Respiration is the process by which organisms burn food to produce energy. The starting material of cellular respiration is the sugar glucose, which has energy stored in its chemical bonds. You can think of glucose as a kind of cellular piece of coal: chock-full of energy, but useless when you want to power a stereo. Just as burning coal produces heat and energy in the form of electricity, the chemical processes of respiration convert the energy in glucose into usable form.
Adenosine triphosphate (ATP) is the usable form of energy produced by respiration. ATP is like electricity: it contains the same energy as coal, but it’s easier to transport and is just what’s needed when the cell needs some power to carry out a task.
ATP
ATP is a nucleic acid similar to RNA. It has a ribose sugar attached to the nitrogenous base adenine. However, instead of the single phosphate group typical of RNA nucleotides, ATP has three phosphate groups. Each of the ATP phosphate groups carries a negative charge. In order to hold the three negative charges in such proximity, the bonds holding the phosphate groups have to be quite powerful. If one or two of the bonds are broken and the additional phosphates are freed, the energy stored in the bonds is released and can be used to fuel other chemical reactions. When the cell needs energy, it removes phosphates from ATP by hydrolysis, creating energy and either adenosine diphosphate (ADP), which has two phosphates, or adenosine monophosphate (AMP), which has one phosphate.
Respiration is the process of making ATP rather than breaking it down. To make ATP, the cell burns glucose and adds new phosphate groups to AMP or ADP, creating new power molecules.
There are actually two general types of respiration, aerobic and anaerobic. Aerobic respiration occurs in the presence of oxygen, while anaerobic respiration does not use oxygen. Both types of cell respiration begin with the process of glycolysis, after which the two diverge. We’ll first discuss aerobic respiration and then move to anaerobic.
Aerobic Cell Respiration
Aerobic respiration is more efficient and more complicated than anaerobic respiration. Aerobic respiration uses oxygen and glucose to produce carbon dioxide, water, and ATP. More precisely, this process involves six oxygen molecules for every sugar molecule:
6O2 + C6H12O6
6CO2 + 6H2O + ATP energy
This general equation for aerobic respiration (which you should know for the test) is actually the product of three separate stages: glycolysis, the Krebs cycle, and the electron transport chain. Typically, the SAT II Biology only asks questions about the starting and ending products of each stage and the location where each takes place. Understanding the internal details of stages will help you remember these key facts and prepare you in case the testers throw in a more difficult question, but the details of all the complex reactions will probably not be tested by the SAT II.
