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 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 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:

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.

Glycolysis is the first stage of aerobic (and anaerobic) respiration. It takes place in the cytoplasm of the cell. In glycolysis (“glucose breaking”), ATP is used to split glucose molecules into a three-carbon compound called pyruvate. This splitting produces energy that is stored in ATP and a molecule called NADH. The chemical formula for glycolysis is:

As the formula indicates, the cell must invest 2 ATP molecules in order to get glycolysis going. But by the time glycolysis is complete, the cell has produced 4 new ATP, creating a net gain of 2 ATP. The 2 NADH molecules travel to the mitochondria, where, in the next two stages of aerobic respiration, the energy stored in them is converted to ATP.

The most important things to remember about glycolysis are:

After glycolysis, the pyruvate sugars are transported to the mitochondria. During this transport, the three-carbon pyruvate is converted into the two-carbon molecule called acetate. The extra carbon from the pyruvate is released as carbon dioxide, producing another NADH molecule that heads off to the electron transport chain to help create more ATP. The acetate attaches to a coenzyme called coenzyme A to form the compound acetyl-CoA. The acetyl-CoA then enters the Krebs cycle. The Krebs cycle is called a cycle because one of the molecules it starts with, the four-carbon oxaloacetate, is regenerated by the end of the cycle to start the cycle over again.

The Krebs cycle begins when acetyl-CoA and oxaloacetate interact to form the six-carbon compound citric acid. (The Krebs cycle is also sometimes called the citric acid cycle.) This citric acid molecule then undergoes a series of eight chemical reactions that strip carbons to produce a new oxaloacetate molecule. The extra carbon atoms are expelled as CO₂ (the Krebs cycle is the source of the carbon dioxide you exhale). In the process of breaking up citric acid, energy is produced. It is stored in ATP, NADH, and FADH₂. The NADH and FADH₂ proceed on to the electron transport chain.

The entire Krebs cycle is shown in the figure below. For the SAT II Biology, you don’t have to know the intricacies of this figure, but you should be able to recognize that it shows the Krebs cycle.

It is also important to remember that each glucose molecule that enters glycolysis is split into two pyruvate molecules, which are then converted into the acetyl-CoA that moves through the Krebs cycle. This means that for every glucose molecule that enters glycolysis, the Krebs cycle runs twice. Therefore, for one glucose molecule running through aerobic cell respiration, the equation for the Krebs cycle is:

For the SAT II Biology, the most important things to remember about the Krebs cycle are:

A great deal of energy is stored in the NADH and FADH₂ molecules formed in glycolysis and the Krebs cycle. This energy is converted to ATP in the final phase of respiration, the electron transport chain:

The electron transport chain consists of a set of three protein pumps embedded in the inner membrane of the mitochondria. FADH₂ and NADH are used to power these pumps. Using the energy in NADH and FADH₂, these pumps move positive hydrogen ions (H⁺) from the mitochondrial matrix to the intermembrane space. This creates a concentration gradient over the membrane.

In a process called oxidative phosphorylation, H⁺ ions flow back into the matrix through a membrane protein called an ATP synthase. This channel is the opposite of the standard membrane pumps that burns ATP to transport molecules against their concentration gradient: ATP synthase uses the natural movement of ions along their concentration gradient to make ATP. All told, the flow of ions through this channel produces 34 ATP molecules. The waste products from the powering of the electron transport chain protein pumps combine with oxygen to produce water molecules. By accepting these waste products, oxygen frees NAD⁺ and FAD to play their roles in the Krebs cycle and the electron transport chain. Without oxygen, these vital energy carrier molecules would not perform their roles and the processes of aerobic respiration could not occur.

For the SAT II Biology, the most important things to remember about the electron transport chain and oxidative phosphorylation are:

Aerobic respiration requires oxygen. However, some organisms live in places where oxygen is not always present. Similarly, under extreme exertion, muscle cells may run out of oxygen. Anaerobic respiration is a form of respiration that can function without oxygen.

In the absence of oxygen, organisms continue to carry out glycolysis, since glycolysis does not use oxygen in its chemical process. But glycolysis does require NAD⁺. In aerobic respiration, the electron transport chain turns NADH back to NAD⁺ with the aid of oxygen, thereby averting any NAD⁺ shortage and allowing glycolysis to take place. In anaerobic respiration, cells must find another way to turn NADH back to NAD⁺.

This “other way” is called fermentation. Fermentation’s goal is not to produce additional energy, but merely to replenish NAD⁺ supplies so that glycolysis can continue churning out its slow but steady stream of ATP. Because pyruvates are not needed in anaerobic respiration, fermentation uses them to help regenerate NAD⁺. While employing the pyruvates in this way does allow glycolysis to continue, it also results in the loss of the considerable energy contained in the pyruvate sugars.

There are two principle forms of fermentation, lactic acid fermentation and alcoholic fermentation. For the SAT II Biology, remember that no matter what kind of fermentation occurs, anaerobic respiration only produces 2 net ATP in glycolysis.

In lactic acid fermentation, pyruvate is converted to a three-carbon compound called lactic acid:

In this reaction, the hydrogen from the NADH molecule is transferred to the pyruvate molecule.

Lactic acid fermentation is common in fungi and bacteria. Lactic acid fermentation also takes place in human muscle cells when strenuous exercise causes temporary oxygen shortages. Since lactic acid is a toxic substance, its buildup in the muscles produces fatigue and soreness.

Another route to NAD⁺ produces alcohol (ethanol) as a by-product:

Alcoholic fermentation is the source of ethyl alcohol present in wines and liquors. It also accounts for the bubbles in bread. When yeast in bread dough runs out of oxygen, it goes through alcoholic fermentation, producing carbon dioxide. These carbon dioxide bubbles create spaces in the dough and cause it to rise.

Like lactic acid, the ethanol produced by alcoholic fermentation is toxic. When ethanol levels rise to about 12 percent, the yeast dies.