A Biological Energy Crisis

Biology concepts –  mitochondria, aerobic respiration, anaerobic respiration, glycolysis, fermentation, mitosome

Birds in flight use an astounding amount of energy, and the smallest birds use the most energy. Hovering hummingbirds must flap their wings 50-80 times a second, which requires a lot of energy. To meet this demand, they use 10x the amount of oxygen that a person uses (per gram of body weight)! To move this much oxygen in their blood when flying, their hearts must beat over 1200 times per minute. At that rate, a red blood cell can traverse the bird’s entire circulatory system in less than one second!

It is a vicious circle; the hummingbird must eat constantly in order to have the energy to hover, and it must hover in order to eat constantly. Hummingbirds convert their carbohydrate intake into cellular energy (ATP) on the fly, using the sugars ingested only a few minutes earlier to support up to 90% of their need. Contrast that to humans; elite athletes can draw only about 15% of their needed energy from the sugars they ate recently. 

So how is all this energy made? Since we have been talking about the mitochondria on and off for several weeks, you would be right to guess that this organelle is involved, but it doesn’t start there.

Dietary glucose ends up in the cytoplasm after it is eaten and transported through the blood to each and every cell in the body. In the cytoplasm, the sugar is broken down in a process called glycolysis (gly = glucose and lysis = splitting). This process takes the carbohydrate from a six-carbon sugar down to two three-carbon sugars (pyruvate). In the process, there is a net gain of two ATP molecules (four are actually made per glucose but you have to invest 2 ATPs to get the process rolling). That isn’t much of a payoff. There must be something more, and this is where the mitochondria figure in the process.

The pyruvates are taken into the mitochondria and a second process begins to consume them. First there is a carbon and two oxygens removed from each pyruvate to form acetyl-CoA in what is called a linking reaction, since it links glycolysis to the next step - the citric acid cycle (Kreb’s cycle).   In this cycle, a series of reactions takes place to sequentially remove carbons from the sugar, leaving a four-carbon molecule (oxaloacetate) that then joins to the acetyl-CoA produced from another pyruvate. The series of reactions results in 2 ATPs and 6 NADH’s formed. This latter molecule (long name = nicotinomide adenine dinucleotide + hydrogen) will become important in the final step.

Remember that the mitochondrion has two membranes, and the inner membrane is folded into many cristae, in order to increase its surface area. The NADH’s produced during the Krebs cycle work with a series of proteins embedded in the inner mitochondrial membrane (called the electron transport chain) to create a proton gradient.

When the NADH is broken down, a hydrogen ion (the same thing as a proton) is pushed into the inner membrane space. This is against its gradient and creates a high-energy situation, since it wants to move back into the matrix (the space inside the inner membrane). The ATP synthase allows the proton to move back in, but uses the energy of the gradient to convert ADP into ATP. One ATP is made for every proton that is pushed out and then allowed back into the matrix by oxidative phosphorylation.

The driving force behind NADH’s release of an electron and a proton (hydrogen atom) is that some atom must be waiting to scoop the extra electron, and this something is oxygen (this is why it is called oxidative phophorylation). This is why we have to breathe, the oxygen is a big magnet (metaphorically speaking) for the electron. The oxygen plus the electron plus two hydrogens bind together to form water. This is the metabolic water that is so important to many animals that don’t drink water. 

All told, the electron transport chain produces 36 ATP molecules per glucose, much more than the paltry 2 resulting from glycolysis (called substrate level phosphorylation as opposed to oxidative phosphorylation). It is a good thing that hummingbirds have mitochondria to wring so much energy out of their food (not so bad for us either).

And herein lies the exception, some eukaryotes have decided to try to live without mitochondria. It isn’t as though they just never underwent endosymbiosis; recent evidence is showing us that all eukaryotes had mitochondria at some point in their evolution. These exceptional organisms just worked out another way to produce energy, and allowed their mitochondria to disappear or change over time.

The human gut pathogen Giardiaintestinalis (or lamblia) is a good example. Look as long as you like, but you won’t find a mitochondrion in this protozoan. Until 2003, scientists hypothesized that the lack of mitochondria in G. lamblia meant that it was a very early eukaryote, diverging from other eukaryotes before the endosymbiotic event that created mitochondria. But, then we discovered it was an even bigger exception.

Instead of mitochondria, Giardia has 2-50 cryptons, also called mitosomes. These are mitochondrial remnant organelles (crypton = cryptic mitochondrion), with no genome of their own. They are completely reduced; all of their DNA has been transferred to the nucleus or lost, so mitosomes do not replicate on their own.

In Giardia, the mitosomes line up and down the sides of the organism’s two nuclei, with some between the nuclei. Yes, you're right -  Giardia doesn’t have any mitcohondria, but it has two nuclei – go figure. This specific and repeated arrangement suggests a specific function for these organellar remants. We aren’t sure what the functions might be, but it is not energy production. G. intestinalis produces its energy by glycolysis and by fermentation– the same process that yeast use to produce alcohol.

In alcohol fermentation of yeast, the 3-carbon pyruvates from glycolysis are converted to 2-carbon ethanol and some NADH is converted back to NAD+. This prevents a critical shortage of NAD+ in the cell. The amount of NAD+ in the cell is limited, so if glycolysis is to continue there must be NAD+ must be recycled from NADH.  The conversion of NADH back to NAD+ is the main purpose behind fermentation; it doesn’t produce any more energy than glycolysis alone.

By the way - you ferment too. Yes, you. When oxygen is scarce, mammals will resort to fermentation, we just don’t produce alcohol. Instead, our waste product is lactic acid. In 1929, Nobel laureate Archibald Hill stated that it was the buildup of lactic acid in the muscles that caused muscle soreness after exercise, but his experiment was flawed. It wasn’t until just a few years ago that we discovered that lactic acid is crucial in keeping the muscles working (and brain) working when they are taxed. Lactic acid isn’t the problem, it is part of the solution.

But back to Giardia. Unlike yeast, G. lamblia doesn’t have a choice, it undergoes alcohol fermentation all the time. Make that almost all the time. Without oxygen (even though it doesn’t use it to make ATP) most of the pyruvate is converted to alanine, an amino acid, during fermentation. With even a little bit of oxygen, this switches over to alcohol production.  But there is another way Giardia can make some energy.

A mechanism called the arginine dihydrolase pathway has been seen only in prokaryotes and two eukaryotic anaerobes (Giardia and Trichomonas vaginalis). This speaks to the primitive nature of Giardia; no wonder scientists thought that it didn’t ever have mitochondria, like prokaryotes. In the arginine dihydrolase pathway, a whole bunch of steps lead to a little bit of ATP formation. It must make a difference for the organism’s survival, otherwise they wouldn't invest the energy in maintaining the pathway.

Giardia isn’t the only eukaryote to choose mitosomes over mitochondria. Entamoeba histolytica also causes diarrhea when it takes up residence in your gastrointestinal tract. I think this suggests that we are providing them with all the carbohydrates they need so that glycolysis and fermentation pay off. Was there less diarrhea before twinkies and french fires? Could be – there is probably grant money available for that study.

E. histolytica was also thought to be an ancient eukaryote that never had a mitochondrion, but mitosomes were discovered in this pathogen way back in 1999; the good old days. Another pathogen, Cryptosporidium parvum is also a mitosome-containing amitochondriate. Again, this is an intestinal parasite that causes diarrhea. I think that living in the gut must have turned these organisms into mutants, like the 1950’s animals exposed to radiation in great old movies like Them! and Godzilla.

C. parvum is closely related to the organism that causes malaria (Plasmodium falciparum), but they make ATP in different ways. P. falciparum  has mitochondria and can carry out oxidative phosphorylation via the electron transport chain. So how can they be related?

Here’s how: P. falciparum might have mitochondria, but they look like they are on their way out. They only have a few genes, and at least one principal enzyme is completely missing. In one stage of the infection, Plasmodium survives only by fermentation (although it goes to lactate, not alcohol), so maybe these two parasites are not so different after all. They have another similarity, but we will talk about that in a couple of weeks when we discuss plants without chloroplasts.