Metabolism and Respiration Overview

Metabolism Overview

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 microbes get their energy from eating dirt, which is probably not the tastiest meal on the planet. The way an organism obtains energy is called its metabolism. Metabolism is not just the way an organism gets food—for example, did it eat tacos or soak up sunlight?—but it also includes the breakdown of food into chemical energy. 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, produced through glycolysis, the citric acid cycle, and oxidative phosphorylation, is exactly 38 molecules of ATP. That is a pretty good payout for one molecule of glucose.

Cellular respiration takes place in three steps:

1. Glycolysis
    
2. Citric acid cycle, also known as the Krebs cycle
    
3. Oxidative phosphorylation
    

To follow along during our behind-the-scenes tour of cellular respiration, it helps to be familiar with oxidation and reduction reactions (also known as redox reactions). Redox reactions are responsible for many of the changes that occur during cellular respiration. They are not related to any of the Clorox brand products. Putting bleach on your cells is a bad, bad idea.

Redox reactions involve losing or gaining electrons.

• In oxidation, an atom loses an electron
   
• In reduction, an atom gains an electron
   

This can be confusing, because why would you call something reduced if it is actually gaining an electron?

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 is short for Loss of Electrons is Oxidation, and GER is short for Gain of Electrons is Reduction.



Redox reactions can be an important source of energy—in fact, redox reactions happen during fires and other types of combustion, 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, as is the case during cellular respiration.

In cellular respiration, glucose (C₆H₁₂O₆) is oxidized in a series of steps that release energy, little by little. The electrons that glucose loses as it is oxidized are usually picked up by NAD⁺, a coenzyme that acts as an electron carrier. In redox terms, NAD⁺ oxidizes glucose, and glucose reduces NAD⁺. NAD⁺ is short for nicotinamide adenine dinucleotide. Feel free to use that fun tidbit at parties if you don't ever want to have any friends.

When it is reduced, NAD⁺ becomes NADH because it gets an extra hydrogen atom and an extra electron; one proton goes off by itself (H⁺).

During oxidative phosphorylation, the last electron acceptor is called the terminal electron acceptor. In organisms that breathe oxygen, such as our lovely human selves, oxygen is the terminal electron acceptor. It is the last molecule to accept electrons, and is therefore reduced, in the whole process. In environments where oxygen is not present, organisms have to use something else as their terminal electron acceptor. Stay tuned for more details.

Brain Snack

For a summary of cellular respiration set to a catchy tune, check out this video.