Cells and Energy - Overview



Living cells require energy to support metabolism, those chemical and physical activities that go on within the cell and make life possible.

Metabolism consists of anabolic reactions, which make large molecules from smaller ones, and catabolic reactions, which break down large molecules. The most important of the catabolic processes are those involved in the production of energy from food molecules.


Glucose: The Energy in One Food Molecule

The primary source of energy for living cells is glucose. Glucose can combine with oxygen to produce carbon dioxide, water, and energy -- an exergonic process.

The equation below summarizes the process of aerobic respiration

C6H12O6 + 6O2 6CO2 + 6H2O + energy

Equation 1


Chemical reactions that involve oxygen are called oxidation reactions. More specifically, in oxidation reactions electrons are transferred from one atom to another. the atom that loses the electrons is oxidized, and the atom that receives the electrons is reduced. Because oxidation and reduction always occur simultaneously, they are frequently referred to as redox reactions.


Glycolysis: The First Pathway

The energy that is available in a molecule of glucose is removed and transferred to the high-energy bonds of ATP by a series of chemical reactions that occur in the cell. The first series of reactions is called glycolysis. In glycolysis, a molecule of glucose is split into 2 molecules of pyruvate. The process requires the input of 2 molecules of ATP and produces 4 molecules of ATP, for a net gain of 2 ATPs. In addition to the ATP and pyruvate, a pair of NAD+ molecules are reduced to NADH, and the electrons that they carry can be used to produce more energy if oxygen is present. The hydrogen carried by NADH can be used to reduce oxygen to water.

Glucose is initially converted to 2 molecules of phosphoglyceraldehyde which then undergoes the following reaction.

Equation 2


The Krebs Cycle (Citric Acid Cycle)

Each molecule of pyruvate can be oxidized and converted into an acetyl group by the removal of a carboxyl group, which diffuses out of the cell as carbon dioxide. The acetyl group is bound to a large molecule called coenzyme A (acetyl CoA) and enters a series of cyclic reactions called the Krebs or citric acid cycle.

In the Krebs cycle, the 2-carbon acetyl group combines with the 4-carbon oxaloacetic acid to form the 6-carbon citric acid, which is the first molecule of the cycle. During the cycle, 2 more carbons are released in the form of carbon dioxide (from the original pyruvate molecule). Almost all of the carbon dioxide that we exhale while breathing comes from this and the previous process. Four more pairs of electrons are also removed during the cycle. Three pairs are used to reduce 3 more molecules of NAD+, and 1 pair is used to reduce a molecule of FAD. Finally, a molecule of GTP, which is equivalent to ATP, is formed.


Electron Transport

The electrons that are carried by the reduced molecules,NADH and FADH2 are released to the inner membrane of the mitochondrion and pass through the electron transport chain. This system uses oxygen as the ultimate acceptor of the electrons that are carried by the reduced molecules. As the electrons move through the series of enzymes in the chain, they activate a proton pump that creates a steep proton gradient. This free energy gradient powers the formation of ATP from ADP and phosphate. Each pair of electrons introduced by NADH can generate enough energy to produce 3 ATPs and each pair of electrons introduced by FADH2 can generate enough energy to produce 2 ATPs.

The net gain by the entire process is 36 ATPs.

If we compare the energy available in a molecule of glucose with the energy that is transferred to ATP, we can account for about 38 percent of the available energy. The remaining 62 percent of the energy available in glucose is lost in the from of heat energy. This process of producing energy from glucose in the presence of oxygen is called cellular respiration.

Glucose is not the only molecule that can be used to produce energy by this process. Other carbohydrates are generally converted to glucose before they enter catabolic pathways. Proteins are broken down into amino acids, which then enter the pathways at various points.Fatty acids can be converted into acetyl CoA (coenzyme A) so that they can enter the Krebs cycle.



Many cells can produce energy in the absence of oxygen by a process known as fermentation (anaerobic respiration). Fermentation does not produce ATP directly but it helps a cell under anaerobic condition by regenerating NAD+ so that glycolysis can continue.

This process is useful if the cell has low energy requirements or if only short-term energy production is called for (generally when oxygen is in short supply). When our muscles lack sufficient oxygen they build up an oxygen debt (lactic acid concentrations build - see below).

The several different types of fermentation are distinguished by their end products. Two of the more common types are alcoholic fermentation, which produces ethanol as an end product, and lactic acid fermentation, which produces lactic acid.

Equation 3