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| Aerobic Respiration |
What is Cellular Respiration
Cellular Respiration is present in every type of cell that breaks glucose producing energy that is consumed by the cell itself. Cellular respiration is of two types Aerobic and anaerobic respiration.
Aerobic respiration requires oxygen to break down
glucose molecules obtained from food into carbon dioxide and water releasing
energy in the form of ATP. This energy is consumed by living cells themselves to grow.
Anaerobic respiration takes place without oxygen, which breaks down glucose, producing lactic acid, ethanol, and energy.
Anaerobic respiration produces insufficient energy for cells to grow because it
takes place in the absence of oxygen and partially breaks down glucose molecules.
Here, in this article, I will discuss the process of aerobic respiration and the three main stages of aerobic respiration, providing a fresh perspective on our knowledge in 2023.
Aerobic Respiration and its types
Aerobic respiration takes place in the cytoplasm and
mitochondria of the cell. This type of respiration is present in animals and
plants including humans and mammals. it is a type of cellular respiration that involves several interconnected stages, including Glycolysis, Citric acid cycle or TCA, and oxidative phosphorylation.
Glycolysis
Glycolysis occurs in the cytoplasm of the cell where
glucose is metabolized to pyruvate releasing energy in the form of ATP. There
are two phases of glycolysis the first phase is the energy investment phase which takes
2 ATP molecules and the second is the energy pay-off phase which generates 4
molecules of ATP. A net total of 2 ATP molecules are produced. For one cycle of glycolysis, 4 molecules of ATP are produced.
Mechanism
In the first step, the Glucose molecule takes ATP and phosphorous group and converts to glucose-6-phosphate
in the presence of enzyme hexokinase. Glucose-6-phosphate by using an isomer
phosphoglucoisomerase is converted to fructose-6-phosphate. Phosphofructokinase
catalyzes the formation of fructose-6-phosphate to fructose 1,6-bisphosphate by
using another molecule of ATP. Fructose 1,6-bisphosphate is converted to
glyceraldehyde-3-phosphate and dihydroxyacetone phosphate in the presence of enzyme aldolase.
These both products are inter- convertible therefore
dihydroxyacetone phosphate will be converted to glyceraldehyde-3-phosphate in
the presence of an isomer triosephosphate. Glyceraldehyde-3-phosphate enters the energy payoff phase where it is oxidized by phosphate dehydrogenase and
converted to 1,3-bisphosphoglycerate by reducing a molecule of an electron to
produce 2 molecules NADH.
1,3-bisphosphoglycerate is converted to
3-phosphoglycerate with the help of the enzyme phosphoglycerokinase producing the first
ATP molecule in glycolysis. 3-phosphoglycerate, with the help of enzyme mutase, is converted to 2-phosphoglycerate. Enolase will catalyze 2-phosphoglycerate to
phosphoenolpyruvate releasing a molecule of water. Phosphoenolpyruvate will
give its phosphate molecule producing another molecule of ATP and will convert
to pyruvate by pyruvate kinase.
Glycolysis is a process
of breaking down six-carbon molecule glucose into three-carbon compound
pyruvate. When we respire, food in the form of glucose is broken down in the presence of oxygen producing carbon dioxide and water and releasing energy. For
each respiration, the cycle occurs twice producing two pyruvate molecules.
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Glycolysis |
Krebs Cycle
This pyruvate molecule, when entering the
Krebs cycle is oxidized by the coenzyme acetyl CoA producing NADH and releasing
carbon dioxide. This cycle is also involved in the breakdown of other molecules such as fatty acid and amino acids.
Mechanism
Now, this acetyl CoA enters the Krebs cycle, combining with four carbon compound oxaloacetic acid to produce the six-carbon
compound citric acid. This reaction is catalyzed by citrate synthase. The
enzyme aconitase will convert citric acid to isocitric acid. Isocitric acid is
oxidized in the presence of the enzyme isocitrate dehydrogenase and converted to
alpha-ketoglutarate. During this reaction NAD is reduced, accepting electron-producing NADH, and carbon dioxide is released.
This
alpha-ketoglutarate, 5-carbon molecule with the help of the enzyme
alpha-ketoglutarate synthase is oxidized and decarboxylated producing succinyl
coA which is a 4-carbon compound. NADH is produced and another molecule of
carbon dioxide is released. Succinyl coA synthase produces succinate from
succinyl coA by replacing the phosphate group from the coenzyme and producing GTP. Succinate
is oxidized by transferring two protons to FAD, reducing it to FADH2 producing
fumarate in the presence of succinate dehydrogenase. With the addition of a
water molecule, fumarate transforms into malate by the fumarase enzyme.
Malate is oxidized by
reducing NAD to NADH in the presence of malate dehydrogenase to produce oxaloacetic
acid. Oxaloacetate, a six-carbon compound is regenerated and the cycle goes on.
Now as we observe here during each Krebs cycle, a total of 3 molecules of
carbon dioxide, 3 NADH, 1GTP, and 1 molecule of FADH2 are produced.
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Krebs cycle |
Oxidative Phosphorylation
Oxidative
phosphorylation is a process of transferring electrons from the inner membrane
to the outer membrane, thus reducing NADH and FADH2 to NAD+ and FAD+. It takes
place in the inter-mitochondrial membrane known as the electron transport chain and in the mitochondrial matrix known as chemiosmosis.
Process of Oxidative Phosphorylation
In eukaryotes,
oxidative phosphorylation occurs inside the mitochondrial membrane where
electrons are transferred through a series of steps. It consists of five
protein complexes. Complex I known as NADH dehydrogenase transfers
electrons by reducing NADH to NAD+. During this reaction, protons are
transferred from the inner to the outer membrane.
Complex II transfers
electrons to Coenzyme Q where succinate dehydrogenase converts succinate to
fumarate oxidizing FADH2 to FAD. This
complex transfers the electrons to coenzyme Q which then pumps protons to complex III.
Complex III called
Cytochrome C oxidoreductase pump protons from a high energy level to a low energy level
establishing an electric-charged gradient. Cytochrome C transfers electrons to
complex IV known as cytochrome oxidase where the O2 molecule breaks into half oxygen
ions accepting 2H+ ions and producing water molecules.
These complexes I, II,
III, and IV are present in the electron transport chain. Complex I, III, and IV
transfer electrons to establish an electrochemical gradient. Complex V is that complex where chemiosmosis
takes place. As protons are pumped from one transport channel to another
because they cannot pass through the phospholipid membrane of mitochondria due
to their hydrophobic nature.
When the protons reach
channel V which is the ATP synthase complex. Complex V turns on by the pumping of
hydrogen ions and it controls the flow of protons by pumping back the protons
to the mitochondrial matrix. The enzyme ATP synthase in complex V synthesizes energy
by taking inorganic phosphate (Pi) from the electrochemical gradient developed by protons and
forming ATP. Protons flow back to the mitochondrial membrane and the process
continues.
Recent advancements
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Oxidative Phosphorylation |
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