How many main stages of catabolism are there

Aminopeptidases in the intestinal juice remove amino acids from the N-terminal end of peptides and proteins possessing a free amino group. The amino acids that are released by protein digestion are absorbed across the intestinal wall into the circulatory system, where they can be used for protein synthesis.

This diagram illustrates where in a peptide the different peptidases we have discussed would catalyze hydrolysis the peptide bonds. A hormone secreted in this region stimulates the gallbladder to discharge bile into the duodenum. The principal constituents of bile are the bile salts, which emulsify large, water-insoluble lipid droplets, disrupting some of the hydrophobic interactions holding the lipid molecules together and suspending the resulting smaller globules micelles in the aqueous digestive medium.

These changes greatly increase the surface area of the lipid particles, allowing for more intimate contact with the lipases and thus rapid digestion of the fats. Another hormone promotes the secretion of pancreatic juice, which contains these enzymes.

The monoglycerides and fatty acids cross the intestinal lining into the bloodstream, where they are resynthesized into triglycerides and transported as lipoprotein complexes known as chylomicrons.

Phospholipids and cholesteryl esters undergo similar hydrolysis in the small intestine, and their component molecules are also absorbed through the intestinal lining. The further metabolism of monosaccharides, fatty acids, and amino acids released in stage I of catabolism occurs in stages II and III of catabolism. During digestion, carbohydrates are broken down into monosaccharides, proteins are broken down into amino acids, and triglycerides are broken down into glycerol and fatty acids.

Most of the digestion reactions occur in the small intestine. In what section of the digestive tract does most of the carbohydrate, lipid, and protein digestion take place? Using chemical equations, describe the chemical changes that triglycerides undergo during digestion. What are the expected products from the enzymatic action of chymotrypsin on each amino acid segment?

Learning Objectives To describe how carbohydrates, fats, and proteins are broken down during digestion. Summary During digestion, carbohydrates are broken down into monosaccharides, proteins are broken down into amino acids, and triglycerides are broken down into glycerol and fatty acids. Concept Review Exercises Distinguish between each pair of compounds. What are the primary end products of each form of digestion?

Answers Pepsinogen is an inactive form of pepsin; pepsin is the active form of the enzyme. Complex organic molecules like proteins, lipids, and polysaccharides are catabolized to their smaller components or monomers, outside cells. These complex molecules are unabsorbable in their complex state and hence, for their absorption, it is essential that these basic and essential molecules break down into easily absorbable and smaller monomers.

The smaller molecules or the monomers are the absorbable form and are taken up by cells and are further converted to smaller molecules like, the acetyl-coenzyme A acetyl-CoA , and releasing energy in the process. Finally, the acetyl group of the CoA is oxidized to water and carbon dioxide in the citric acid cycle and electron transport chain. The purpose of the two branches of metabolism namely. The anabolic processes are building processes of the metabolism wherein simple molecules are converted to complex molecules whereas the catabolic process is the breakdown processes wherein complex molecules are broken down into simple molecules along with the release of energy.

The major differences between catabolism and anabolism are enlisted in the Table below. Catabolism is the breakdown mechanism in the metabolic processes. Multiple essential enzymes are involved in the catabolic processes. These are:. What are examples of catabolism? Essentially, during catabolism, complex molecules like, proteins, polysaccharides, and fats are broken into small molecules like amino acids, monosaccharides, and fatty acids.

Some of the major or key catabolic processes are as follows:. Citric acid cycle, glycolysis, lipolysis, oxidative deamination, and oxidative phosphorylation are the key catabolic reactions examples that occur in all the eukaryotic cells. Kreb cycle, named after scientist Sir Hans Krebs who discovered it, is also known as the tricarboxylic acid TCA cycle.

Kreb cycle is an 8 step cyclic reaction occurring in the mitochondrial matrix of eukaryotes and cytoplasm of prokaryotes. TCA cycle is the common oxidation pathway for carbohydrates, proteins, and fats. It is also sometimes classified as an amphibolic pathway as it is part of both the catabolic pathway as well as anabolic pathways.

The process of replenishment of the intermediates of the Kreb cycle is known as anaplerosis. Glycolysis is the catabolic process that occurs in all the eukaryotic cells. Breakdown or lysis of glucose to pyruvic acid in aerobic conditions whereas in anaerobic conditions glucose is converted to lactic acid. When the cellular ATP level is low, glycolysis is initiated in the cytosol of the cell. Glycolysis is further divided into two stages:.

Glucose metabolism using this pathway occurs in all the cells of the body. Aerobic glycolysis occurs in the brain whereas anaerobic glycolysis occurs in RBC due to the absence of mitochondria. The breakdown of glycogen initiates the process of glycolysis in human muscles. However, vigorous muscular contractions and exercise induce anaerobic glycolysis.

Lipolysis is the breakdown of triglycerides to yield energy. In this process, triacylglycerol TAG , stored in cellular lipid droplets, undergo hydrolytic cleavage generating non-esterified fatty acids.

These non-esterified fatty acids are subsequently utilized as a substrate for energy production, essential precursors for lipid and membrane synthesis, or cell signaling processes mediators. Lipids or triglycerides are hydrolyzed to free fatty acids and glycerol. The resultant glycerol subsequently becomes the part of glycolysis while the fatty acids formed are further are cleaved by beta-oxidation to release acetyl-CoA.

This acetyl Co-A is the key component of the citric acid cycle. Oxidation of fatty acids releases more energy than carbohydrates. This is because carbohydrates contain more oxygen in their structures. This process has key importance in energy and lipidic homeostasis of the body. The primary enzymes involved in the process of lipolysis are lipoprotein lipase and hormone-sensitive lipase.

Epinephrin , glucagon , or adrenocorticotropic hormone ACTH are the key hormones that stimulate lipolysis. Completely oxidation of fatty acids, particularly the triglycerides, gives the maximum amount of ATP energy per gram basis , and therefore fatty acid is the primary storage form of fuel in most animals. Catabolism of amino acids occurs via transamination and oxidative deamination of amino acid that results in the formation of the metabolizable form of the amino acid.

Oxidative deamination and Transamination are the two key steps involved in protein or amino acid catabolism. Separation of an amino group from the carbon skeleton of amino acids is carried out in the transamination process.

The transamination process is carried out by transaminases or aminotransferases and coenzyme pyridoxal phosphate. The resultant carbon skeleton is eventually utilized in the anabolic process. While in oxidative deamination, removal of the amine group in amino acid results in the formation of a corresponding keto acid.

This reaction occurs in the liver. The amine functional group is replaced by the ketone group and ammonia is formed as a by-product. Eventually, this toxic ammonia is neutralized into urea via the urea cycle.

This process is known as Oxidative phosphorylation and is a chief source of ATP in aerobic organisms. It can also be used to construct the amino acid alanine, and it can be converted into ethanol. Pyruvic acid supplies energy to living cells through the citric acid cycle also known as the Krebs cycle when oxygen is present aerobic respiration ; when oxygen is lacking, it ferments to produce lactic acid.

Pyruvate is an important chemical compound in biochemistry. It is the output of the anaerobic metabolism of glucose known as glycolysis. One molecule of glucose breaks down into two molecules of pyruvate, which are then used to provide further energy in one of two ways. Pyruvate is converted into acetyl- coenzyme A, which is the main input for a series of reactions known as the Krebs cycle. Pyruvate is also converted to oxaloacetate by an anaplerotic reaction, which replenishes Krebs cycle intermediates; also, oxaloacetate is used for gluconeogenesis.

These reactions are named after Hans Adolf Krebs, the biochemist awarded the Nobel Prize for physiology, jointly with Fritz Lipmann, for research into metabolic processes.

The cycle is also known as the citric acid cycle or tri-carboxylic acid cycle, because citric acid is one of the intermediate compounds formed during the reactions.

If insufficient oxygen is available, the acid is broken down anaerobically, creating lactate in animals and ethanol in plants and microorganisms. Pyruvate from glycolysis is converted by fermentation to lactate using the enzyme lactate dehydrogenase and the coenzyme NADH in lactate fermentation. Alternatively it is converted to acetaldehyde and then to ethanol in alcoholic fermentation. Pyruvate is a key intersection in the network of metabolic pathways. Pyruvate can be converted into carbohydrates via gluconeogenesis, to fatty acids or energy through acetyl-CoA, to the amino acid alanine, and to ethanol.

Therefore, it unites several key metabolic processes. Privacy Policy. Skip to main content. Microbial Metabolism. Search for:. Types of Catabolism Catabolism is the set of metabolic processes that break down large molecules. Learning Objectives Summarize various types of catabolism included in metabolism catabolism of carbohydrates, proteins and fats. Key Takeaways Key Points The purpose of the catabolic reactions is to provide the energy and components needed by anabolic reactions.

Microbes simply secrete digestive enzymes into their surroundings, while animals only secrete these enzymes from specialized cells in their guts.