This process is essentially the opposite of glycogenesis with two exceptions:. Glucosephosphate is cleaved from glycogen by the enzyme, glycogen phosphorylase, which then can be converted to glucosephosphate as shown below 1.

If a person is in a catabolic state or in need of energy, such as during fasting, most glucosephosphate will be used for glycolysis. Glycolysis is the breaking down of one glucose molecule 6 carbons into two pyruvate molecules 3 carbons.

The figure below shows the stages of glycolysis, as well as the transition reaction, citric acid cycle, and electron transport chain that are utilized by cells to produce energy. They are also the focus of the next 3 sections. If a person is in a catabolic state, or needs energy, how pyruvate will be used depends on whether adequate oxygen levels are present.

If there are adequate oxygen levels aerobic conditions , pyruvate moves from the cytoplasm, into the mitochondria, and then undergoes the transition reaction. If there are not adequate oxygen levels anaerobic conditions , pyruvate will instead be used to produce lactate in the cytoplasm.

We are going to focus on the aerobic pathway to begin with, then we will address what happens under anaerobic conditions in the anaerobic respiration section.

The transition reaction is the transition between glycolysis and the citric acid cycle. We are going to continue to consider its use in an aerobic, catabolic state need energy. The following figure shows the citric acid cycle. This leaves alpha-ketoglutarate 5 carbons.

GTP is readily converted to ATP, thus this step is essentially the generation of 1 ATP. The first video does a good job of explaining and illustrating how the cycle works. The second video is an entertaining rap about the cycle. Under aerobic conditions, these molecules will enter the electron transport chain to be used to generate energy through oxidative phosphorylation as described in the next section.

The electron transport chain is located on the inner membrane of mitochondria. The electron transport chain contains a number of electron carriers. This creates a proton gradient between the intermembrane space high and the matrix low of the mitochondria.

ATP synthase uses the energy from this gradient to synthesize ATP. Oxygen is required for this process because it serves as the final electron acceptor, forming water. Collectively this process is known as oxidative phosphorylation.

The following figure does a nice job of illustrating how the electron transport chain functions. The first video does a nice job of illustrating and reviewing the electron transport chain. The second video is a great rap video explaining the steps of glucose oxidation.

The table below shows the ATP generated from one molecule of glucose in the different metabolic pathways. Notice that the vast majority of ATP is generated by the electron transport chain. Remember that this is aerobic and requires oxygen to be the final electron acceptor. But the takeaway message remains the same.

The electron transport chain by far produces the most ATP from one molecule of glucose. Conditions without oxygen are referred to as anaerobic. In this case, the pyruvate will be converted to lactate in the cytoplasm of the cell as shown below.

What happens if oxygen isn't available to serve as the final electron acceptor? However, anaerobic respiration only produces 2 ATP per molecule of glucose, compared to 32 ATP for aerobic respiration.

The biggest producer of lactate is the muscle. Through what is known as the Cori cycle, lactate produced in the muscle can be sent to the liver.

In the liver, through a process known as gluconeogenesis, glucose can be regenerated and sent back to the muscle to be used again for anaerobic respiration forming a cycle as shown below.

It is worth noting that the Cori cycle also functions during times of limited glucose like fasting to spare glucose by not completely oxidizing it.

Search site Search Search. Go back to previous article. Sign in. Monosaccharide Metabolism Galactose and fructose metabolism is a logical place to begin looking at carbohydrate metabolism, before shifting focus to the preferred monosaccharide glucose.

Fructose Unlike galactose, fructose cannot be used to form phosphorylated glucose. GlucosePhosphate Within hepatocytes or myocytes muscle cells , glucosephosphate can be used either for glycogenesis glycogen synthesis or glycolysis breakdown of glucose for energy production.

Glycogenesis The synthesis of glycogen from glucose is a process known as glycogenesis. MY PROFILE. Access Sign In Username. Sign In. Create a Free Access Profile Forgot Password? Forgot Username? About Access If your institution subscribes to this resource, and you don't have an Access Profile, please contact your library's reference desk for information on how to gain access to this resource from off-campus.

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Animals can make most of the fatty acids they need. Triglycerides can be both made and broken down through parts of the glucose catabolism pathways. Glycerol can be phosphorylated to glycerolphosphate, which continues through glycolysis.

Fatty acids are catabolized in a process called beta-oxidation, which takes place in the matrix of the mitochondria and converts their fatty acid chains into two-carbon units of acetyl groups. The acetyl groups are picked up by CoA to form acetyl CoA that proceeds into the citric acid cycle.

Figure 2. Glycogen from the liver and muscles, as well as other carbohydrates, hydrolyzed into glucosephosphate, together with fats and proteins, can feed into the catabolic pathways for carbohydrates.

The processes of photosynthesis and cellular metabolism consist of several very complex pathways. If these cells reproduced successfully and their numbers climbed steadily, it follows that the cells would begin to deplete the nutrients from the medium in which they lived as they shifted the nutrients into the components of their own bodies.

This hypothetical situation would have resulted in natural selection favoring those organisms that could exist by using the nutrients that remained in their environment and by manipulating these nutrients into materials upon which they could survive.

Selection would favor those organisms that could extract maximal value from the nutrients to which they had access. Another type of anoxygenic photosynthesis did not produce free oxygen because it did not use water as the source of hydrogen ions; instead, it used materials such as hydrogen sulfide and consequently produced sulfur.

It is thought that glycolysis developed at this time and could take advantage of the simple sugars being produced but that these reactions were unable to fully extract the energy stored in the carbohydrates.

Living things adapted to exploit this new atmosphere that allowed aerobic respiration as we know it to evolve. When the full process of oxygenic photosynthesis developed and the atmosphere became oxygenated, cells were finally able to use the oxygen expelled by photosynthesis to extract considerably more energy from the sugar molecules using the citric acid cycle and oxidative phosphorylation.

The breakdown and synthesis of carbohydrates, proteins, and lipids connect with the pathways of glucose catabolism. The simple sugars are galactose, fructose, glycogen, and pentose. These are catabolized during glycolysis.

The amino acids from proteins connect with glucose catabolism through pyruvate, acetyl CoA, and components of the citric acid cycle. Cholesterol synthesis starts with acetyl groups, and the components of triglycerides come from glycerolphosphate from glycolysis and acetyl groups produced in the mitochondria from pyruvate.

Would you describe metabolic pathways as inherently wasteful or inherently economical? Show Solution. They are very economical. The substrates, intermediates, and products move between pathways and do so in response to finely tuned feedback inhibition loops that keep metabolism balanced overall.

Intermediates in one pathway may occur in another, and they can move from one pathway to another fluidly in response to the needs of the cell. Privacy Policy. Skip to main content. Cellular Respiration. Search for:. Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways Learning Objectives By the end of this section, you will be able to do the following: Discuss the ways in which carbohydrate metabolic pathways, glycolysis, and the citric acid cycle interrelate with protein and lipid metabolic pathways Explain why metabolic pathways are not considered closed systems.

Connections of Other Sugars to Glucose Metabolism Glycogen, a polymer of glucose, is an energy storage molecule in animals. Connections of Proteins to Glucose Metabolism Proteins are hydrolyzed by a variety of enzymes in cells.

Connections of Lipid and Glucose Metabolisms The lipids connected to the glucose pathway include cholesterol and triglycerides. Evolution Connection Pathways of Photosynthesis and Cellular Metabolism The processes of photosynthesis and cellular metabolism consist of several very complex pathways.

Section Summary The breakdown and synthesis of carbohydrates, proteins, and lipids connect with the pathways of glucose catabolism. glucosephosphate fructose-1,6-bisphosphate dihydroxyacetone phosphate phosphoenolpyruvate.

Glucagon in the liver stimulates glycogenolysis when the blood glucose is lowered, known as hypoglycemia. Adrenaline stimulates the breakdown of glycogen in the skeletal muscle during exercise. Glycogenesis refers to the process of synthesizing glycogen.

The pentose phosphate pathway is an alternative method of oxidizing glucose. Fructose must undergo certain extra steps in order to enter the glycolysis pathway.

Lactose, or milk sugar, consists of one molecule of glucose and one molecule of galactose. Many steps of carbohydrate metabolism allow the cells to access energy and store it more transiently in ATP. Typically, the complete breakdown of one molecule of glucose by aerobic respiration i.

involving glycolysis, the citric-acid cycle and oxidative phosphorylation , the last providing the most energy is usually about 30—32 molecules of ATP. Hormones released from the pancreas regulate the overall metabolism of glucose.

The level of circulatory glucose known informally as "blood sugar" , as well as the detection of nutrients in the Duodenum is the most important factor determining the amount of glucagon or insulin produced. The release of glucagon is precipitated by low levels of blood glucose, whereas high levels of blood glucose stimulates cells to produce insulin.

Because the level of circulatory glucose is largely determined by the intake of dietary carbohydrates, diet controls major aspects of metabolism via insulin. Regardless of insulin levels, no glucose is released to the blood from internal glycogen stores from muscle cells.

Carbohydrates are typically stored as long polymers of glucose molecules with glycosidic bonds for structural support e. chitin , cellulose or for energy storage e. glycogen , starch. However, the strong affinity of most carbohydrates for water makes storage of large quantities of carbohydrates inefficient due to the large molecular weight of the solvated water-carbohydrate complex.

In most organisms, excess carbohydrates are regularly catabolised to form acetyl-CoA , which is a feed stock for the fatty acid synthesis pathway; fatty acids , triglycerides , and other lipids are commonly used for long-term energy storage.

The hydrophobic character of lipids makes them a much more compact form of energy storage than hydrophilic carbohydrates. Gluconeogenesis permits glucose to be synthesized from various sources, including lipids.

In some animals such as termites [20] and some microorganisms such as protists and bacteria , cellulose can be disassembled during digestion and absorbed as glucose. Contents move to sidebar hide. Article Talk. Read Edit View history.

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A review". The Canadian Veterinary Journal. Bibcode : Natur. Journal of General Microbiology. Metabolism , catabolism , anabolism.

Metabolic pathway Metabolic network Primary nutritional groups. Purine metabolism Nucleotide salvage Pyrimidine metabolism Purine nucleotide cycle. Pentose phosphate pathway Fructolysis Polyol pathway Galactolysis Leloir pathway.

Glycosylation N-linked O-linked. Photosynthesis Anoxygenic photosynthesis Chemosynthesis Carbon fixation DeLey-Doudoroff pathway Entner-Doudoroff pathway. Xylose metabolism Radiotrophism.

Fatty acid degradation Beta oxidation Fatty acid synthesis. Steroid metabolism Sphingolipid metabolism Eicosanoid metabolism Ketosis Reverse cholesterol transport. Metal metabolism Iron metabolism Ethanol metabolism Phospagen system ATP-PCr. Metabolism map.

Carbon fixation. Photo- respiration. Pentose phosphate pathway. Citric acid cycle. Glyoxylate cycle. Urea cycle. Fatty acid synthesis. Fatty acid elongation.

Beta oxidation. beta oxidation. Glyco- genolysis. Glyco- genesis. Glyco- lysis. Gluconeo- genesis. Pyruvate decarb- oxylation. Keto- lysis. Keto- genesis.