Respiration and energy transfer
Respiration is a set of chemical reactions in cells that transfers energy for life processes.
Aerobic respiration
Aerobic respiration uses oxygen. Its overall word equation is glucose + oxygen → carbon dioxide + water. Cells use the energy transferred for movement, growth, active transport and other processes. Some energy transfers to the surroundings by heating.
Much of aerobic respiration takes place in mitochondria, small structures inside cells. Plants respire as well as animals. Respiration continues day and night. Breathing moves air; respiration is the chemical process in cells.
Without enough oxygen
Anaerobic respiration transfers energy without using oxygen. In muscles, glucose is broken down to lactate, often called lactic acid at this level. It transfers less usable energy per glucose molecule than aerobic respiration.
In yeast, anaerobic respiration produces ethanol and carbon dioxide. This is fermentation. The carbon dioxide helps bread rise. Aerobic and anaerobic processes can occur at the same time in active muscles.
Measure respiration
A respirometer can measure oxygen uptake. Carbon dioxide is absorbed so that the gas change can be linked to oxygen being used. Keep temperature controlled, because gases expand when warmed.
For a fair comparison, use the same mass of living material or calculate a rate per gram. Repeat measurements to check variation. A yeast gas-collection experiment measures carbon dioxide output, not the energy transferred directly.
Worked example
Two yeast samples produce 12 cm³ and 18 cm³ of carbon dioxide in 60 s. Their mean collection rates are 12 ÷ 60 = 0.20 cm³/s and 18 ÷ 60 = 0.30 cm³/s. Compare equal yeast masses under the same conditions.
Read the detailed explanation
Living cells require continuous energy transfers for movement, synthesis, active transport and other processes. Respiration is a set of chemical reactions transferring energy from organic molecules. It takes place in plants as well as animals and continues when no movement is visible. Breathing supplies oxygen but is not the chemical process itself.
Follow the aerobic reaction
In the simplified overall description, aerobic respiration uses glucose and oxygen and produces carbon dioxide and water. Energy is transferred for cellular processes and to surroundings. The word equation summarises many reactions; glucose does not burn in one uncontrolled step inside a cell. Much aerobic respiration is associated with mitochondria, although the full pathway is not confined to one location.
An energy transfer is not the creation of energy. Energy held in chemical stores is redistributed through reactions and cellular processes. Cells use these transfers to build larger molecules, maintain internal conditions, contract muscles and move substances against concentration differences. Some energy eventually transfers to surroundings by heating.
Plants use glucose made through photosynthesis or obtained from stored materials. Their cells respire in daylight and darkness. Photosynthesis depends on light and produces organic molecules; respiration transfers energy from those molecules. The processes are connected but are not the same mechanism running backwards in every detail.
Compare anaerobic pathways
When oxygen supply does not meet demand in working muscles, anaerobic respiration can contribute to energy transfer. In the school-level summary, glucose is converted to lactate, often described as lactic acid, without oxygen being a reactant. This transfers less usable energy per glucose than complete aerobic breakdown. Aerobic respiration can continue alongside the anaerobic contribution.
In yeast, anaerobic breakdown produces ethanol and carbon dioxide. Fermentation supports food and drink production under controlled conditions. Yeast and muscle cells therefore do not have identical anaerobic products. Name the organism when explaining which products form.
During recovery from vigorous exercise, ventilation and circulation can remain elevated. Oxygen supports recovery processes, including restoration of energy stores and processing of lactate. Lactate can be used as fuel or transported for conversion through metabolic pathways. Lactate is processed during recovery; it does not remain permanently in the muscles.
Interpret practical evidence
A respirometer can use gas-volume or pressure changes to investigate oxygen uptake when carbon dioxide is absorbed in a suitable supervised setup. Temperature changes also alter gas volume, so equilibration and an appropriate control are essential. Germinating seeds have living cells with respiratory demand; a comparison must account for the amount and condition of material.
A yeast experiment measuring carbon dioxide compares gas output, not energy transfer directly. Keep sugar concentration, temperature, yeast quantity and collection time suitable for the question. More gas over a fixed interval can support a greater fermentation rate under stated conditions, but leaks or dissolved gas complicate measurement.
Compare rates using a defined time interval and, where necessary, mass of living material. A larger sample can produce more total gas without each gram respiring faster. Repeated observations reveal variability. Use the gas readings to describe that organism’s activity under the conditions tested.
Another worked example
Two yeast mixtures collect 12 cm³ and 18 cm³ carbon dioxide in 60 s. Mean collection rates are 0.20 and 0.30 cm³/s. With yeast quantity and other conditions controlled, the second has a greater measured output rate. These data do not directly measure energy transferred per glucose molecule.
Try these questions
Write your answer before opening the model answer, then check which points you included. These written answers are not automatically marked or saved.
Question 1
Explain why a plant kept in darkness can still release carbon dioxide.
Show model answer and points to check
Plant cells respire in darkness, using sugars stored or made earlier. Aerobic respiration produces carbon dioxide. Photosynthesis needs light, but respiration does not. A plant can therefore release carbon dioxide when photosynthesis is not taking it in.
- Identify continuing respiration
- Identify stored molecules
- Distinguish light requirements
Question 2
A yeast flask produces more carbon dioxide after twice as much yeast is added. Does that establish a greater rate per gram?
Show model answer and points to check
No. Twice as many yeast cells can produce more total gas without each gram working faster. Measure gas over the same time, divide by yeast mass and keep other conditions the same. This compares output per gram.
- Distinguish total and mass-specific rate
- Explain more cells
- Compare gas volume per gram over the same time
Question 3
Explain why temperature control matters when a respirometer measures changing gas volume.
Show model answer and points to check
Gas expands when warmed and contracts when cooled, even if no oxygen is used. These changes could hide or imitate respiration. Let the apparatus reach a steady temperature and use a suitable control. Repeat readings to check variation.
- Identify the physical gas-volume effect
- Link it to misleading readings
- Allow temperature to settle or use a suitable control