Written by: Jake Khalde
Ever wondered why we breathe oxygen in and carbon dioxide out? It is such a simple activity that we all do without giving much thought. Wherever we are, we breathe in and out. While we perform the seemingly monotonous task of breathing, every cell in our bodies performs a fascinating task that is essential to all life: cellular respiration.
Not to be confused with breathing, cellular respiration is the process where every living thing makes usable energy. Cells convert glucose (sugar) into adenosine triphosphate (ATP).
There are two types of cellular respiration: aerobic and anaerobic. Aerobic cellular respiration involves oxygen, while anaerobic cellular respiration does not. Both produce ATP, but in different quantities and with different steps.
Aerobic cellular respiration has three stages: glycolysis, the Krebs cycle, and oxidative phosphorylation.
Glycolysis is where one glucose molecule breaks into two pyruvate molecules and synthesizes two ATP molecules, two NADH2 molecules (a kind of electron carrier molecule), and water (the byproduct of any lyse). Glycolysis occurs in the cytoplasm of the cell. To remember glycolysis, associate the first letter, g, with go, as it is the start of cellular respiration. The word glycolysis gives some helpful hints about the process: glyco- meaning glucose and -lysis meaning breaking. Together, as the name suggests, glycolysis means glucose breaking.
The second stage of aerobic cellular respiration is the Krebs cycle, also called the citrus acid cycle. This cycle occurs in the mitochondria, the powerhouse of the cell. The Krebs cycle is an eight-step process in which the pyruvate products from glycolysis are used to produce carbon dioxide, the same that we breathe out, 2 ATP molecules, and electron transport molecules. These electron transport molecules are FADH2 and NADH.
Speaking of oxidative phosphorylation, this process is the last stage of aerobic cellular respiration; there are two stages in oxidative phosphorylation: electron transport chain and chemiosmosis. Oxidative phosphorylation occurs in the mitochondria of the cell. During the electron transport chain, electrons go through an enzyme chain where electrons pass from molecule to molecule, and energy is released. This energy is used in a hydrogen pump or released as heat. The hydrogen pump makes an electrochemical gradient. At the end of the electron transport chain, oxygen molecules accept the electrons, pick up protons, split them in half, and take in hydrogen molecules, making water. If oxygen does not do this, the electron transport chain stops, and ATP won’t be able to be produced - that is why we need oxygen! Chemiosmosis is where ATP is synthesized using the energy from the electrochemical gradient. Oxidative phosphorylation makes about 32-34 ATP molecules and water. Oxygen is necessary for this process, but what about if the body does not have oxygen?
Cells can undergo anaerobic cellular respiration where oxygen is not used. This process has two steps rather than three, like in aerobic cellular respiration. These steps are glycolysis and fermentation. Anaerobic cellular respiration glycolysis is the exact same as in aerobic cellular respiration, where two ATP, two pyruvate molecules, electron carrier molecules, and water are produced.
Fermentation occurs in the cytoplasm and produces different things in plants and animals. In animals and some bacteria, the pyruvate molecules from glycolysis, with the help of catalyst enzymes, reduce to lactic acid. The enzyme catalyst responsible is lactate dehydrogenase, found throughout the body. In plants, fungi, and some bacteria, fermentation produces alcohol; this happens when a carboxyl group is removed from the pyruvate molecules and released as carbon dioxide, synthesizing a molecule by the name of acetaldehyde. The NADH electron carrier molecule passes its electrons to this acetaldehyde, making ethanol (alcohol) and carbon dioxide. Anaerobic respiration makes two ATP molecules.
Cellular respiration is a necessity of life. From humans to rare bacteria, every living thing must convert sugar to ATP to perform cell tasks. While both anaerobic and aerobic cellular respiration synthesizes ATP, aerobic cellular respiration makes more thanks to oxidative phosphorylation.
Sources:
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