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5.7 Respiration - Coggle Diagram
5.7 Respiration
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Anaerobic Respiration
Respiration can also occur in the absence of oxygen - this is called anaerobic respiration. In mammals, glucose can be converted into lactate (aka lactic acid) which releases a small amount of energy in the form of ATP.
The first step of anaerobic respiration is the same as aerobic respiration: glycolysis. Glucose is converted into pyruvate with the net release of 2 ATP molecules. 2 molecules of reduced NAD are also formed. In the second step, reduced NAD donates hydrogen (and electrons) to pyruvate, producing lactate and NAD. This regenerates more oxidised NAD for glycolysis. This enables anaerobic respiration to continue and ensures that small amounts of energy can still be made in the absence of oxygen, allowing biological reactions to keep ticking over.
Continued anaerobic respiration results in the build-up of lactate, which needs to be broken down. Cells can convert lactate back into pyruvate, which is then able to enter aerobic respiration at the Krebs cycle. In addition, liver cells can convert lactate into glucose, which can then be respired aerobically (if oxygen is now present) or stored for later use.
In plants and yeast, anaerobic respiration is a little different. Pyruvate produced in glycolysis is converted into ethanol and carbon dioxide.
Glycolysis
The first stage of aerobic respiration is glycolysis, which takes place in the cytoplasm. Glycolysis converts glucose, a six-carbon molecule, into two smaller three-carbon molecules called pyruvate. This stage doesn’t require oxygen, so it is an anaerobic process and is involved in both aerobic and anaerobic respiration pathways.
Glucose is phosphorylated using the phosphate groups from two molecules of ATP. ATP is hydrolysed into ADP and inorganic phosphate. This forms a molecule which is unstable and immediately breaks down into two three-carbon molecules called triose phosphate (TP). Hydrogen is removed from TP to convert it into pyruvate. The hydrogen is transferred to a coenzyme called NAD to form reduced NAD (NADH). The removal of hydrogen from TP oxidises it. The reduced NAD is used in the last stage of aerobic respiration, oxidative phosphorylation, whereas the pyruvate moves into the mitochondria for the next stage of respiration, the link reaction.
The conversion of triose phosphate to pyruvate produced four molecules of ATP. Since two molecules were used for the phosphorylation of glucose in the first step, this means there is a net gain of two ATP molecules in glycolysis.
The Link Reaction
The link reaction takes place in the mitochondrial matrix and converts pyruvate into a molecule called acetyl coenzyme A (acetyl CoA). This stage does not produce any energy in the form of ATP but does produce reduced NAD and acetyl CoA. Reduced NAD will be used in oxidative phosphorylation while the acetyl CoA will be used in the next stage of aerobic respiration, the Krebs cycle.
During the link reaction, a carbon atom is removed from pyruvate, forming carbon dioxide. This converts pyruvate into a two-carbon molecule called acetate. Hydrogen is also removed from pyruvate in the conversion into acetate, which is picked up by the coenzyme NAD to form reduced NAD. The acetate is combined with coenzyme A (CoA) to form acetyl CoA.
Since one glucose molecule is converted into 2x pyruvate, the link reaction happens twice for every glucose molecule. This means that each molecule of glucose produces two molecules of acetyl CoA (along with 2x carbon dioxide and 2x NADH).
The Krebs Cycle
The Krebs cycle (also known as the citric acid cycle) is a series of reactions which generate reduced NAD and a similar molecule called reduced FAD which are needed for oxidative phosphorylation. Acetyl CoA from the link reaction reacts with a four-carbon molecule called oxaloacetate.
The coenzyme A portion of acetyl CoA is removed and returns to the link reaction to be reused. A 6-carbon molecule called citrate is produced. Carbon and hydrogen are removed from citrate, forming carbon dioxide and reduced NAD. The citrate is converted into a 5-carbon compound. Decarboxylation and dehydrogenation occur once more, which converts the 5-carbon compounds into the 4-carbon molecule oxaloacetate which we started with. ATP, 2 molecules of reduced NAD, one molecule of FAD and carbon dioxide are also formed in this step. This cycle takes place twice for each glucose molecule that is respired aerobically.
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Respiratory Substrates
As well as respiring glucose, cells also respire other substrates, including carbohydrates, proteins and lipids. Different respiratory substrates release different amounts of energy in respiration: Lipids release the most, followed by proteins and carbohydrates release the least.
Because the majority of ATP is made using the proton gradient that flows through ATP synthase (in oxidative phosphorylation), the more hydrogens a respiratory substrate has, the more energy it can release. Lipids contain the most hydrogens per unit mass while carbohydrates contain the least.
Respiratory Quotient
The respiratory quotient (RQ) is the volume of carbon dioxide produced during respiration, divided by the volume of oxygen consumed for a particular respiratory substrate in a given time.
Respiratory quotients tell us what respiratory substrate is being used and whether aerobic or anaerobic respiration is taking place. RQ values greater than 1 indicate a short oxygen supply since the organism is respirating both aerobically and anaerobically.
Aerobic Respiration
Aerobic respiration is made of four stages: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. During aerobic respiration, glucose is effectively burned inside our bodies (it reacts with oxygen) to produce carbon dioxide, water and lots of energy in the form of ATP.