Respiration is not breathing. It is a set of chemical reactions inside every living cell that break down nutrient molecules such as glucose and release the energy stored in them. Muscles, nerves, growing roots and dividing cells all depend on it. Because the reactions are controlled by enzymes, temperature changes the rate, and yeast lets you measure that in a flask.
Respiration is the chemical reactions in cells that break down nutrient molecules to release energy. Organisms use that energy for muscle contraction, protein synthesis, cell division, active transport, growth, the passage of nerve impulses and the maintenance of a constant body temperature. Respiration is controlled by enzymes, so in yeast its rate rises with temperature up to an optimum and then falls as the enzymes are denatured.
In the Respiration tab a glucose molecule enters the cell and is broken down; the yellow sparks show the energy released. Watch the "Energy used for" card: it cycles through the seven uses you must be able to state.
| Use of energy | Example |
|---|---|
| Muscle contraction | leg muscles in running; the heart beating; the diaphragm in breathing |
| Protein synthesis | joining amino acids into enzymes, antibodies and haemoglobin |
| Cell division | new cells for growth and repair, and gametes |
| Active transport | root hair cells taking up nitrate ions against a gradient |
| Growth | making new cytoplasm, membranes and cell walls |
| Passage of nerve impulses | impulses travelling along neurones |
| Constant body temperature | mammals and birds staying near 37 °C in cold surroundings |
Respiration releases energy that is already stored in glucose. Never write that respiration "makes" or "produces" energy, and do not confuse respiration (a chemical process in cells) with breathing (ventilation, moving air in and out of the lungs).
Yeast is a single-celled fungus. Given sugar solution it respires and gives off carbon dioxide, which collects in the balloon over the flask. In the Yeast tab choose a water bath temperature, press Leave for 10 minutes and read the volume of carbon dioxide into the results table. Repeat for each temperature.
| Water bath / °C | 10 | 20 | 30 | 40 | 50 | 60 |
|---|---|---|---|---|---|---|
| CO2 in 10 min / cm3 | 20 | 40 | 80 | 123 | 18 | 1 |
The temperature axis is the independent variable and the carbon dioxide volume is the dependent variable. Do not say the yeast "dies at the optimum": the optimum is where respiration is fastest; it is above the optimum that the enzymes denature.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the Respiration tab, which of these appears on the "Energy used for" card?
In the Yeast tab, which water bath gave the largest volume of carbon dioxide in 10 minutes?
Why did the yeast at 60 °C give almost no carbon dioxide?
In the yeast investigation, what does the size of the balloon measure?
The same idea works with germinating seeds or small invertebrates such as maggots or woodlice in a sealed tube. They give out carbon dioxide and heat, and they use up oxygen.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Which list contains only uses of energy released by respiration in humans?
Why do mammals need energy from respiration even when lying still in a cold room?
A student measures yeast respiration at 15, 25, 35 and 45 °C. Which is a control variable?
Germinating peas in a vacuum flask become warmer, but boiled peas do not. The best explanation is:
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