Material Cycling Carbon and Water Cycles for the ESAT

Understanding the Carbon Cycle

All life on Earth is based on carbon. Because there is a finite amount of carbon on the planet, it must be recycled continuously. The carbon cycle involves the movement of carbon between the atmosphere, the soil, and living organisms. There are four primary processes that you must understand for the ESAT: photosynthesis, respiration, combustion, and decomposition.

i. Photosynthesis

Photosynthesis is the process by which producers, such as green plants and algae, remove carbon dioxide ($CO_2$) from the atmosphere. Using light energy, they convert $CO_2$ and water ($H_2O$) into glucose ($C_6H_{12}O_6$), which is an organic carbon compound. This is the only process in the cycle that decreases the concentration of atmospheric carbon dioxide. The chemical energy stored in glucose is then passed along food chains as herbivores eat plants and carnivores eat herbivores.

ii. Respiration

All living organisms, including plants, animals, and microbes, perform respiration to release energy from food. This process involves the reaction of glucose with oxygen ($O_2$), producing carbon dioxide and water as waste products. The $CO_2$ is released back into the atmosphere. This balances the removal of carbon by photosynthesis. The equation for aerobic respiration is: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$.

iii. Combustion

Combustion is the burning of organic material in the presence of oxygen. This includes the burning of wood or, more significantly, fossil fuels such as coal, oil, and natural gas. These fossil fuels are the remains of ancient organisms that did not fully decompose; they act as a carbon sink. When we burn them, the stored carbon is oxidised and released into the atmosphere as $CO_2$. This process is a major contributor to the rise in global carbon levels.

iv. Decomposition

When plants and animals die, or when animals produce waste products (faeces and urea), the organic matter contains carbon. Decomposers, primarily bacteria and fungi, break down this organic material to obtain energy. As these microorganisms respire, they release $CO_2$ into the atmosphere. Without decomposition, nutrients would remain trapped in dead matter and the carbon cycle would stop.

The Role of the Water Cycle in Biology

Water is essential for the survival of every living organism. It moves through the environment via the water cycle, which includes evaporation, transpiration, condensation, and precipitation. For the ESAT, you must understand why this cycle and water itself are so important to life.

Why is the water cycle important to living things?

1. Solvent for Metabolic Reactions: Water is the medium in which almost all chemical reactions occur within a cell. Nutrients must be dissolved in water to be absorbed and used by the cytoplasm.

2. Transport Medium: In animals, water is the primary component of blood, which transports oxygen, glucose, and hormones. In plants, water in the xylem and phloem transports mineral ions and sugars throughout the organism.

3. Temperature Regulation: Living things use the high latent heat of vaporisation of water to cool down. For example, humans sweat and plants transpire. As water evaporates from the surface, it removes heat energy from the organism.

4. Structural Support: In plants, the vacuole fills with water to create turgor pressure. This pressure pushes the cell membrane against the cell wall, making the cells turgid. This rigidity allows the plant to remain upright and keep its leaves spread out to capture sunlight.

5. Raw Material for Photosynthesis: Water is one of the two essential reactants required for plants to produce food.

Worked Example: Impact of Deforestation

Consider a scenario where a large area of tropical rainforest is cleared and the wood is burned. How does this affect the carbon and water cycles?

1. Carbon Cycle Impact: The removal of trees stops photosynthesis, meaning less $CO_2$ is removed from the atmosphere. The combustion of the wood releases stored carbon immediately as $CO_2$. Finally, the lack of trees reduces the amount of organic matter being returned to the soil for decomposition.

2. Water Cycle Impact: Trees release huge volumes of water vapour through transpiration. Without trees, the local atmosphere becomes drier, leading to reduced cloud formation and less precipitation. This can eventually lead to the desertification of the area.