What role do photosynthesis and cellular respiration play in the cycling of carbon among the biosphere atmosphere hydrosphere and geosphere?

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The time it takes carbon to move through the fast carbon cycle is measured in a lifespan. The fast carbon cycle is largely the movement of carbon through life forms on Earth, or the biosphere. Between 1015 and 1017 grams (1,000 to 100,000 million metric tons) of carbon move through the fast carbon cycle every year.

Carbon plays an essential role in biology because of its ability to form many bonds—up to four per atom—in a seemingly endless variety of complex organic molecules. Many organic molecules contain carbon atoms that have formed strong bonds to other carbon atoms, combining into long chains and rings. Such carbon chains and rings are the basis of living cells. For instance, DNA is made of two intertwined molecules built around a carbon chain.

The bonds in the long carbon chains contain a lot of energy. When the chains break apart, the stored energy is released. This energy makes carbon molecules an excellent source of fuel for all living things.

During photosynthesis, plants absorb carbon dioxide and sunlight to create fuel—glucose and other sugars—for building plant structures. This process forms the foundation of the fast (biological) carbon cycle. (Illustration adapted from P.J. Sellers et al., 1992.)

Plants and phytoplankton are the main components of the fast carbon cycle. Phytoplankton (microscopic organisms in the ocean) and plants take carbon dioxide from the atmosphere by absorbing it into their cells. Using energy from the Sun, both plants and plankton combine carbon dioxide (CO2) and water to form sugar (CH2O) and oxygen. The chemical reaction looks like this:

CO2 + H2O + energy = CH2O + O2

Four things can happen to move carbon from a plant and return it to the atmosphere, but all involve the same chemical reaction. Plants break down the sugar to get the energy they need to grow. Animals (including people) eat the plants or plankton, and break down the plant sugar to get energy. Plants and plankton die and decay (are eaten by bacteria) at the end of the growing season. Or fire consumes plants. In each case, oxygen combines with sugar to release water, carbon dioxide, and energy. The basic chemical reaction looks like this:

CH2O + O2 = CO2 + H2O + energy

In all four processes, the carbon dioxide released in the reaction usually ends up in the atmosphere. The fast carbon cycle is so tightly tied to plant life that the growing season can be seen by the way carbon dioxide fluctuates in the atmosphere. In the Northern Hemisphere winter, when few land plants are growing and many are decaying, atmospheric carbon dioxide concentrations climb. During the spring, when plants begin growing again, concentrations drop. It is as if the Earth is breathing.

The ebb and flow of the fast carbon cycle is visible in the changing seasons. As the large land masses of Northern Hemisphere green in the spring and summer, they draw carbon out of the atmosphere. This graph shows the difference in carbon dioxide levels from the previous month, with the long-term trend removed.

This cycle peaks in August, with about 2 parts per million of carbon dioxide drawn out of the atmosphere. In the fall and winter, as vegetation dies back in the northern hemisphere, decomposition and respiration returns carbon dioxide to the atmosphere.

(Graph by Marit Jentoft-Nilsen and Robert Simmon, using data from the NOAA Earth System Research Laboratory. Maps by Robert Simmon and Reto Stöckli, using MODIS data.)

Develop a model to illustrate the role of photosynthesis and cellular respiration in the cycling of carbon among the biosphere, atmosphere, hydrosphere, and geosphere.

Standard Breakdown

This standard has 4 specific levels that we will look at in detail:

Biosphere

The biosphere includes everything on Earth, both living and non-living. Carbon cycles through both living and non-living parts of the biosphere. Starting in the atmosphere, carbon dioxide is used by plants to create glucose. Glucose travels through the food web, where it is used for energy. Some of the carbon is released as carbon dioxide back into the atmosphere. Other carbon remains bound in bonds – some of it becoming petroleum as it sinks into the pressurized environment of the Earth’s crust. Carbon also makes its way from the atmosphere and into the hydrosphere and geosphere – all of which are considered “carbon-sinks”, or places where carbon can be stored.

Atmosphere

The atmosphere is where a majority of the gaseous forms of carbon end up. Within the context of photosynthesis and respiration – this is mainly carbon dioxide. However, human activities also produce molecules like carbon monoxide and methane, both of which are also greenhouse gases. In the atmosphere, many carbon-based molecules serve as greenhouse gases, which reflect heat back onto the Earth. Too many greenhouse gases can drastically change how much heat is trapped, affecting global weather patterns and plant growth. However, the carbon in the atmosphere is also transferred to the hydrosphere and geosphere.

Hydrosphere

The hydrosphere is all of the interconnected water sources on the globe. Water is a universal solvent – because water is a polar molecule it can easily dissolve many other solutes. As such, water also allows a certain amount of carbon dioxide to be dissolved into water sources. Without humans, the oceans may have just acted as a reservoir that kept carbon dioxide at a more reasonable level. However, because of how much carbon dioxide and other greenhouse gases humans are creating, the oceans are now soaking up far too much carbon dioxide. In water, carbon dioxide dissolves and can form carbonic acid, which may be responsible for higher rates of coral and mollusk mortality in many areas. This is the same phenomenon that creates acid rain.

Geosphere

The geosphere includes inorganic carbon sinks that exist, such as the soil (lithosphere) and various types of porous rocks that can store carbon. One of the largest carbon reservoirs from the geosphere that we use is fossil fuels. Essentially, fossil fuels represent the carbon stored by plants and algae for hundreds of millions of years. These carbons, which were not released by respiration, make their way into the rock layers of the earth. With the right amounts of heat, compression, and time, the carbons joined together into longer molecules packed with energy. By burning these carbons, we are essentially releasing carbon that plants and algae stored millions of years ago.

A little clarification:

The standard contains this clarification statement:

Examples of models could include simulations and mathematical models.

Let’s look at this clarification a little closer:

Simulations and Mathematical Models

There are many video simulations online that can help explain this standard in a visual and auditory way. For more hands-on learning, you can do an experiment where you show the accumulation of carbon in water. This basic experiment works by using lime water (adding calcium hydroxide to water) as an indicator of carbon dioxide. As you breathe into the lime water, it becomes cloudy as the carbon dioxide reacts with the calcium hydroxide to precipitate calcium carbonate – chalk!

Good mathematical models involve measuring and analyzing the contributions of greenhouse gases to the atmosphere, as well as a carbon-for-carbon analysis of the processes of photosynthesis and respiration, which shows mathematically how the processes complement each other.

What to Avoid

This NGSS standard also contains the following Assessment Boundary:

Assessment does not include the specific chemical steps of photosynthesis and respiration.

Here’s a little more specificity on what that means:

Specific Chemical Steps:

For this standard, the actual processes of photosynthesis and respiration do not have to be memorized in detail. Instead, it is more important to show the movement and transformation of carbon as it weaves its way through the biological and inorganic world. While the chemical steps can help explain how carbon is changed and transformed, they should not be assessed for this standard.