Cellular Respiration
Cellular respiration is a series of metabolic processes that all living things use to convert energy stored in organic molecules, such as glucose. The process can seem complicated but is actually pretty straightforward.
Photosynthetic Producers
Plants and other photosynthetic producers (autotrophs) create glucose molecules by utilizing sunlight, water, and carbon dioxide to form a chemical bond between glucose and oxygen. These chemical bonds store energy, which organisms need to survive and reproduce.
Glucose as a Source of Energy
Glucose is the major source of cellular energy for all eukaryotic organisms, including humans! All eukaryotic organisms can catabolize glucose into energy-storing molecules called ATP and carbon dioxide.
Glycolysis
Glycolysis is the first step in cellular respiration, and it occurs in all living cells, regardless of type. It does not require oxygen and is therefore anaerobic (processes that do require oxygen are called aerobic).
In this stage, a carboxyl group from the molecule of glucose is removed and replaced with a hydroxyl group; then it is oxidized to an acetyl group. This acetyl group is picked up by NAD+, forming NADH, which then picks up the high-energy electrons to generate ATP.
The Citric Acid Cycle
The citric acid cycle or the Krebs cycle is the second pathway in cellular respiration. This pathway takes place in the mitochondria. It produces a net gain of CO2 and ATP, along with 3 NADH and 1 FADH2, each time it completes a turn.
This is a closed loop, so the cycle continues until all of the needed compounds are produced. The rate of this cycle is controlled by ATP concentration.
Oxidative Phosphorylation
Oxidative phosphorylation is the final stage in cellular respiration, and it occurs on the inner mitochondrial membrane. This is an anaerobic pathway, which means it does not require oxygen; however, it can still occur in the presence of extraneous oxygen.
During oxidative phosphorylation, protons are diffused from the outer mitochondrial membrane into the mitochondrial matrix to bind with cytochrome c, producing two NADH and one FADH2. This redox reaction transfers the energy from NADH and FADH2 to as many as 34 more ATP molecules.
These ATP molecules can then power a variety of reactions throughout the body. They also help cells withstand the effects of stress, such as a cold or a fever.
Biochemical Steps and Redox Reactions
Cellular respiration involves a series of biochemical steps, most of which are redox reactions. These redox reactions break down large molecules into smaller ones, releasing their free energy as ATP.
Redox Reactions of Cellular Respiration
The three redox reactions involved in cellular respiration include glycolysis, transformation of pyruvate, and the citric acid cycle. They are controlled by enzymes called phosphofructokinases, pyruvate dehydrogenases, and isocitrate dehydrogenases.
Each molecule of glucose undergoes a series of carefully controlled redox reactions to break down its chemical bonds and release its free energy as ATP. The reactions are controlled by rate-determining enzymes.
Location and Importance of Cellular Respiration
The reactions of cellular respiration take place in the cytoplasm and mitochondria of all cells, and they involve the oxidation of nutrients to produce ATP. These reactions are the most common and important ones of all because they sustain life by transferring energy to other biological molecules that carry out cellular functions.
