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All organisms need energy to live. Humans like to sit down to three square meals a day (even if they are on round plates), but other living things have drastically different ways of dealing with their energy needs. Some creepy crawlies get their energy from eating dirt, which is probably not the tastiest meal on the planet. And plants sunbathe, make their own food, and then chow down. The point is we all have to eat.
But energy and food are not the same thing when we're discussing the cellular level. Metabolism is the way we get to the good stuff, the chemical energy, in the molecules of the food we eat. Chemical energy is boss when it comes to getting things done: it powers every process needed for life.
Cellular respiration overview
Cellular respiration is also called aerobic respiration because it takes place when oxygen is present. The purpose of cellular respiration is to make usable energy for the cell. Instead of Red Bull or Monster Energy, cellular energy takes the form of a compound called ATP (short for adenosine triphosphate). ATP is often called the energy currency of the cell.
The end product of cellular respiration is exactly 38 molecules of ATP. That is a pretty good payout for one molecule of glucose.
Cellular respiration takes place in three steps:
To follow along during our behind-the-scenes tour of cellular respiration, it helps to be familiar with oxidation and reduction reactions. Redox reactions (as they're known) are responsible for many of the changes that occur during cellular respiration. Redox reactions involve either losing or gaining electrons.
This can be confusing, because why would you call something reduced if it's actually gaining electrons? Good question.
The answer lies in the charge of the atom. Since electrons are negatively charged, gaining an electron also means gaining a -1 charge, reducing the overall charge of the atom.
You can remember oxidation and reduction with a simple trick: LEO the lion says GER.LEO stands for Loss of Electrons is Oxidation, andGER stands for Gain of Electrons is Reduction.
Electron donors are oxidized, and electron acceptors are reduced.
Redox reactions are an important source of energy and happen during all kinds of combustion reactions, such as the burning of methane to heat a stove or the heating of gasoline to make a car run. In biology, redox reactions are common and extremely important, such as during cellular respiration.
In fact, cellular respiration is one of the processes that allows us to stay warm and cozy in our skin. Heat released from these funky reactions is what keeps our bodies at a constant temperature, even when it's chilly outside.
In cellular respiration, glucose (C6H12O6) is oxidized in a series of steps that releases energy little by little. The electrons that glucose loses as it's oxidized are picked up by NAD+ or FAD2+ molecules that act as electron carriers. In redox terms, glucose is oxidized, and the NAD+ and FAD2+ molecules are reduced to form NADH and FADH2.
During oxidative phosphorylation, the last molecule in the chain to accept an electron is called the terminal electron acceptor. In organisms that breathe oxygen, such as our lovely human selves, oxygen is the terminal electron acceptor; therefore, it is reduced in the whole process.
For a summary of cellular respiration set to a catchy tune, check out this video.