A living cell cannot store significant amounts of free energy. Excess free energy would result in an increase of heat in the cell, which would result in excessive thermal motion that could damage and then destroy the cell.

Rather, a cell must be able to handle that energy in a way that enables the cell to store the energy safely and release it for use only as needed.

Living cells accomplish this by using the compound adenosine triphosphate ATP. It functions similarly to a rechargeable battery. When ATP is broken down, usually by the removal of its terminal phosphate group, energy is released.

The cell uses the energy to do work, usually by the released phosphate binding to another molecule, activating it. For example, in the mechanical work of muscle contraction, ATP supplies the energy to move the contractile muscle proteins.

Recall the active transport work of the sodium-potassium pump in cell membranes. ATP alters the structure of the integral protein that functions as the pump, changing its affinity for sodium and potassium.

In this way, the cell performs work, pumping ions against their electrochemical gradients. At the heart of ATP is a molecule of adenosine monophosphate AMP , which is composed of an adenine molecule bonded to a ribose molecule and a single phosphate group Figure 4.

Ribose is a five-carbon sugar found in RNA, and AMP is one of the nucleotides in RNA. The addition of a second phosphate group to this core molecule results in the formation of adenosine di phosphate ADP ; the addition of a third phosphate group forms adenosine tri phosphate ATP.

The addition of a phosphate group to a molecule requires energy. Phosphate groups are negatively charged and thus repel one another when they are arranged in series, as they are in ADP and ATP.

This repulsion makes the ADP and ATP molecules inherently unstable. The release of one or two phosphate groups from ATP, a process called dephosphorylation, releases energy.

Even exergonic, energy-releasing reactions require a small amount of activation energy to proceed. However, consider endergonic reactions, which require much more energy input because their products have more free energy than their reactants.

Within the cell, where does energy to power such reactions come from? The answer lies with an energy-supplying molecule called adenosine triphosphate, or ATP. ATP is a small, relatively simple molecule, but within its bonds contains the potential for a quick burst of energy that can be harnessed to perform cellular work.

This molecule can be thought of as the primary energy currency of cells in the same way that money is the currency that people exchange for things they need.

ATP is used to power the majority of energy-requiring cellular reactions. Excess free energy would result in an increase of heat in the cell, which would denature enzymes and other proteins, and thus destroy the cell.

Rather, a cell must be able to store energy safely and release it for use only as needed. Living cells accomplish this using ATP, which can be used to fill any energy need of the cell. It functions as a rechargeable battery.

Rather, a cell must be able to store energy safely and release it for use only as needed. Living cells accomplish this using ATP, which can be used to fill any energy need of the cell. It functions as a rechargeable battery.

When ATP is broken down, usually by the removal of its terminal phosphate group, energy is released. This energy is used to do work by the cell, usually by the binding of the released phosphate to another molecule, thus activating it. For example, in the mechanical work of muscle contraction, ATP supplies energy to move the contractile muscle proteins.

Ribose is a five-carbon sugar found in RNA and AMP is one of the nucleotides in RNA. The addition of a second phosphate group to this core molecule results in adenosine di phosphate ADP ; the addition of a third phosphate group forms adenosine tri phosphate ATP. The addition of a phosphate group to a molecule requires a high amount of energy and results in a high-energy bond.

Phosphate groups are negatively charged and thus repel one another when they are arranged in series, as they are in ADP and ATP. This repulsion makes the ADP and ATP molecules inherently unstable. The release of one or two phosphate groups from ATP, a process called hydrolysis, releases energy.

You have read that nearly all of the energy used by living things comes to them in the bonds of the sugar, glucose. Glycolysis is the first step in the breakdown of glucose to extract energy for cell metabolism.

Many living organisms carry out glycolysis as part of their metabolism. Glycolysis takes place in the cytoplasm of most prokaryotic and all eukaryotic cells.

Glycolysis begins with the six-carbon, ring-shaped structure of a single glucose molecule and ends with two molecules of a three-carbon sugar called pyruvate.

In order to provide a cell with energy, these molecules have to pass across the cell membrane, which functions as a barrier — but not an impassable one. Like the exterior walls of a house, the plasma membrane is semi-permeable.

In much the same way that doors and windows allow necessities to enter the house, various proteins that span the cell membrane permit specific molecules into the cell, although they may require some energy input to accomplish this task Figure 2.

Figure 2: Cells can incorporate nutrients by phagocytosis. This amoeba, a single-celled organism, acquires energy by engulfing nutrients in the form of a yeast cell red.

Through a process called phagocytosis, the amoeba encloses the yeast cell with its membrane and draws it inside. Specialized plasma membrane proteins in the amoeba in green are involved in this act of phagocytosis, and they are later recycled back into the amoeba after the nutrients are engulfed.

Figure Detail. Complex organic food molecules such as sugars, fats, and proteins are rich sources of energy for cells because much of the energy used to form these molecules is literally stored within the chemical bonds that hold them together.

Scientists can measure the amount of energy stored in foods using a device called a bomb calorimeter. With this technique, food is placed inside the calorimeter and heated until it burns.

The excess heat released by the reaction is directly proportional to the amount of energy contained in the food. Figure 3: The release of energy from sugar Compare the stepwise oxidation left with the direct burning of sugar right.

Through a series if small steps, free energy is released from sugar and stored in carrier molecules in the cell ATP and NADH, not shown. On the right, the direct burning of sugar requires a larger activation energy.

In this reaction, the same total free energy is released as in stepwise oxidation, but none is stored in carrier molecules, so most of it will be lost as heat free energy. This direct burning is therefore very inefficient, as it does not harness energy for later use.

In reality, of course, cells don't work quite like calorimeters. Rather than burning all their energy in one large reaction, cells release the energy stored in their food molecules through a series of oxidation reactions. Oxidation describes a type of chemical reaction in which electrons are transferred from one molecule to another, changing the composition and energy content of both the donor and acceptor molecules.

Food molecules act as electron donors. During each oxidation reaction involved in food breakdown, the product of the reaction has a lower energy content than the donor molecule that preceded it in the pathway.

At the same time, electron acceptor molecules capture some of the energy lost from the food molecule during each oxidation reaction and store it for later use.

Eventually, when the carbon atoms from a complex organic food molecule are fully oxidized at the end of the reaction chain, they are released as waste in the form of carbon dioxide Figure 3. Cells do not use the energy from oxidation reactions as soon as it is released. Instead, they convert it into small, energy-rich molecules such as ATP and nicotinamide adenine dinucleotide NADH , which can be used throughout the cell to power metabolism and construct new cellular components.

In addition, workhorse proteins called enzymes use this chemical energy to catalyze, or accelerate, chemical reactions within the cell that would otherwise proceed very slowly. Enzymes do not force a reaction to proceed if it wouldn't do so without the catalyst; rather, they simply lower the energy barrier required for the reaction to begin Figure 4.

Figure 4: Enzymes allow activation energies to be lowered. Enzymes lower the activation energy necessary to transform a reactant into a product. On the left is a reaction that is not catalyzed by an enzyme red , and on the right is one that is green. In the enzyme-catalyzed reaction, an enzyme will bind to a reactant and facilitate its transformation into a product.

Consequently, an enzyme-catalyzed reaction pathway has a smaller energy barrier activation energy to overcome before the reaction can proceed.

Figure 5: An ATP molecule ATP consists of an adenosine base blue , a ribose sugar pink and a phosphate chain. The high-energy phosphate bond in this phosphate chain is the key to ATP's energy storage potential.

Figure Detail The particular energy pathway that a cell employs depends in large part on whether that cell is a eukaryote or a prokaryote. Eukaryotic cells use three major processes to transform the energy held in the chemical bonds of food molecules into more readily usable forms — often energy-rich carrier molecules.

Adenosine 5'-triphosphate, or ATP, is the most abundant energy carrier molecule in cells. This molecule is made of a nitrogen base adenine , a ribose sugar, and three phosphate groups. The word adenosine refers to the adenine plus the ribose sugar. The bond between the second and third phosphates is a high-energy bond Figure 5.

The first process in the eukaryotic energy pathway is glycolysis , which literally means "sugar splitting. Glycolysis is actually a series of ten chemical reactions that requires the input of two ATP molecules.

This input is used to generate four new ATP molecules, which means that glycolysis results in a net gain of two ATPs. Two NADH molecules are also produced; these molecules serve as electron carriers for other biochemical reactions in the cell.

Glycolysis is an ancient, major ATP-producing pathway that occurs in almost all cells, eukaryotes and prokaryotes alike. This process, which is also known as fermentation , takes place in the cytoplasm and does not require oxygen. However, the fate of the pyruvate produced during glycolysis depends upon whether oxygen is present.