So if only one molecule of ATP is produced by substrate level phosphorylation in the TCA cycle then where does all the other energy come from?
Answer: oxidative phosphorylation.

From the metabolism of one molecule of glucose 10 electrons are carried by NADH molecules (donÕt forget the NADH formed in the glycolytic pathway!!) and two by FADH2 (formed in the TCA cycle).

These are the key intermediates in the formation of the rest of the energy. So how is energy transferred in the form of electrons (redox potential) to ATP (chemical) production?

Electron transport
The reason is is that they are contained within a molecule that has been reduced (i.e. gained an electron). Remember: reduced compounds contain more energy than an oxidised one in redox pairs. Therefore, as they are reduced they are now able to donate this electron to some compound that has a lower potential energy (i.e. they are reducing agents). A convenient way to consider electron transfer is to think of an energy tower (see Figure 12:1). Here the tower represents a range of reduction potentials for various oxidation-reduction couples, from the most negative (i.e. the highest energy; NAD+/NADH) at the top to the most positive (i.e. least energy; O2/H2O) at the bottom. Essentially what this means is that the redox potential energy in NADH can flow downhill to O2. Or in other words, O2 is a better electron acceptor that NAD+.

Therefore, in the aerobic metabolism of glucose, electrons enter the chain in the form of NADH and FADH2, and eventually reduce O2. Consequently, it is called aerobic respiration.

Electron transport chain is a series of oxidation-reduction reactions that move an electron down the chain (see Figures 12:2 and 12:3). Each intermediate has a lower reduced energy potential than the oxidation-reduction intermediate immediately above it. Therefore, electrons are donated down the chain. These intermediates are found in the inner membrane in bacteria (in eucaryotes they are found in the mitochondria) and their composition varies between different organisms. However, all electron transport chains contain flavoproteins, cytochromes, and quinones.

The Proton Motive Force - Chemiosmosis
The production of ATP is directly linked to the establishment of the proton motive force across the membrane, and the electron transport reactions help establish this energized membraneous state. The electrons enter the membrane through the reduced form of NAD+ (i.e. NADH). Therefore, they enter initially as a hydrogen atom. To produce an electron from a hydrogen atom also yields a proton.

H --------> H+ + e-

The electrons so liberated then zig-zag through the membrane using the quinones etc, and as they zig-zag H+ ions are driven from the inside of the cell to the outside. This then sets up a proton gradient across the membrane.

Protons are required to reduce oxygen:

O2 + 4H+ ---------> 2H2O

Therefore, H+ ions are needed in the cytoplasm. However, the movement of electrons drives them to the outside of the cell, and because a H+ ion is charged, it cannot freely diffuse across the membrane. Consequently, what is set up is:

i) proton gradient (pH gradient) and

ii) electrochemical gradient

What this essentially means is that there is considerable energy stored up with these gradients (measured in volts) within the membrane that can be used to generate chemical energy.

For that to occur, the H+ ions on the outside of the cell must be driven back across the membrane to the cytoplasm through a multisubunit enzyme called ATP synthase (see Figure 12:4) which catalyses the reversible reaction:

ADP + Pi --------> ATP

In doing so, some of the electrical energy has now been transferred into the high energy bonds in ATP.

Due to this pathway, 38 molecules of ATP can be derived from one molecule of glucose using aerobic respiration, compared to 2 molecules of ATP via anaerobic metabolism (see Figure 12:5). Therefore this pathway is more efficient at generating energy.

Photosynthesis
For photoheterotrophs, energy is captured from light, and converted to ATP. Two types of bacteria use this pathway, the green sulphur bacteria and the purple sulphur bacteria (see Figure 12:6). Photosynthesis is just a fancy way of harnessing energy from light and creating chemical energy in the form of ATP (see Figure 12:7). However, there are several very important implications with respect to all this.

Two types of reaction generate ATP from light:
i) cyclic photophosphorylation
Here light excites chlorophyll and electrons are excited into a higher energy state and are then passed to an electron transport chain where ATP is made by chemiosmosis. Once the energy has been used from the excited electron it then fall back down an electron transport chain to chlorophyll to be used all over again.

ii) noncyclic photoreduction
Here energy from light is also converted to ATP using electrons from chlorophyll. However, this time, the elecrons are replenished in the chlorophyll molecule by oxidising water which yields O2:

H2O --------> 2e- + 1/2 O2 + 2H+

Furthermore, the reduced NADPH which is also formed in this pathway can then enter the so-called dark reactions which allows carbohydrates to be made from CO2 (see Figure 12:8). Here CO2 is reduced to carbohydrate using NADPH as electron donor (also uses ATP formed during photophosphorylation).

Metabolism of Fats and Proteins
For most prokaryotes glucose is the major energy source. However, all organic carbon sources can be used for energy. Microorganisms can also use fats and proteins for energy. Basically, these molecules are broken down into components in the glycolytic and TCA pathways (see Figure 12:9). If an organism needs to make lipids or proteins, many of these reaction pathways are reversible. Think of it as just some huge interconvertible network where parts can be exchanged either for energy formation (catabolism) or for building things (anabolism).

Return to Bios 213 Home page
Return to Dr. Hill's Home Page
Return to the Biology Home page.