Life Science: Session 7
What is cell respiration?
Many scientists would agree that life runs on sugar. Sugar, which is a type of carbohydrate, comes from the breakdown of food. Almost all living things, even those that photosynthesize, “burn” sugar as a source of energy. The process that does this is called cell respiration. The following is the chemical reaction for cell respiration:
CH2O + O2 -> CO2 + H2O + energy
sugar + oxygen -> carbon dioxide + water + energy
In cell respiration, sugar is broken down to form carbon dioxide, which is respired as a gas. In this reaction, the hydrogen in sugar combines with oxygen to form water. The energy that is released is stored temporarily in another molecule called ATP (adenosine triphosphate), which delivers and transfers the energy where it is needed. This reaction is similar to the burning of the marshmallow seen in the video in that heat is a by-product. The main difference is the nature of the reaction — cell respiration is much more controlled.
In Session 7, Dr. Aaron Ellison placed a pitcher plant in a chamber and measured the level carbon dioxide inside. When he covered the plant, preventing light from reaching it, the level increased. This would be expected as photosynthesis, which uses carbon dioxide, stopped, and cell respiration, which releases carbon dioxide, continued. Cell respiration is always occurring in living cells.
Like photosynthesis, cell respiration occurs in a special organelle. It is called the mitochondrion. As with chloroplasts, mitochondria had their origins as bacterial symbionts.
How does cell respiration reflect the properties of energy?
In cell respiration, the chemical energy in sugar is transferred to ATP. In the process, some of its chemical energy is changed to heat. As with photosynthesis, the amount of energy that enters the system in the chemical bonds of sugar equals the amount transferred to ATP plus the amount changed to heat. The second law of thermodynamics applies to cell respiration too. Heat is a very low-quality form of energy compared to chemical energy. Even the ATP molecules represent a decrease in organization, as they are much smaller than sugar molecules.
Some people notice that photosynthesis and cell respiration are “opposites” in terms of their chemical equations. With energy, this is true in the sense that photosynthesis stores energy while cell respiration releases it. It’s important to note, however, that energy flow is not cyclical. Energy changes forms from light to chemical to heat energy. There is no “looping back” – or, as some say, “You can’t eat heat!”
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