Problem 8 - Campbell Biology

Question

Scientific evidence indicates that the \(\mathrm{CO}_{2}\) added to the air by the burning of wood and fossil fuels is contributing to global warming, a rise in global temperature. Tropical rain forests are estimated to be responsible for approximately \(20 \%\) of global photosynthesis, yet the consumption of large amounts of \(\mathrm{CO}_{2}\) by living trees is thought to make little or no net contribution to reduction of global warming. Explain why this might be the case. (Hint: What processes in both living and dead trees produce \(\mathrm{CO}_{2} ?\) )

Step-by-Step Solution

Answer

The \( \text{CO}_2 \) absorbed during photosynthesis by living trees is balanced by \( \text{CO}_2} \ released through respiration and decomposition of dead trees.

Step 1: Identify the Main Processes

Recognize that photosynthesis and respiration are two key processes that contribute to the carbon cycle in trees. Photosynthesis absorbs \( \text{CO}_2 \) while respiration releases \( \text{CO}_2 \).

Step 2: Understand Photosynthesis and Respiration in Trees

Living trees remove \( \text{CO}_2 \) from the atmosphere through photosynthesis: \[ \text{6 CO}_2 + \text{6 H}_2\text{O} + \text{light} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + \text{6 O}_2 \]. Simultaneously, trees release \( \text{CO}_2 \) back into the atmosphere through cellular respiration: \[ \text{C}_6\text{H}_{12}\text{O}_6 + \text{6 O}_2 \rightarrow \text{6 CO}_2 + \text{6 H}_2\text{O} + \text{energy} \].

Step 3: Decomposition of Dead Trees

When trees die, decomposers like bacteria and fungi break down the biomass of the tree, releasing \( \text{CO}_2 \) through decomposition. This is another process that returns \( \text{CO}_2 \) to the atmosphere: \[ \text{Organic Matter} \rightarrow \text{CO}_2 + \text{Other Byproducts} \].

Step 4: Carbon Equilibrium in Forests

Understand that even though tropical rainforests are responsible for significant photosynthesis, their net impact on atmospheric \( \text{CO}_2 \) is minimized because the \( \text{CO}_2 \) absorbed by living trees is approximately balanced by \( \text{CO}_2 \) released through respiration and decomposition of dead trees.

Key Concepts

Photosynthesis

Photosynthesis is a process that takes place in plant leaves. It helps convert carbon dioxide (CO₂) and water (H₂O) into sugars (glucose) and oxygen (O₂) using light energy from the sun. This process is represented by the equation:
\[ 6 \text{CO}_2 + 6 \text{H}_2 \text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6 \text{O}_2 \]
Through photosynthesis, trees remove CO₂ from the atmosphere and store the carbon in their tissues as glucose. This is vital for the growth and energy needs of the tree.
However, while photosynthesis reduces atmospheric CO₂, it is part of a larger cycle.

Respiration

Respiration is the process by which living organisms, including trees, convert glucose and oxygen back into carbon dioxide, water, and energy. The energy produced is utilized for all cellular activities. The equation for respiration is:
\[ \text{C}_6\text{H}_{12}\text{O}_6 + 6 \text{O}_2 \rightarrow 6 \text{CO}_2 + 6 \text{H}_2 \text{O} + \text{energy} \]
Trees, like all living organisms, need energy to survive and grow. They release CO₂ back into the atmosphere when they respire. This means that while trees absorb CO₂ during the day through photosynthesis, they also release it during respiration. This back-and-forth exchange is crucial in understanding the overall carbon balance in an ecosystem.

Decomposition

Decomposition involves the breakdown of dead organic matter by decomposers like bacteria and fungi. This process releases stored carbon from dead trees back into the atmosphere in the form of CO₂. The breakdown of organic matter can be summarized by:
\[ \text{Organic\underline{\phantom{xxx}}Matter} \rightarrow \text{CO}_2 + \text{Other\underline{\phantom{xxx}}Byproducts} \]
When trees die and fall to the forest floor, decomposers consume the dead material. In doing so, they release CO₂ over time. This helps recycle nutrients back into the ecosystem, but it also means that the carbon captured by the tree during its lifetime is eventually released back into the atmosphere.

Global Warming

Global warming refers to the rise in Earth's average temperatures, mainly due to increased levels of greenhouse gases like CO₂ in the atmosphere. While tropical rainforests are responsible for a significant portion of global photosynthesis, their overall effect on reducing atmospheric CO₂ is limited. This is because the CO₂ absorbed during photosynthesis is balanced by CO₂ released during respiration and decomposition.
Burning fossil fuels and deforestation contribute additional CO₂, exacerbating global warming. Thus, understanding the carbon cycle— including photosynthesis, respiration, and decomposition—helps us see why cutting down rainforests may not solve the CO₂ problem alone. Instead, reducing fossil fuel use and protecting forests are both crucial steps in combating global warming.