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Take a deep breath in and gently exhale. Every day, all day and all night, our lungs inhale and exhale air. Although calming, the goal of this process is to bring oxygen into the body. But ,why do we really need oxygen? Most students say to breathe, but this isn't quite accurate. We breathe to get oxygen, not the other way around. As it turns out, oxygen is the essential ingredient for making energy in a process called cellular respiration.
Cellular respiration is the process cells use to make energy. Our body cells need oxygen to do this process, although other organisms, like yeast or bacteria, don't always need it. They have other ways of making energy. But since the focus of this particular lesson is on the role of oxygen, we'll stick with animal cells, like the ones in our body.
All body cells engage in cellular respiration. They use oxygen and glucose, a sugar found in the foods we eat and convert them to ATP (adenosine triphosphate), or cellular energy, and carbon dioxide. Although this process can be represented by a single equation, there are actually many small steps that take place before we actually use oxygen to get ATP. Let's examine each of the three main steps in detail next.
Cellular respiration has three main steps: glycolysis, the citric acid cycle, and oxidative phosphorylation, where oxygen is used.
Step 1: Glycolysis
Glycolysis is the first step in cellular respiration, and it occurs in the main compartment of the cell: the cytoplasm. Cells let glucose in from the blood--the glucose comes from the food we eat. Next, cells convert glucose through several different compounds to make two ATP molecules and a molecule called pyruvate. A compound called NADH (nicotinamide adenine dinucleotide + hydrogen) is also created. This molecule stores electrons harvested from the glucose, which will be used later to create a larger amount of ATP.
Step 2: Citric Acid Cycle
Next, the cell takes the pyruvate made in glycolysis and converts it to a molecule called acetyl Co-A. This happens in the powerhouse of the cell, the mitochondria. Acetyl Co-A is also converted to several different compounds but, ultimately, acetyl Co-A is regenerated, hence the 'cycle' part of the citric acid cycle. The citric acid cycle also creates another molecule of ATP, additional NADH, and the molecule FADH (flavin adenine dinucleotide + hydrogen), which also transports electrons.
Step 3: Oxidative Phosphorylation
The entire point of cellular respiration up until now has been to get a few ATP, but now it focuses on the electrons housed in the NADH. The NADH is taken to the mitochondrial membrane, or barrier of the mitochondria. There are actually two membranes--an inner and an outer membrane--and a small space in between called the intermembrane space. Here, electrons are transferred between proteins in the membrane in the electron transport chain. The proteins act like factory workers, passing down the electrons in a chain. As the electrons pass through, four proteins use the energy stored in the electrons to move hydrogen ions into the intermembrane space.
At the end of the chain is the ultimate electron acceptor: oxygen. Oxygen loves electrons more than any of the other proteins in the chain, so the proteins keep passing them down so oxygen can have them all. When oxygen finally gets the electrons, it also picks up two hydrogen ions. When the electrons, hydrogen ions and oxygen combine, they make water! After the oxygen is used up, the electrons have no place to go at the last protein, and the chain stops. As a result, the other steps stop, too, like backed up traffic at a light. The cell is no longer able to make energy and dies.
You might still be wondering where all the ATP gets made. So far, we only made a couple in glycolysis and the citric acid cycle, but the real pay day has yet to come. Let's look at how this happens.
In the intermembrane space, the hydrogen ions build up like water behind a dam. When we have water behind a dam, the water can flow through a designated space, and we harvest the energy released as the water moves to make electricity.
The cell has a similar method in place for the hydrogen ions. An important protein called ATP synthase acts like the dam. It has space for the hydrogen ions to flow into the mitochondria. As the hydrogen ions flow, ATP synthase harvests the energy stored and uses it to make ATP. Then, that energy can be used for all processes in the cells. Everything we do needs energy, which is ultimately made using oxygen and glucose.
Cellular respiration is the process cells use to make energy. Cells in our body combine glucose and oxygen to make ATP and carbon dioxide. Cellular respiration starts with glycolysis, where glucose enters the cell, is converted to pyruvate, and makes a few ATP and NADH. Next, the pyruvate moves into the citric acid cycle, as acetyl Co-A and creates more ATP and NADH. Finally, during the third step, oxidative phosphorylation, the NADH moves to the inner mitochondrial membrane to transfer electrons to proteins in the electron transport chain.
As the electrons are transferred between the proteins, the proteins pump hydrogen ions into the intermembrane space. After each of the four proteins, the electrons end with oxygen. Oxygen combines with the electrons and two hydrogen ions to make water. Lastly, the hydrogen ions flow through ATP synthase to make ATP.
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