Aerobic Respiration: Definition, Steps, Products & Equation

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  • 0:00 Aerobic Respiration
  • 0:45 Definition of Aerobic…
  • 1:35 The Steps of Aerobic…
  • 5:15 The End Products and Equation
  • 5:51 Lesson Summary
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Lesson Transcript
Instructor: Meredith Mikell
Aerobic respiration is the process most living things undergo to use food energy. Here, we will investigate the definition, the steps of the process, what goes in and what comes out of the process, and the chemical formula. Get ready to breathe!

Aerobic Respiration

Take a deep breath...now exhale. Do you feel it? Every time you breathe in oxygen and exhale carbon dioxide, you are exchanging gases that are a crucial part of your energy metabolism. We don't normally equate oxygen and breathing with metabolizing food, yet the latter cannot occur without the former.

Most living organisms undergo this process, from single-celled bacteria to the multi-celled blue whale. While many microorganisms can accomplish this task within their single cell, we larger macroorganisms have evolved body organs dedicated to efficiently obtaining enough oxygen, and eliminating enough carbon dioxide, to undergo enough aerobic respiration to fuel our large, complex selves.

Definition of Aerobic Respiration

Respiration is the metabolic process of most living things in which food molecules or glucose are turned into usable energy for the cell, called ATP. Respiration is the anti-process to photosynthesis, the process in which plants use sunlight and carbon dioxide to build food molecules releasing oxygen as a waste product.

During aerobic respiration, oxygen is present and helps the process to crank out energy very efficiently. Some organisms can also undergo anaerobic respiration, in which oxygen is absent, and a somewhat less efficient method of metabolism takes place. While photosynthesis takes place in the chloroplasts of plant and algae cells, aerobic respiration takes place in the cytoplasm, or the gooey inner cell space and mitochondria of all eukaryotic cells.

The Steps of Aerobic Respiration

It all starts with a sugar! An organism takes in carbohydrates for energy, and the digestion process breaks the carbs down into their smallest units, glucose, a type of sugar molecule. Cells then make energy by breaking the glucose molecule down and releasing its electrons, which are later used to help crank out ATP. There are three main steps in this process.

There are three main steps in this process. It begins with glycolysis. During glycolysis, the 6-carbon glucose molecule undergoes a series of reactions that break it down into two 3-carbon pyruvate molecules. The purpose of this process is to release electrons from the bonds in the glucose, which are scooped up by an acceptor molecule called NAD+, turning it into NADH when it accepts the electrons. In this process, two molecules of ATP are made. This step occurs in the cytoplasm, and the pyruvate and NADH molecules then enter the mitochondria for the next step.

The next phase of aerobic respiration is the citric acid cycle, also known as the Kreb's cycle, named for the biochemist who discovered it. To prepare for this stage, the pyruvate molecules from glycolysis are converted to a 2-carbon compound called Acetyl CoA. What happened to the third carbon? You just exhaled it in the form of carbon dioxide!

With each turn of the cycle, the Acetyl CoA is broken down and rebuilt into carbon chains. The purpose is to extract electrons from them and generate more ATP, similar to the more simple process of glycolysis. NAD+ is used again to pick up the electrons released, as is another acceptor molecule, FADH, which becomes FADH2 when reduced. These acceptor molecules get loaded up with electrons, like cargo trucks, and carbon dioxide is released as the carbon chains are broken down and new Acetyl CoA comes in. Exhale!

Where do the cargo trucks go once they are loaded up? The FADH2 and NADH molecules take their electrons to the inner mitochondrial membrane for the final stage of aerobic respiration, oxidative phosphorylation. Here, using the power of a concentration gradient, a very large amount of ATP is generated. The electrons from the citric acid cycle are dropped off and used to force hydrogen atoms that were released when the acceptor molecules picked up electrons in Steps 1 and 2, to pump against their concentration gradient. Proteins embedded in the membrane undergo active transport to push all these hydrogens into a highly concentrated area, just so they can then rush downward through an enzyme, the ATP Synthase, which turns like a gear to crank out about 32 ATP!

Think of it like a dam: electricity or the ATP is produced when water, or in this case hydrogens ,flow through a turbine. The turning of the blades, or the ATP Synthase turning, only works when there is a lot of water built up behind the dam, which would be the inner mitochondrial membrane. It may take some energy to pump water from the lower side of the dam to the reservoir, or, in this example, the electrons used on membrane proteins, but it is far less than the net gain of energy produced by the dam's turbines.

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