11 February 2005
Reading, Chapter 5
Cells that are not photosynthetic acquire the energy they need by oxidizing carbohydrates and other "food" molecules to carbon dioxide. They use the energy difference between the food molecules (reduced) and carbon dioxide (oxidized) to make ATP. This controlled burning is called "cell respiration".
Cell respiration consists of three steps: glycolysis, the Krebs cycle, and respiratory electron transport.
The first step by which cells make ATP from food is glycolysis. Imagine eating a donut. The starches and sugars of the donut are converted to glucose (blood sugar) in your mouth and stomach by digestive enzymes. The glucose is taken up by your blood in your intestines and distributed throughout your body to all its cells. Cells that need glucose take it up through glucose transport proteins in their membranes and glycolysis begins.
Glycolysis is a series of chemical reactions performed by enzymes in the cytosol of all cells. They convert the glucose, which is a 6 carbon sugar, into two molecules of pyruvate, which has 3 carbons. In the process, two molecues of ATP are made, as are a couple of NADH molecules, which are reductants and can donate electrons to various reactions in the cytosol.
Glycolysis requires no oxygen. It is an anaerobic type of respiration performed by all cells, including anaerobic cells that are killed by oxygen. For these reasons, glycolysis is believed to be one of the first types of cell respiration and a very ancient process, billions of years old.
In the present, many cells add a step to glycolysis if oxygen is not available. In the case of yeast cells, this extra step is the conversion of pyruvate to ethanol (alcohol) and carbon dioxide (CO2). This extra step is called "fermentation". It is the process by which baking yeasts cause bread to rise and brewing yeasts add alcohol to beer and wine.
Your muscle cells also add a fermentation step to glycolysis when they don't have enough oxygen. They convert pyruvate to lactate. This lactate can cause inflammation of muscle tissues, which is why muscles can be sore after vigorous exercise.
Why do some cells add fermentation steps in the absence of oxygen? Fermentation steps act to increase the rate of glycolysis.
2. The Krebs cycle
If oxygen is present, pyruvate from glycolysis is sent to the mitochondria. The pyruvate is transported across the two mitochondrial membranes to the space inside, which is called the mitochondrial matrix. There it is converted to many different carbohydrates by a series of enzymes. This process is called the Krebs cycle. The Krebs cycle consumes pyruvate and produces three things: carbon dioxide, a small amount of ATP, and two kinds of reductant molecules called NADH and FADH.
The CO2 produced by the Krebs cycle is the same CO2 that you exhale. The electron carriers NADH and FADH are sent to the final step of cell respiration, which is respiratory electron transport. The Krebs cycle does not use oxygen, though it does stop in the absence of oxygen because it runs out of NAD and FAD.
Many of your body's cells can also use fatty acids in the Krebs cycle. Fatty acids are the major components of fats. When fats are being used to make ATP, fatty acids are released into the blood by fat cells, taken up by other cells, sent to the mitochondria, and consumed by the Krebs cycle. This use of fatty acids by the Krebs cycle generates CO2, a small amount of ATP, and the electron carrier molecules NADH and FADH just as use of pyruvate does.
3. Respiratory electron transport
When ATP is used for energy, it is converted to ADP and P (P stands for phosphate, in this case). ADP and P are remade into ATP by the third step of cell respiration, respiratory electron transport.
Respiratory electron transport is a current of electrons that passes through proteins in the inner mitochondrial membrane. This is similar in many ways to the electric current of photosynthetic electron transport in chloroplasts.
Respiratory electron transport starts when NADH and FADH donate electrons to the first of these proteins. In doing this, they become NAD and FAD and return to the Krebs cycle for more electrons. The electrons taken from NADH and FADH are passed to several other proteins in the inner mitochondrial membrane. Ultimately, they are passed to oxygen (O2). O2 becomes two molecules of water (H2O) after receiving four electrons. This is how oxygen is used by cell respiration. It is a safe endpoint for the electric current that drives the synthesis of ATP.
The electron transport between proteins of the inner mitochondrial membrane makes ATP (adenosine triphosphate) from ADP (adenosine diphosphate) by adding the third phosphate. It does not do this directly, however. Instead, the electron transport acts to move protons (H+) across the inner mitochondrial membrane. The accumulation of H+ on one side of the inner membrane is like water behind a dam. It can be used to drive a tiny turbine.
Recall that charged solutes cannot pass easily across a membrane, even very small charged solutes like H+. Recall also that solutes move so as to create even distribution and electrical neutrality. This means that the H+ accumulating on one side of the inner mitochondrial membrane will try to move back across the membrane. It can only do so through the appropriate transport protein, however. In this case, the appropriate transport protein is the ATP synthase, a protein that acts like a tiny turbine. As H+ pass through it across the membrane, part of it spins and the energy of its spinning adds the third phosphate to ADP.
In summary, respiratory electron transport is a flow of electrons from NADH and FADH to oxygen that produces water and ATP. The ATP is exported out of the mitochondria to fuel cell activities.