Introduction Once the energy that was in sunlight is transformed into chemical energy, the organism has to now convert the chemical energy into a usable form. It may seem a bit odd for they " re still to be more steps. After all, when we eat a candy bar isn't the sugar in the candy bar "burnt" by the body to provide energy? Well the answer is yes and no. First of all when we burn something normally in the air we combine that substance with oxygen releasing energy from the substance. Indeed, an analogous process does happen in our bodies.

The process is analogous in the sense that the food we eat is oxidized and energy released that we can use. By oxidation, we mean that electrons are lost by the molecules we take in from our food and given to other molecules. The electrons though aren't given up right away but instead the energy they contain is used to produce an important energy-carrying molecule: adenosine tri-phosphate, more commonly called ATP. Thus, what goes on in living things is not really like burning because the molecules from which we harvest energy give up their energy in a controlled fashion rather than all at once as what happens in a fire. If we think of our car, all the energy in the gas tank when we get in our car is not released all at once but rather in small bursts, which allow us to control the car's movement.

In the same way cells take the energy from the "food" and package that energy into manageable bursts that provide just the right amount of energy for the organism's activities be those activities driving a car or flashing a light to attract a mate. We " ll examine two main types of respiration: first aerobic respiration, which involves oxygen and then fermentation, and anaerobic respiration, which does not involve oxygen. Why ATP? What's so special about ATP that the cell spends so much time making it? There are several things: 1. When ATP releases energy, the energy release only involves breaking the last of three phosphate bonds in the molecule.

This results in the production of a small controlled amount of energy that is just the right amount for most of the energy using processed of the cell. 2. Energy release since it involves breaking just the one phosphate bond means that ATP can easily be put together by taking ADP and adding phosphate to it. Thus the cell to make new ATP can recycle the products of energy release from ATP. If this sort of cycling between ADP+Phosphate and ATP were not possible our bodies would require huge amounts of ATP. Given below is the table for ATP requirements for a "simple" bacteria cell: Biosynthesis in E.

coli modified from Ensign (1998) Cell constituent Number of molecules per cell Molecules synthesized per second Molecules of ATP required per second for synthesis DNA 1 0. 00083 60, 000 RNA 15, 000 12. 5 75, 000 Polysaccharides 39, 000 32. 5 65, 000 Lipids 15, 000, 000 12, 500. 0 87, 000 Proteins 1, 700, 000 1, 400. 0 2, 120, 000 In the above table the number of DNA molecules per cell excludes plasmids.

Also we find how expensive molecular synthesis is. A single DNA molecule should take approximately 72, 289, 156 ATP molecules to synthesize. This is a good illustration of why ATP is recycled. 3. Energy release is almost spontaneous.

It does not take much energy for the cells to get at the energy in that third phosphate bond in the ATP molecule. Other molecules have more energy, but the cell has to "spend" a lot more energy than its worth to get at the energy. The Electron Transport System The electron transport system takes electrons that it receives from NADH and FADH 2 and passes those electrons from one protein to another. The energy from those electrons is used to pump hydrogen ions into the mitochondrion's inter membrane space. The reason for doing this is that the hydrogen ions then diffuse back to the inner compartment through special channels in a protein structure called ATP synthase. The hydrogen ions transfer energy to the ATP synthase which uses the energy to make ATP from ADP + inorganic phosphate.

In many cells including those of the human body this type of electron transport phosphorylation is the main way ATP is made. NAD+, NADH, FAD+, FADH 2, NAD+ and FAD are electron acceptors that carry electrons that are largely from the energy containing molecules that enter the Kreb's cycle. These electron acceptors pick up two electrons along with hydrogen ions yielding NADH and FADH 2. Kreb's cycle: - The Kreb's cycle is the source of the hydrogen ions and the electrons needed to make ATP from the electron transport system. The Kreb's cycle takes place in the mitochondrion's inner compartment often called the matrix As the electrons travel through the ETS the energy contained in the electrons is used to pump hydrogen ions into the mitochondrion's outer compartment or inter membrane space, the inter membrane space. The electron paths are shown in yellow.

Hydrogen pumps The hydrogen pumps are shown in green integrated into the electron transport system. The path of hydrogen ions from the matrix to the inter membrane space is shown in black. Hydrogen ions The pumping of hydrogen ions into the outer compartment sets up a concentration gradient. Naturally the hydrogen ions should diffuse back into the mitochondrion's matrix. The black arrows represent the paths of hydrogen ions involved in the production of ATP using the electron transport system. Inner mitochondria membrane The inner mitochondria membrane is generally impermeable to the hydrogen ions except for ATP synthase channels (Blue flask shaped structure at right) that allow the hydrogen ions to diffuse through is a combination channel for diffusion of the hydrogen ions back to the matrix and enzyme for making ATP.

ATP Synthase As the hydrogen ions diffuse through the ATP synthetase (Blue flask shaped structure at right) energy is transferred from the ions to the rest of the ATP synthase molecule. This energy is used to power the production of ATP from ADP and inorganic phosphate. The process of making ATP in this way is called electron transport phosphorylation or chemi o-osmosis. Most of the ATP we use is made in this way. Inorganic phosphate ATP production by electron transport phosphorylation uses inorganic phosphate from the cytoplasm as the source of phosphate.

In contrast substrate level phosphorylation uses phosphate transferred from other organic compounds to the ADP to make ATP. The role of oxygen As the electrons in the ETS are used to do work, the electrons lose energy and reach a point at the end of the ETS where they have to be gotten rid of. The scheme the cell uses to do this is to combine the electrons with hydrogen ions and oxygen to produce water. This is a sensible thing for our cells to do since oxygen is an excellent electron acceptor.

In anaerobic respiration, as opposed to aerobic respiration, other electron acceptors are used to accept electrons from the electron transport systems. Alternate Metabolic Paths in Respiration Since we don't just eat glucose. The other foods we eat are broken down by digestion and in the cell into smaller subunits, which can feed into cellular respiration. The diagram below shows the fate of some of the major organic compounds metabolized by the body. The key thing to note here is that the breakdown products are fed into cellular respiration at different points in the process. Thus, in the case of amino acids, the amino group is separated in the cell as waste material and is used to make urea, which is excreted in urine.

The remaining carbon skeletons from the amino acids are broken down further and fed into respiration as indicated by the arrow. Fatty acids are broken down into smaller two-carbon units join CoA to form acetyl CoA that can feed directly into the Kreb's cycle. This is one of the reasons why fats have the most energy per unit weight. Fermentation and Anaerobic Respiration.

If oxygen is absent, many cells are still able to use glycolysis to produce ATP. Two ways this can be done are through fermentation and anaerobic respiration. Fermentation is the process by which the electrons and hydrogen ions from the NADH produced by glycolysis are donated to another organic molecule. The reason this is done is to produce NAD+, which in tern is needed to keep glycolysis going.

Remember that unless the cell has some sort of electron transport system, the NADH is not usable. At the same time NAD+ is needed for glycolysis and its much less expensive in terms of energy for the cell to simply take the NADH that would normally go to the mitochondrion and use it to regenerate the NAD+. There are a number of fermentation pathways that different cells use. Yeast cells produce ethyl alcohol by fermentation.

Certain cells of our body, namely muscle cells, use lactic acid fermentation, while depending on the organism some of the other products of fermentation include acetic acid, formic acid, acetone and isopropyl alcohol. In our bodies certain muscle cells, called fast twitch muscles, have less capability for storing and using oxygen than other muscles. When we run and these muscles run short of oxygen, the fast twitch muscles begin using lactic acid fermentation. This allows the muscle to continue to function by producing ATP by glycolysis. The muscles get enough ATP for quick spurts or shall we say sprints, but quickly become fatigued as their stores of glycogen are used up.

Eventually we cramp. This is in part because the muscles lack sufficient ATP to continue contracting. Also, lactic acid builds up and must be metabolized by the liver. If we want to see what these muscles are like, when we eat chicken or turkey the white meat is fast twitch muscle. The dark meat is what is called slow twitch muscle. This meat is dark because it contains oxygen holding protein called myoglobin.

The slow twitch muscles tend to be wing and leg muscles where long term endurance is required. The fast twitch muscles tend to be more common in the breast where quick response but not necessarily endurance is needed. Also, wild animals tend to have slower twitch muscle than their domestic counterparts. Respiration in microorganisms: Other paths. Microorganisms, especially bacteria are metabolically much more diverse than plants or animals. For instance different species of bacteria, as noted before, can carry out a number of different fermentation pathways as part of anaerobic respiration.

Even within a species, the bacteria may produce different fermentation products. Bacteria in the genus Leuconostoc can produce lactic acid and ethanol plus carbon dioxide. With anaerobic respiration many bacteria are able to produce more ATP than from glycolysis alone. They do this by using some other compound as an electron acceptor. Remember that this is the role of oxygen in aerobic respiration. Some of the electron acceptors used include nitrate, sulfate and carbon dioxide.

Thus, in anaerobic respiration at least part of the electron transport system is used to power the production of ATP via electron transport phosphorylation. However, since not all the ETS is involved, anaerobic respiration produces less ATP than does aerobic respiration. When carbon dioxide is used as an electron acceptor, the resulting product is either methane or acetic acid, depending on the organism involved. Much if not all the methane produced in our gut or by cows or in swamps is produced by this process. We have these types of anaerobic bacteria in our gut is well. So when you eat too many beans...

blame the anaerobic bacteria instead!