(f) Respiration
- Syllabus
- 2024
- Topic
- —
- Level
- —
Learning objectives
2.34Respiration and ATPUnderstand how the process of respiration produces ATP in living organisms.2.35ATP provides energy for cellsKnow that ATP provides energy for cells.2.36Aerobic and anaerobic respirationDescribe the differences between aerobic and anaerobic respiration.2.37Aerobic respiration equationsKnow the word equation and the balanced chemical symbol equation for aerobic respiration in living organisms.2.38Anaerobic respiration equationsKnow the word equation for anaerobic respiration in plants and in animals.2.39Respiration products practicalPractical: investigate the evolution of carbon dioxide and heat from respiring seeds or other suitable living organisms.
Transfer energy from respiration into ATP
Respiration is a series of enzyme-controlled reactions that transfers chemical energy from glucose into ATP in living cells.
Breaking down glucose releases energy in controlled steps. Cells capture part of this energy by making ATP rather than releasing it all at once as heat. Aerobic respiration uses oxygen and transfers more energy to ATP than anaerobic respiration.
A cell's respiration rate rises when its ATP demand rises. For example, active muscle cells need more ATP, so they consume glucose and—during aerobic respiration—oxygen more rapidly.
Respiration is not the same as breathing: breathing moves air, whereas respiration is a chemical process in cells. ATP is the immediate energy carrier made using energy transferred from glucose.
Use ATP to power cell processes
ATP provides cells with small, immediately usable transfers of energy for energy-requiring processes.
| Cellular process | Why ATP is needed |
|---|---|
| active transport | moves substances across membranes against a concentration gradient |
| synthesis of large molecules | joins smaller units to support growth and repair |
| muscle contraction | powers movement of contractile proteins |
| cell division and movement | powers chromosome movement and other cellular work |
When ATP releases energy it is converted to ADP and phosphate. Energy transferred during respiration is then used to regenerate ATP, linking glucose breakdown to the cell's changing energy demands.
ATP is not a long-term energy store like glycogen or lipid. It is continually used and regenerated, so reduced respiration can quickly limit active transport, synthesis and movement.
Compare aerobic and anaerobic respiration
Aerobic and anaerobic respiration both transfer energy from glucose to ATP, but they differ in oxygen use, completeness of breakdown, energy yield and products.
| Feature | Aerobic respiration | Anaerobic respiration |
|---|---|---|
| oxygen | required | not required |
| glucose breakdown | complete | incomplete |
| ATP yield per glucose | greater | smaller |
| products in animals | carbon dioxide and water | lactic acid |
| products in plants and yeast | carbon dioxide and water | ethanol and carbon dioxide |
During vigorous exercise, oxygen delivery may not meet muscle demand, so anaerobic respiration supplies some ATP and lactic acid accumulates. Afterwards, breathing remains elevated while the body deals with the lactic acid and repays the oxygen debt.
Anaerobic does not mean no energy is transferred; it means less ATP is obtained from each glucose molecule because breakdown is incomplete.
Write both equations for aerobic respiration
Aerobic respiration uses glucose and oxygen and produces carbon dioxide and water while transferring energy to ATP.
ext{glucose}+ ext{oxygen}\longrightarrow ext{carbon dioxide}+ ext{water}
\mathrm{C}{6}\mathrm{H}{12}\mathrm{O}{6}+6\mathrm{O}{2}\longrightarrow6\mathrm{CO}{2}+6\mathrm{H}{2}\mathrm{O}
The balanced equation has six carbon, twelve hydrogen and eighteen oxygen atoms on each side. Energy transferred to ATP is a consequence of the reaction, not a chemical atom that is balanced into the symbol equation.
Distinguish anaerobic respiration products
Without sufficient oxygen, cells can transfer some energy from glucose by anaerobic respiration, but animals and plants form different products.
| Organisms | Word equation |
|---|---|
| animals | glucose ⟶ lactic acid |
| plants and yeast | glucose ⟶ ethanol + carbon dioxide |
The products still contain chemical energy because glucose is only partly broken down. This explains why anaerobic respiration transfers less energy to ATP than aerobic respiration.
Animals do not produce ethanol and carbon dioxide in this pathway, and plant/yeast anaerobic respiration does not produce lactic acid in the syllabus equations. Oxygen is absent from both reactant sides.
Detect carbon dioxide and heat from respiration
Respiring seeds provide measurable evidence that respiration releases carbon dioxide and transfers some energy as heat.
| Product | Experimental comparison | Evidence |
|---|---|---|
| carbon dioxide | place equal masses of germinating seeds and boiled or dry control seeds in separate sealed apparatus; keep temperature and time equal | a carbon-dioxide sensor rises, or gas from germinating seeds turns limewater cloudy |
| heat | place equal masses of germinating and boiled control seeds in insulated flasks with thermometers; use the same starting temperature | a larger temperature rise in the germinating-seed flask shows heat release |
Use the same seed species, age or stage, mass, moisture, oxygen supply and measurement time. Repeat each condition and compare mean changes. A control separates respiration from temperature drift or other environmental change.
Disinfect seeds to reduce microbial respiration, but do not kill the experimental seeds. Carbon dioxide production alone does not distinguish aerobic from anaerobic respiration, so ensure oxygen is available when claiming aerobic respiration.