Link | Breathing supplies the oxygen for respiration and removes the carbon dioxide it produces. | |
More activity, more respiration
When you exercise, your muscle cells need much more energy, so they respire faster. That means they need glucose and oxygen delivered faster and carbon dioxide removed faster. Your body responds by breathing faster and deeper and by making your heart beat faster to pump blood around more quickly. You also get hot and sweaty, because more respiration releases more heat.
Testing for carbon dioxide
Limewater is a clear solution of calcium hydroxide. When carbon dioxide bubbles through it, a white solid forms and the limewater turns milky. This is the standard test for carbon dioxide.
When oxygen runs short
During a flat-out sprint your blood can't deliver oxygen fast enough. Muscle cells can still release a little energy from glucose without oxygen, called anaerobic respiration. It releases far less energy and makes lactic acid, which builds up in the muscles. That's why you can only sprint for a short time, and why you keep breathing hard after you stop: you are taking in extra oxygen to deal with the lactic acid.
Yeast does something similar. Without oxygen it turns sugar into alcohol and carbon dioxide. The carbon dioxide bubbles are what make bread rise.
Test yourself
Cellular respiration mainly takes place in the
Mitochondria are where glucose and oxygen react to release energy.
Which are the two products (besides energy) of cellular respiration?
Glucose + oxygen → carbon dioxide + water + energy.
Limewater turning milky shows the presence of
This is the standard test for carbon dioxide.
Which organisms carry out cellular respiration?
Every living cell respires, including plant cells and bacteria.
Your heart rate goes up during exercise mainly to
Faster respiration in muscle cells needs faster delivery of its reactants.
Breathing or cellular respiration?
In cellular respiration, reacts with to release energy. The waste gas, , is carried by the blood to the lungs. When there is not enough oxygen, muscle cells make . Much of the energy released is given off as .
A student's breathing rate is 14 breaths per minute while sitting and 40 breaths per minute after running. Explain this change, referring to cellular respiration.
When running, the muscle cells need much more energy to contract. They get it from cellular respiration in their mitochondria, where glucose reacts with oxygen to release energy, producing carbon dioxide and water. To release more energy, the cells respire faster, so they need more oxygen and produce more carbon dioxide. The student breathes faster to take in the extra oxygen and to breathe out the extra carbon dioxide.