Energy Pathways: Respiration

Energy is conserved quantity that can be interconverted between its types, including kinetic energy, potential energy, chemical energy, and electromagnetic energy. Energy is sometimes defined as "the capacity to do 'work''. Energy can have several forms, some of which are easily convertible and can be changed to another form useful for work.

Living things require energy.  But from where does this energy come?  We know that consumers eat producers and decomposers recycle raw material, but from where do the producers get their energy?  The source of energy for the earth is the sun!  The sun warms the earth, heats the waters and provides the energy needed for living things.  But how?  The answer lies in the pathways of photosynthesis. The Energy of the sun (electromagnetic) is converted to chemical Energy usually in the form of Glucose. The useful chemical Energy in Glucose is found in the electrons forming the bonds between Carbon and Hydrogen atoms.

How many Carbon-Hydrogen Bonds do you see in the Glucose molecule? (click your answer) 6 - 9 - 10 - 12

Living systems depend on an input of this chemical energy (Glucose) to fuel the reactions that constitute life (aka metabolism) and several molecules are involved. Glucose contains a large amount of Energy and is used as a short term Energy storage molecule. ATP is the molecule that cells use for immediate energy. ATP carries a useful amount of energy within its molecular structure, and this energy can be released when the terminal phosphate of ATP is removed by hydrolysis to form ADP. So our question is: How do cells get a supply of ATP to fuel their biochemical reactions?

Cellular respiration is a catabolic metabolic pathway that uses oxygen to release stored energy from carbohydrates. These chemical reactions are the only reason you need to breathe or need a Heart. During respiration, Energy is released slowly, step-wise, through many enzymatic reactions in different parts of the cell. The released energy is ultimately used to add Pi to ADP to regenerate ATP (more on this on the next page).

ATP and ADP are constantly cycled. Energy is added to ADP with Pi to form energy-rich ATP; energy is released when Pi is removed from ATP to re-form ADP. It is estimated that single working muscle cell recycles ATP at  the rate of 10 million molecules per second. This Energy was initially stored in the potential energy of electrons found in reduced organic compounds .

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REDOX reactions - One way Energy is transferred from one molecule to another via the transfer of electrons in redox reactions. Redox reactions (oxidation-reduction reactions) involve two molecules, one of which, the reducing agent, donates one or more electrons to the other, the oxidizing agent. During redox reactions, Hydrogen ions are often transferred along with electrons to keep charges neutral.

Highly reduced molecules are energy-rich (like Glucose). During cellular respiration, these reduced molecules are slowly oxidized to release their stored energy. The electrons (along with H+) are usually passed temporarily to the coenzyme NAD+, which carries them to an electron transport chain, where their energy can be captured to make ATP. NAD+ is most common electron acceptor in respiration, but there are others, such as FAD, which is very similar in structure, in the cell.

The coenzyme electron-carrying NAD is also know as vitamin B.

During Respiration, electrons are systematically removed from organic fuel molecules and shuttled to O₂, the ultimate oxidizing agent in the process. This process is accomplished by a set of metabolic reactions, so all the reactions are catalyzed and regulated by enzymes. As we examine the steps of respiration, remember that each step is probably catalyzed by a different Enzyme.

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