- Assignment 3.1

Living systems depend on an input of energy to fuel the reactions that constitute life. Remember that ATP is the molecule that cells use for short-term 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 new supply of ATP to fuel their biochemical reactions?

Cellular respiration (aerobic respiration) is a catabolic metabolic pathway that uses oxygen to release stored energy from carbohydrates, lipids, and proteins. Energy is released slowly, step-wise, through many enzymatic reactions in the cytoplasm and the mitochondria of the cell. The released energy is ultimately used to add Pi to ADP to regenerate ATP.

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 .

In general, 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 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.

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 pathways, 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.

The reactions of respiration