Lesson 1
Carbon Cycle - Cellular Respiration (Lesson + Lab)


This lab activity is to introduce students to the carbon cycle and how plants and animals are related in this process and therefore dependent on one another.


The Carbon Cycle refers to the movement of carbon (atomic number = 6,atomic mass = 12.01 amu, molar mass = 12.01 g/mol) through the environment between abiotic and biotic sinks and the processes that cause this movement.

Abiotic Sinks refer to the non-living systems in the earth where carbon is stored (sunken into sink). These sinks can be broken up into three categories.

Atmosphere: Carbon is mainly stored in the air as carbon dioxide (CO2), but also as methane (CH4) from swamps, cow belches, and landfills, as well as many, many other carbon containing gases.

Lithosphere: Carbon is also stored in the ground in decomposing plants and animals, fossil fuels, and minerals such as calcium carbonate.

Hydrosphere: Carbon is stored in the water as well. As carbon dioxide mixes with water it forms carbonic acid, which then lowers the pH of the water. Carbon is also in any decomposing material that is the water as well.

Biotic Sinks refer to the living systems on the earth that house carbon. Since carbon is the molecular backbone of all biological molecules (carbohydrates [sugars, starch, cellulose], lipids [fats, oils], proteins, and nucleic acids [DNA, RNA]), any living organism is a carbon sink. Carbon is also incorporated into long term sinks by living organisms such as corals and mollusks when they for the coral reefs and shells. These reefs and shells can build up over time and form large reservoirs of carbon.

Carbon Cycle Processes: Carbon can move from one reservoir (sink) to another through a variety of processes.

Erosion and geological processes: the processes of wind and water erosion, as well as general weathering stresses like the freeze fracture cycle can move Carbon that is stored in soil or rock. One large but irregular source of carbon dioxide is the eruption of a volcano. Spewing large amounts of material into the atmosphere, these materials can spread for thousands of miles onto the surrounding lands and waterways.

Sedimentation: carbon that has moved into the water as particulate matter (soil particles, dead plants, animals, and algae) can settle to the bottom of the body of water and become part of the sediment. Sediment, given enough time and pressure, can become hardened into rock. Biological matter can undergo decomposition or it may eventually become fossil fuels.

Decomposition: through the action of decomposers (bacteria and fungi) biological material is broken down releasing carbon dioxide in oxygen rich environments or methane in oxygen poor environments (swamps and landfills).

Combustion: natural fires will rapidly decompose biological materials releasing large amounts of carbon dioxide into the atmosphere. The burning of fossil fuels (coal, oil, gasoline) also releases large amount of carbon dioxide.

Photosynthesis: photosynthetic organisms such as plants, algae, cyanobacteria (blue green algae), and many protista, use the energy of the sun to fix (bind) carbon dioxide to form sugars. These sugars are then used to make energy for the organisms to grow and reproduce. Much of this sugar is converted into stored food (starch and oil) and for structural components of the cells (cellulose for cell walls). Many plants, such as trees, can store tons of carbon in the form of wood for decades to centuries, making them a vital carbon sink for excess carbon dioxide.

Chemosynthesis: some types of bacteria, are able to use the energy in other chemicals, such as hydrogen sulfide (H2S), instead of the energy in sunlight, to fix carbon dioxide into sugars. These organisms are critical for the primary production of food where sunlight is not available as seen at the bottom of the ocean near hydrothermal vents.

Cellular Respiration: as organisms use food to make energy in the process of cellular respiration, carbon dioxide is produced. All types of biological molecules can be processed in cellular respiration to generate energy and, therefore, carbon dioxide. The preferred food source for this process is sugar. Note: plants also carry out cellular respiration. Once they have made their food, they use cellular respiration to break it down to provide energy for the cellular processes. Not all organisms use the same type of cellular respiration and therefore do not produce the same amounts of carbon dioxide.

Decomposition: this is really cellular respiration carried out by decomposers. However, its role in the ecosystem is so vital that it is generally listed as a separate process. Not only do decomposers return carbon dioxide to the atmosphere and water through cellular respiration, they return all the other nutrients (nitrogen, phosphorus, sulfur, and trace minerals) as well. Without the decomposers the plants would run out of the other nutrients and die causing the food chain to collapse. (See Composting Activity)

Begin/Anticipatory Set/Launch:

  1. Have students hold their breath for as long as possible, while timing them. Students could be timed using a stopwatch and the teacher could count out loud while having the students listen to the time for when they breathe out. Students then write down their individual times. Or, students could time themselves using a classroom clock.
  2. Students should be given time to recover (breathing returns to normal)and then take 10 very deep breaths. Students hold their breaths again and record the time.
  3. Students should do something active (run in place, jumping jacks, push ups) for 30 seconds and then hold their breaths again. The time is again recorded.

QUESTION: What is happening? How did your body feel as you held your breath? Was it easier after you took the 10 deep breaths? Was it harder after the 30 seconds of activity? Why? What is going on in your body?

Direct the discussion to the fact that they need to breathe in fresh air because it contains oxygen and they need to breathe out old air because it contains carbon dioxide.

QUESTION: Where does the oxygen go? Where does the carbon dioxide come from?

In order for us to live, our bodies need to generate energy. The energy is made by breaking down the food we eat. Our bodies digest the food into small molecules, which our cells absorb. Those molecules are then broken apart allowing our cells to make energy, which we use to move, think, and grow.

However, in order for our cells to do this, they must have oxygen from the air.

As the food molecules are broken down, it is converted into carbon dioxide, which we have to breathe out. When we have too much carbon dioxide in our bodies, our brains tell us to breathe harder and faster. (The feeling of wanting to breathe out when holding the breath is our brain telling us to breathe!)

Specific Detail: The regulation of the breathing cycle is due to the balance of the pH of our blood. As carbon dioxide is released from the cells it reacts with the water in the blood to form carbonic acid. The normal blood pH is between 7.38 and 7.42. If the level of carbon dioxide in the blood increases (exercise, holding the breath, agitated emotions), the pH will drop below 7.38. This drop in pH is sensed by the brain (chemical receptors in the medulla), which then sends signals to the lungs to cause greater contraction of the diaphragm (muscle below the lungs) and the muscles between the ribs (intercostal muscles). This increases the volume of the rib cage pulling in more air and expelling more carbon dioxide until normal pH levels return.

The process of breaking down food to make energy is called cellular respiration, or cell breathing. The cells take in oxygen and get rid of carbon dioxide, just like our lungs.

This process can be represented as a sentence,

Sugar + Oxygen makes Water, Carbon Dioxide, and Energy

Or a Chemical Equation,

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