Carbon: The Element of Surprise

The delicate balance of chemical changes involved in recycling carbon between living and nonliving parts of the biosphere is the pervasive theme of this module. With the dependence of life on carbon and the ongoing debate over global warming and the greenhouse effect, it is essential that students have a basic understanding of the impact of these chemical processes on their environment and life itself.

Most middle school students know that photosynthesis is the process used by plants to make food, but they do not understand or realize the profound impact that it has on the rest of the biosphere. The chemical reactions that are part of the carbon cycle either remove carbon dioxide gas from the atmosphere, the soil, or aquatic ecosystems or add it back to them. Through photosynthesis producers such as plants, algae, and some bacteria remove carbon dioxide, an inorganic compound, and convert it into glucose for use by themselves and other living things. Glucose is the simplest organic compound and contains carbon, hydrogen, and oxygen. Glucose, often referred to as food, is used by cells and tissues in the plant as a source of chemical energy and as a source of molecules with which to build other needed compounds that make up the structure of living things. For example, some of this glucose is changed to cellulose and used to build plant tissues; some is changed to starch and stored in leaves, stems, or roots as an energy reserve. Other glucose molecules combine with nitrogen from the soil to form amino acids, proteins, or nucleic acids.

Understanding photosynthesis requires some knowledge of where and how the process occurs in plants. Producers have their own mini carbon cycle. A key element of photosynthesis is the chemical chlorophyll. Most chlorophyll is located within the leaf inside cellular structures called chloroplasts. Chlorophyll is a green pigment molecule that absorbs energy from sunlight. This energy removes hydrogen atoms from water that enters through the plant roots and releases carbon from carbon dioxide. The carbon dioxide enters the leaves through small openings called stomata. The carbon is then trapped within the plant by energy-rich chemical bonds and, along with hydrogen, combines to form glucose. Light energy from the sun has now been transformed into chemical bond energy. Upon its release from the water molecules, oxygen exits the leaves through the stomata.

Plants, animals, and most other living things combine oxygen with glucose during aerobic respiration, another group of important chemical reactions. These chemical reactions return carbon dioxide to the atmosphere or aquatic systems. Because carbon dioxide is a gas that moves from one place to another throughout the atmosphere, the carbon cycle is global. The carbon dioxide that a plant takes in may have been released by an organism's respiration in another location far away.

On a larger scale, carbon cycles by way of many individual organisms as it moves through food chains and food webs. This fast track can take minutes to years to complete. Carbon moves from the soil, water or atmosphere through living things by photosynthesis, respiration, and decomposition and back to the soil, water, or atmosphere. The demand for carbon is great but only producers can convert carbon dioxide gas into carbon compounds through photosynthesis. Producers form the basis of terrestrial, marine, and aquatic food chains and food webs. Consumers and decomposers that are unable to use carbon dioxide depend on producers and/or other consumers for their carbon compounds. Decomposers break down the carbon compounds when organisms die and release carbon back into the atmosphere as carbon dioxide. Once carbon compounds are inside the organism other chemical reactions can change them into a variety of carbon compounds. Living things also use atmospheric oxygen during the process of respiration to break down carbon compounds for energy, and in the process, release more carbon dioxide. The same carbon atoms are used over and over in an endless cycle. Respiration and photosynthesis are opposite reactions and are the key recyclers of carbon through living things. Photosynthesis traps carbon into compounds and respiration releases it from compounds.

But most of the carbon on Earth is recycled through a slow track that can take millions of years. As marine organisms die, their shells and skeletons become buried under layers of silt on the ocean floor and their carbon becomes part of sedimentary rock. When sediments covered marine organisms before they decomposed, the resulting heat and pressure caused huge deposits of petroleum (oil) to form. Millions of years ago the remains of plants became buried in sediments under swamps where they were compacted into coal. Today, these fossil fuels are the major energy source used by humans. Burning these fuels releases carbon back into the atmosphere as carbon dioxide that is then available for the fast track. Carbon dioxide is also released from rock through weathering and erosion and as volcanoes erupt into the air. The atmospheric carbon dioxide can then be channeled through the fast track carbon cycle.

Humans can impact the fast and slow tracks of the carbon cycle in both positive and negative ways. An awareness and understanding of the cycle and its interrelationships with living things can help us act in more responsible ways. Slight seasonal changes in atmospheric concentrations of carbon dioxide occur because plants remove more carbon dioxide during the summer when photosynthesis is greater than they do in winter. When it is summer in the Northern Hemisphere (which has more land), there are more plants removing carbon dioxide. This results in a decrease of global concentrations of carbon dioxide. The opposite occurs during winter in the Northern Hemisphere. Globally, carbon dioxide removal by photosynthesis is balanced by its release through respiration. However, humans increase the amount of carbon dioxide in the atmosphere by burning wood and fossil fuels. Deforestation removes trees that could absorb some of the carbon dioxide from the atmosphere. Because carbon dioxide is one of the greenhouse gases that absorbs radiant energy (heat) from the sun, many scientists argue that human actions have caused the amount of atmospheric carbon dioxide to steadily increase, resulting in warming temperatures. These increases in temperature may be causing climate changes, which can upset the delicate balance between carbon removal and carbon release. Oceans play a major role in the carbon cycle by absorbing carbon dioxide. Carbon dioxide dissolved in water forms an acid that dissolves limestone (CaCO3). These chemical reactions help absorb carbon dioxide and regulate some of the carbon dioxide in the atmosphere. Many scientists argue that the oceans cannot regulate carbon dioxide levels fast enough and that humans must help preserve this ecosystem. Other scientists report that planting trees, restoring habitats, and burning less fossil fuel have helped remove carbon dioxide from the carbon cycle and restore the balance so that a global warming problem is not occurring.

The formulas for photosynthesis

6CO₂ + 6H₂O + radiant energy > C₆H₁₂O₆ + 6O₂

and respiration

C₆H₁₂O₆ + 6O₂ > 6CO₂ + 6H₂O