Metabolism/Energy Overview Glycolysis Citric Acid Cycle  Elmhurst College
Cell Structure Energy Storage - ATP FAD, CoQ, CoA  Chemistry Department
Energy Overview NAD+ Electron Transport  Virtual ChemBook

Energy Storage


The processes of catabolism provide energy which must be made available for performing useful work. The energy cannot be in the form of heat because cells function at constant temperature. Heat energy is only useful when transferred from a hot body to a cold body. The energy from catabolism must be conserved or transformed into chemical storage. The major chemical storage is in the form of adenosine triphosphate (ATP). ATP is the link between exothermic reactions and endothermic reactions. ATP is made of adenosine and ribose bonded to three phosphate groups through phosphate ester bonds.

ATP, ADP, AMP - Chime in new window

The importance of ATP centers on the storage of about 7 kcal/mole of energy in the phosphorus-oxygen bond between the first and second phosphate group. The relationship of energy and the formation and hydrolysis of ATP is illustrated in the following equations: P = PO4-3; ADP = adenine diphosphate.

a) Hydrolysis: ATP + H2O --> ADP + P + energy

b) Formation: ADP + P + energy --> ATP + H2O

Under certain conditions ATP may be hydrolyzed directly to AMP (adenine monophosphate).

ATP + H2O --> AMP + PP + energy

There are other metabolic phosphate molecules which store or give off energy as needed. One further example is the hydrolysis of creatine phosphate in muscle cells which also releases energy.

Quiz: Which reaction above is exothermic and endothermic?
What do the letters ATP stand for? ADP?  

Click for larger image 

Quiz: What is the definition of a coupled reaction?
Which reaction is endo- and exothermic?  
When CoQ converts to CoQH2, is this oxidation or reduction?  
QUES : Write the energy term on the proper side of the equation.
Reactions a) and b) are coupled.
 a) ADP + P -----> ATP + H2O
 b) ATP + H2O -----> ADP + P
 c) creatine-H + HPO4-3 -----> creatine-PO3 + HOH  

Coupled Reactions:

Many biochemical reactions result in energy being given off (exothermic), while many others require energy (endothermic). In order for both processes to be carried out efficiently, they must be "coupled". Usually a coupled reaction will involve ATP or some similar molecule. A coupled reaction is where two reactions occur nearly simultaneously. The first reaction must be exothermic and give off energy. The second reaction is endothermic and immediately uses the energy produced from the first reaction.

An example of a coupled reaction is the hydrolysis of ATP and the contraction of muscle tissue. Two proteins, actin and myosin, form a loose complex called actomyosin. When ATP is added to isolated actomyosin, the protein fibers contract. The hydrolysis of ATP releases energy which is used by muscles to contract. The coupled reaction is:

a) ATP + H2O --> ADP + P + energy

b) relaxed muscle + energy --> contracted muscle

When the ATP is used up by the muscles, a further supply of energy is released from creatine phosphate. Another example of a coupled reaction is the hydrolysis of creatine phosphate to release energy which in turn is used for the formation of more ATP. The coupled reaction is:

a) creatine --- PO3 + H2O --> creatine H + HPO4-3 + energy

b) ADP + HPO4-3 + energy --> ATP + H2O

During periods of low muscular activity, the reactions are reversed to replenish the supplies of ATP and creatine phosphate. The energy for the formation of ATP is supplied by other metabolic reactions.

Click for larger image 

ATP Synthesis:

Cells use a proton-pumping system made up of proteins inside the mitochondria to generate ATP. Synthesis of ATP is coupled with the oxidation of NADH and the reduction of O2 in Electron Transport (both future topics). There are three key steps in this process:

1.Electrons are transferred from NADH, through a series of electron carriers, to O2.
2.Transfer of electrons by these carriers generates a proton (H+) gradient across the inner mitochondrial membrane.
3.When H+ spontaneously diffuses back across the inner mitochondrial membrane, ATP is synthesized.

ATP synthetase converts the free energy of the proton gradient to chemical energy in the form of ATP. It consists of two main components:

ATP synthetase (multicolors) has two components: a proton channel (magenta) which allows diffusion of protons down a concentration gradient, from the intermembrane space, and a catalytic component (F1) to catalyze the formation of ATP in six alpha beta subunits (multicolors) in graphic on the left.

Every time three protons are pumped through the channel, the F1 subunit rotates 120 degrees. The actual synthesis (formation of the bond between ADP and Pi is catalyzed by conformational changes of the enzyme that occur as a consequence of the rotation.

The key point is that the rotation moves the beta subunit that contains ADP + Pi to a new position. In this new position the beta subunit would rather bind ATP, and thus catalyzes the formation of a ATP from the bound ADP and Pi. The newly-formed ATP is released with the transport of an additional proton. Three protons must be transported to make one ATP. Look at one or more of the animations below for a more complete description.

ATP Synthetase - Chime in new window

Animation of ATP synthesis - Thomas M. Terry, The University of Connecticut
Block animation of ATP synthesis - Carnegie Mellon University
Animation Move - Great but 6MB size - Thomas M. Terry, The University of Connecticut