Organisms use energy to build complex biological molecules, maintain their structure and move.

There are two sources of energy available to organisms, electromagnetic and chemical. Electromagnetic energy is, of course, light.

Phototrophs, organisms that extract energy from light, capture light particles, photons, and transform their electromagnetic energy into chemical energy that they can use.

One way they store this energy is in concentration gradients across membranes. Another way is within the structure of molecules.

In fact, energy is stored in the molecular bonds. Different bonds store different amounts of energy, and the coupled breaking and formation of bonds is central to the synthesis and degration of molecules.

We have already discussed one of the major forms of chemical energy within a cell, ATP.

In ATP energy is stored in the structure of the molecule and released when the molecule is cleaved to form ADP and phosphate.

review redox reactions

Generally, the reduction of one molecule is coupled to the oxidation of another. For this reason, reactions of this type are refered to as redox reactions.

There are many inorganic compounds in the nature world from which organisms can extract energy.

Lithotrophs or rock eaters extract energy from compounds, such as H₂, CH₄, CO, S, H₂S and NH₄.

Most higher organisms are either phototrophs, the plants, or chemo-organotrophs, which extract energy from organic molecules. You yourself are a chemo-organotroph.

The larger the H of a reaction, the more energy is released when the reaction occurs.

Such reactions are termed exothermic when they release heat and exergonic when they release other forms of energy.

Consider wood, which is mainly composed of the carbohydrate polymer cellullose.

The reaction of wood burning can be written cellulose + O₂ CO₂ + H₂O

This reaction is highly exothermic, but its activation energy is high, which is why books do not spontaneously burst into flames.

Once started, however, the large amounts of energy released act to maintain the reaction -- the reaction becomes self-sustaining.

Energy is stored by the addition of two electrons and a proton to nicotinamide adeninine dinucleotide (NAD⁺) to form NADH or by the addition of two electrons and two protons to flavin adenine dinucleotide (FAD) to form FADH₂.

Both NADH and FADH₂ are high energy compounds, like ATP.

When electrons are removed, that is when they are oxidized, they release their energy and regenerate NAD⁺ and FAD.

The energy carried by NADH and FADH₂ is used to drive a number of cellular reactions.

How does the cell use the energy stored in NADH and FADH₂?

The answer came from the work of a rather eccentric British scientist, Peter Mitchell. It is known as the chemoisomotic hypothesis.

Mitchell proposed that in bacteria, the energy held in NADH and FADH₂ is used to generate a H⁺ concentration gradient across the plasma membrane.

Electrons are removed from NADH and FADH₂ and passed through an electron transport chain.

The energy is "bled-off' in a series of small steps in which the oxidation of one compound is linked to the reduction of another.

As the electrons move through the electron transport chain, H⁺s are pumped out of the cell, forming H⁺ gradient across membrane.

H⁺s then move down their concentration gradient, through the ATP synthase complex.

The ATP synthase is a membrane pump running backward. As H⁺s move through it, and across the membrne, their energy is captured by the synthesis of ATP.

The same basic process is used by eukaryotic cells and underlies most forms of photosynthesis.

The ATP synthase is a complex protein whose structure is remarkably similar in all organisms.

The similarities of electron transport and ATP synthesis between organisms is yet another piece of evidence for the common origin of living things.

ATP synthases consists of two parts, the F_o membrane channel and the F₁ ATP synthase.

These can be separated from one another and reassembled into a functional complex.

Running in one direction, the complex couples the movement of H⁺s down their concentration gradient with ATP synthesis.

Running in the opposite direction, the complex couples the hydrolysis of ATP to the pumping of H⁺s.