GED Science Practice Test: Metabolic Processes: Respiration

As previously mentioned, metabolic processes fall into one of two categories:  breaking down larger molecules, or building up larger molecules. Carbohydrates, lipids, proteins, and nucleic acids are the molecules involved in metabolic processes, and they are organic, meaning they contain carbon.  Two of the most essential metabolic processes in living things include respiration and photosynthesis.  These two processes are actually the reverse of each other.  Respiration breaks down glucose to produce carbon dioxide and water.  Photosynthesis builds glucose up from carbon dioxide and water.   While it easy to say that respiration happens in animals, and photosynthesis happens in plants, this is oversimplified.  Plants also undergo respiration, when they use the glucose they make for fuel.  The following diagram shows how the two processes are related:


Most metabolic processes in cells need a source of energy. Cells initially obtain chemical energy from biomolecules such as carbohydrates, lipids, and proteins, but in order to use this energy, they must convert it into adenosine triphosphate, or ATP. You can think of ATP as the energy “currency” that cells uses;  just like you can only use American dollars (as opposed to, say Euros, British pounds) as money in the United States, cells can only use energy in its ATP form. Both respiration and photosynthesis use ATP.

ATP is a molecule that is comprised of a molecule called adenosine that is bonded to a chain of three phosphate molecules. The chemical bond between the second and third phosphate is a special, high-energy bond that is the direct source of energy for cellular activities.


Cellular respiration is the process by which cells convert biochemical energy into ATP to be used to power cellular processes. There are two main types of respiration; aerobic and anaerobic respiration.

Anaerobic respiration is a type of respiration that does not need oxygen to occur. Anaerobic respiration is not very efficient at converting biochemical energy into ATP, and is used primarily by microorganisms such as certain types of bacteria. Certain types of anaerobic respiration are used to make some well-known products that we use; for example, a step of anaerobic respiration called fermentation is used to create cheese, yogurt, alcoholic beverages, black tea, and a variety of other food products.

Aerobic respiration, which uses oxygen, is a much more efficient method of converting biochemical energy into ATP; while anaerobic respiration can only extract 2 ATP per molecule of glucose, aerobic respiration can extract up to 38 ATP molecules from a molecule of glucose.  Remember that ATP is the “energy currency” of cells.  If possible, an organism would rather use aerobic respiration for its energy needs.  Some organisms, such as yeast, are called facultative anaerobes, meaning that they will do anaerobic respiration if oxygen is not present.

The overall chemical reaction for aerobic respiration is:



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