3.6: Cellular Respiration

In the previous lesson, you learned how plants use photosynthesis to get and store energy. Once that energy is stored (typically in the form of glucose), eukaryotes have several ways to turn it back to usable energy (ATP). Topic 3.6 explains both the aerobic (with oxygen) and anaerobic (without oxygen) ways cells produce ATP.

Vocab List

• Aerobic respiration
  • Mitochondria
    • Matrix
    • Intermembrane space
  • Cellular respiration
    • Glycolysis
    • Pyruvate
    • NAD⁺ / NADH
    • Link Reaction
    • Acetyl-CoA
    • Krebs Cycle
    • FAD / FADH₂
    • Electron transport chain
    • Electrochemical gradient
    • ATP Synthase
    • Oxidative phosphorylation
    • Chemiosmosis
• Anaerobic respiration
  • Fermentation
    • Alcoholic
    • Lactic acid
• Redox reaction

Written Explanation

Aerobic Respiration:

When you walk around at a light pace, your respiratory system manages to keep all your cells having enough oxygen to use aerobic respiration to convert glucose to ATP (aerobic meaning "requiring or using oxygen"). Specifically, cells use the process of cellular respiration. In cellular respiration, photosynthesis is reversed, releasing the stored energy in glucose (which came from the sun).

Most of cellular respiration occurs in the mitochondria

The first step of cellular respiration is glycolysis, which occurs in the cytosol of the cell. It uses the input energy of two ATP molecules to split one glucose molecule (a 6 carbon molecule). This forms two pyruvate molecules (3 carbon molecules), and also releases a large amount of energy. Some of this energy goes into phosphorylating four ADP into ATP, and the rest goes towards reducing (adding electrons to) two electron carriers. These electron carriers are known as NAD⁺ before having an electron, and NADH after.

The next step of cellular respiration is the Link Reaction. First, pyruvate is moved from the cytosol to the mitochondrial matrix. In the reaction, one CO₂ molecule is removed from the pyruvate, and one NAD⁺ is reduced to NADH, resulting in a two-carbon structure called acetyl-CoA.

After that, the Krebs Cycle begins, also in the matrix. This process may also be called the citric acid cycle. Every acetyl-CoA causes one "turn" in the cycle, which produces 2 ATP, 6 NADH, 2 FADH₂ (a different electron carrier which starts as FAD), and 4 CO₂. By the end of the cycle, all of the initial carbons in glucose have been released from the body as the waste product carbon dioxide.

The next step of cellular respiration is the electron transport chain. The electrons stored in FADH₂ and NADH travel between membrane proteins in the inner mitochondrial membrane, powering the pumping of H⁺ ions across the inner mitochondrial membrane. This creates an electrochemical gradient between the matrix and intermembrane space. The electron carriers are reduced to FAD and NAD⁺ in the process. Additionally, these electron carriers release some electrons when being oxidized, which contributes to the movement of H⁺ across the membrane. The electrons travelling through the ETC are attracted to the final electron acceptor, electronegative O₂, and then bind with H⁺ to form water (notice the similarity to photosynthesis?). Using that oxygen to power the pumping of protons across the mitochondrial membrane is the reason cellular respiration is aerobic.

And now … finally … the last step of cellular respiration is chemiosmosis, the use of the electrochemical gradient to produce ATP. The intermembrane space has a high positive charge created by the presence of many positive H⁺ ions. Because similar charges repel each other, these positive ions want to cross back across the membrane. However, the mitochondrial membrane is selectively permeable and doesn't let H⁺ freely move across it, so the H⁺ ions must go through an integral protein called ATP Synthase. ATP Synthase uses the movement of H⁺ through it to synthesize ATP, by phosphorylating ADP. This process, in combination with the ETC, is also known as oxidative phosphorylation (using oxygen to phosphorylate). In total, the ETC and chemiosmosis produce between 30 and 38 ATP per glucose molecule input.

Similarities to Photosynthesis:

While cellular respiration and photosynthesis are complete opposites in their formulas and goals, they follow a remarkably similar process. Additionally, they create a continuous cycle in which carbon dioxide and oxygen are exchanged back and forth, and energy is passed from producer to consumer. Both processes are also redox reactions, meaning they involve the exchange of electrons between molecules.

Anaerobic Respiration:

When you start running, you might start to be short of breath. That means your cells might not be getting enough oxygen for cellular respiration. This makes your cells take the other energy generation pathway: fermentation. Fermentation is much simpler than cellular respiration, and also evolved first. The goal of fermentation is oxidizing NADH, allowing glycolysis to be repeated indefinitely. While there are many types of fermentation, it's useful to focus on the following two types.

First, alcoholic fermentation begins with glycolysis. It then converts the two pyruvates into ethanol, releases CO₂, and oxidizes two NADH to NAD⁺. This is the process famously used by yeast.

Second, lactic acid fermentation also starts with glycolysis, and forms lactate as a waste product. This is the process used by humans when they are out of oxygen, and the buildup of lactic acid is what causes cramps while running.