Cellular Respiration

Cellular (or internal) respiration is the process by which energy rich molecules, such as glucose, are broken down with oxygen to generate ATP (the body uses this as an energy source for almost all energy-requiring processes), carbon dioxide and water.  There are three major parts to cellular respiration:

Glycolysis

Courtesy of Wikipedia

This is the breakdown of 6- carbon sugar (glucose) molecules into two 3-carbon pyruvate molecules.  This process occurs in the the cytosol and requires the dephosphorylation of two ATP molecules (consuming energy).  It generates four ATP molecules (net gain of 2 ATP).  It also releases two hydrogen atoms which join to NAD (nicotinamide adenine dinucleotide) to form NADH, which is used later (see below).

Glycolysis, while providing a gain of two ATP, is not effective in providing for the body’s energy requirements.  As such, the body uses the Kreb’s cycle and Oxidative Phosphorylation to generate the majority of the ATP.

Pyruvate to Acetyl-CoA

Pyruvate is actively transported into the mitochondria via a transport protein.  In the mitochondrial matrix, it undergoes decarboxylation, producing acetate and carbon dioxide (and a hydrogen ion to join with another NAD).  Acetate then joins with its coenzyme to form acetyl-coA, which is one of the molecules of the …

Kreb’s (Citric acid) cycle

This cycle of chemical reactions is catalysed by the enzymes of the mitochondrial matrix.

English: Tricarboxylic acid cycle (also known as the citric acid cycle) and some preceding steps (Photo credit: Wikipedia)

The ins and outs of each step is not crucial to know about but there are some general points of particular interest:

1. Two carbons are released, one at a time, from the 6 carbon citrate formed at the start of the cycle.  This process allows for the recycling of oxaloacetate and the coA enzyme.These are released as CO2 into the blood.  NB the oxygen used to form CO2 is NOT from respiration but from oxygen involved in the reactions.
2. Hydrogen atoms are also ‘kicked off’ at four points in the cycle.  As a result, three NADH complexes and one FADH2 complex is formed.  These will be important later in the electron transport system in the mitochondrial membrane.
3. One more molecule of ATP is produced for every Acetyl coA that enters the cycle.  The energy is actually, initially captured by the phosphorylation of guanosine diphosphate (GDP) to GTP.  It is then transferred to ATP.
4. NB A single glucose molecule provides 2 acetate molecules and thus completes two ‘turns’ of the cycle.

Oxidative Phosphorylation

So far, the body has only gained 6 ATP per glucose, but there is plenty of energy still available that the body needs.  To acquire this, the body’s cells carry out a process known as oxidative phosphorylation.  This occurs at the inner mitochondrial membrane, and involves capturing the energy of high-energy electrons as they are transferred onto oxygen.

1. The high-energy electrons are taken from the hydrogens in NADH and FADH2 and are transferred onto another electron carrier molecule as part of the electron transport chain.
   1. This releases H+ and NAD+/FAD+, freeing the latter to pick up more hydrogen in the Kreb’s cycle.
2. The electrons fall to lower energy levels as they are processed through complexes and enzymes found in the mitochondrial membrane (NB I don’t feel it is that important for me to know the ins and outs of these complexes, but if you want to know what role each one plays, wikipedia has a pretty good page going through each one- http://en.wikipedia.org/wiki/Oxidative_phosphorylation#NADH-coenzyme_Q_oxidoreductase_.28complex_I.29).
   1. The release of energy here is partly used to transport H+ across the membrane into the intermembrane space.  This creates a concentration gradient.  H+ ions move back down this gradient via ATP synthase to drive ATP synthesis.  This process is called chemi-osmosis.
      1. For every NADH molecule, 2.5 ATP are generated, and for every FADH2, 1.5 ATP are generated.  Given that there are 10 NADH and 2 FADH2 per glucose molecule, oxidative phosphorylation produces 28 ATP molecules per glucose molecule.
3. Finally, the electrons combine with oxygen, negatively charging it.  This O2- combines with H+ released earlier to form water.