Photosynthesis: Capturing Atmospheric Carbon
Photosynthesis is the process by which green plants absorb carbon dioxide (CO₂) from the atmosphere and convert it into glucose (C₆H₁₂O₆), a form of chemical energy. This process requires sunlight and chlorophyll and can be summarised by the equation:
6CO₂ (g) + 6H₂O (l) → C₆H₁₂O₆ (aq) + 6O₂ (g) (in the presence of light)
By converting inorganic carbon into organic molecules, photosynthesis forms the base of the carbon cycle, enabling energy flow through ecosystems. For example, a tree absorbs CO₂ during the day, storing carbon in its wood and leaves.
- Plants remove CO₂ from the atmosphere.
- Glucose produced stores chemical energy and carbon.
- Oxygen is released as a by-product.
Respiration: Returning Carbon to the Atmosphere
Respiration occurs in all living organisms, including plants and animals. It breaks down glucose to release energy for cellular activities, producing CO₂ and water as waste products. The general equation is:
C₆H₁₂O₆ (aq) + 6O₂ (g) → 6CO₂ (g) + 6H₂O (l) + energy
This process returns carbon to the atmosphere, balancing the carbon fixed by photosynthesis. For instance, when animals breathe out, they release CO₂ formed during respiration.
- Glucose is oxidised to release energy.
- CO₂ is produced and exhaled or diffused back into the atmosphere.
- Respiration occurs continuously in living organisms.
Feeding: Carbon Transfer Through Food Chains
Carbon moves through ecosystems as animals consume plants or other animals. When an herbivore eats a plant, it incorporates the plant’s carbon compounds into its own body. Carnivores then obtain carbon by eating herbivores. This transfer of carbon through feeding links organisms and maintains the flow of carbon within ecosystems.
For example, a rabbit eating grass obtains carbon compounds, which are then passed to a fox that eats the rabbit.
- Carbon compounds move from plants to animals.
- Energy stored in carbon compounds supports growth and activity.
- Feeding connects different trophic levels in the carbon cycle.
Decomposition: Recycling Carbon into the Environment
When plants and animals die, decomposers such as bacteria and fungi break down their bodies. This process releases carbon back into the soil as organic matter and into the atmosphere as CO₂ through microbial respiration. Decomposition is essential for recycling carbon and nutrients, maintaining soil fertility.
For example, fallen leaves decomposing on the forest floor release carbon compounds that enrich the soil and CO₂ that returns to the air.
- Decomposers break down dead organic matter.
- Carbon is released as CO₂ or incorporated into soil.
- Decomposition sustains nutrient cycles and soil health.
Fossil Fuels: Long-Term Carbon Storage
Over millions of years, some dead organic matter is buried under sediment and subjected to heat and pressure, transforming into fossil fuels such as coal, oil, and natural gas. These fuels store carbon that was once part of living organisms, effectively removing it from the active carbon cycle for long periods.
For example, ancient swamp plants compressed over geological time formed coal deposits rich in carbon.
- Organic matter is converted into fossil fuels underground.
- Carbon is stored for millions of years.
- Fossil fuels represent a carbon sink outside the short-term cycle.
Combustion: Releasing Stored Carbon
Combustion is the burning of fossil fuels or biomass, which releases stored carbon back into the atmosphere as CO₂. This process provides energy for human activities but increases atmospheric CO₂ levels, contributing to climate change.
The general combustion reaction for a hydrocarbon fuel is:
Fuel + O₂ → CO₂ + H₂O + energy (heat) (e.g., CH₄ + 2O₂ → CO₂ + 2H₂O) For example, burning petrol in a car engine releases CO₂ formed from fossil carbon.
- Burning fossil fuels releases CO₂ rapidly.
- This adds to atmospheric greenhouse gases.
- Combustion links human energy use to the carbon cycle.
At-a-glance summary
Comparison table
| Step | Detail | Example | Exam Use |
|---|---|---|---|
| Photosynthesis | Plants absorb CO₂ and convert it to glucose using sunlight | A tree absorbing CO₂ during the day | Explain how carbon enters the food chain |
| Respiration | Organisms break down glucose, releasing CO₂ | Animals exhaling CO₂ | Describe how carbon returns to the atmosphere |
| Feeding | Carbon compounds move through food chains | Rabbit eats grass, fox eats rabbit | Outline carbon transfer between organisms |
| Decomposition | Decomposers break down dead matter, releasing CO₂ and enriching soil | Fallen leaves decomposing in forest | Explain carbon recycling in ecosystems |
| Fossil Fuel Formation | Organic matter compressed over millions of years forms coal/oil | Coal formed from ancient swamp plants | Describe long-term carbon storage |
| Combustion | Burning fossil fuels releases CO₂ into atmosphere | Car engine burning petrol | Explain human impact on carbon cycle |
Active recall
Quick check
1. What process do plants use to remove carbon dioxide from the atmosphere?
Photosynthesis, where plants convert CO₂ into glucose using sunlight.
2. How is carbon returned to the atmosphere by living organisms?
Through respiration, which breaks down glucose and releases CO₂.
3. How does carbon move from plants to animals?
By feeding, animals consume plants and incorporate their carbon compounds.
4. What role do decomposers play in the carbon cycle?
They break down dead organisms, releasing carbon back into the soil and atmosphere.
5. How are fossil fuels formed?
From the long-term burial and compression of dead organic matter over millions of years.