Cellular Respiration-Glycolysis, Kreb's cycle, Electron Transport Chain

Cellular respiration is the process of oxidizing food molecules, like glucose, to carbon dioxide and water.

C₆H₁₂O₆ + 6O₂ + 6H₂O → 12H₂O + 6 CO₂

The energy released is trapped in the form of ATP for use by all the energy-consuming activities of the cell.
The process occurs in two phases:

• glycolysis, the breakdown of glucose to pyruvic acid
• the complete oxidation of pyruvic acid to carbon dioxide and water

In eukaryotes, glycolysis occurs in the cytosol.  The remaining processes take place in mitochondria.

Mitochondria

Mitochondria are membrane-enclosed organelles distributed through the cytosol of most eukaryotic cells. Their number within the cell ranges from a few hundred to, in very active cells, thousands. Their main function is the conversion of the potential energy of food molecules into ATP.
Mitochondria have:

• an outer membrane that encloses the entire structure
• an inner membrane that encloses a fluid-filled matrix
• between the two is the intermembrane space
• the inner membrane is elaborately folded with shelflike cristae projecting into the matrix.
• a small number (some 5–10) circular molecules of DNA

This electron micrograph (courtesy of Keith R. Porter) shows a single mitochondrion from a bat pancreas cell. Note the double membrane and the way the inner membrane is folded into cristae. The dark, membrane-bounded objects above the mitochondrion are lysosomes.

The number of mitochondria in a cell can

• increase by their fission (e.g. following mitosis);
• decrease by their fusing together.

(Defects in either process can produce serious, even fatal, illness.)

The Outer Membrane

The outer membrane contains many complexes of integral membrane proteins that form channels through which a variety of molecules and ions move in and out of the mitochondrion.

The Inner Membrane

The inner membrane contains 5 complexes of integral membrane proteins:

• NADH dehydrogenase (Complex I)
• succinate dehydrogenase (Complex II)
• cytochrome c reductase (Complex III; also known as the cytochrome b-c₁ complex)
• cytochrome c oxidase (Complex IV)
• ATP synthase (Complex V)

The Matrix

The matrix contains a complex mixture of soluble enzymes that catalyze the respiration of pyruvic acid and other small organic molecules.
Here pyruvic acid is

• oxidized by NAD⁺ producing NADH + H⁺
• decarboxylated producing a molecule of
  • carbon dioxide (CO₂) and
  • a 2-carbon fragment of acetate bound to coenzyme A forming acetyl-CoA

The Citric Acid Cycle

• This 2-carbon fragment is donated to a molecule of oxaloacetic acid.
• The resulting molecule of citric acid (which gives its name to the process) undergoes the series of enzymatic steps shown in the diagram.
• The final step regenerates a molecule of oxaloacetic acid and the cycle is ready to turn again.

Summary:

• Each of the 3 carbon atoms present in the pyruvate that entered the mitochondrion leaves as a molecule of carbon dioxide (CO₂).
• At 4 steps, a pair of electrons (2e^-) is removed and transferred to NAD⁺ reducing it to NADH + H⁺.
• At one step, a pair of electrons is removed from succinic acid and reduces the prosthetic group flavin adenine dinucleotide (FAD) to FADH₂.

The electrons of NADH and FADH₂ are transferred to the electron transport chain.

The Electron Transport Chain

The electron transport chain consists of 3 complexes of integral membrane proteins

• the NADH dehydrogenase complex (I)
• the cytochrome c reductase complex (III)
• the cytochrome c oxidase complex (IV)

and two freely-diffusible molecules

• ubiquinone
• cytochrome c

that shuttle electrons from one complex to the next.
The electron transport chain accomplishes:

• the stepwise transfer of electrons from NADH (and FADH₂) to oxygen molecules to form (with the aid of protons) water molecules (H₂O);(Cytochrome c can only transfer one electron at a time, so cytochrome c oxidase must wait until it has accumulated 4 of them before it can react with oxygen.)
• harnessing the energy released by this transfer to the pumping of protons (H⁺) from the matrix to theintermembrane space.
• Approximately 20 protons are pumped into the intermembrane space as the 4 electrons needed to reduce oxygen to water pass through the respiratory chain.
• The gradient of protons formed across the inner membrane by this process of active transport forms a miniature battery.
• The protons can flow back down this gradient only by reentering the matrix through ATP synthase, another complex (complex V) of 16 integral membrane proteins in the inner membrane. The process is called chemiosmosis.

Chemiosmosis in mitochondria

The energy released as electrons pass down the gradient from NADH to oxygen is harnessed by three enzyme complexes of the respiratory chain (I, III, and IV) to pump protons (H⁺) against their concentration gradient from the matrix of the mitochondrion into the intermembrane space (an example of active transport).

As their concentration increases there (which is the same as saying that the pH decreases), a strong diffusion gradient is set up. The only exit for these protons is through the ATP synthase complex. As in chloroplasts, the energy released as these protons flow down their gradient is harnessed to the synthesis of ATP. The process is called chemiosmosis and is an example of facilitated diffusion.
One-half of the 1997 Nobel Prize in Chemistry was awarded to Paul D. Boyer and John E. Walker for their discovery of how ATP synthase works.Link to some of the details.

It is tempting to try to view the synthesis of ATP as a simple matter of stoichiometry (the fixed ratios of reactants to products in a chemical reaction). But (with 3 exceptions) it is not.How many ATPs?

Most of the ATP is generated by the proton gradient that develops across the inner mitochondrial membrane. The number of protons pumped out as electrons drop from NADH through the respiratory chain to oxygen is theoretically large enough to generate, as they return through ATP synthase, 3 ATPs per electron pair (but only 2 ATPs for each pair donated by FADH₂).
With 12 pairs of electrons removed from each glucose molecule,

• 10 by NAD⁺ (so 10x3=30); and
• 2 by FADH₂ (so 2x2=4),

this could generate 34 ATPs.
Add to this the 4 ATPs that are generated by the 3 exceptions and one arrives at 38.
But

• The energy stored in the proton gradient is also used for the active transport of several molecules and ions through the inner mitochondrial membrane into the matrix.
• NADH is also used as reducing agent for many cellular reactions.

So the actual yield of ATP as mitochondria respire varies with conditions. It probably seldom exceeds 30.

The three exceptions

A stoichiometric production of ATP does occur at:

• one step in the citric acid cycle yielding 2 ATPs for each glucose molecule. This step is the conversion of alpha-ketoglutaric acid to succinic acid.
• at two steps in glycolysis yielding 2 ATPs for each glucose molecule.

Mitochondrial DNA (mtDNA)

The human mitochondrion contains 5–10 identical, circular molecules of DNA. Each consists of 16,569 base pairs carrying the information for 37 genes which encode:

• 2 different molecules of ribosomal RNA (rRNA)
• 22 different molecules of transfer RNA (tRNA) (at least one for each amino acid)
• 13 polypeptides

The rRNA and tRNA molecules are used in the machinery that synthesizes the 13 polypeptides.
The 13 polypeptides participate in building several protein complexes embedded in the inner mitochondrial membrane.

• 7 subunits that make up the mitochondrial NADH dehydrogenase (complex I)
• cytochrome b, a subunit of cytochrome c reductase (complex III)
• 3 subunits of cytochrome c oxidase (complex IV)
• 2 subunits of ATP synthase (complex V)

Each of these protein complexes also requires subunits that are encoded by nuclear genes, synthesized in the cytosol, and imported from the cytosol into the mitochondrion. Nuclear genes also encode ~1,000 other proteins that must be imported into the mitochondrion