Coaching Centre
Cellular respiration
The stages of aerobic respiration, ATP, what the energy is used for, anaerobic respiration in muscles and yeast, net gas exchange and the compensation point, shown with indicator tube experiments.
Right now, in every one of your trillions of cells, glucose is being "burned" without any flames. It's the same overall reaction as a campfire: fuel plus oxygen gives carbon dioxide, water and energy. Your cells just do it slowly, in dozens of small steps, so the energy can be captured instead of escaping as heat and light.
Aerobic respiration in more detail
In Year 8 (2B) you met the respiration equation. When oxygen is available, cells carry out aerobic respiration:
glucose + oxygen → carbon dioxide + water + energy
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
It doesn't happen in one step. The glucose is first split in half in the cytoplasm, which releases a small amount of energy and needs no oxygen. The two halves then go into the mitochondria, where they are broken down completely to carbon dioxide, and the hydrogen they carried is finally combined with oxygen to make water. That last stage is where most of the energy is released, and it is why you need a constant supply of oxygen.
The released energy isn't used directly. It is stored in a small molecule called ATP, which acts like a rechargeable battery: cells "spend" ATP wherever energy is needed, then recharge it by respiration. Your body recycles roughly its own body mass of ATP every day. Cells that work hard, such as heart muscle and sperm cells, are packed with mitochondria.
Breathing (moving air in and out of the lungs) is not the same thing as respiration (the chemical reaction in cells). Breathing exists to supply the oxygen and remove the carbon dioxide.
What the energy is used for
- Building molecules: making proteins, DNA and new cells for growth and repair.
- Movement: muscle contraction, from running to your heartbeat.
- Transport: pumping substances into and out of cells.
- Communication: sending nerve signals and making hormones.
- Keeping warm: much of the energy ends up as heat, which keeps mammals and birds at a steady body temperature. This is also why energy is "lost" at every step of a food chain.
Without oxygen
When oxygen runs short, some cells can release a little energy from glucose without it. This is anaerobic respiration. It releases far less energy than aerobic respiration (about one-sixteenth as much per glucose molecule).
- In hard-working muscles: glucose → lactic acid + a little energy. This is what happens in a sprint finish. Your heavy breathing afterwards supplies the extra oxygen needed to deal with the lactic acid.
- In yeast (fermentation): glucose → alcohol (ethanol) + carbon dioxide + a little energy. The carbon dioxide makes bread rise.
Net gas exchange and the compensation point
A plant photosynthesises only in light but respires all the time, so what you measure from a leaf is the difference between the two processes: its net gas exchange.
- In the dark, only respiration happens: the plant takes in oxygen and gives out carbon dioxide.
- In dim light, photosynthesis uses some of the carbon dioxide from respiration, but not all of it. The plant still gives out carbon dioxide overall.
- At the compensation point, photosynthesis exactly balances respiration. There is no overall gas exchange, and the plant makes no net sugar: everything it makes, it uses.
- Above the compensation point, the plant takes in carbon dioxide and gives out oxygen overall, and the extra sugar can be used to grow.
A plant must spend more time above its compensation point than below it, or it slowly starves. Plants of the rainforest floor have a very low compensation point: they respire slowly and can make a profit from light so dim that a sun-loving plant would starve in it. This is why a wheat crop dies in a shady corner where ferns do well.
The same idea applies to whole ecosystems. Over a year, a growing forest takes in more carbon dioxide than it gives out and acts as a carbon sink (3A). A mature forest in balance takes in about as much as its plants, animals and decomposers release.
You can test for carbon dioxide with limewater, which turns milky when CO₂ bubbles through it, or with hydrogencarbonate indicator, which changes colour as CO₂ levels change.
- Predict the colour of all four tubes in bright light, then run it. Were you right?
- Repeat in the dark. Why does the pondweed tube change?
- Find the light level where the pondweed tube hardly changes. What is happening?
- Why is the empty tube needed?
Test yourself
Aerobic respiration mainly takes place in the
Mitochondria release energy from glucose.
Yeast respiring without oxygen produces
Fermentation makes ethanol and CO₂.
A sealed tube contains pondweed and hydrogencarbonate indicator and is left in the dark. The indicator will turn
No light, no photosynthesis; respiration adds CO₂.
Why do you keep breathing hard after a sprint?
Extra oxygen is needed to break down the lactic acid.
Label the compensation point graph.
What is the plant's net gas exchange?
Cells store the energy released by respiration in a molecule called . The waste products of aerobic respiration are carbon dioxide and . In muscles without enough oxygen, acid is made. Carbon dioxide turns milky. The light level where photosynthesis and respiration are equal is the point.
A gardener moves a sun-loving tomato plant into the deep shade under a tree, next to a fern from the rainforest floor. After a few weeks the tomato is dying but the fern is healthy. Explain this using the compensation point.
The compensation point is the light intensity at which a plant's rate of photosynthesis equals its rate of respiration, so it makes no net sugar. A sun-loving plant such as a tomato has a high compensation point. In deep shade the light is below that level for most of the day and all night, so the tomato uses more glucose in respiration than it makes in photosynthesis. It has to draw on its stores, can't grow, and slowly starves. A rainforest-floor fern respires slowly and photosynthesises efficiently in dim light, so its compensation point is much lower. Even in deep shade it spends much of the day above its compensation point and makes a small net profit of sugar to grow with.