The Carbon and Water Cycles
This topic covers how essential materials like carbon and water are continuously recycled through ecosystems. ESAT questions test your understanding of the key biological and physical processes involved, and how they interact to maintain a balance.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The carbon cycle describes the movement of carbon atoms between the atmosphere, oceans, land, and living organisms.
- Photosynthesis, carried out by plants and algae, is the primary process that removes carbon dioxide from the atmosphere, converting it into organic compounds (glucose).
- Respiration, performed by ALL living organisms (including plants), releases energy from organic compounds and returns carbon dioxide to the atmosphere.
- Decomposition by bacteria and fungi breaks down dead organic matter, releasing CO2 through respiration.
- Combustion of fossil fuels and wood rapidly releases large amounts of carbon that were stored for millions of years, disrupting the cycle's balance.
- The water cycle is vital as water acts as a universal solvent for metabolic reactions, a transport medium within organisms, and a habitat.
Formulae
6CO2 + 6H2O --(light energy)→ C6H12O6 + 6O2 To represent photosynthesis. Shows carbon dioxide being converted into glucose (an organic carbon store).
C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP (energy) To represent aerobic respiration. Shows glucose being broken down to release energy, returning carbon dioxide to the atmosphere.
Definitions
- Photosynthesis
- The process in which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water, producing glucose and oxygen.
- Aerobic Respiration
- The chemical process in all living cells that breaks down glucose with oxygen to release energy for life processes. Carbon dioxide and water are waste products.
- Decomposition
- The breakdown of dead organic material by microorganisms (decomposers), which respire and release carbon dioxide and mineral nutrients.
- Combustion
- The process of burning something, such as fossil fuels. This rapidly oxidises carbon compounds, releasing carbon dioxide and water into the atmosphere.
- Transpiration
- The evaporation of water from a plant's surface, primarily through the stomata in the leaves, which draws water up from the roots.
Worked example
A sealed, transparent container holds a healthy plant, a snail, and some soil decomposers. It is placed in direct sunlight. Which statement best describes the net change in gas concentrations inside the container after several hours?
- 1
Step 1:
Identify all processes occurring.
The plant photosynthesises (uses CO2, produces O2) and respires (uses O2, produces CO2).
The snail respires.
The decomposers respire.
- 2
Step 2:
Consider the condition:
'direct sunlight'.
This means the rate of photosynthesis in the plant will be significantly higher than its rate of respiration.
- 3
Step 3:
Sum the gas changes.
All three types of organisms are producing CO2 through respiration.
However, the plant is consuming CO2 for photosynthesis at a much faster rate.
- 4
Step 4:
Determine the net effect.
The high rate of photosynthesis will consume more CO2 than is produced by all organisms combined.
Simultaneously, it will produce more O2 than is consumed by all organisms combined.
- 5
Step 5:
Conclude the net change.
Therefore, the concentration of carbon dioxide will decrease, and the concentration of oxygen will increase.
Answer: The concentration of CO2 will decrease, and the concentration of O2 will increase.
Common mistakes
- ×Forgetting that plants respire. Plants photosynthesise to create food and respire to release energy from that food. In the dark, plants are net producers of CO2.
- ×Misinterpreting cycle diagrams. Always follow the direction of the arrow. An arrow from 'Animals' to 'Atmosphere' represents a process that releases a substance (like CO2) into the air, such as respiration.
- ×Confusing short-term and long-term carbon cycles. Respiration and decomposition are part of the active, short-term cycle. Combustion of fossil fuels releases carbon from a long-term geological store, which is the primary cause of modern atmospheric CO2 increase.
No-calculator tips
- ✓View the carbon and water cycles as closed loops. For any question about balance, consider the processes that add a substance to a reservoir (e.g., CO2 to the atmosphere) and those that remove it. An imbalance occurs when rates of addition and removal are not equal.
- ✓Recognise that photosynthesis and respiration are reverse processes in terms of their overall chemical equations. This helps you quickly recall the inputs and outputs of each without memorisation.
- ✓For questions involving water potential (e.g., plants in salty water), remember water moves by osmosis from a region of high water potential (less negative, dilute solution) to a region of low water potential (more negative, concentrated solution).