When you are hungry, how do you feel? If you are like most people, you feel sluggish, a little dizzy, and weak. Weakness is your body's way of telling you that your energy supplies are low. Food serves as your source of energy. But how does the food you eat get converted into a usable form of energy for your cells? The answer is cellular respiration.

When you eat, your body digests the food into smaller chemical compounds like sugars (glucose), fats, and proteins. These nutrients enter your cells and are converted into adenosine triphosphate (ATP). ATP is a source of usable energy for cells and is the key energy molecule for all biological organisms.

Directions: Watch the video Energy Consumption: An Overview for a look at the different cellular processes responsible for generating and consuming energy. At this point, try not to worry about the names of compounds or the details of the processes shown. Watch for a general overview. It's actually quite amazing.

Equation for Cellular Respiration

Cellular respiration is often expressed as a chemical equation:

This equation shows that during cellular respiration, one glucose molecule is gradually broken down into carbon dioxide and water. Along the way, ATP (energy for cells) is produced.

Glycolysis is the first set of reactions that occur during cellular respiration. Glycolysis is an anaerobic process, meaning it occurs without oxygen. Glycolysis takes place in the cytoplasm of the cell. 

Directions: Watch Glycolysis: An Overview to see how glucose is broken down during the process of glycolysis.

Under aerobic conditions (i.e., oxygen is present), the pyruvate and NADH molecules made during glycolysis move from the cytoplasm into the matrix of the mitochondria. There pyruvate feeds into the next stage of respiration, which is called the citric acid cycle (or Krebs cycle). Carbons are broken down and released as carbon dioxide while ATP is made and electrons are passed to electron carriers, NADH and FADH_2.

Directions: Watch The Citric Acid Cycle: An Overview to see how pyruvate is broken down during the citric acid cycle.

The electron transport chain (ETC) is the final stage of cellular respiration. A large amount of ATP is generated during this stage—32 ATP molecules to be exact! 

Directions: Watch Cellular Processes: Electron Transport Chain and Cellular Processes: ATP Synthase to learn how electrons are passed through proteins in the electron transport chain and ATP is produced.

Now that we have studied each stage of cellular respiration in detail, let's take another look at the equation that summarizes cellular respiration and see how various processes relate to it: