Photosynthesis transforms light energy into chemical energy stored in glucose, while cellular respiration transforms the chemical energy in glucose into a form that cells can use. Together, these processes transfer and transform matter while energy flows through an ecosystem and is eventually lost as heat.
This is SL and HL content in Subtopic 2.2, Energy and biomass in ecosystems.
The Mechanism
In photosynthesis, producers such as plants and phytoplankton absorb light. They use this energy to transform carbon dioxide and water into glucose and oxygen:
The transformation has two connected effects: inorganic matter becomes organic carbon compounds, and light energy becomes chemical energy. Some glucose is converted into biomass, transferring stored chemical energy to consumers through feeding.
In cellular respiration, organisms break down glucose using oxygen:
This releases chemical energy in a form that cells can use for active processes. However, respiration is inefficient, so some energy is transformed into heat and dissipated to the surroundings, consistent with the second law of thermodynamics.
| Process | Transformation of matter | Transformation of energy |
|---|---|---|
| Photosynthesis | Carbon dioxide and water become glucose and oxygen | Light energy becomes chemical energy |
| Cellular respiration | Glucose and oxygen become carbon dioxide and water | Chemical energy becomes usable chemical energy and heat |
A common misconception is that energy is recycled. Matter cycles, because carbon dioxide and water can be reused, but energy flows through ecosystems and cannot be recycled after it dissipates as heat. For example, photosynthesis in the Amazon rainforest stores carbon in biomass, while respiration returns carbon dioxide to the atmosphere; deforestation alters these flows and affects societies dependent on forest ecosystem services.
Exam Technique
For an explain question, identify both the energy and matter transformations and link them causally. Avoid merely writing the equations: state that photosynthesis stores energy, respiration releases it, matter cycles, and energy flows.