Cellular respiration is the process by which cells release energy from glucose. It happens in every living cell, in plants as well as animals, all the time — which is the point most often confused with photosynthesis in examinations.
This page sets out what respiration requires, how aerobic and anaerobic respiration differ, and how each requirement is demonstrated in the standard school practicals.
Requirements at a Glance
What cellular respiration requires.
- Glucose — the respiratory substrate, from food or from photosynthesis
- Oxygen — for aerobic respiration only
- Enzymes — the whole process is enzyme-controlled
- A suitable temperature, because those enzymes have an optimum
- Living cells with functioning mitochondria, for aerobic respiration
- A means of transporting glucose and oxygen to the cells, and removing waste
- Water, as the medium in which the reactions take place
Raw Materials Required for Respiration
The balanced equation for aerobic respiration: glucose plus oxygen produces carbon dioxide, water and energy. In symbols, C6H12O6 + 6O2 → 6CO2 + 6H2O + energy. Note that this is the reverse of the photosynthesis equation, which is why the two are so easily confused.
Respiration is not breathing. Breathing, or ventilation, is the physical movement of air into and out of the lungs. Respiration is the chemical release of energy inside cells. Losing marks for using the two interchangeably is extremely common.
The energy is released as ATP, not as heat alone. ATP is the usable energy currency of the cell, and heat is a by-product.
Glucose comes from digested food in animals, and in plants from photosynthesis or from stored starch converted back to glucose.
Oxygen reaches the cells through the breathing system and the blood in animals, and by diffusion through stomata and air spaces in plants.
Plants respire continuously, day and night, while photosynthesis only occurs in light. At night a plant only respires, which is why it takes in oxygen and releases carbon dioxide at night.
Aerobic and Anaerobic Requirements Compared
Aerobic respiration requires oxygen and releases a large amount of energy from each glucose molecule. It takes place mainly in the mitochondria, and its products are carbon dioxide and water.
Anaerobic respiration takes place without oxygen and releases far less energy per glucose molecule, because the glucose is only partly broken down.
In human muscle, anaerobic respiration produces lactic acid. This happens during vigorous exercise when the blood cannot deliver oxygen fast enough. The lactic acid accumulation causes muscle fatigue, and the oxygen debt is repaid afterwards by continued heavy breathing.
In yeast, anaerobic respiration produces ethanol and carbon dioxide. This is fermentation, and it is the basis of brewing and of bread making — the carbon dioxide makes the dough rise and the ethanol evaporates during baking.
Cells with high energy demands have more mitochondria — muscle cells, liver cells and sperm cells are the usual examples.
Temperature affects the rate because respiration is enzyme-controlled. The rate rises to an optimum and then falls sharply as the enzymes denature.
Products and Where the Energy Goes
Products and their fate.
- Energy as ATP — used for muscle contraction, active transport, cell division, growth and the synthesis of new molecules
- Heat — a by-product, and the source of body heat in mammals and birds
- Carbon dioxide — a waste product, removed through the lungs in animals and the stomata in plants
- Water — produced in aerobic respiration and used or excreted
- Lactic acid — produced in anaerobic respiration in muscle, causing fatigue until it is broken down
- Ethanol and carbon dioxide — produced in anaerobic respiration in yeast
- A far smaller energy yield from anaerobic respiration, because the glucose is only partly broken down
How the Requirements Are Tested in Practicals
Showing carbon dioxide is released: pass the exhaled or evolved air through limewater. Limewater turns milky in the presence of carbon dioxide. Comparing inhaled and exhaled air through two limewater tubes is the standard set-up.
Hydrogencarbonate indicator is the more sensitive alternative, changing from red to yellow as carbon dioxide concentration rises.
Showing heat is released: place germinating seeds in a vacuum flask with a thermometer, alongside a control flask of dead boiled seeds that have been disinfected. The temperature rises only in the flask of living seeds. The dead seed control is essential, and it is where the marks are.
The seeds must be disinfected in the control, so that any temperature rise cannot be attributed to microorganisms rather than to the seeds. Examiners ask why.
Showing oxygen is used: use a respirometer, with soda lime or potassium hydroxide to absorb the carbon dioxide produced. The liquid in the manometer moves as oxygen is consumed.
Showing fermentation in yeast: mix yeast, glucose and warm water under a layer of oil to exclude air, and pass the gas through limewater. Bubbles and milky limewater show carbon dioxide is produced without oxygen.
Always name your controls and constants. Temperature, volume, concentration and time held constant, and a suitable control — these carry marks in every practical question.
Frequently Asked Questions
What does respiration require?
Glucose, enzymes, a suitable temperature and living cells — plus oxygen for aerobic respiration. Anaerobic respiration proceeds without oxygen but releases far less energy.
Is respiration the same as breathing?
No. Breathing is the physical movement of air into and out of the lungs. Respiration is the chemical release of energy inside cells. Using the terms interchangeably loses marks.
Do plants respire?
Yes, continuously, day and night, in every living cell. Photosynthesis only occurs in light, which is why a plant takes in oxygen and releases carbon dioxide at night.
Why are dead boiled seeds used as a control?
So that any temperature rise can be attributed to respiration in living seeds rather than to something else. The seeds are also disinfected so microorganisms cannot explain the result.
Curriculum content, prescribed practicals and examination requirements are set in the national Life Sciences curriculum and assessment policy and are revised. Confirm the current requirements with your teacher and the Department of Basic Education’s published curriculum documents.