I was going to discuss a paper today, but I was finding it difficult to explain without using a lot of scientific jargon.  I finally realized why: I had to describe the process of cellular respiration before I could talk about the paper for it to make sense to a wide audience.  As a result, the post kept getting longer and longer and I hadn’t even gotten the good parts.  So, I am going to split the post in half!  Today I’ll cover respiration in general and tomorrow I’ll go over the paper.  If you know the details of how respiration works, feel free to skip over this post because it’s going to be review.  If you’re like most people, however, and it’s been a while since you took high school biology or intro biology in college, then this post will help make tomorrow’s post a little more clear.  Consider this your respiration refresher!

Diagram of a simple insect tracheal system.

The word “respiration” describes two different but related processes in biological organisms.  At the organismal scale, it describes the process by which an animal (or plant or fungus) obtains the oxygen it requires from the environment.  For example, most vertebrates use two different organ systems to deliver oxygen to their cells, the respiratory system and the circulatory system.  These animals breathe by inhaling air into the lungs.  Oxygen then passes out of the lungs, into red blood cells, and is transported to cells via the bloodstream.  Carbon dioxide wastes then follow the opposite path and are released into the atmosphere.  Insects, as I’ve described before, use a different system.  Instead of lungs, they have a series of branching tubes (trachea and tracheoles) through which oxygen flows from the atmosphere to individual cells or small groups of cells.  CO2 then travels back out along the same tubes.  A variety of respiratory systems can thus achieve the same thing: transporting oxygen from the environment to the cells and moving respiratory waste products from the cells to the environment.

The structure of a simple animal cell. The flagellum is absent in most multicellular animals. (Image in public domain, available at http://en.wikipedia.org/wiki/File: Animal_cell_structure_en.svg)

The purpose of organismal respiration is to provide cells with the oxygen required for energy production in the mitochondria via cellular respiration.  If you recall from your biology classes, eukaryotic cells (i.e. cells from living organisms other than bacteria) are full of little organelles that have various duties within the cell.  The nucleus is the “brain” of the cell and contains the DNA necessary for cellular replication and several cellular processes.  The endoplasmic reticulum is the primary site of protein synthesis while the golgi apparatus packages and processes proteins.  Then there are the mitochondria, the double membraned “power plants” of the cells.  Mitochondria contain a small amount of DNA (important to note for tomorrow’s post) and are the site of cellular respiration.  Oxygen is thus delivered via organismal respiration to the mitochondria within cells so that cellular respiration can occur.

Cellular respiration is a rather complex process that converts nutrients obtained by the organism from its food (or via photosynthesis in plants) into energy and waste products (CO2 and water).  So, how does cellular respiration work?  There are four different processes that occur altogether: glycolosis, pyruvate oxidation, the citric acid cycle, and the respiratory chain.  I’ll summarize each step here, though to keep things as simple as possible, I’m not going to include all the gory details.  If you’re interested in more information, I’ve included the link to each step on Wikipedia so you can read more.

1) Glycolosis.  Glycolosis involves 10 different chemical reactions, but is ultimately important because it converts glucose into pyruvate, which is essential in the next step.  The other products of glycolosis are a few electrons (necessary later on) and 2 molecules of adenosine triphosphate (ATP), energy rich molecules that power many reactions within the cell.  This step takes place outside the mitochondria.

2) Pyruvate decarboxylation.  In this step, pyruvate is converted into acetate, which is then activated by a coenzyme.  Carbon dioxide is also produced during this step.  As CO2 is not needed for anything, it is a waste product that is eliminated from the cells via organismal respiration.  Pyruvate decarboxylation occurs in the space between the two membranes of the mitochondria, the intermembrane space.

3) Citric acid cycle.  The citric acid cycle takes the acetate formed during pyruvate oxidation and oxidizes it (strips electrons from the molecule), producing CO2 and transferring electrons to a carrier molecule (NAD+) that is involved in the next step.  This process takes place within the interior of the mitochondria.

4) Respiratory chain.  This is the step that is important for tomorrow’s post and the final step of cellular respiration.  Here, the reduced form of NAD generated in the citric acid cycle transfers electrons into a series of reactions that occur along the inner membrane of the mitchondria.  Electrons are passed from one carrier molecule within the membrane to the next.  As the they move, they cause the active transport of protons into the intermembrane space.  The protons then flow back across the membrane into the interior of the mitochondria to form ATP, the fuel that powers cellular processes.  This step of cellular respiration is also where oxygen comes into play and why organismal respiration is so important.  Oxygen molecules pick up the free electrons as they come out of the respiratory chain, grab a few protons, and transform into water, a harmless waste product.

Cellular respiration is an incredibly important process in most eukaryotic organisms (all living things other than bacteria), so it is essential that the four processes described here work properly.  For example, if a cell has a faulty respiratory chain, it can’t produce enough ATP to function and produces harmful free radicals rather than water.  Low ATP and high free radical production signal other chemical pathways that lead to programmed cell death, eliminating the faulty cell from the organism or killing a developing embryo before it fully develops.  Tomorrow, I’ll discuss a paper that proposes an intriguing new idea about how variation in respiratory chain efficiency might contribute to reproductive success, fertility, and life span.  I think it’s a fascinating paper that proposes a very interesting new idea, so I hope you’ll check back again tomorrow!

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